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Computer-Related Incidents with Commercial Aircraft

The American Airlines B757 Accident in Cali

20 December 1995

Synopsis The Boeing B757 aircraft is not `fly-by-wire', but relies on an FMGS and other computer systems for its normal operation. The accident report has not yet appeared. I dedicate this section to computer scientist Paris Kanellakis, who perished with his family in the aircraft. This CFIT (Controlled Flight Into Terrain) accident is the first accident for a B757 in a decade and a half of service.

The Aircraft Accident Report (the final report) from the Colombian Aeronautica Civil was released by the NTSB on 27 September, 1996. It is included here in two parts, the text with Appendix A and the Appendices B-F. The NTSB Recommendations to the FAA were published on October 16, 1996. I thank Barry Strauch, Chief of the Human Performance Division of the NTSB, for sending me copies of the final report and the recommendations; and Marco Gröning for engineering the pictures from Appendices B-F.

The paper Analysing the Cali Accident with a WB-Graph contains a WB-Graph causal analysis of the events and states in the Cali Report, prepared by Thorsten Gerdsmeier, Peter Ladkin and Karsten Loer. WB-analyses determine the causal relations between the events of an accident according to a rigorous formal semantics, and may provide insight into the accident. These analyses are presented in the form of a graph whose nodes are critical events and states. The Cali WB analysis-exposes some fundamental causal factors that were mentioned in the report, and also addressed in the NTSB's recommendations to the FAA, but not included in the report's list of probable causes and contributory factors.

Early News The NTSB issued a Press Release containing factual data, whose text contains the press release signed by the Head of the Columbian Oficina de Control y Seguridad Aerea.
The two relevant arrival and approach navigation plates are the Cali VOR/DME/NDB Rwy 19 Instrument Approach Procedure (http://www.jeppesen.com/cali-1.html) and the Cali Rozo One Arrival Procedure (http://www.jeppesen.com/cali.html).
The specialist weekly Flight International included reports and comment in its January editions. Computer-relevance appears in the crew's handling of the FGMS in concert with other procedures. However, they descended below the cleared altitude, and there appear to be other procedural inadequacies in their flying (see also Wally Roberts's TERPS Page for a further comment on this). The FAA is conducting a review of training at AA.

The short paper Comments on Confusing Conversation at Cali by Dafydd Gibbon and Peter Ladkin points out some linguistic features of the ATC - AA965 radio conversation immediately prior to the accident which might have contributed to the crew's confusion.

For those whose patience or WWW-bandwidth is limited, there is a synopsis of contemporary news concerning the FMC memory readout, giving the probable reason for the left turn away from course (namely that the pilots selected the ROZO beacon based on its identifier, but there is a specified difference between the ROZO beacon identifier and its identifier in the FMC database), the probable causes as contained in the final report, and suggested probable causes contained in American Airlines' submission to the docket in August.

As a result of the investigation of this accident, the NTSB issued a collection of safety recommendations on October 1, 1996, with the concurrence of the Aeronautica Civil of Colombia. These recommendations address various issues such as pilot and aircraft performance after the GPWS warning, specifically the feasibility of (retro-)fitting automatic speedbrake retractors (the pilots failed to retract speedbrakes, and also pulled up too far, momentarily going "through" the stick-shaker warning - some investigators believe that the aircraft could have missed the terrain, had an optimal escape manoeuver been executed: Flight International, 9-15 October, p9), modifications to FMS data presentation, evaluation of an Extended-GPWS system, a requirement to positively cross-check positional information on the FMS, certain enhancements to navigation charts, an ICAO review of navaid naming conventions, a program to enhance English fluency of controllers, and various other measures. (These last two measures address concerns also raised in the Confusing Conversation note.) note on the Puerto Plata and Cali accidents, highlighting the human-computer interface (HCI) issues, appeared in RISKS-18.10, was rebroadcast on Phil Agre's RRE mailing list (May 7th), and became the subject of the what's happening column of ACM Interactions, July/August 1996, p13.

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