1. An Airline Crash Investigation: Physics vs. Lawyers
Michael Dubson
(SSP class of 1973)
Physics Dept
U. Colorado at Boulder

2. (1 flight per day = 40,000 years)
Some Safety Statistics
Odds of dying in any single commercial jet flight:
1 in 14 million
(1 flight per day = 40,000 years)
Odds of dying on drive to the airport:
1.4 fatalities
108 vehicle miles
X 40 miles to Denver Airport =
1 in 1.8 million

3. 5:37 am, July 26, 2002:
FedEx Flight 1478
crashed on landing,
0.7 mile short of runway, Tallahassee Regional Airport,
1.3 hr before sunrise

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

6. Pilots' view on approach to runway
Conditions
Dark, warm, humid
Clear: visibility 8 miles
Pilots' view on approach to runway
PAPI

7. 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

8. 3o glide path 3o 3o , to scale 4 whites 1 red 20' 2 reds 3 reds 4 reds
PAPI
35' 3o
runway
ground
3o , to scale

9. 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 crash and possibly testify.

11. PAPI = Precision Approach Path Indicator

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

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

14. 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

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

16. 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

20. 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.

21. 1 2
screen
lens
mask

22. 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.

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

24. Dew point: Tair at which water vapor precipitates (rain).
Dew point depends on humidity:
high humidity  high dew point
100% humidity  Tdewpoint = Tair
Water condenses onto a surface
when Tsurface < Tdewpoint
Droplet size ~ visible wavelength

25. 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 effects

26. Condensation maker: a steam cleaner

27. PAPI clear

28. PAPI fogged

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

39. Pilots' view on approach to runway
Pinkish-white PAPI lights look white when compared with red runway lights.
PAPI
Pilots' view on approach to runway

40. Conditions were ideal for condensation:
100% humidity (Temperature = dew point)
Clear night sky: Objects cool by radiation.
Time is just before dawn: PAPI and surroundings have had maximum time to cool.
PAPI turned on remotely by pilots only 3 minutes before landing. PAPI front plate surface takes ~ 10 minutes to defog.
Airport employees reported condensation on windows.

41. Does condensation clear when PAPI turned on?
Yes, but slowly.
PAPI on  temp of front plate rises at 1o F / minute.
Need Tsurface > Tdewpoint before dew begins to evaporate.
PAPI turned on remotely by flight crew only 3 minutes before landing.
IR thermometer to check temperature of cover plate

43. Pilot Errors: Chose visual over instrument landing
Miss-identified runway, late lineup, late approach.
Ignored danger signs:
 Wrong descent rate
 Wrong altitude
 Intermittent obscuration of runway

44. My conclusions:
Extremely likely that PAPI's fogged, causing pilots to see 4 whites (too high).
View of 4 whites (safe!) masked danger signs
Pilots made multiple errors, but PAPI fogging was the final link in the error chain leading to the crash
NTSB conclusions:
Crash due solely to pilot errors.
PAPI operated normally.
Arbitration board conclusions:
Crash due solely to pilot errors.

45. Physicist World vs. Lawyer World
Clash of World Views:
Physicist World vs. Lawyer World
I was not asked to testify because my conclusions contradicted pilots' testimony that they saw
2 whites / 2 reds on PAPI.
Physics World: human memory is the least reliable form of evidence
Lawyer World: clients' testimony is the most reliable form of evidence.

46. NTSB Investigation Team
vs. Me

47. Physics involved: Geometrical Optics Mie Scattering
Heat transfer mechanisms
Meteorology
Color vision

48. NTSB conclusions:
My conclusions:
~10 min
wrong

49. NTSB says:
Dubson says:
Very unlikely

51. The Philosophy of Science
The final test of the validity of any idea is
experiment.