Optics, Meteorology, and an Airline Crash Investigation

Michael Dubson

Physics Dept, U. of Colorado at Boulder

(SSP class of 1973)

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)

Boeing 727

BEFORE

AFTER

Crew of 3:pilot, co-pilot, flight engineer.All 3 survived.

Conditions•Dark, hot, humid•Clear, visibility 8 miles

Pilots' view on approach to runwayPAPI

PAPIPrecision Approach Path Indicator~keeps plane on 3o glidepath

Very HighHighGoodLowVery Low

3o glide path

3o

20'

35'PAPI

runway ground

4 whites

1 red

3 reds

4 reds

2 reds

3o , actually

• 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.

PAPI = Precision Approach Path Indicator

PAPI schematic:

It's a simple slide projector:

lens slide light source

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

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 sourcefar 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 vanishesB) Image 2 vanishesC) Something else happens.

1 2 screen

masklens

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.

Mechanisms of Heat Transfer

• Conduction

• Convection

• Radiation

Direct touching of atoms

Bulk motion of hot matter

Energy carried by electromagnetic radiation

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

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

PAPI clear

PAPI fogged

Human Perception of Color

The human brain perceives color largely by comparing different colors in the visual field.

Pink PAPI lights can look white when compared with red runway or cockpit lights.

Pilots' view on approach to runwayPAPI

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.

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.)

Physics involved:

• Geometrical Optics

• Scattering by diffraction

• Heat transfer mechanisms

• Meteorology

• Perception of color