temp inversion

AIRBUS 11th Performance and Flight Operations Support Operations Conference

Chapter 25 Page 1

TEMPERATURE INVERSION DURING TAKEOFF

By Francis PAYEUR

Department Manager A300 / A310 Operational Standards

1. INTRODUCTION In a standard atmosphere, the outside air temperature decreases as altitude increases (some 2C per 1,000 ft). The engine performance is influenced by various parameters such as outside air pressure (altitude), aircraft speed, outside air temperature and bleed demand. Under normal conditions, an increase of altitude brings a combination of two effects. The decrease of air pressure decreases thrust. The decrease of temperature tends to increase thrust. Combination of both is a net decrease of thrust, because of the influence of pressure is dominant However, weather characteristics and geographical environment may affect the lower layer of the atmosphere in such a way that the standard atmosphere is not encountered during each takeoff. Amongst those cases, an increase of temperature can be met when altitude increases. That is the temperature inversion. Under such circumstances, an increase of altitude will bring a decrease of thrust that is substantial than usual, because the effect of pressure and temperature both contribute to the decrease. The aim of this article is to present the effect on the engine, and consequently, the aircraft performance (full thrust takeoff, flex / derate takeoff) of a temperature inversion for all Airbus aircraft types and to provide the Airbus recommendations in this matter.

2. ENGINE BEHAVIOR IN A STANDARD ATMOSPHERE

2.1. General Depending on the aircraft you operate and on the engine type installed on your aircraft, different thrust control devices are installed. These devices meter the fuel to the engine in order to provide adequate thrust according to the thrust lever position. For takeoff, the engine control is as follows: - On A300 B2 / B4 models, the engines are hydro-mechanically controlled. Above 60 Kt on the runway,

the target N1 is frozen and variation of OAT are no longer taken into account for adjustment. However, due to the temperature and altitude effect during the takeoff, the N1 and consequently the thrust will be affected.

- On A310 / A300-600 models not fitted with a FADEC, the engines are controlled by a combined

electronic and hydro-mechanical system. These engines are controlled in the same way as a FADEC engine as described below. The electronic control can be selected OFF. In this case, the engine is only hydro-mechanically controlled similarly to A300 B2 / B4 models.

- For last generation engines fitted with a FADEC, the FADEC refreshes computation of engine power

setting parameters (N1 or EPR) as flight conditions change along the takeoff path. The regulation system is a sophisticated system and provides complex regulation. Whether the engine control unit is a hydro-mechanical or an electronic device, the engine regulation depends on various parameters such as: - flight phase


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- selected operating mode (Max takeoff / Go around, Flex takeoff, Max climb, Max cruise, Max continuous, Idle)

- pressure altitude - TAT - Mach number - bleed demand According to these parameters, the fuel schedule as controlled by the engine control device provides a fan speed, thus, a given thrust.

2.2. The flat rate concept For takeoff (and Go around), the engine thrust is defined in order to meet the aircraft performance requirements but also to maintain the EGT below the maximum certified EGT. Some margin to the maximum EGT is applied to account for engine degradation during its life. The engine is designed to provide a given thrust level up to a given OAT, this is the flat rate concept. This temperature is usually named the Flat Rating Temperature, and is the indicated T.REF on the takeoff charts. T.REF is defined as a differential temperature to ISA depending on the engine model as follows:

ENGINE T.REF (all temperatures in C)

ENGINE T.REF (all temperatures in C)

A300 B2 / B4 A319/320/321 GE CF6-50 ISA + 15 CFM 56-5A1 / A3 ISA + 15

PW JT9D-59A ISA + 15 CFM 56-5B1(2)(3) ISA + 15 A310 / A300-600 CFM 56-5B5 / B6 ISA + 30

GE CF6-80A3 and

80C2A1/A3/A5

ISA + 15

CFM 56-5B4 / B7

ISA + 29 below 2 000 ft ISA + 18 between 5000 and 10000 ft

ISA + 23 at or above 15 000ft GE CF6-80C2A2 ISA + 34 at 2000ft

ISA + 29 at SL ISA + 25 at 2000 ft ISA + 20 at 4000 ft ISA + 16 at 5300 ft

ISA + 15 at or above 5650 ft

CFM 56-5A4

CFM 56-5A5

ISA + 30 at or below 10 000ft

ISA + 23 at 15 000 ft

ISA + 22 or below 10 000ft ISA + 15 at 15 000 ft

GE CF6-80C2A8 ISA + 24 at 2000ft ISA + 20 at SL

ISA + 15 at or above 2000 ft

IAE V2527A5

ISA + 33 at 1000ft ISA + 31 at SL

ISA + 25 at or above 5 000 FT PW 7R4-D1 ISA + 26 at 2000ft

ISA + 18 at SL ISA + 15 at or above 5000 ft

IAE V2522-A5 IAE V2524-A5

ISA + 40 at or below 8 000ft ISA + 30 at or above 13 000 ft

PW 7R4-E1 ISA + 26 at 2000ft ISA + 18 at SL

ISA + 15 between 5000 and 12000 ft ISA + 20 at or above 15 000ft

IAE V2530-A5 IAE V2533-A5

ISA + 19 at 2 000ft ISA + 15 at SL

ISA + 13 at 8 000 ft (V2530 only) ISA + 18 at 10 000 ft ISA + 19 at 11 000 ft

ISA + 26 at or above 14 500 ft PW 7R4-H1 ISA + 23 at 2000ft

ISA + 15 at or above SL IAE V2500-A1 ISA + 18 at 1000ft

ISA + 15 at or above SL

PW 4152

PW 4156A

ISA + 30 at 1000ft ISA + 15 at 5000 ft

ISA + 23 at 10 000 ft ISA + 21 at 12 200ft ISA + 24 at 14 100ft

ISA + 20 at 1000ft

ISA + 18 at SL ISA + 15 at or above 1 200 ft

IAE V2527EA5 IAE V2527MA5

ISA + 35 at 2 000ft ISA + 31 at SL

ISA + 26 at 2 000 ft ISA + 17 at 5 000 ft

ISA + 15 between 6000 and 8000ft ISA + 18 at 10 000 ft ISA + 19 at 11 000 ft

ISA + 26 at or above 14 500 ft

PW 4158 ISA + 19 at 1000ft ISA + 15 at or above SL


Chapter 25 Page 3

A330 A330 (contd) GE 80 E1 PW 4000

ISA + 15 ISA + 20 at 2000 ft

ISA + 15 at or above SL

RR TRENT 772B ISA + 22 at or below 2000 ft ISA + 15 between 5000 and 8000 ft

ISA + 10 at or above 10 000 ft RR TRENT 768 ISA + 15 A340 RR TRENT 772 ISA + 15 at or below 8000 ft

ISA + 10 at or above 10000 ft CFM 56-5C ISA + 15

Below this T.REF the engine provides a rather constant thrust for a given altitude and Mach, above this T.REF, thrust is decreased and aircraft performance is adjusted accordingly. The reason for decreasing the thrust above the T.REF is to prevent an engine over-temperature, ie, to maintain a rather constant EGT level below the red line. With the temperature increasing, more thrust would be needed to meet the aircraft performance. Consequently, more fuel would be needed to increase N1 and thrust. Resulting from this increased fuel burnt, the EGT would be increasing. However, to prevent deterioration of the hot section and rotating parts of the engine, the EGT is limited and the thrust is consequently reduced when the ambient temperature is above the T.REF in order to satisfy the certified EGT limit as illustrated below. A margin between the actual EGT and the EGT red line not only accounts for engine transient characteristics, altitude effect and engine deterioration but also temperature inversion. The engine thrust setting for takeoff is determined according to the defined thrust levels to comply with these thrust and EGT requirements.

Pressure altitude 0 ft THRUST

ISA T.REF

Pressure altitude > 0 ft

EGT

ISA T.REF

RED LINE

MARGIN Deterioration

(With linear variation in-between the values)


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N1 or EPR for power management schedule is defined depending on the temperature in order to maintain the thrust constant below the T.REF and to reduce the thrust above this point as schematically shown below. In fact, N1 parameter is not directly used for engine thrust management. N1 is corrected as a function of the TAT (depending on aircraft speed). This corrected N1 is also called N1K. There is a direct relationship between N1K and thrust. Below the T.REF, N1K is consequently constant while N1 is decreasing. However, since N1 is the main parameter used for engine thrust control by pilots and auto-thrust system, this article will refer to N1 as necessary. Similarly to N1K, EPR is function of the TAT. There is a direct relationship between EPR and thrust. EPR remains constant below the T.REF. When the crew selects engine bleed for air conditioning or anti ice, a decrement is applied to the power setting parameters. This decrement permits to keep a constant EGT and EGT margin compared to the bleed off operations at the same takeoff conditions.

2.3. Takeoff without temperature inversion During the takeoff phase, the thrust computer continuously computes the N1 / EPR target based on the pressure altitude, the temperature and the aircraft speed. Therefore, resulting from the evolution of the pressure altitude and the aircraft speed which increase, but also resulting from the temperature which decreases, the engine parameters will change in the following way during the takeoff: - Thrust decreases particularly due to the pressure altitude and aircraft speed effect - N1 / EPR and N2 slightly increases - EGT also increases For instance, on an A330 aircraft fitted with CF6-80E1A4 engines, during a maximum takeoff thrust from an airport altitude 0ft and OAT 15C (below T.REF) the engine parameters will change as follows: Altitude 0ft (Mach 0.1) / temp 15 C Altitude 1500 ft (Mach 0.3) / temp 12 C

N1 : 108.84 % N1 : 109.38 % N2 : 106.64 % N2 : 107.09 % EGT : 872 C EGT : 885.2 C Thrust : 27 951 daN Thrust : 23 730 daN

THRUST

ISAT.REF

N1

N1K / EPR


Chapter 25 Page 5

Similar behavior will be noticed whatever the engines and whether the temperature is below or above T.REF. However, this effect on the engine behavior and parameters is the result of the combined effect of the altitude, the temperature and the speed evolutions. In order to better assume the effect of a temperature inversion, this article focuses on the sole effect of the temperature during the takeoff. The two other parameters, i.e., the altitude and the speed will vary rather in the same way, whatever the temperature evolution.

2.3.1. Effect of temperature on thrust Considering the only effect of the temperature, the thrust is rather constant below the T.REF. Above the T.REF, the thrust varies with the temperature. It will increase with a decreasing temperature. For instance, on A320 fitted with CFM 56-5B4, for an altitude 0ft, the static thrust will vary from 10 838 daN at 50C to 11 256 daN at 44C (T.REF) and will remain somewhat constant below this temperature. As a general rule, each degree decrease in OAT results in a thrust increase by about 0.75 % (above T.REF).

2.3.2. Effect of temperature on N1 / EPR Above T.REF, N1 / EPR will increase with a decreasing temperature to maintain power management curve still keeping a rather constant EGT. Below T.REF, N1K / EPR will remain constant to maintain a rather constant thrust (N1 will decrease). For instance, on A310 fitted with PW 4152 engines, at sea level, EPR will vary from 1.387 at 55C (above T.REF) to 1.476 at 42C (at or below T.REF). On A310 fitted with GE CF6-80C2A8, at sea level, N1 will vary from 105.8 % at 55C to 108.3% at 30C (T.REF) then to 95.1 at 40C (N1K will remain constant at or below T.REF).

2.3.3. Effect of temperature on EGT Because power management is established to maintain a constant EGT above T.REF as indicated above, the EGT will remain constant above T.REF and will decrease below T.REF since the thrust is maintained rather constant.

T.REF

N1K / EPR

Initial temperature

N1

THRUST

T.REF

Initial temperature

ISA

ISA


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As a general rule applicable to all engine models and to illustrate the effect of the temperature on the EGT, it is considered that the EGT will decrease by approximately 3C for each degree decreased below T.REF. This summarizes the engine behavior with regards to a normal temperature decrease during the takeoff phase. However, weather characteristics and geographical environment may affect the lower layer of the atmosphere in such a way that the normal standard temperature evolution is not encountered during each takeoff.

3. TEMPERATURE INVERSION, THE WEATHER PHENOMENON

3.1. General In meteorology, air temperature at the earths surface is normally measured at a height of about 1.20 meter (4ft) above the ground. From that temperature, which is reported by Air Traffic Control, takeoff performance will be defined. All along the takeoff flight path, aircraft performance is computed considering the altitude gained, the speed increase, but also implicitly considering a standard evolution of temperature, i.e. temperature is considered to decrease by 2C for each 1000 ft. However, although most of the time, temperature will decrease with altitude in quite a standard manner, specific meteorological conditions may lead the temperature evolution to deviate from this standard rule. With altitude increasing, marked variations of the air temperature from the standard figure may be encountered. In that way, air temperature may decrease in a lower way than the standard rule or may be constant or may even increase with altitude. In this last case, the phenomenon is called a temperature inversion. As described below, this may particularly affect the very lower layer of the atmosphere near the earths surface. There are many parameters, which influence air temperature and may lead to a temperature inversion. Close to the ground, air temperature variations mainly result from the effects of: - seasonal variations - diurnal / nocturnal temperature variations - weather conditions (effect of clouds and wind) - humidity of the air - geographical environment such as:

- mountainous environment - water surface (sea) - nature of the ground (arid, humid) - latitude - local specificity

EGT

T.REF

Initial temperature

ISA


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Therefore, it is not intended to describe here all the meteorological conditions which may lead to a temperature inversion but your own experience and the local weather reports are obviously the best way of knowing if a particular area in which you operate is subject to frequent temperature inversions. However, as general rule, valid for everywhere, low wind conditions and clear skies at night, will lead to rapid cooling of the earth and a morning temperature inversion at ground level.

3.2. Morning temperature inversion In the absence of wind or if the wind is very low, the air, which is in contact with a cold earth surface will cool down by heating transfer from the warm air to the cold ground surface. This transfer of heat occurs by conduction only and consequently leads to a temperature inversion which is limited in altitude. This process needs stable weather conditions to develop. Schematically, during the day, the air is very little heated by solar radiation and the earth is very much. But the lower layer of the atmosphere is also heated by contact with the ground, which is more reactive to solar radiation than the air, and by conduction between earth and atmosphere. At night, in the absence of disturbing influences, ground surface cools down due to the absence of solar radiation and will cool the air near the ground surface. In quiet conditions, air cooling is confined to the lowest levels. Typically, this effect is the biggest at the early hours of the day and sunshine subsequently destroys the inversion during the morning. Similarly, wind will mix the air and destroy the inversion.

3.2.1. Magnitude of temperature inversion This kind of inversion usually affects the very lowest levels of the atmosphere. The surface inversion may exceed 500 ft but should not exceed 1000 to 2000 ft. The magnitude of the temperature inversion cannot be precisely quantified. However, a temperature inversion of about +10C is considered as quite an important one. Usually, within a temperature inversion, temperature regularly increases with altitude until it reaches a point where the conduction has no longer any effect.

3.2.2. Where can they be encountered? This kind of inversion may be encountered worldwide. However, some areas are more exposed to this phenomenon such as arid and desert regions. It may be also encountered in temperate climate particularly during winter season (presence of fog). Tropical regions are less sensitive due to less stable weather conditions. In some northern and continental areas (Canada, Siberia) during winter in anticyclonic conditions, the low duration of sunshine during the day could prevent the inversion from destruction. Thus, the temperature of the ground may considerably reduce and amplify the inversion phenomenon. In a lower extent, this may also occur in temperate climate during winter, if associated with cold anticyclonic conditions. Another important aspect of an inversion is wind change. The airmass in the inversion layer is so stable that winds below and above, tend to diverge rapidly. Therefore, the wind change, in force and direction, at the upper inversion surface may be quite high. This may add to the difficulty of flying through the inversion surface. In some conditions, the wind change may be so high as to generate a small layer of very marked turbulence.

3.3. Other types of temperature inversion The process described in the above paragraph 3.2 is considered as the most frequent and the most sensitive. However, as discussed above, other meteorological conditions, of a less frequent occurrence and magnitude, may lead to temperature inversions.


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For instance, the displacement of a cold air mass over a cold ground surface may lead to turbulence resulting in a transfer of heat to the lower levels of this mass, thus, also creating a temperature inversion in the lower levels of the atmosphere below this air mass. Usually, this kind of inversion has lower magnitude than the previous case described above. In any case, your experience, weather reports or pilot reports will be the best way in identifying such weather conditions.

4. ENGINE BEHAVIOR IN A TEMPERATURE INVERSION During the takeoff phase, the thrust computer continuously computes the N1 / EPR target based on the current temperature. Thus, the effect of a temperature inversion on the engine parameters will be in the reverse way as the effect described in the above paragraph 2.2. However, the effect of a sudden increase in ambient temperature on the engine control during takeoff will depend on the type of takeoff being performed and on the magnitude of the temperature increase. The three following cases are considered to assess the effect of a temperature inversion:

4.1. Temperature inversion during a maximum takeoff below T.REF If a maximum (or a derated) takeoff is being performed below T.REF and if a temperature increase occurs which is still below the T.REF, the N1 target will increase to maintain thrust for the higher temperature. N1K and EPR will remain constant. Resulting from the higher temperature, the EGT will increase versus the no inversion case but will be maintained below the maximum EGT. Because still lower to T.REF, i.e., on the flat part of the thrust curve, then, no effect on thrust will occur compared with the standard temperature evolution as illustrated below: A = No temperature inversion B = Temperature inversion In both cases, the thrust level will remain the same. For instance, on an A330 aircraft fitted PW4168A engines, during a maximum takeoff thrust from an airport altitude 0ft at ISA, the thrust will be as follows: - Altitude 0ft (Mach 0.1) / OAT 15 C: Thrust = 27 498 daN (EGT 530 C) - Altitude 1500 ft (Mach 0.3) / with no temperature inversion: Thrust = 24 060 daN (EGT 540 C)

/ with a temperature inversion of +10C : Thrust = 24 060 daN (EGT 570 C)

Consequently, this shows that a temperature inversion has not effect on the engine performance as long as the ambient temperature does not reach T.REF. The only effect will be an increase of the EGT compared to the normal case with no temperature inversion.

THRUST Max takeoff thrust at 0ft

Max takeoff thrust at 1500ft

A B

ISAT.REF


Chapter 25 Page 9

4.2. Temperature inversion during a maximum takeoff thrust above the T.REF If a maximum (or a derated) takeoff is being performed and if a temperature increase occurs which is beyond the T.REF, the N1 / EPR target will decrease to maintain N1 / EPR power management curve and to maintain a rather constant EGT. This concept is used in order to protect the engine against EGT exceedance. Resulting from this, the thrust level will decrease compared to the case with no temperature inversion in order to maintain a rather constant EGT. EGT will remain approximately constant versus the no inversion case. The higher the temperature level above T.REF, the greater the reduction on thrust will be as illustrated below: A = No temperature inversion B = Temperature inversion As general rule and to illustrate the effect of the temperature inversion on the thrust, a temperature inversion of + 10 C will result in a thrust reduction of about 10 % (between 8 and 12 % depending on the engines). This applies whatever the engine. For instance, on an A330 aircraft fitted with RR Trent 772B engines, a temperature inversion of +10 C will result in the following thrust levels: - Altitude 0ft (Mach 0.1) / OAT 40 C: Thrust = 27 086 daN - Altitude 1500 ft (Mach 0.3) / with no temperature inversion: Thrust = 23 035 daN

/ with a temperature inversion of +10C : Thrust = 20 685 daN ( -10.2 % compared to the case with no inversion)

However, this loss of thrust applies at the maximum magnitude of the inversion (in this example, 10 degrees). As discussed in the above paragraph 3, the temperature should regularly increase with the altitude. Consequently, the thrust should be regularly reduced compared with the normal case from nothing at the ground level to approximately 10 % when reaching 1500ft. The consequences of this thrust reduction on the aircraft performance will be developed in the paragraph 5 of this article.

4.3. Temperature inversion during a flex takeoff If a flex takeoff is being performed, and a temperature inversion occurs, the two following cases have to be considered: - If the temperature increase stays below the selected flex temperature, then no effect on thrust will

occur. The engine being regulated and thrust adapted according to the selected flex temperature, the thrust will remain constant compared to the case with no temperature inversion. The effect on the engine parameters will be mainly an increase of the EGT due to the higher ambient temperature.

THRUST Max takeoff thrust at 0ft

Max takeoff thrust at 1500ft A

B

ISA

T.REF

THRUST REDUCTION


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A = Temperature inversion during the takeoff. No effect since thrust is still limited by the flex takeoff thrust.

- If the temperature increase goes above the selected flex temperature, then the flex temperature

takeoff will be de-selected and will revert to the maximum takeoff thrust. A reduction of thrust relative to the magnitude of the temperature increase above T.REF and above the selected flexible temperature will occur (similarly to the case described for a maximum takeoff thrust above T.REF).

However, this case is less likely to occur than the previous case described for flex takeoff since it requires an already high temperature on the ground with a limited amount of reduced thrust and a temperature inversion higher than the difference between the actual temperature and the selected flex temperature. As discussed in the paragraph 3 of this article, a relatively important temperature inversion should not develop or should be destroyed when the temperature is high. Consequently, this shows that a temperature inversion during takeoff has no effect on the engine performance when it occurs during a maximum takeoff thrust below T.REF or when performing a flex takeoff with the OAT at or below T.REF.

THRUSTthrust at 0ft

thrust at 1500ft A

ISA

T.REF

SELECTED FLX TEMP

FLX TO THRUST

OAT

THRUSTthrust at 0ft

thrust at 1500ft

ISA

T.REF SELECTED FLX TEMP

THRUST REDUCTION


Chapter 25 Page 11

5. THE EFFECT ON AIRCRAFT PERFORMANCE AND RECOMMENDATIONS As previously discussed, a temperature inversion will result in a reduction of the thrust mainly when performing a maximum takeoff thrust during hot days, i.e., the actual ambient temperature is above T.REF. This is the result of a design concept in order to prevent EGT exceedance by maintaining the thrust at a constant level above T.REF. Allowing an increase of thrust above T.REF would lead to reduction of EGT margins, thus, resulting in rapid engine deterioration. It would also result in frequent or even regular EGT limit exceedance, thus, calling for manual thrust reduction in accordance with the procedures.

5.1. Effect on aircraft performance The certified takeoff performance is based on a constant ISA during the climb. In the event of temperature inversion, the climb performance will be affected in the cases where the thrust is affected as described in the above paragraph 4. However, to affect the aircraft performance, a temperature inversion must be combined with other factors. During a normal takeoff with all engines operative, the inversion will have no effect since the actual aircraft performance is already far beyond the minimum required performance. Then, the actual aircraft performance could be affected only in the event of an engine failure at takeoff. However, conservatism in the aircraft certified performance is introduced by the FAR/JAR Part 25 rules, to take account for inaccuracy of the data that are used for performance calculations. Although not specifically mentioned, temperature inversions can be considered as part of this inaccuracy. Therefore, a temperature inversion could become a concern during the takeoff only in the following worst case with all of these conditions met together: - The engine failure occurs at V1,and - Takeoff is performed at maximum takeoff thrust, and - OAT is close to or above T.REF, and - The takeoff weight is limited by obstacles, and - The temperature inversion is such that it results in the regulatory net flight path margin cancellation

and leads to fly below the regulatory net flight path. In all other cases, even if the performance is affected (inversion above T.REF), the only detrimental effect will be the climb performance to be lower than the nominal one. The minimum climb gradient required at the point 35 ft above the runway for the second segment one engine inoperative is: - 2.4 % for twin engine aircraft. - 3 % for four engine aircraft. The margin between the net and the gross flight path is: - 0.8 % for twin engine aircraft. - 1 % for four engine aircraft. Assuming a 10C temperature inversion (above T.REF) between the ground and 1500 ft, the effect on the aircraft performance will be as described in the following graph. The first graph applies to an A320 fitted with CFM engines. However, the effect of the temperature inversion on the engine thrust is quite similar whatever the engine type as described in the paragraph 4.2 (about 10 % thrust loss with a 10 C inversion). Thus, the effect on the climb performance, in terms of climb gradient, will be similar whatever the twin engine aircraft model. With an engine failure at V1, the graph shows the gross trajectory (curve A) limited by the minimum required second segment climb gradient with a normal temperature evolution with the altitude (-3C between ground and 1500 ft). The curve B shows the relevant net flight path. The curve C shows the gross trajectory with a 10C inversion from the ground to 1500 ft.


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0

2 0 0

4 0 0

6 0 0

8 0 0

1 0 0 0

1 2 0 0

1 4 0 0

1 6 0 0

1 8 0 0

2 0 0 0

0 20 0 0 4 0 0 0 6 0 0 0 8 0 00 1 0 0 0 0 1 2 0 00 1 4 0 0 0 1 60 0 0 1 8 0 00 20 0 0 0 2 2 0 00 2 4 0 0 0

D is tan c e fro m b ra k e re le a s e (m )

Hei

ght a

bove

sur

face

(ft)

Similarly, for an A340 aircraft, the following graph shows the effect of the temperature inversion during takeoff:

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

Distance from BRP (m)

Hei

ght a

bove

runw

ay (f

t)

These graphs show that for conservative conditions and particularly an engine failure at V1 and a temperature inversion of 10C, although the gross climb gradient is affected it should not become a concern. Should the engine failure occur later during the takeoff, it will provide an additional margin since providing more time and more climb capacity with all engines operating.

A

B

C

A

B

C


Chapter 25 Page 13

These graphs also show that there is a margin between the gross flight path (with inversion) and the net flight path (computed without inversion) which still remains available for obstacle clearance. Obviously, the margin reduces with the distance due to the inversion. However, it is more likely that an immediate return to the departure airport will be initiated following an engine failure at V1 while only very remote obstacles would be a concern. An extrapolation of the above graphs will show that the actual gross climb gradient (with a 10C temperature inversion) will reach the required net gradient (calculated without inversion) at a distance of approximately 42 000 m for twin engine aircraft or 38 800 m for A340 models. This situation could become a concern but again, this is still assuming an engine failure at V1, a climb gradient limited by very remote obstacles, no immediate return to the departure airport, takeoff performed on a hot day condition (while inversion should not develop) and a temperature inversion with a great magnitude. This has a very low probability of occurrence. Consequently, this paragraph has shown that a temperature inversion has no significant effect during a normal takeoff with all engines operative, which is obviously the very large majority of takeoffs performed. For the whole Airbus fleet since the entry into service of the first A300 in 1972, the number of engine failure cases at takeoff is 73. This figure includes all cases of engine failure after V1 including those occurring while the aircraft has already reached some tens or even some hundreds feet after rotation. This figure has to be compared with the number of takeoffs performed during this period, i.e., 22 271 900 (until mid Jan 2001)! Thus, the probability of an engine failure during the takeoff phase is very remote, i.e. approximately 3.25 X 10-6. However, although very remote, an engine can fail at takeoff and from the regulatory point of view, this occurrence must be taken into account in the performance determination. In this case, assuming an engine failure at V1, the temperature inversion may have an effect. However, this effect may become significant when the magnitude of the temperature inversion is such that the regulatory margin between the gross and the net gradient is cancelled.

5.2. Recommendations

5.2.1. Expected temperature inversions during takeoff From a regulatory point of view, there is no requirement to take into account the temperature inversion for required takeoff performance determination. Due to the low probability of an engine failure at the most critical case, i.e. V1, together with all the conditions leading to a thrust reduction as described in the paragraph 5.1, we believe that this very remote case does not need to be taken into account in addition to the regulatory performance conservatism. This approach is well reinforced by the experience in operations, as no event due to the effect of a temperature inversion occurring during takeoff has ever been reported Additionally, it must be kept in mind that a temperature inversion with a great temperature magnitude remains an exceptional event. Temperature inversions with low magnitude may be more frequently encountered but we have seen that their effect on the aircraft performance is quite negligible. However, more than a general rule, the experience of individual airlines according to the specific weather conditions they encounter on the airport frequently operated should be the main criteria in decision making for accounting of temperature inversions. Some airlines operating in desert regions being subject to frequent temperature inversions have established with their local meteorology agency a policy with regard to the temperature inversions. The inversions being regularly published by the meteorology agency during the day, these operators take them into account in the takeoff performance determination. Pilot reports can be also used for inversions encounter report.


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Although temperature inversions are of a particular concern only when associated with additional conditions such as high OAT, performance and remote obstacles limited takeoff weight and engine failure, large temperature inversions can degrade the takeoff performance. Therefore, if frequently exposed to large temperature inversions, and when they are reported, it is still advisable to take them into account for performance determination particularly if obstacle limited and OAT at or close to T.REF. This permits, as an additional measure, to keep the required margin on the takeoff performance in its whole in the event of an engine failure. This can be made by adding the temperature inversion value to the OAT in order to correct the temperature to be used for performance determination. This precaution could be also considered if your engines are EGT limited at high temperature in order to recover an adequate margin to the EGT red line. If flex takeoff is performed, the flex takeoff can be normally made provided the inversion does not exceed the maximum possible flex temperature for the actual takeoff weight.

5.2.2. Unexpected temperature inversion during takeoff If not reported, there is obviously no way to account for the effect of a possible temperature inversion. If an engine fails during the takeoff while an inversion condition is present, there is no requirement for application of any specific procedure. The low probability of having all the detrimental conditions previously described met together and no possibility of return to the departure airport reinforces this. The abnormal procedures for engine failure will have to be followed and we believe that, during this particular and increased workload situation, there is no room for pilots to speculate for a possible temperature inversion and no way to regain a part of the thrust. This is particularly true for aircraft fitted with a FADEC fully managing the thrust according to the selected Thrust Lever Angle (TLA). For A310 and A300-600 aircraft models without FADEC but with an electronic trim, increase of thrust is still possible but would require the remaining operative engine to be first set at idle in order to deselect the ENG TRIM to prevent an engine overboost. This obviously cannot be recommended when this engine is the only one providing thrust during the takeoff while the aircraft is still at a low altitude. At the very most, and in accordance with the recommended procedures for engine failure during takeoff, in the case where flexible takeoff was used, the performance may be improved if required, by setting the operative engine to the full takeoff thrust.

Actual OAT

Max Flex temp

Actual TOW

Max TOW

Temp inversion Normal Flex

takeoff can be performed without restriction


Chapter 25 Page 15

For A300 B2/B4 which are equipped with pure hydro-mechanical controls, the N1 / EPR can be increased during takeoff if not already at the maximum setting but this would be detrimental to the other parameters (N2 and EGT). The FCOM chapter Procedures and Techniques (see chapter 8.03.14) has a specific procedure for temperature inversion during takeoff. This procedure permits an increase of the N1 / EPR while monitoring the other parameters in order to prevent limits exceedance. Such a procedure should be considered as an additional precaution which is acceptable on aircraft operated with 3 crewmembers while one crew member may have the dedicated task to closely monitor the engine parameters during a takeoff with an engine inoperative. However, whatever the aircraft, there is no requirement for monitoring the temperature (SAT, TAT) in case of an engine failure at takeoff in an attempt to detect any unlikely temperature inversion.

6. CONCLUSION This article has highlighted the effect of the temperature on the engine performance at takeoff. The reduction of thrust when the outside air temperature increases beyond the T.REF is a design choice and permits to reach a compromise between the engine (and aircraft) performance and the engine life span. This is the result of the flat rate concept. Thanks to this concept, the engine is protected against EGT limits exceedance with some margins and engine deterioration is limited. The aircraft performance is determined in accordance with this flat rate concept. The takeoff performance is based on a constant reduction of the temperature with the altitude. However, as discussed in this article, specific weather conditions may lead to temperature inversions. There is no doubt that temperature inversions have a direct effect on the engine and the aircraft performance during the takeoff climb. This effect can be completely ignored when all engines are operative. When of a great magnitude and when combined with other severe conditions such as an engine failure at V1, high OAT and performance limited by remote obstacles, it may become a concern. But combination of all these events is unlikely to occur. Despite that there is no regulation requiring the taking into account of such an effect for takeoff performance determination, temperature inversions with a great magnitude, when known, should be considered. This is particularly true if you are operating in areas frequently affected by inversions with a great magnitude. On this subject, in the light of this article, your experience and your knowledge of the environmental conditions you frequently operate will be the best clue in deciding for account of temperature inversions in the takeoff performance determination.

temp inversion

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