1. Optics, Meteorology, and an Airline Crash Investigation
Michael Dubson
Physics Dept, U. of Colorado at Boulder
(SSP class of 1973)

2. 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)

4. Boeing 727 BEFORE AFTER Crew of 3: pilot, co-pilot, flight engineer.
All 3 survived.

5. Pilots' view on approach to runway
Conditions
Dark, hot, humid
Clear, visibility 8 miles
Pilots' view on approach to runway
PAPI

6. Precision Approach Path Indicator ~keeps plane on 3o glidepath
PAPI
Precision Approach Path Indicator
~keeps plane on 3o glidepath
Very Low
Very High
Good
High
Low

7. 3o glide path 3o 3o , actually 4 whites 1 red 20' 2 reds 3 reds 4 reds
PAPI
35' 3o
runway
ground
3o , actually

8. 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.

9. PAPI = Precision Approach Path Indicator

10. PAPI schematic:
It's a simple slide projector:
slide
light source
lens

11. The PAPI Optical System
Glass cover plate
Red transparency plate
Empty space below red plate acts as white part of "slide"
Reflector throws light forward to produce brighter beam
2 lenses act effectively as one good lens
We have verified correct operation of our reconstructed PAPI system:
brightness
angular width of beam
sharpness of white/red transition
aviation red color

14. Focal Length of a Lens focal point parallel rays focal length f > 0
short f
long f

15. 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?
Eye
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

16. Parallel bundle of rays is created by point source at infinity:
far from source,
small bundle of rays
is nearly parallel

17. 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? 1 2
screen
lens
mask
A) Image 1 vanishes
B) Image 2 vanishes
C) Something else happens.

18. 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?
Yes
No, because this would violate Conservation of Energy
No, because this would violate the 2nd Law of Thermodynamics
No, because of some other reason.

19. Mechanisms of Heat Transfer
Conduction
Convection
Radiation
Direct touching of atoms
Bulk motion of hot matter
Energy carried by electromagnetic radiation

20. Dewpoint: temperature of air at which water vapor will precipitate (rain).
Dewpoint depends on humidity:
high humidity  high dewpoint
100% humidity  Tdewpoint = Tair
Water condenses onto a surface when
Tsurface < Tdewpoint

21. 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

22. PAPI clear

23. PAPI fogged

24. Human Perception of Color
The human brain perceives color largely by comparing different colors in the visual field.

27. Pilots' view on approach to runway
Pink PAPI lights can look white when compared
with red runway or cockpit lights.
Pilots' view on approach to runway
PAPI

28. 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.

30. 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.)

31. Physics involved: Geometrical Optics Scattering by diffraction
Heat transfer mechanisms
Meteorology
Perception of color