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

5:37 am, July 26, 2002: FedEx Flight 1478 crashed on landing, ½ mile short of runway, Tallahassee Regional Airport, 1.3 hr before sunrise

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

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

Precision Approach Path Indicator ~keeps plane on 3 o glidepath Very HighHighGoodLowVery Low

3 o glide path 3o3o 20' 35' PAPI runwayground 4 whites 1 red 3 reds 4 reds 2 reds 3 o, to scale

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.

PAPI = Precision Approach Path Indicator

PAPI schematic: It's a simple slide projector: lens slidelight source

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

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

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

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

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

A hot copper cube initially at T i = 40 o C, is suspended from a thread outside. The air temperature is a constant T = 20 O C. The cube cools. Is it possible that the cube can cool to a final temperature T f < 20 O C? A) Yes B) No, because this would violate Conservation of Energy C) No, because this would violate the 2 nd Law of Thermodynamics D) No, because of some other reason.

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

Dew point: T air at which water vapor precipitates (rain). Dew point depends on humidity: high humidity  high dew point 100% humidity  T dewpoint = T air Water condenses onto a surface when T surface < T dewpoint

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

My condensation maker: a steam cleaner

PAPI clear

PAPI fogged

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

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

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.

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

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. PAPI operated normally. Arbitration board conclusions: Crash due solely to pilot errors.

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

Physics involved: Geometrical Optics Scattering by diffraction Heat transfer mechanisms Meteorology Color vision

NTSB Investigation Team vs. Me

NTSB conclusions: My conclusions: wrong ~10 min

Dubson says: NTSB says: Very unlikely Why should you believe me?

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