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Computer-Related Incidents with Commercial Aircraft

The A320 Accident in Warsaw

14 September 1993

Synopsis A Lufthansa A320 landed at Warsaw airport in a thunderstorm. The landing appeared to be normal, smooth, even though somewhat fast. The pilots were unable to activate any of the braking mechanisms (spoilers, reverse thrust, wheelbrakes) for 9 seconds after `touchdown', at which point the spoilers and reverse thrust deployed. The wheelbrakes finally became effective 13 seconds after touchdown. The aircraft was by this time way too far along the runway to stop before the runway end. It ran off the end, and over an earth bank near the end of the runway, before stopping. Both pilots were very experienced A320 operators. The captain was returning to duty after illness and the first officer was a senior Airbus captain and training officer, who was monitoring the captain's flying skills on his return to service. The first officer died in the accident, as did a passenger who was overcome by smoke and didn't evacuate the aircraft, which burned.

The text of the Accident Report from the Polish authorities is reproduced here, along with
selected Appendices, namely
Section 4.2, CVR transcripts,
Section 5, Documentation of the Braking System, and
Section 6, Performance and Procedures Documentation.

The paper Analysing the 1993 Warsaw Accident with a WB-Graph contains a WB-Graph causal analysis of the events and states in the Warsaw Report, prepared by Michael Höhl and Peter Ladkin. 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 Warsaw WB analysis-exposes some fundamental causal factors that were mentioned in the report, but not included in the report's list of probable causes and contributory factors.

Clive Leyman, formerly reponsible for A320 landing-gear engineering at British Aerospace, and now a Visiting Professor at City University, London, has prepared an Engineering Analysis of the Landing Sequence which analyses the effects of all the factors on the stopping distance of DLH2904. Referenced in the analysis are graphs he plotted of Airspeed on Approach, Altitude and Windspeed in the Final Phases, Flare and Derotation Details, Calculated vs. Actual Distances, Stopping Distances, Ground Deceleration, and Runway Friction.

Questions from computer scientists and system experts focused on why the braking systems didn't deploy as expected by the pilots. The RISKS comments are:
Lufthansa in Warsaw by Peter Ladkin;
More News... by Peter Ladkin;
Re: Lufthansa Airbus ... by Udo Voges;
Lufthansa Warsaw Crash--A Clarification by Peter Ladkin;

More and more technical literature is discussing this accident for one reason or another.
The X-31 and A320 Warsaw Crashes: Whodunnit? by Peter Ladkin discusses causes and how to try to ensure more complete coverage of causal relations. The X-31 accident and the Warsaw A320 accident are analysed as examples.
A Model for a Causal Logic for Requirements Engineering, by Jonathan Moffett, Jon Hall, Andrew Coombes and John McDermid suggests a logical theory of causality for engineering and applies that to analyse braking in the Warsaw accident.
Reasons and Causes by Peter Ladkin discusses those notions in general, and comments extensively on the proposal of Moffett et al.


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