oral basic gas turbine (a&p)
TRANSCRIPT
Oral Basic Gas Turbine
Following are the questions usually asked in orals, they will be updated from time to time, so keep on visiting regularly. Thank you
Questions
Standard Definitions:
MASS: Mass is a quantity of matter in a body. Units MKS – kg; CGS – gm; FPS – lb(mass). Scalar quantity.
MATTER: Anything that occupies space (volume) and have mass
RATIO: One magnitude divided by another magnitude of the same kind
MOLECULE: Molecule is a combination of two or more atoms.
COMPOUND: It is a chemical combination of molecules. E.g. NACL
MIXTURE: It is a physical combination of molecules. E.g. Brine
ALLOY: An alloy is a partial or complete solid solution of one or more elements in a metallic matrix . Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may be homogeneous in distribution depending on thermal (heat treatment) history. Alloys usually have different properties from those of the component elements.
ION: Ions are the charged particle
PLASMA: Plasma is an ionized state. It is a combination of liquid and solid state of matter and occurs at very high temperature
FLUID: Anything that can flow.
COHESION: It is a force of attraction between same type of atoms and molecules
ADHESION: It is a force of attraction between different type of atoms and molecules.
SPECIFIC GRAVITY:
It is the ratio of a density of a substance to the density of water. Density of water is 1000 kg/m3
1 NAUTICAL MILE:
The arc along the circle of the earth if the angle substanded is equal to one minute.
1 KNOT: A knot is one nautical mile covered in one hour. It is a unit of speed and is used in aviation.
MINUTE: In astronomy, the minute is a unit of angle, the minute of right ascension. It is equal to 1/60th of an hour of right ascension and can be further divided into 60 seconds of right ascension.
DISPLACEMENT:
Shortest distance between two locations or points. It is a vector quantity.
DISTANCE: It is the amount of travel. Scalar quantity.
SPEED: Distance covered in a unit time or rate of change of distance. Speed = Distance / time. Units MKS – m/s; CGS – cm/s; FPS – ft/s. Scalar quantity
VELOCITY: Rate of change of displacement. Units MKS – m/s; CGS – cm/s; FPS – ft/s. Vector quantity. V=s/t(Difference between speed and velocity is of distance and displacement)
INSTANTANEOUS VELOCITY:
Velocity at some instance.
ACCELERATION:
Rate of change of velocity. Units MKS – m/s2; CGS – cm/s2; FPS – ft/s2. Vector quantity. a= vf-vi /t.
REST: If a body doesn’t change its position wrt to its surrounding the body is said to be in rest
MOTION: If a body changes its positin wrt to its surrounding the body is said to be in motion.
TYPES OF MOTION:1. Translatory Motion: A motion in which every particle of a body is being displaced by the same amount is called Translatory motion. E.g. a car.2. Rotational Motion: A motion in which a body rotates about a fixed point or axis. E.g. a fan.3. Oscillatory or Vibratory Motion: to and fro motion of a body about a mean position. E.g. Pendulum.
EQUATIONS OF MOTION:1. Vf = Vi + at2. S = Vit + ½ at23. 2as = Vf2 – Vi2
NEWTON’S LAWS OF MOTION:1. First Law of motion: A body at rest will remain at rest and a body in motion will continue its state of uniform motion unless some force is applied. 2. Second Law of motion: When an external force acts on a body it accelerates the body in the direction of force. Acceleration of an object is directly proportional to the force acting on it and inversely proportional to the mass of the object.3. Third Law of motion: To every action there is an equal and opposite reaction.
FORCE: Force is that agent which produces or tends to produce, stops or tends to stop motion in a body. Units MKS – Newton (kg.m/s2); CGS – Dyne (g.cm/s2); FPS – Pound (force) (lb(mass). ft/s2. Vector quantity. F=ma.
WEIGHT: Weight is a force with which earth attracts a body towards its center. Units MKS – Newton (kg.m/s2); CGS – Dyne (g.cm/s2); FPS – Pound (force) (lb(mass). ft/s2. Vector quantity. W=mg.
NEWTON’S LAW OF GRAVITIATION:
Everybody in the universe attracts every other body with a force which is directly proportional to the product of their masses and inversely proportional to the square of distance between them. F=G m1.m2/r2. G=6.67x10 power -11.
SCALER QUANTITIES:
Scalar quantities are those quantities that have magnitude only but no direction. E.g. mass, speed, time, volume, temperature, work, density, distance. Scalars can be added subtracted, multiplied and divided according to ordinary arithmetic rules.
VECTOR QUANTITIES:
Vector quantities are those quantities that have both magnitude and direction. E.g. Force, velocity, weight, displacement, acceleration, momentum. Graphically a vector can be added or subtracted by head to tail rule.
TRIGNOMETERY:1. Sin theta = Prependicular / Hypotenuse2. Cos theta = Base / Hypotenuse3. Tan theta = Perpendicular / Base
ENERGY: Energy is the capacity to do work. It is inherent Power
POWER: Power is the rate of doing work. P= F x D T It is calculated in foot pounds per second Or Watts.
WORK: Act of performing a productive operation by some mechanical means.
TORQUE: Torque is the twisting or rotary force exerted by the engine to turn the Propeller.
RPM: Number of Revolutions per minute
BOYLE’S LAW: Boyle’s law states that at constant temperature, the absolute pressure and the volume of a gas are inversely proportional. PV = K
CHARLES LAW: At constant pressure, the volume of a given mass of an ideal gas is directly proportional to its absolute temperature. V @ T
PRESSURE: Pressure is a force per unit area. Its unit in MKS is N/m2 and FPS is Psi. P=F/A
KINDS OF PRESSURES:
Absolute pressure is the sum of gauge pressure & atmospheric pressure. It is the actual pressure of a fluid on surface because of the force exerted by the molecules. It is zero referenced against a perfect vacuum. It is never negative.
· Gauge pressure is equal to absolute pressure minus atmospheric pressure. It is zero referenced against ambient air pressure, so Negative signs are usually omitted. It can show the negative reading.
· Atmospheric Pressure is the pressure of the outside air which is 14.7 psi at sea level.
· Differential pressure is the difference in pressure between two points.
· Ram or Dynamic Pressure is the pressure of the air or gas cause by motion is called Ram or Dynamic pressure.
· Static Pressure in fluid dynamics, static pressure is the pressure at a nominated point in a fluid. Static pressure is the true pressure of a gas. Aneroid barometer measures static pressure.
· Total Pressure is the sum of Dynamic pressure and Static Pressure.
· Hydrostatic Pressure is the pressure due to the height of the fluid. Its unit in MKS is Pascal (N/m2) and FPS is Psi, also bar. P=egh.
IDEAL GAS LAW: The combined gas law or general gas equation is formed by the combination of the three laws, and shows the relationship between the pressure, volume and temperature for a fixed mass of gas:
Three earlier gas laws:
Boyle's law (1662, relating pressure and volume):
,
Charles' law or law of volumes (1787, relating volume and temperature):
Pressure law or Third gas law (Gay-Lussac in 1809, relating temperature and pressure)
The combined gas law or general gas equation is formed by the combination of the three laws, and shows the relationship between the pressure, volume and temperature for a fixed mass of gas:
There is also Avogadro's Law, which is particularly useful in chemistry: For any gas, the ratio of Liters of the gas to moles of the gas is:
, and uses the molar volume of a gas: 22.4 Liters.
With the addition of Avogadro's law, the combined gas law developed into the ideal gas law:
This law has the following important consequences:
1. If temperature and pressure are kept constant, then the volume of the gas is directly proportional to the number of molecules of gas.
2. If the temperature and volume remain constant, then the pressure of the gas changes is directly proportional to the number of molecules of gas present.
3. If the number of gas molecules and the temperature remain constant, then the pressure is inversely proportional to the volume.
4. If the temperature changes and the number of gas molecules are kept constant, then either pressure or volume (or both) will change in direct proportion to the temperature.
IDEAL GAS: An ideal gas is a theoretical gas composed of a set of randomly-moving point particles that interact only through elastic collisions. The ideal gas concept is useful because it obeys the ideal gas law, a simplified equation of state, and is amenable to analysis under statistical mechanics.
ENTROPY: It is the degree of molecular disorder.
TEMPERATURE:It is the degree of hotness or coldness of a body.
TEMPERATURE SCALES:
Marks Degree Celsius Degree Fahrenheit Degree Kelvin
Min 0 32 273
Max 100 212 373
Parts 100 180 100
Conversion Degree Celsius Degree Fahrenheit Degree Kelvin
Celsius 5/9 (F-32) +273
Fahrenheit 9/5 C + 32 +273
Kelvin +273 +273
Degree Celcius – Degree Farenhiet – Degree Kelvin
COEFFICIENT OF LINEAR EXPANSION: It is change in length per unit length per degree rise in Kelvin.
COEFFICIENT OF VOLUME EXPANSION:It is change in volume per unit volume per degree rise in Kelvin.
DALTON’S LAW of Partial Pressures:
The pressure of a mixture of gases simply is the sum of the partial pressures of the individual components.
PASCAL’S LAW: When a fluid is confined in a container or a system and its pressure is increased or decreased by means of a piston or some other mean, then it is observed that the pressure at every point within the system is changed by the same amount.
PASCAL: Pascal is a force of one Newton on an area of one meter square. N/m2. 14.7psi = 1.103 x 10 power 5 Pascal. One bar is equal to 10 power 5 Pascal.
HOOK’S LAW: Hooke's law of elasticity is an approximation that states that the extension of a spring is in direct proportion with the load added to it as long as this load
does not exceed the elastic limit. Materials for which Hooke's law is a useful approximation are known as linear-elastic or "Hookean" materials.
Mathematically, Hooke's law states that
where
x is the displacement of the end of the spring from its equilibrium position;
F is the restoring force exerted by the material; and
k is the force constant (or spring constant).
OHM’S LAW:In electrical circuits, Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them, provided that the temperature remains constant.[1]
The mathematical equation that describes this relationship is:[2]
KRISCHOFF’S LAW:Kirchhoff's circuit laws are two equalities that deal with the conservation of charge and energy in electrical circuits, and were first described in 1845 by Gustav Kirchhoff. Widely used in electrical engineering, they are also called Kirchhoff's rules or simply Kirchhoff's laws (see also Kirchhoff's laws for other meanings of that term).
COLOUMB’S LAW:The magnitude of the electrostatic force between two point electric charges is directly proportional to the product of the magnitudes of each of the charges and inversely proportional to the square of the distance between the two charges
BUOYANCY FORCE:Buoynacy force is an upward thrust. It depends upon the weight of the volume of the fluid displaced by the body. It is used in hydrometer to determine the specific gravity of the liquids. E.g. battries. Floating bodies have a greater buoyancy force then their weight. Aircrafts that fly due to buoyancy force are balloons and airships.
STREAM LINE FLOW:
When a fluid flows through a duct in such a way that there is no turbulence in the flow, the flow is said to be streamline.
VENTURI: Venturi is a streamline duct through which air will flow without turbulence. The bore of venture converges upto throat and diverges towards the outlet.
CONTINUITY OF FLOW:
Fluids in steady motion pass each cross-section of the streamline duct in identical amount in each second.Continuity of flow is when the mass flow rate is constant
m = eAV e=density, A=Area, V=Velocity.
BERNAULI’S THEOREM:
In streamline flow of ideal fluid (invicid – nonviscous fluid), the sum of Kinetic Energy, Potential Energy and Pressure Energy remains same.
BRAYTON CYCLE:The Brayton cycle is a thermodynamic cycle that describes the workings of the gas turbine engine, basis of the jet engine and others. It is named after George Brayton (1830–1892), the American engineer who developed it, although it was originally proposed and patented by Englishman John Barber [1] It is also sometimes known as the Joule in 1791. cycle.The term Brayton cycle has more recently been given to the gas turbine engine. This also has three components:
A gas compressor A burner (or combustion chamber) An expansion turbine
Ideal Brayton cycle:
isentropic process - Ambient air is drawn into the compressor, where it is pressurized.
isobaric process - The compressed air then runs through a combustion chamber, where fuel is burned, heating that air—a constant-pressure process, since the chamber is open to flow in and out.
isentropic process - The heated, pressurized air then gives up its energy, expanding through a turbine (or series of turbines). Some of the work extracted by the turbine is used to drive the compressor.
isobaric process - Heat Rejection (in the atmosphere).
Actual Brayton cycle:
adiabatic process - Compression. isobaric process - Heat Addition. adiabatic process - Expansion. isobaric process - Heat Rejection.
HORSE POWER: Horse Power is a unit of Power. One Horse Power is 550 foot-pounds of work accomplished in one second. (375 mile pound per hour = 550 ft. lbs / sec = 33,000 ft. lbs / min). Both time and distance are necessary to compute Horse Power.
The term, Horse Power is not used for turbo-fan or turbo-jet engines because time and distance elements are not always involved, since when a turbo-jet or turbo-fan is not moving forward as like a plane standing on ground with engines running, time and distance elements are zero.
POWER OF A GAS TURBINE ENGINE:
Power of a gas turbine engine can only be calculated if the aircraft is moving, when thrust is opposing drag & propelling the aircraft at a constant speed. Power of a gas turbine engine is given by:Power = Drag (lbs) X Aircraft speed (ft / sec)OR
Since at a constant aircraft speed Thrust = Drag so,Thrust Horse Power = Thrust (lbs) x Aircraft Speed (ft / sec) 550 ft. lbs / sec
375 mile pound per hour = 550 ft. lbs / sec = 33,000 ft. lbs / min
Power of a Gas Turbine engine is calculated as THRUST HORSE POWER, because this engine delivers power through thrust generated by the reaction force.
GAS TURBINE ENGINES:
Gas Turbine Engines are simple heat engines that convert heat energy of fuel into mechanical work.
They are machines which give momentum to the mass of air & fuel.
GAS GENERATOR:
A gas generator is a gas producing section of a gas turbine engine. It excludes inlet duct, propelling nozzle of a turbo jet & propeller shaft and reduction gear of turbo-prop engines.
THRUST: Thrust is a forward acting force and is a reaction force to the force applied to accelerate the mass of air rearward in case of a gas turbine engine. It is measured in pounds.
MOMENTUM THRUST:
Momentum Thrust is the Majority of the thrust. It is obtained by the change of momentum of gasses within an engine.
PRESSURE THRUST:
Pressure thrust is an additional thrust obtained when the engine operates with the propelling nozzle in a choked condition. It is obtained by the pressure difference at the propelling nozzle and the outside atmosphere.
Pressure Thrust = (Pressure at the jet nozzle Pj – Ambient Pressure Pam) x Jet nozzle area.
GROSS THRUST:
Pressure Thrust added to Momentum thrust provides Gross Thrust.
THRUST HORSE POWER:
Thrust Horse Power is defined that at an aircraft speed of 550 ft. lbs / sec the thrust of one pound is equal to one horse power.
Power of a Gas Turbine engine is calculated as THRUST HORSE POWER, because this engine delivers power through thrust generated by the reaction force.
For a Turbo-jet & turbo-fan engine:
Thrust Horse Power = Thrust (lbs) x Aircraft Speed (ft / sec) 550 ft. lbs / sec
375 mile pound per hour = 550 ft. lbs / sec = 33,000 ft. lbs / min
Thrust Horse Power is proportional to both engine thrust and aircraft speed. If the aircraft speed is zero THP is also zero. Likewise if aircraft speed for a given thrust is doubled, THP is also doubled.
For a Turbo-prop engine:
Thrust Horse Power = Shaft Horse Power x Propeller efficiency
Thrust horse power developed by a turbo-prop will always be less than its Shaft horse power because the propeller is less than 100 percent efficient (it is usually 80 % efficient), converting only part of the horse power output into thrust.
SHAFT HORSE POWER:
Shaft Horse Power is the measure of power supplied to the propeller in a turbo-prop-engine.One SHP supplied to the propeller is assumed to produce 2.5 pounds of thrust.
EQUIVALENT SHAFT HORSE POWER:
Equivalent Shaft Horse Power is the power produced by a Turbo-prop engine.
Equivalent Shaft Horse Power is Shaft Horse Power supplied to the propeller plus the amount of thrust produced by the engine.
The static Equivalent Shaft Horse Power is the static jet thrust in pounds divided by 2.5 plus the SHP supplied to the propeller.
ESHP (static) = SHP + Fn (Jet) 2.5
EFFICIENCY: Efficiency is the effectiveness with which a machine, piece of equipment, process or a person operates.
It is the ratio of the energy obtained from a machine to the energy put into the machine.
ENGINE EFFICIENCY:
Engine efficiency is the over-all efficiency of an engine which is usually between 70-80%. Each part of an engine such as compressor, combustion chamber, turbine, jet nozzle has its own efficiency. All of these combine to produce one single Over-all or Engine efficiency. To assess the engine or over-all efficiency of an engine Thermal or internal efficiency and Propulsive or external efficiency of the installed engine must be considered. Over –all Efficiency is a product of Thermal efficiency and Propulsive efficiencyOver-all efficiency = Thermal efficiency x Propulsive efficiency
MECHANICAL EFFICIENCY:
Mechanical efficiency is the ratio of the useful work output of a machine to the work or energy input.
Mechanical efficiency = Useful work output Work Input
The difference between the two values is chiefly due to the mechanical frictional losses and losses like air leakages etc.
THERMAL EFFICIENCY:
Thermal efficiency of an engine is the ratio of useful work output to the heat of combustion of the fuel.
Thermal efficiency = Useful work output Heat of combustion of the fuel
Thermal efficiency is affected by the temperature drop across the turbine.The more is the Turbine Entry Temperature (TET), the more will be the energy to do work or more will be the Thermal Efficiency.
Thermal efficiency (Or Turbine Entry Temperature TET) is a function of (or depends
on): 1. Pressure Ratio 2. Mass of Airflow 3. Temperature to which the air is heated.
OPERATING CYCLE EFFICIENCY:
Operating cycle efficiency is the ratio of the amount of useful work obtained from a jet engine’s actual cycle to the amount of useful work obtained from the same ideal cycle.Actual efficiency is always less than the ideal.
PROPULSIVE EFFICIENCY:
Propulsive efficiency can be defined as that proportion of the engine work that can be converted into aircraft work.
Propulsive efficiency can also be defined as the amount of thrust developed by the jet nozzle compared with the energy supplied to it in the usable form.
Propulsive efficiency is related to an engine installed on the airframe.
Propulsive efficiency indicates how effective an engine is as a propelling unit. If the aircraft is stationary, regardless of the amount of thrust produced, the fuel consumed is wasted as far as the aircraft propulsion is concerned, infact the propulsive efficiency would be zero. But if aircraft speed becomes equal to the jet speed the Propulsive efficiency would be 100%. Thrust will be zero in that case.
Propulsive efficiency increases as the difference between the aircraft speed and the jet velocity decreases. The faster the aircraft flies the closer the jet speed and the aircraft become, and the energy put into the jet stream performs more useful work.
Propulsive efficiency is said to increase when at higher throttle settings the nozzle becomes choked so there is a very little increase in the jet velocity (Vj), so the gap between the jet velocity and the aircraft speed narrows.
Propulsive efficiency = Work done on the aircraft x 100% Work done on the gas stream
Or,
Propulsive efficiency = twice aircraft speed x 100% aircraft speed + Jet speedprop. eff = 2Vi/(Vj+Vi)
Question: Why does Propulsive efficiency increases when difference between the aircraft speed and Jet speed decreases? Answer: Because the purpose of the engines is to propel the aircraft as much as it can. The more they are closer to their purpose the more will be the propulsive efficiency.
SPECIFIC FUEL CONSUMPTION (SFC) OR THRUST SPECIFIC FUEL CONSUMPTION (TSFC):
Specific Fuel Consumption (SFC) is the amount of fuel required to generate one pound of thrust in one hour for a turbo-jet & turbo-fan engines.TSFC = fuel flow in lbs / lb of thrust / hour
Specific Fuel Consumption (SFC) is the amount of fuel required to generate one Shaft Horse Power (SHP) in one hour for a turbo-prop engine.SFC = fuel flow / SHP / hour
Specific Fuel Consumption (SFC) is the measure of an engine’s thermal efficiency and performance. Low Specific Fuel Consumption (SFC) means better thermal efficiency. It depends on compressor / turbine efficiencies.
EQUIVALENT SPECIFIC FUEL CONSUMPTION (ESFC):
It is the rate of fuel flow in pounds per hour divided by a turbo-prop’s ESHP.
Turbo-props cannot be compared on the basis of TSFC, EQUIVALENT SPECIFIC FUEL CONSUMPTION (ESFC) is therefore used instead
FACTORS AFFECTING THE SPECIFIC FUEL CONSUMPTION:
· SFC & RPM:SFC is high at reduced RPM, because the compressor / turbine efficiency is poor.SFC improves with the increasing RPM and is lowest in the designed cruising speed RPM.
SFC & Forward speed of an Aircraft:As the aircraft speed increases the Intake momentum drag also increases thus decreasing the thrust, but on the other hand there is more MAF due to Ram Pressure, and necessities more fuel energy to compress the mass, consequently SFC rises.
SFC & Altitude:With an increase in the altitude the SFC improves or decreases, because the compressor and turbine efficiencies are high at higher altitude. At higher altitude there is less density and lesser Intake momentum drag. Thus an aircraft needing 80% throttle setting to maintain a speed of 350 knots at 15,000 ft. may only require 65% of the throttle setting at 28000 ft. to keep the same speed.When the conditions are colder at higher altitude, for the same Pressure Ratio, the compressor has to do a lesser work on the MAF that has a little volume thus less fuel is required.
SFC & Pressure Ratio:SFC decreases as the Pressure Ratio increases. With more Pressure at the outlet of the compressor means more addition of the fuel, but a higher thrust is obtained as compared to the addition of the fuel. The pounds of thrust obtained by the addition of the pounds of fuel is high.
SFC & Temperature:For the same thrust to be obtained as on a standard day (15 deg. C), on a colder day, the
compressor has to do a lesser work on MAF for the required Pressure rise thus less fuel is required, SFC improves, but on a hot day the compressor will need more MAF to compress and has to do more work for the required Pressure rise, thus more fuel is required, SFC increases.
BYPASS ENGINE:
Bypass engine is a dual flow system engine in which the single incoming flow of air is divided in two flows, one flow passes through the core engine and the other is bypassed through a bypass duct which is an annular space between the core and the outer casing.
Advantages of a bypass engine:
· Higher propulsive and overall efficiency
· Low specific fuel consumption
· Lower noise level due to reduced velocity of the jet stream
· Lighter core engine. Less mass flow to handle
Dis-advantages of a bypass engine:
· Extra weight involved in the LP section that has to handle a greater mass airflow in order to develop the same thrust.
Question: How does a Turbo-fan has a better propulsive efficiency than a Turbo-jet engine?Answer: Turbo-fan has a better propulsive efficiency than a turbo-jet because it has a lighter core that handles a lesser mass airflow which gives reduced jet stream velocity, while most of the thrust is developed by the fan which handles a greater mass moving it rearward slowly.
BYPASS RATIO: The ratio of the cold stream to the hot stream. The ratio of the secondary air to the Primary air by weight is called Bypass Ratio.
FACTORS AFFECTING MASS OF AIRFLOW:
· Design of the compressor
· Compressor RPM or speed (more RPM more MAF)
· Density (more Density more MAF)
· Temperature (more Temperature less MAF)
· Altitude (more Altitude less Density less MAF)
· Forward speed of the aircraft (more Forward speed more Ram effect and more MAF)
FACTORS AFFECTING ACCELERATION OF AIRFLOW:
· Amount of fuel burnt in the combustion chamber. (more fuel burnt more RPM and hence more acceleration imparted to the MAF by the fast moving compressor)
· Choking or the limitation of compressor to handle the MAF. No more acceleration can be imparted to the MAF after the compressor begins to choke.
FACTORS AFFECTING THRUST:
· Forward speed of the aircraft:Net thrust decreases with the forward speed of an aircraft due to intake momentum drag.Net Thrust = Gross Thrust – Intake Momentum DragNet thrust is Maximum when aircraft is stationary with engines running, or it is said to be equal to Gross Thrust. Gross Thrust does not take into account the Intake Momentum Drag.Also at higher throttle settings the nozzle becomes choked and there is a vey little increase in the jet velocity (Vj) in comparison with the forward aircraft speed (Va). The difference between the two velocities narrows and the thrust output decreases, while on the other hand the propulsive efficiency increases.The nozzle becomes choked because of the high EGT that increases the speed of sound and thus the exhaust gas velocity reach sonic thus choking the nozzle.
· Jet Nozzle Velocity:Thrust increases as the Jet Velocity (Vj) increases, however at high throttle settings the nozzle does becomes choke due to the high EGT that increases the speed of sound and thus the exhaust gas velocity becoming sonic thus choking the nozzle. After this an increase in the Jet velocity is only possible if the EGT is further increased.
· Engine RPM:Thrust increases with the increase in RPM. In the higher RPM range when the compressor and turbine efficiencies are high, i.e. In the designed operating range, there is a large increase in thrust for a relatively small RPM increase.
Majority of the thrust is obtained in the last 25% of the RPM change.
· Mass of Airflow:Thrust increases with the increase in MAF
· Ram Effect:The pressure of the air or gas cause by motion is called Ram pressure or Ram Effect.As the aircraft speed increases the dynamic or Ram pressure of the air in the engine intake increases. With the increases in air density the MAF through the engine will rise. The result will be an increase in GROSS THRUST as the forward speed increases.
With the increase in the forward speed of an aircraft the Intake momentum Drag and Ram Effect both raise at the same time and will tend to cancel each other. Ram also compensates the loss of thrust due to intake momentum drag because of reduced pressure at high altitude. Net thrust increases as the forward speed rises above 300 mph. Under subsonic
conditions, Ram does not affect the engine thrust as much as it affects the thrust in supersonic conditions.
· Altitude:Thrust decreases with increasing altitude.There are two things with the increase in altitude:1. the density decreases thus reducing the thrust.2. The temperature drop thus increasing the MAF and thus increasing the thrust.The overall compensating effect is that the thrust falls with an increase in altitude.
Though the reduction of thrust is compensated by the decrease in temperature till only 36090 ft. after which the temperature remains constant while density continues to fall thus making the fall ion thrust more pronounced.
If only engine is considered, this makes 36000 ft the optimum altitude for long range cruising at nominal airspeeds.
§ Air Density and the effect of Temperature and Pressure.Thrust increases as density increases. Density fall with the increases in altitude and increase in Temperature. Density is the number of molecules per cubic feet or mass per unit volume.Density affects thrust proportionally.
Thrust increases with the fall in temperature i.e. on cold days and on higher altitudes, but decreases on a hot day when temperature is high. Higher temperature means reduced air density, & Lower temperature means higher density.
Thrust increases with an increase in Pressure.
Unless an engine has a variable inlet-area, the MAF into the engine at a given RPM is determined by the density of air going into the compressor.
When the pressure (Ram) increases with an increase in the airspeed or decreased altitude, density increases.
Ambient Temperature:1. Reduced Ambient Temperature – Thrust IncreasesWith the reduced OAT the density of air rises thus increasing the weight of the air or the MAF. This will put the compressor under load and it will run at reduced RPM but at constant thrust.To maintain constant RPM, under this condition we need more fuel flow and thrust will increase. (Note that the MAF is also high)2. Increased Ambient Temperature – Thrust DecreasesWith the increased ambient temperature the density of air decreases thus decreasing the weight of the air or the MAF. This will put the compressor off-load and it will run at higher RPM and constant thrust.To maintain (decrease) the RPM to constant, we need to decrease the fuel flow. This will cause the thrust to drop. (Note that the MAF is also less).
RAM EFFECT: The pressure of the air or gas cause by motion is called Ram pressure or Ram Effect.
RAM RATIO: The pressure rise in the intake due to Ram Effect. It is the Ratio of the Compressor Inlet pressure to the Ambient Pressure.
Ram Ratio = P1 P0
RAM RECOVERY:
The amount of Static Pressure recovered from the moving airstream or Ram by the Intake is called Ram recovery.
TOTAL RAM RECOVERY OR TOTAL PRESSURE RECOVERY:
If all of the available Ram pressure is converted into Static Pressure, it is called Total Ram Recovery.
RAM TEMPERATURE RISE:
The temperature rise due to Ram Compression Effect is called Ram Temperature Rise.
FULLY RATED ENGINE:
A Fully rated engine is one which will give maximum thrust at a specified OAT, after this specified OAT the thrust will decrease. Lets say that it will give maximum thrust at 25 degree C. Fully Rated Engines will give maximum Thrust at ISA condition, when the throttle is full. In fully rated the EGT does not exceed its maximum limit at full throttle.
FLAT RATED ENGINE:
A flat rated engine gives maximum thrust upto a specified band of OAT by increasing the fuel flow and after the range of the band, the thrust starts decreasing. Its RPM cannot be further increased by a further increase in fuel flow because of the surge factor.
By adjusting fuel flow (decreasing it) in the flat rated engine, it could be derated from a higher thrust at a given band of temperature to a lower thrust for the same band of temperature. This is done to enhance the engine life. In flat rated engines the EGT may exceed than its prescribed limit at full or rapid throttle movement.
FREE TURBINE: It is a turbine which alone rotates the single staged Fan.
VARIABLE GEOMETERY DUCT:
A Variable Geometery Duct is used in very high speed supersonic aircrafts. In such type of duct the inside area or the configuration of the duct is changed by a mechanical device as the speed of the aircraft increases or decreases.
The geometry may be changed by a movable spike within the duct, or by incorporating some form of movable restriction such as ramp or wedge inside the duct, or by the use of variable airflow bypass system which extracts part of inlet airflow from the duct ahead of the engine.
Devices of this type are rigged to operate without the attention of the pilot. Variable geometry method eliminates Buzz.
SHOCKWAVE: Shockwave is a thin region of discontinuity in a flow of air or gas, during which the speed, pressure, density and temperature undergo a sudden change.
A shockwave is intentionally setup in the supersonic flow of air entering the duct by means of some restriction of small obstruction. The shockwave results in the diffusion of the airflow and its velocity is decreased.
BUZZ: Buzz is airflow instability and occurs when a shockwave is alternately swallowed and regurgitated by the inlet. It occurs at high Mach numbers and can be avoided by changing the amount of inlet area variation that takes place when variable geometry inlet system is in operation.
A Buzz can cause violent fluctuations in pressure through the inlet which may result in damage to the inlet structure or possibly to the engine.
CASCADE EFFECT:
In Cascade effect before the air can completely separate from the trailing edge of one blade is affected by the leading edge of blade which follows. Cascade effect is the primary consideration in determining the aerofoil section, angle of attack, and the spacing between blades to be used for the compressor blade design for any given axial compressor.
COMPRESSOR MAP:
A compressor Map is a tool to visualize compressor performance by the designer & the operator. Pressure Ratio developed across the compressor is plotted against the corrected weight of airflow (MAF) through the unit.
COMPRESSOR SURGE & BLADE STALL & CHOKING:
· Compressor Blade Stall: Compressor Blades are tiny aerofoil just like the aerofoil of a wing. An aircraft wing stalls when the aircraft flies below the stalling speed. That is the Relative Airflow Speed is too (MAF is low) low. We need to increase the angle of attack to avoid stalling upto 15 degrees with the help of flaps and slats.
Similarly, when the MAF in a compressor is too low the blade stalls. The angle of attack tends to increase above 15 degree where the lift is completely destroyed.
The word stall applies to the instability of airflow on a compressor blade or a stage.
· Compressor Surge (Axi-Symmetric Stall):Axi-Symmetric stall or Compressor or pressure surge is a complete breakdown in compression within a compressor resulting in a reversal of air flow.
Surge results from a condition of instable airflow within a compressor. This condition occurs when there is less MAF within the compressor while the RPM are very high. The air piled up in the rear stages of the compressor tends to have a reverse flow following the low MAF in the front stages of the engine. The situation is such that flame in the combustion chamber also finds an area of low pressure in the front stages of the compressor and tends to move towards that area, resulting in a fire situation in the compressor. It’s Oxygen in the air within the compressor that burns.
· Choking: When the compressor is not operating at its optimum rpm while the MAF within it is very high. The forward compressor blades are not be able to bite off enough air to be able to compress it sufficiently and to force it on through the rear stages of the compressor.
Commonly Asked Questions:
Q. WHAT DO WE DO IN THE COCKPIT BEFORE AN ENGINE REMOVAL?
We pull out the squib CB and pull the fire handle. The fire handle is pulled so that no fuel or hydraulic or electrical supply be available to the engine, and Squib CB is pulled so that fire bottle be not discharged on pulling the fire handle.
Q. WHAT ARE THE GENERAL PRECAUTIONS FOR AN ENGINE REMOVAL?
1. BEFORE ATTEMPTING MAINTENANCE OPERATIONS ON THE FUEL SYSTEM MAKE CERTAIN THAT FIRE EXTINGUISHING EQUIPMENT IS READILY AVAILABLE IN PROXIMITY TO WORK AREA.
2. CHECK THAT LANDING GEAR GROUND SAFETIES INCLUDING WHEEL CHOCKS ARE IN POSITION.
3. IN ORDER TO AVOID DAMAGE TO LINES, PNEUMATIC DUCTS AND ELECTRICAL CONNECTORS AND TO PREVENT COMTAMINATION BY FOREIGN BODIES, BLANK OFF THE LINES, DUCTS AND CONNECTORS WITH PROTECTIVE CAPS.
4. B_E_F_O_R_E_ _P_R_O_C_E_E_D_I_N_G_ _W_I_T_H_
_M_A_I_N_T_E_N_A_N_C_E_ _W_O_R_K_ _O_N_ _O_R_ _N_E_A_R_ MECHANICAL FLIGHT CONTROLS OR PRIMARY FLIGHT CONTROL SURFACES, LANDING GEARS, ASSOCIATED DOORS OR ANY MOVING COMPONENT, MAKE CERTAIN THAT GROUND SAFETIES AND/OR WARNING NOTICES ARE IN CORRECT POSITION TO PREVENT INADVERTENT OPERATION OF CONTROLS.
5. B_E_F_O_R_E_ _P_O_W_E_R_ _I_S_ _S_U_P_P_L_I_E_D_ _T_O_ _T_H_E_ _A_I_R_C_R_A_F_T_ MAKE CERTAIN THAT ELECTRICAL CIRCUITS UPON WHICH WORK IS IN PROGRESS ARE ISOLATED.
6. B_E_F_O_R_E_ _P_R_E_S_S_U_R_I_Z_I_N_G_ _H_Y_D_R_A_U_L_I_C_ _S_Y_S_T_E_M_S_, MAKE CERTAIN THAT HYDRAULIC SYSTEM UNDER MAINTENANCE HAS BEEN ISOLATED.
7. MAKE SURE THAT PYLON AFT HOIST FITTINGS ARE IN GOOD CONDITION BEFORE YOU INSTALL AND AFTER YOU REMOVE THE BOOTSTRAP REAR MAIN BEAM.
8. OBSERVE SAFETY PRECAUTIONS WHEN WORKING ON THE HYDRAULIC SYSTEM. LONG EXPOSURE TO HYDRAULIC FLUID CAN CAUSE SKIN DEHYDRATION AND CHAPPING. REF. 29-00-00.
9. CHECK AIRCRAFT STABILITY. 05-57-00
Q1. WHAT ARE THE TWO POSITIONS OF A THRUST REVERSER?
Stow and Deployed
Q2. IF A THRUST REVERSER IS NOT PROPERLY RIGGED. WHAT WILL HAPPEN?
A. The FADAC will be sensing that the Thrust Reverser is in deployed position and will not allow an power output by the engine when throttle is moved forward.
Q3. WHAT IS THE DIFFERENCE BETWEEN DRY MOTORING AND WET MOTORING?
In Dry motoring an engine is cranked via the bleed air taken from the APU to the engine. And in wet motoring HP fuel valve is opened momentarily during cranking just to pressurize the fuel lines for the purpose of leak test.
Q4. WHAT IS MIC?
Major Item change sheet.
Q5. WHAT DOES A NACELLE COMPRISES OF?
AN engine nacelle comprises of air-inlet, fan cowl, reversal cowl. core cowl.
Q6. WHAT PRECAUTIONS BE TAKEN BEFORE THE REMOVAL OF NOSE COWL, FAN COWL, FAN-REVERSER COWL AND EXHAUST NOZZLE?
Q7. WHICH LIGHTS ILLUMINATE IN STOW AND DEPLOYED POSITION OF THRUST REVERSERS?
Deploy Position – Green LightStow Position – No light.
Q8. PRECAUTIONS WHEN BRINGING AN AIRCRAFT IN HANGAR?
A good care must be take during towing an aircraft to hanger ..... 2 mechanics on each wing ... 1 to the tail.....1 in cockpit ...........
the steering system to be unlocked and breaks should be working properly.
nose steering by-pass pin... landing gear grnd lock pin... pressurize hyd B for brakes... anti-collision light
Q9. WHAT ARE WARNINGS, CAUTIONS AND NOTES?
W_A_R_N_I_N_G_ : CALLS ATTENTION TO USE OF MATERIAL, PROCESSES, METHODS, PROCEDURES OR LIMITS WHICH MUST BE FOLLOWED PRECISELY TO AVOID INJURY OR DEATH TO PERSONS.
- C_A_U_T_I_O_N_ : CALLS ATTENTION TO METHODS AND PROCEDURES WHICH MUST BE FOLLOWED TO AVOID DAMAGE TO EQUIPMENT.
- N_O_T_E_ : Calls attention to methods which make the job easier or provide supplementary or explanatory information
Q10. WHAT ARE SCHEDULED AND UN-SCHEDULED MAINTENANCE CHECKS
S_c_h_e_d_u_l_e_d_ _M_a_i_n_t_e_n_a_n_c_e_ _C_h_e_c_k_s_ (05-20-00)
The initial Scheduled Maintenance Checks are those prescribed by the Maintenance Review Board Report (MRBR).
U_n_s_c_h_e_d_u_l_e_d_ _M_a_i_n_t_e_n_a_n_c_e_ _C_h_e_c_k_s_ (05-50-00)
The Unscheduled Maintenance Checks section covers Maintenance Checks to be performed whenever a flight in abnormal conditions has been reported by the flight crew. This section has been divided into two categories of information :
- Inspections
- Checks
Q11. WHICH CHAPTER OF THE AMM TELLS US ABOUT THE TIME LIMITS AND MAINTENANCE CHECKS?
Chapter 00
Q12. LIST THE ATA CHAPTERS?
AIRCRAFT GENERAL CHAPTER
Time Limits/Maintenance Checks .......................... 5Dimensions & Areas ...................................... 6Lifting and Shoring ..................................... 7Leveling & Weighing ..................................... 8Towing & Taxiing ........................................ 9Parking & Mooring ....................................... 10Placards & Markings ..................................... 11Servicing ............................................... 12AIRFRAME SYSTEMSStandard Practices - Airframe ........................... 20Air Conditioning ........................................ 21Auto Flight ............................................. 22Communications .......................................... 23Electrical Power ........................................ 24Equipment/Furnishings ................................... 25Fire Protection ......................................... 26Flight Controls ......................................... 27Fuel .................................................... 28Hydraulic Power ......................................... 29Ice & Rain Protection ................................... 30Indicating/Recording Systems ............................ 31Landing Gear ............................................ 32Lights .................................................. 33Navigation .............................................. 34Oxygen .................................................. 35Pneumatic ............................................... 36Water/Waste ............................................. 38Airborne Auxiliary Power ................................ 49
STRUCTUREStructures .............................................. 51Doors ................................................... 52Fuselage ................................................ 53Nacelles/Pylons ......................................... 54Stabilizers ............................................. 55Windows ................................................ 56Wings ................................................... 57
POWER PLANTStandard Practices - Engines ............................ 70Power Plant ............................................. 71Engine .................................................. 72Engine Fuel and Control ................................. 73Ignition ................................................ 74Air ..................................................... 75Engine Controls ......................................... 76Engine Indicating ....................................... 77Exhaust ................................................. 78Oil ..................................................... 79Starting ................................................ 80
Q13. WHAT IS A CHAPTER BREAKDOWN POLICY?
Each Chapter/System is broken down into sections/subsystems (combinations of functional/physical groups).
Example :
- 29-00-00 - Hydraulic Power - General
- 29-10-00 - Main
- 29-20-00 – Auxiliary
29-00-00 CHAPTER (hydraulic)
29-10-00 SECTION / SUBSYSTEM (combinations of functional/physical groups).
29-11-00 SUB-SUBSYSTEM (main)
29-11-11 SUBJECT (unit or component)
Each Section/subsystem is broken down into sub-subsystems (Installations/Circuits). Example :
- 29-10-00 - Main
- 29-11-00 - Green Main Hydraulic Power
- 29-12-00 - Blue Main Hydraulic Power
- 29-13-00 - Yellow Main Hydraulic Power
Each sub-subsystem is divided into subjects. Each subject represents a unit or component. Example :
- 29-11-00 - Green Main Hydraulic Power
- 29-11-11 - Green Assembly Reservoir
- 29-11-12 - Hydraulic Reservoir
- 29-11-13 - Green Hydraulic Pump
N_O_T_E_ : The subjects 01 to 08 are used in the Illustrated Parts Catalog (IPC) only in order to split the sub-subsystems of the aircraft into zones.
Q14. WHAT IS DTMSRAICAD?
(D) Description and Operation – Page 001-099(T) Trouble shooting (Refer to TSM) – Page 101-199(M) Maintenance Practices – Page 201-299(S) Servicing – Page 301-399(R) Removal and Installation – Page 401-499(A) Adjustment Tests - Page 501-599(I) Inspection Checks – Page 601-699(C) Cleaning and Painting – Page 701-799(A) Approved Repairs – Page 801-899(D) Deactivation and Reactivation - Page 901-999
N_O_T_E_ : When the quantity of pages for any one sub-heading will exceed 99 the next pages will be numbered :
- 99, A00, A1, A2, A3 etc...
- 599, A500, A501, A502, A503 etc...
Q15. Functional Item Numbers (FIN)
Equipment on the aircraft is generally allocated a unique identifier known as a Functional Item Number (FIN).
(2)Mechanical FIN
Mechanical equipment is identified by 6 numerals, the first two are the
ATA Chapter/System prefix and the last four the equipment number.
A typical mechanical FIN is 271198, where :
27 : ATA Chapter/System (Flight Controls in this example)
1198 : equipment number
N_O_T_E_ : The third numeral may identify a specific system to which an equipment belongs.
e.g. 291XXX = Hydraulic System - Green
292XXX = Hydraulic System - Blue
293XXX = Hydraulic System - Yellow
Q16. TYPES OF ADJUSTMENT TESTS?
e)Adjustment/Test (A/T) (Pages 501 to 599)
Test information is divided into three categories - operational test,
functional test, system test.
Below are definitions of the three categories :
1_ Operational test
This test is required to ascertain that an item (system, subsystem component) is fulfilling its intended purpose. It does not require quantitative tolerances and it can include readings using aircraft instruments. This test requires no special equipment or facilities other than that installed on the aircraft and is comparable to the tests performed by the flight crews. It is not intended that the operational test of the unit shall meet the specifications and tolerances ordinarily established for overhaul, or major maintenance periods. A test can be carried out where appropriate, with ground hydraulic, electrical and/or air conditioning connections made to the aircraft.
2_ Functional test
This test is required to ascertain quantitatively that a system or unit is functioning in all aspects in accordance with minimum acceptable system or unit design specifications. This test may require supplemental ground support equipment and be more specific and detailed than an operational test. It contains all necessary information to perform proficiency tests to maintain system or unit reliability at an acceptable level without reference to additional documents.
3_ System test
This test contains all adjustment specifications and tolerances required to maintain system and/or unit performance at maximum efficiency and design specifications. It is self-contained and may duplicate other tests.
Q17. WHAT EFFECT OF OAT ON ENGINE THRUST?
1. Reduced Ambient Temperature – Thrust IncreasesWith the reduced OAT the density of air rises thus increasing the weight of the air or the MAF. This will put the compressor under load and it will run at reduced RPM but at constant thrust.To maintain constant RPM, under this condition we need more fuel flow and thrust will increase. (Note that the MAF is also high)2. Increased Ambient Temperature – Thrust DecreasesWith the increased ambient temperature the density of air decreases thus decreasing the weight of the air or the MAF. This will put the compressor off-load and it will run at higher RPM and constant thrust.To maintain (decrease) the RPM to constant, we need to decrease the fuel flow. This will cause the thrust to drop. (Note that the MAF is also less).
Q18. WHAT IS A HEAT ENGINE? WHAT ARE ITS TYPES?
A heat engine is a device which converts thermal energy into mechanical output. Gas Turbine Engines are simple heat engines that convert heat energy of fuel into mechanical work.
Q19. WHAT ARE ANTI-STALL DEVICES?
VBV, VSV, Twin Spool, IGV’s, Bleed valves
Q20. HOW DOES VBV’s AND VSV’s WORK? WHAT IS THE FEEDBACK OF THEIR OPERATION?
VBV’s and VSV’s work automatically and in relation with each other. At high power setting when there is a need of more airflow the VBV’s are closed so as to provide as much of the airflow to the rear compressor stages as it can while the VSV’ are at their optimum angle (open) to facilitate the airflow to the later stages.
But at low power settings VBV’s are open to bleed off the excessive airflow to avoid surging and VSV’s are closed (means at the zero degrees of their angle).
Q21. WHAT IS A RAM JET, PULSE JET AND ROCKET MOTOR? HOW DO THEY WORK?
Ram Jet, Pulse Jet and Rocket motors are not gas turbine engines but they work on the same principle that is the reaction propulsion. They could be called members of reaction family engines.
Ram Jet: Ram jet is the simplest jet engine and does not have any moving parts. It is only a large open-ended piece of pipe with a fuel injection and fuel metering system. Ram jet relies upon the ram effect to build up the pressure of the air entering the engine to the amount that will enable the engine to operate. Hence a Ram jet must be carried aloft and accelerated to operating speed by some means other than its own thrust. It may ride piggy-back on a rocket to operational altitude or it may be borne to the proper height and speed as a dropable external store on a conventional airplane, which might be the case for an air-to-air and air-to-ground ram-jet missile.
It is a breathtaking engine.
Pulse Jet: A pulsejet is a ramjet with an air inlet which is provided with a set of shutters that is spring loaded to remain in the closed position. After a pulse jet is launched, ram air pressure forces the shutters to open, fuel is injected into the combustion chamber, and is burned. Ignition is intermittent and goes on and off as the shutters open and close. The gases produced by combustion are forced out of the jet nozzle by the pressure that has build up within the combustion chamber. The acceleration of the gases through the nozzle generates thrust.
When the pressure in the combustion chamber is less than the ram pressure the shutters open admitting more air and the cycle repeats itself.
Pulse jets may be started and operated at a considerably lower speeds than Ramjets, and it is possible to design a Pulse jet that would hardly require any initial velocity. Pulse Jet was used as Buzz Bombs by the Germans in the second world war.
It is a breathtaking engine.
Rocket Motor: A rocket motor or a rocket operates on jet propulsion principle and carries its own fuel and an oxidizer to burn with the fuel within itself or aboard the vehicle that the rocket propels. A rocket motor is not a breathtaking engine and can operate in complete independence in outer space.The fuel and the oxidizing agent together, constitute the propellant. Solid fuel or propellant, motors carry the propellants stored in their combustion chamber, while in case of liquid propellant, it may be stored in tanks that is piped to the combustion chamber.
Q22. FUNCTIONS OF NOSE COWL.
First, it must be able to recover as much of the total pressure of the free airstream as possible and deliver this pressure to the front of the compressor with minimum loss.
Secondly, it must deliver air to the compressor inlet under all flight conditions with as little turbulence and pressure variation as possible.
Also, it must hold minimum drag, which it itself creates.
Q23. WHAT IS THE PURPOSE OF A COMPRESSOR? HOW DOES IT WORKS?
Purpose of compressor is the compression of air. Because mixing of fuel with uncompressed air will not create enough expansion of the gases to do any useful work for the engine operation.
A compressor stage consists of a set of rotating blades called rotor followed by a set of stationary blades called stator. What the rotors do, they speed up the air going through the engine, while the stators they actually slow down the air. What happens is the transfer of velocity energy into pressure energy.
Q24. TYPES OF COMBUSTION CHAMBERS?
Can Type combustion chamber (Small Turbo-props)Annular Type – (Large Turbo-fan engines)Can Annular
Q25. STATE THE ADVANTAGES OF CAN, ANNULAR AND CAN-ANNULAR TYPE COMBUSTION CHAMBERS?
Can Type:1. Structural strength (Due to smaller size and lesser diameter)2. Light weight structure3. Individual units can be removed from the engine for inspection.4. Use on centrifugal compressor type engines.
Disadvantages - Due to their shorter dia, they are made larger in length. – Improper gas distribution on the face of the turbine.
Annular:1. It uses limited space without any increase in length or dia.2. Better mixing of fuel and air within a relatively simple structure.3. Proper distribution of hot gases at the face of the turbine. 4. Use in high bypass turbo-fan engines
Disadvantages – Fuel spray patterns with the combustors are difficult to achieve. Degradation within the combustor liners require a major engine disassembly to rectify.
Can-Annular:1. It is more efficient in respect of power output.2. Also the length of cans is reduced.3. Easy removal of cans from the engine without major disassembly for inspection or repair.4. Use on large turbo-jet & turbo-fan engines.
Q26. WHAT DO WE DO IN THE COCKPIT BEFORE AN ENGINE REMOVAL?
We pull out the squib CB and pull the fire handle. The fire handle is pulled so that no fuel or hydraulic or electrical supply be available to the engine, and Squib CB is pulled so that fire bottle be not discharged on pulling the fire handle.
Q27. WHY AN AIRCRAFT IS INTO THE WIND FOR RUNUP? ALSO ITS EFFECTS ON ENGINE PARAMETERS?
An aircraft is faced to the wind direction for run-up because:
1. maximum amount of mass of airflow is available through the Ram pressure for the engine.
EPR - will increaseN1 & N2 RPM - N1 and N2 Rpm will be within the operator’s designed limits for normal operation and the difference between the speed of two compressors will not be greater
EGT - will be within the operator’s designed limits for normal operation
2. also if an engine is tested in tail wind there is probably a chance of exhaust gas ingestion through the intake again and will lead the engine to surge.
EPR - will decrease
N1 & N2 RPM - N1 and N2 will be offloaded due to warmer air ingestion that has a lesser density and lesser MAF, but the difference between their speed will be much greater.
EGT - will experience an abnormal rise.
3. Reverse wind-milling may also happen if engine is tested in the tail wind.
Q28. SAFETY PRECAUTIONS OF ENGINE RUN-UP?
Safety precautions
• Anti-collision beacons must be switched on throughout the engine ground run
• Aircraft maintenance organisations must ensure that all personnel, equipment and cargo is well clear of the rear of the aircraft during an engine ground run
• A supervisor must be appointed over the engine ground run to ensure the safety of the operation and all airside users in the vicinity. The engine ground run must be stopped immediately if a dangerous situation arises
• To improve visibility for airside drivers, all ground service equipment must be moved well away from the aircraft during the operation
• Before commencement of aircraft ground run activity at Site 1 (Terminal and Freight Apron areas), warning signs must be placed on the edge of the Apron Service Road directly behind each aircraft wing tip to warn other apron users that aircraft ground run activity is in progress.
The signs should state, ‘Caution: Engine Ground Run in Progress’.
Engine ground run signs must be removed immediately following the end of the aircraft ground run activity to signal to apron users that it is safe to pass behind the aircraft.
Q29. WHAT ARE THE REQUIREMENTS OF A MAINTENANCE ORGANIZATION COMMENCING A GROUND RUN?
Requirements of Maintenance Organisations
• To ensure that appropriate maintenance personnel are aware of the ground running sites, and of the conditions relating to their use, the maintenance organisation must take such steps as necessary to publish details of the sites and procedures in whatever form of internal documentation is most appropriate.
A copy of all documentation must be supplied to the Airside Standards Supervisor.
• Maintenance organisations must ensure the person towing an aircraft to a ground running location is a holder of a current Airside Driver Authority Level 3 and the vehicle has a current Authority for Airside Use (Airside Vehicle Permit).
Q30. WHAT IS THE DIFFERENCE BETWEEN DRY MOTORING AND WET MOTORING?
In dry motoring we run the HPC via bleed through the APU to the starter. In wet motoring we do the same but with HP fuel valve open for not more than 30 seconds to pressurize the fuel lines and thus do a leak check.
Q31. WHY DO WE DO A DRY MOTORING?
To clear off any accumulated fuel to clear off an engine after failed attempt to normal start.
Q32. WHAT IS MAXIMUM MOTORING SPEED?
The maximum motoring speed is defined as the rate of increase in N2 rpm is less than 1% in approximately 5 seconds.
Q33. AT WHAT N2 SPEED DOES THE STARTER CUTS OUT?
45-47% rpm.
Q34. IN CASE OF FIRE INDICATION (INTERNAL ENGINE FIRE OR TAIL PIPE FIRE) WHEN DO WE OPERATE FIRE EXTINGUISHER?
DRY MOTOR THE ENGINE TO EXTINGUISH AN INTERNAL ENGINE FIRE OR TAILPIPE FIRE. USE A FIRE EXTINGUISHER ON THESE FIRES ONLY IF THE DRY MOTOR CANNOT CONTROL OR EXTINGUISH THE FIRE. THE ENGINE WILL BE DAMAGED IF A FIRE EXTINGUSHER IS USED, AND YOU MUST REMOVE THE ENGINE
Q35. WHY DO WE GIVE A TIME OF 5 MINUTES AT IDLE BEFORE ENGINE SHUT DOWN?
Because turbine case and turbine rotor do not cool at the same rate after shut down. The turbine case that cools faster may shrink down on the on the rotating turbine blades, if the engine is too hot. In extreme cases the blades squeal and sieze.
Q36. CAN ENGINE OPERATION WITH OPEN COWLS BE DONE TO OPERATE THRUST REVERSERS?
No
Q37. WHAT DETERMINES EPR OR N1 BEFORE A FLIGHT FOR TAKE-OFF, CLIMB & CRUISE AND DESCENT? WHAT FACTORS ARE TAKEN INTO CONSIDERATION?
The required EPR is determined by the flight operations just before the flight consulting a Flight Manual or Operation Manual. The factors are considered are as follows:
· Aircraft Type
· Engine Types
· Outside Air Temperature
· Wind Direction
· Runway selected
· Humidity
· Altitude
· Pay Load (Cargo, Passenger and Fuel)
Q.38. IN CASE OF A TURBO PROP, IN WHICH RANGE IS THE POWER CONTROL LEVER CONTROLLED MANUALLY?
· In Beta range, which is used for ground handling and in-flight approach. At all other times a constant propeller speed is maintained automatically by the PCU for any given Propeller control lever setting.
Q39. WHAT WILL YOU DO AFTER ENGINE FIRE BOTTLE DISCHARGE?
· Washing with engine being dry motored and water with detergent Zok-27 sprayed at the inlet.
Q40. FROM INTAKE TO EXHAUST…WHAT HAPPENS WITH THE AIR?
· Air Intake:Well, speaking in subsonic terms, when air enters the intake it is diffused due to the divergent shape of the intake where its velocity is decreased along with an increase in the static pressure.Compressor: After it enters the compressor a continuous steady rise in the pressure is witnessed but as compared to the compressor inlet and outlet there is a slight decrease in pressure due to frictional losses. Then there is a steady decrease in the velocity through the compressor, though the axial velocity is maintained constant. Diffuser: The air then enters the diffuser. It has a divergent shape and due to this shape the static pressure rises while the velocity is further decreased so that it could sustain the flame in the combustion chamber.Combustion Chamber: Combustion occurs at constant Pressure. Though practically a slight decrease in pressure has been observed due to air friction and turbulence. The velocity keeps on decreasing here. The temperature is at its highest in the Primary section of combustion chamber. The outlet of a combustion chamber is convergent in shape and this is to provide expansion as the air leaves the combustion chamber following with an increase in velocity.NGVs: There is a sudden expansion of gasses as they pass through the nozzle guide vanes where the velocity is at its highest.Turbine: After the NGV as the air passes on the turbine a large drop in pressure occurs as the pressure energy is converted into mechanical work. Similarly Velocity drops as the Kinetic energy is again converted into mechanical work, and also the Temperature drop occurs as heat energy is again converted into mechanical work.Exhaust: After the turbine the gas passes through the exhaust section where the Pressure in the exhaust duct drops slightly due to frictional losses and a further drop in pressure occurs in the propelling nozzle where the pressure energy is converted into Kinetic energy. Here the velocity highly increases and the temperature drops.
After the air leaves the
Q41 INSPECTION SEQUENCE?
· To permit simultaneous inspection of several areas of the aircraft the inspection has been divided into a number of _Packages_.
· The inspections are divided into three phases :
· - Phase 1 : is a general inspection for primary damage and indications of
· remote damage and is mainly external.
· - Phase 2 : is a more detailed inspection and is mainly internal. Some component
· removal may be called up.
· - Phase 3 : is a very detailed inspection involving component removal and
· strip down.
Q42. WHAT IS MAXIMUM POWER ASSURANCE TEST? STATE ITS PURPOSE? WHEN DOES A POWER ASSURANCE TEST BECOMES MANDATORY?
AMM 71-00-00
(3) Power assurance check.
Power assurance check is a functional test
· It determines that the engine can go to takeoff power while the EGT and engine RPM stay in operation limits.
· This test is not performed to accept or reject an engine
· This check becomes necessary after engine maintenance that could change engine operation, like after engine installation, modular installation etc.
· This check compares engine performance to other power assurance runs. The difference in power assurance run data tells you if there are large changes in engine operation margin and if the margin where the engine operates is permitted.
(a)The function of the power assurance check is to make sure that the engine can go to takeoff power while the EGT and N2 speed stay in operation limits. Do this check after engine maintenance occurs that could change engine operation. This check compares engine performance to other power assurance runs. The difference in power assurance run data tells you if there are large changes in engine operation margin and if the margin where the engine operates is permitted. The check can also examine flight crew concerns.
(b)Because a power assurance run is usually not sufficiently stable, other quality check procedures can give a more accurate estimate of margin. These other quality checks include a test stand run as given in the Engine Manual, and/or review of flight data received during takeoff and analysis done with algorithms from the OEM.
The Power Assurance Check is not a good test for the performance analysis of the engine. Do not use only the Power Assurance Check to accept or reject an engine. The Power Assurance run is usually not sufficiently stable to accurately calculate the engine’s health.
You can get a more reliable performance analysis by doing a test cell operation or on-wing performance trend monitoring.
Q43. CAN RECOMMENDED GRADE OF FUEL BE MIXED?
Yes
Q44. CAN ALL OF THE FUEL BE TRANSFERRED FROM ONE FUEL TANK TO ANOTHER DURING FLIGHT?
No
Q45. CAN OIL OF DIFFERENT GRADES BE MIXED?
No
Q46. WHAT IS ENGINE TRIMMING? AND WHY IS IT DONE?
The fuel control adjustment is called engine trimming. It is done because at the time of manufacture there are manufacturing tolerances due to which two engines may produce different level of thrust at the same RPM setting. So by adjusting the fuel control the Thrust is maintained the same with a slight change in the RPM.
Q47. WHAT IS ENGINE TRIM SPEED? IS TRIM SPEED AND DATA PLATE SPEED IS SAME?
The adjusted compressor RPM , corrected to standard day conditions at seal level is known as the engine trim speed. This is done by an adjustment on the fuel control that governs the N2 speed.
Yes, both are same. Data plate speed is determined at the time of the manufacture when the engines are adjusted to produce their exact rated thrust on calibrated engine test stand. Data plate speed is then stamped on engine data-plate in terms of both actual RPM and percent RPM.
Q48. WHAT IS ENGRAVED ON ENGINE DATA PLATE? AND WHERE IS IT INSTALLED ON AN ENGINE?
1. Engine Trim Speed
2. Engine Gross weight
3. Engine Serial Number
4. Engine Rated Thrust
Q49. WHAT IS SHUNT?
Shunt is installed to give a false EGT signal
Q50. WHAT IS FLAT RATE CORNER POINT?
Flat rate corner point in flat rated engines is the OAT limit after which the Thrust will tend to fall, we cannot increase the Thrust or the EGT will go beyond its limit. EGT though becomes constant after this point.
Q51. WHAT ARE THE POSSIBLE CAUSES & EFFECTS OF A HUNG AND A HOT START?
Hung Start:1. Inadequate bleed pressure to the starter.2. A faulty starter3. Ceased Main engine bearing.4. Inadequate fuel flow.5. Abnormal signals to FCU of CIT (high CIT), BP (high burner Pressure signal), Ambient Pressure (Low ambient pressure).
Effects: Deterioration of the Starter
Hot Start:1. Accumulated fuel in the combustion chamber.2. Faulty fuel nozzles.
Effects: Deterioration of the NGV’s and the Turbine area.
Q52. WHAT IS TAKE OFF THRUST?
This is the maximum thrust that can be used without over boasting the engine. This rating is normally
continued to only 5 minutes time period and is to be used for take-off only.
Q53. WHAT IS MAXIMUM CONTINEOUS?
This is the maximum thrust that may be used continuously and is primarily intended for emergency use at the discretion of the captain.
Q54. WHAT IS IDLE?
It is not a thrust rating, but a thrust lever position obtained by fully retarding the thrust lever. Minimum thrust suitable for ground operation is called ground idle and that in air is called flight idle.
Q55. WHAT ARE THE COMPRESSOR SURGE PARAMETERS?
EPR, EGT & RPM
Q56. WHAT IS FOG?
Visibility of less than 3280 ft due to moisture.
Q57. IS ENGINE OPERATION WITH HINGED COWL IS PERMISSIBLE?
Yes it is permissible but its is normally done with closed cowls.
Q58. IN WHICH ENGINE OPERATION MODE, THE ENGINE EXPERIENCES THE MAXIMUM MATERIAL AND THERMAL STRESSES?
During Thrust reverser mode, because it is during when the gas flow moment is changed against it course
Q59. WHERE IS AN ENGINE DATA-PLATE LOCATED? WHAT ARE THE PARTICULARS OF ENGINE DATA-PLATE?
Data-Plate is located on the left side of the fan frame. Fan frame denotes the actual engine
TYPE CERTIFICATE FRENCH CERTIFICATE
Engine M/1M13
MODEL # SERIAL #
CF680-C2A8 695-408
RATING
T/O THRUST MAX. CONT. THRUST
57860 48080
Q60. WHAT SENSE GOES TO MEC THAT ENABLES TO OPEN AND CLOSE VBV AND VSV?
During high thrust operation, the burner pressure will be sensed low by the MEC and it will signal the VBV to close and VSV to open, but in low thrust operation mode, the burner pressure will be sensed high by the MEC and it will signal VBV to open and VSV towards close.
Q61. MEC SIGNALS VSV OR VBV TO CLOSE OR OPEN?
MEC signals VSV to close or open, which is linked with the VBV. VBV itself doesn’t receive signal directly from the MEC
Q62. WHAT IS NORMALLY THE POSITION OF VBV ON GROUND OR WHEN THE ENGINE IS SHUTTED OFF?
Its position is towards open, because on ground there is a low thrust operation mode, so the air is being bled off via VBV…and when the engine is stopped the position of VBV remain the same that is in open.
Q63. WHERE IS FUEL USED IN ENGINE?
1. for ignition in the fuel nozzles2. For cooling in the fuel oil heat exchanger3. For the operation of MEC4. For the hydraulic actuation of VSV
Q64. WHAT ARE THE UNITS OF VBV ON TEST BENCH AND VSVs?
VBV – volt DCVSV – in degrees and volts
Q65. TREND MONITERING (ETMS – Engine Trend Monitering Software) SOFTWARE IN TSE-LM
SAGE – System Analysis for Gas Turbine Engine
Q66. THROUGH WHICH PORTS DO YOU DO THE BSI OF 1ST STAGE NGV?
Through ports in Combustion Chamber.
Q67. How one can reduce / optimize Engine Maintenance Cost per Flight Hour?
There are four basic parameters to calculate severity factor, in order to calculate true Engine Maintenance Cost:1. Annual Utilization-EFH, EFC2. Average OAT3. EFH:EFC Ratio4. %age Derate
Q68. VBV IS AN ANTI-STALL DEVICE, BUT WHEN IT DUMPS THE MAF…IT DECREASES WITH AN INCREASE IN ANGLE OF ATTACK…THIS SHALL TAKE THE ENGINE TO STALL, THEN WHY IS IT CALLED AN ANTI STALL DEVICE?
Actually when it dumps the almost air or mass of airflow, more air rushes into the engine compressor thus increasing the velocity of air…now with an increase in velocity the angle of attack decreases taking away the engine compressor from stall.
Q69. What is important for the weight calculation of Jet A-1 fuel?
A. When weight is to be measured than the specific gravity of the fuel on that particular day considering the pressure and temperature must be known.
Q70. Why is a turbo-prop inefficient at height as above as 30000 to 35000 ft??The higher you go, the faster you must go to maintain the same lift because of air thinning.
Now if you want to go faster, you need to push around air with more thrust.
But you can't just turn the prop faster, as a prop tip speed at the speed of sound induces wave drag which dramatically reduces thrust. ... You'll need to add more blades to the prop. The more blades you add, the more your prop will look like a fan. This is where you find a prop fan design.
The higher/faster you want to go, the more blades you add. At some point, you might as well cowl the blades of your 'prop' to increase the air speed, reduce noise and increase the compression of the blades. Now you have a turbo fan.
You will need to add more blades to the prop. The more blades you add, the more your prop will look like a fan. This is where you find a prop fan design.
The high / faster you want to go, the more blades you add, at some stage , you might as well coel the blades of your prop to increase the speed, reduce noise and increase the compression of the blades. Now you have a turbo-fan.
Q71. IN GROTOR TYPE OF PUMPS, WHICH GEAR HAS MORE TEETH?
Internal Gear
Q72. WHAT IS THE DIFFERENCE BETWEEN PACKING AND SEALS?
Seals are used in stationary parts and packings are used in rotary parts.
Q73. GEAE – CWC stands for: Customer Web Center
Types of GEAE SBs?
1. Spare parts
2. Standard
3. Alert
4. Other
5. N/A
Q.74 WHAT ARE THE CATEGORIES OF SBs?
0-9 Categories
Q75. TYPES OF BEARINGS
Ball bearing: takes thrust and radial loads. It has lesser friction due to point contactRollar bearing: takes radial load only. It has relatively more friction due to line contact. Tapered bearing are roller bearings.
Q76. CLASSIFICATION OF BEARINGS?
One dot
Two dot
Three dot
Q77. PRIMARY MAINTENANCE
1. Hard time maintenance (Preventative)
2. On-condition (Preventative)
3. Condition monitoring (Trend)
Q78. WHAT IS HARD TIME MAINTENANCE?
A.
PRIMARY MAINTENANCE
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| | Preventative Trend | |
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Hard Time On-condition Condition Monitoring
| Engine Health Monitoring: Following Parameters -Engine Oil systems -Engine airborne vibration monitoring
-Boroscope inspections