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Computer-Related Incidents with Commercial Aircraft

The Air Inter A320 Accident near Strasbourg

20 January 1992

Synopsis Approaching Strasbourg Airport on a VOR/DME approach to Runway 05, Air Inter Flight 148 DA initiated a 3,300 fpm descent at 11 DME from the VOR (STR) at 5,000ft altitude. At 9DME, they collided with Mont St.-Odile at an altitude of 2,620ft, at which point they would normally have been at 4,300ft altitude and an 800fpm descent to cross STR at 1320ft on their way to touch down.

The investigation commission used the SHEL model, which provides a conceptual framework for understanding the interfaces between different `subsystems' in operation. SHEL focuses on the four basic subsystems: software, hardware, "environment" and "liveware" (people). No definitive story was determined as to how the extraordinary rate-of-descent was actually initiated. The commission analysed all of the possible alternative scenarios thoroughly and based their conclusions and recommendations on these alternatives.

This accident generated much discussion and controversy within the aviation community, focusing often on the design of the autopilot interface, specifically the mode change between HDG V/S (Heading and Vertical Speed mode) and TRK FPA (Track and Flight Path Angle) modes, which were set by a `toggle'-type switch.

We include the Report of the Commission of Inquiry (in French) in full.

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