optics, meteorology, and an airline crash investigation michael dubson physics dept, u. of colorado...
TRANSCRIPT
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Optics, Meteorology, and an Airline Crash Investigation
Michael Dubson
Physics Dept, U. of Colorado at Boulder
(SSP class of 1973)
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FedEx Flight 1478
Crashed on landing,
½ mile short of runway, Tallahasee Regional Airport,
5:37 am, July 26, 2002.
(1.3 hr before sunrise)
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Boeing 727
BEFORE
AFTER
Crew of 3:pilot, co-pilot, flight engineer.All 3 survived.
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Conditions•Dark, hot, humid•Clear, visibility 8 miles
Pilots' view on approach to runwayPAPI
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PAPIPrecision Approach Path Indicator~keeps plane on 3o glidepath
Very HighHighGoodLowVery Low
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3o glide path
3o
20'
35'PAPI
runway ground
4 whites
1 red
3 reds
4 reds
2 reds
3o , actually
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• NTSB investigation determines that crash was due to pilot errors.
• FedEx fires all 3 crew.
• Union rules allow appeal before a 3-person arbitration board.
• Pilots vs. FedEx: Each side hires an "expert" to investigate causes of crash and possibly testify.
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PAPI = Precision Approach Path Indicator
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PAPI schematic:
It's a simple slide projector:
lens slide light source
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The PAPI Optical System
2 lenses act effectively as one good lens
Glass cover plate
Red transparency plate
Reflector throws light forward to produce brighter beam
Empty space below red plate acts as white part of "slide"
We have verified correct operation of our reconstructed PAPI system:
• brightness• angular width of beam
• sharpness of white/red transition • aviation red color
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Focal Length of a Lens
parallel rays
focal point
focal length f > 0
short f long f
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A bundle of parallel rays approaches the eye and some of the rays enter the eye's pupil. No other rays enter the eye. What does the eye see?
A) A single point of light, surrounded by blackness.B) A uniformly illuminated wall of light, like a white wall.C) Many scattered points of light, like stars in the night sky.D) None of these
Eye
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point sourcefar from source, small bundle of rays is nearly parallel
Parallel bundle of rays is created by point source at infinity:
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Two point sources of light are imaged onto a screen by a converging lens as shown. The images are labeled 1 and 2. A mask is used to cover up the left half of the lens, as shown. What happens to the images on the screen when the mask covers the left half of the lens?
A) Image 1 vanishesB) Image 2 vanishesC) Something else happens.
1 2 screen
masklens
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A hot copper cube initially at Ti = 40o C, is suspended from a thread outside. The air temperature is a constant T = 20O C. The cube cools. Is it possible that the cube can cool to a final temperature Tf < 20O C?
A) YesB) No, because this would violate
Conservation of EnergyC) No, because this would violate the
2nd Law of ThermodynamicsD) No, because of some other
reason.
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Mechanisms of Heat Transfer
• Conduction
• Convection
• Radiation
Direct touching of atoms
Bulk motion of hot matter
Energy carried by electromagnetic radiation
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Dewpoint: temperature of air at which water vapor will precipitate (rain). Dewpoint depends on humidity: high humidity high dewpoint100% humidity Tdewpoint = Tair
Water condenses onto a surface when Tsurface < Tdewpoint
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Condensation on glass cover plate
strongly scatters light
clear glass
fogged glass
Small droplets scatter much more strongly than large droplets:
•You can see through glass with rain drops on it, but not through fogged glass.
•You can see through falling rain, but not through clouds or fog.
Scattering is very strong because droplet size wavelength of light strong diffraction
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PAPI clear
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PAPI fogged
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Human Perception of Color
The human brain perceives color largely by comparing different colors in the visual field.
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Pink PAPI lights can look white when compared with red runway or cockpit lights.
Pilots' view on approach to runwayPAPI
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Conditions were ideal for condensation:
• 100% humidity (Temperature = dew point)• Clear sky: Objects that can see the sky cool by radiation.• Time is just before dawn: PAPI and surroundings have
had all night to cool.• PAPI was turned on remotely by pilots only 3 minutes
before landing. PAPI front coverplate surface takes at least 10 minutes to defog.
• Airport employees reported fogging of windows.
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My conclusions:• Extremely likely that PAPI's fogged, causing pilots
to see 4 whites (too high).• Pilots made multiple errors, but PAPI fogging was
the final contributing factor leading to the crash
NTSB conclusions:• Crash due solely to pilot errors. • PAPIs operated normally.
Arbitration board conclusions:• Crash due solely to pilot errors. (I was not asked to testify because my conclusions contradict pilots' testimony that they saw PAPI 2 whites/ 2 reds.)
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Physics involved:
• Geometrical Optics
• Scattering by diffraction
• Heat transfer mechanisms
• Meteorology
• Perception of color