Pacific Southwest Airlines Flight 182 & Cessna 172 - mid-air collision

Accident details

Date: 25 September, 1978
Location: North Park, San Diego, CA, near San Diego International Airport [SAN]
Accident type: Mid-air collision
Fatalities: 144 including 7 on the ground
Primary causes: Errors made by the pilots of both aircraft and air traffic control [ATC]
coupled with the inherent limitations of 'see and avoid'

Aircraft #1

Operator: Pacific Southwest Airlines [PSA]
Flight number: 182
Aircraft type: Boeing 727-200
Fatalities: 135 of 135

Aircraft #2

Operator: Gibbs Flite Center [sic]
Registration number: N7711G
Aircraft type: Cessna 172
Fatalities: 2 of 2

Introduction

The following is a human factors analysis of the mid-air collision of Flight 182 and Cessna 172 N7711G; factual information of the sequence of events is drawn primarily from the official accident report composed by the National Transportation Safety Board [NTSB] (19791).

Sequence of events

Background information

The PSA 727, Flight 182, was en-route to San Diego International Airport from Sacramento via Los Angeles. The Cessna 172 was an IFR training flight, with the student wearing a hood to limit his vision to the aircraft's interior and hence simulate IFR conditions.

Initial contact with ATC

The San Diego approach controller notified Flight 182 of the Cessna, and the captain replied confirming they had the traffic in sight. Both aircraft were on the same course, with the Boeing catching up to the Cessna from behind, so that the Cessna pilots could not have seen the 727. Furthermore, the Cessna pilots did not maintain the heading given to them by ATC; this took them into the path of Flight 182. The approach controller then radioed Flight 182, telling the crew to maintain visual separation and to change to the tower frequency. San Diego tower then informed the Cessna pilot that the PSA pilots had them in sight, and reminded Flight 182 of the Cessna's position, now at their 12 o'clock, and 1 mile distant.

Confusion of PSA pilots

Flight 182's CVR then recorded confusion among the pilots as to the location of the Cessna. The Captain made an ambiguous radio call, recorded on the CVR as “I think he's pass(ed) off to our right”, recorded on the ATC transcript as “think he's passing off to our right” and according to the controller, “he's passing off to our right”. This lead the controller to believe that Flight 182 still had the Cessna in sight, when they actually did not. A few seconds after the radio transmission, flight 182's first officer asked “Are we clear of that Cessna?”, to which the other three cockpit occupants replied: “Supposed to be”, “I guess”, and “I hope”.

Traffic conflict warning

The approach controller received a traffic conflict warning shortly thereafter but did not act on it, believing that the PSA crew would maintain separation. Furthermore, these traffic conflict warnings occurred frequently without any actual conflict.

Collision

Nineteen seconds later, the PSA 727 caught up to the Cessna and its right wing cut through the light aircraft, destroying it instantly. The 727's right wing and empennage were severely damaged and the pilots could not maintain control of the aircraft. All 135 of flight 182's occupants and the 2 Cessna occupants were killed, along with 7 people on the ground in, at that time, the deadliest plane crash in the United States.

220px-WendtPSA.jpg
PSA Flight 182 shortly after the collision
(Image embedded from Wikipedia on 18 October, 2011)

Causal factors

Cessna pilots

Because the Cessna pilots did not maintain their given heading, it set the two aircraft on a collision course. It is not known why the pilots deviated about 20 degrees from the correct heading. Even so, this fact is relatively minor, as it was still the responsibility of the flight crew of Flight 182 to maintain separation.

Flight 182 pilots

Even when the pilots lost sight of the Cessna, they did not explicitly state as such to ATC. The ambiguous radio call was not sufficient to notify ATC that they were unsure of the Cessna's position and simply confused the controller. The flight crew of Flight 182 were responsible for maintaining separation, and just assuming they had passed the Cessna showed a serious hazardous attitude of invulnerability, and poor decision making skills; "hoping" and "guessing" is not good enough.

ATC

While it was the pilots of Flight 182 who were responsible for maintaining separation, it should be questioned whether visual separation should have been applied by the controller. It would have been possible to radar vector Flight 182 to ensure separation. The controller should have also clarified the ambiguous communications from Flight 182. Not passing on the traffic conflict warning was a significant error but given the many false warnings the system gave it is understandable, and the main cause of this is the latent error of the poor system.

Traffic conflict warning system

The usefulness of a warning system that gives a significant amount false alerts is highly questionable. Leaving this poor system in place was a considerably dangerous latent error, which provided the opportunity for this accident to occur. Had the system been reliable, the warning subsequently passed on, and the pilots initiated evasive maneuvers, the accident most likely would have been averted.

'See and avoid'

The subsequent investigation covered in significant detail the ability of the PSA pilots to 'see and avoid' the Cessna. When the approach controller told the crew of Flight 182 to maintain visual separation, it meant that the pilots had to visually sight the Cessna and keep clear of it. As the Cessna was facing away from the 727, there was no possibility of its pilots seeing the airliner approaching from behind. The investigation determined that the crew of flight 182 could have seen the Cessna at all times during the accident sequence, but they still lost sight of it.

Human limitations

Subsequent studies have shown that in a high-speed jetliner environment, there are many aspects of human vision and information processing that can make it very difficult to prevent collisions by 'see and avoid' principles.

Lack of relative motion

One of the most significant aspects in any mid-air collision is the fact that any two aircraft on a collision course will have essentially no relative motion, and a pilot looking at the other aircraft will only see a motionless speck that quickly grows in size just before the collision (Civil aviation authority [CAA], 20092). It is therefore easy for the aircraft to blend in with the background or even marks on the windscreen and avoid detection. Unfortunately, the human visual system, comprised of the eyes and brain, can pick out moving objects much easier than stationary objects, and the lack of relative motion in this situation would have significantly reduced the pilots' ability to keep the Cessna in sight (CAA, 20092).

Ground clutter

Another consideration is that as the 727 was descending and the Cessna was climbing, the pilots of Flight 182 would have experienced difficulty trying to pick out the relatively motionless Cessna from the 'ground clutter' of residential San Diego. The human visual system detects objects primarily by observing borders between different objects, and in this case between aircraft and the background (Hess, Dakin, Kapoor, & Tewfik, 20003). As the Cessna appeared stationary to the pilots of PSA Flight 182, it blended in and became like one of the many objects on the ground below. The abundance of objects and borders between them would have made it even more difficult for the pilots to successfully identify the Cessna.

Contour interaction

A further disadvantage of viewing the Cessna against the ground clutter is 'contour interaction', which is when objects blend in to their surroundings so that the visual system 'sees' an object, but does not recognise it (Hess et al., 20003). This is the principle on which camouflage works, by breaking up contours so that objects become unrecognisable.

Blind spot

The human eye has a blind-spot where the optic nerve passes through the retina to exit the eyeball. Under normal conditions, the brain fills in this blind spot with information from the surrounding environment, and by comparing information received from each eye (Ewing, 20034). However, when the vision of one eye is obscured, the blind-spot of the other eye becomes apparent because the brain does not have the capability to compare information between eyes. The Boeing 727's main windscreen is comprised of six windows, which necessitates five window-posts, each of which can partially block the vision of an eye and render objects in the blind-spot of the other eye invisible.

Mandelbaum effect

While the blind-spot is a minor factor, it can be exacerbated by the Mandelbaum effect, which occurs when objects close to the eye, such as window-posts, cause the eye to focus at its natural focus point of about 50 cm (Endsley, Garland, Shook, Coello, & Bandiero, 20005; Robson, 20086). This temporary myopia may go unnoticed by the pilot and significantly reduce the chance of spotting other aircraft. The high speed of the PSA 727 gave the pilots less time to spot the Cessna, and no evasive action was attempted before impact.

Conclusion

This accident was caused by an unauthorised course deviation by the Cessna pilots, poor separation techniques by the PSA pilots, controller errors, and a faulty collision warning system. It clearly highlighted a number of problems with the 'see and avoid' principle, especially in high-speed aircraft environments.

References
1. NATIONAL TRANSPORTATION SAFETY BOARD. (1979). Aircraft accident report: NTSB-AAR-79-5. Washington DC: Author.
2. CIVIL AVIATION AUTHORITY. (2009). Vector. Wellington, NZ: Author.
3. HESS, R. F., DAKIN, S. C., KAPOOR, N., & TEWFIK, M. (2000). Contour interaction in fovea and periphery. Journal of the Optical Society of America A, 17(9), 1516-1524.
4. EWING, R. L. (2003). Aviation medicine and other human factors for pilots. Christchurch, New Zealand: Old Sausage.
5. ENDSLEY, M., R., GARLAND, D. J., SHOOK, R. W. C., COELLO, J., & BANDIERO, M. (2000). Situation awareness in general aviation pilots. Marietta, GA: SA Technologies.
6. ROBSON, D. (2008). Human being pilot. Cheltenham, Australia: Aviation theory limited.

Want to know more?

Flight 182 on Wikipedia
Flight 182 on Airdisaster.com
Flight 182 at the Aviation safety network
Full NTSB report on Flight 182
ATC transcript at planecrashinfo.com
'See and avoid' by ICAO on AviationKnowledge

Contributors to this page

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