Figure: Collision geometry as viewed from in front of the DC9, from NTSB Report, Figure 7.

Eddie Sez:

Just because you are on an IFR flight plan doesn't mean you have the sky to yourself, as this mishap clearly illustrates. The Aeroméxico was technically in the right but all aboard perished after a light aircraft encroached on their airspace. The "See and Avoid" concept was found causal, as well as the light aircraft. But sometimes your last line of defense — your eyes — is the only defense you have left.

A lot has happened over the last 20+ years with the mandatory inclusion of TCAS on most aircraft. The problem remains, however, because still not everyone has TCAS. On a VMC day when low to the ground, one pilot's eyes have to be outside. When there is a mix of traffic and not everyone is playing by the IFR rules, you need to have your head on a swivel, your eyes outside the cockpit. Every pilot should know the Big Sky Theory doesn't work.

What follows are quotes from the sources listed below, as well as my comments in blue.


Accident Report

First Aircraft:

Second Aircraft:


Narrative

[NTSB Accident Report 87/07, page v.]

Findings

[NTSB Accident Report 87/07, ¶3.1]

  1. The airplanes collided at a 90° angle, at an altitude of about 6,560 feet, and in visual meteorological conditions. The collision occurred inside the Los Angeles TCA.

  2. Both pilots were required to see and avoid the other airplane. There was no evidence that either pilot tried to evade the collision.

  3. The pilot of the Piper was not cleared to enter the Los Angeles TCA. His entry was inadvertent and was not the result of any physiological disablement.

  4. The unauthorized presence of the Piper in the TCA was a causal factor to the accident.

  5. The positions of the Piper were displayed on the AR-l controller's display by, at the least, an alphanumeric triangle; however, the Piper's primary target may not have been displayed or may have been displayed weakly due to the effects of an atmospheric temperature inversion on the performance of the radar. The analog beacon response from the Piper's transponder was not displayed because of the equipment configuration at the Los Angeles TRACON.

  6. The AR-l controller stated that he did not see the Piper's radar return on his display, and, therefore, did not issue a traffic advisory to flight 498. His failure to see this return and to issue a traffic advisory to flight 498 contributed to the occurrence of the accident.

  7. The Los Angeles TRACON was not equipped with an automated conflict alert system which could detect and alert the controller of the conflict between the Piper PA-28 and flight 498.

Probable Cause

[NTSB Accident Report 87/07, page v.] The National Transportation Safety Board determines that the probable cause of the accident was the limitations of the air traffic control system to provide collision protection, through both air traffic control procedures and automated redundancy. Factors contributing to the accident were (1) the inadvertent and unauthorized entry of PA-28 into the Los Angeles Terminal Control Area and (2) the limitations of the "'see and avoid" concept to ensure traffic separation under the conditions of the conflict.

Figure: Probability of seeing the other aircraft as a function of time, from NTSB Aircraft Accident Report 87/07, Figure 3.

Back in 1986 many of us gave too much credit to our IFR flight plans to provide protection from itinerant aircraft blundering into our flight paths. Now with TCAS, many of us rely on the electronic negotiation between aircraft for the same "warm and fuzzy." But TCAS relies on the other aircraft being so equipped and its pilots reacting correctly to its warnings. Your last line of defense remains your eyes. This accident spurred a study on pilot scanning techniques and reaction times. If you are in a traffic pattern environment, IFR or VFR, your eyes need to be outside. The study shows this practice can make a critical difference. More about this: Normal Procedures & Techniques / Big Sky Theory.


See Also:


References

May Day: Out of Sight, Cineflix, Episode 34, Season 4, 17 May 2007 (Aeromexico 498)

NTSB Aircraft Accident Report, AAR-87/07, Collision of Aeronaves de Mexico, S.A., McDonnell Douglas DC-9-32, XA-JED and Piper PA-28-181, N4891F Cerritos, California, August 31, 1986