What happened
On May 8, 1977, at 12:41 GMT, a Swissair DC-1-30, registration HB-IHF, was preparing for a scheduled flight from Zurich to Tel Aviv. During the takeoff roll on runway 28, the crew experienced a significant impact on the left side of the aircraft at approximately 135-140 knots. This was immediately followed by an engine failure warning.
The captain initiated an aborted takeoff just before reaching the decision speed, V1. Despite the application of full brakes and reverse thrust, the aircraft failed to stop within the runway limits. The aircraft overran the end of the runway at approximately 48 knots, coming to a halt about 80 meters beyond the threshold in a soft, grassy area. While no injuries were reported among the 166 passengers and 15 crew members, the incident caused damage to engines number 1 and 3 due to the ingestion of debris as the landing gear moved into the soft ground. There was also minor damage to ground facilities.
The investigation
The investigation focused on why the aircraft could not stop within the remaining runway distance. Investigators examined the engine failure, the braking performance on the wet surface, and the physical characteristics of the runway. The inquiry included comparative measurements conducted by NASA to analyze ground performance.
Technical analysis of the engines revealed that a faulty compressor inlet temperature (CIT) sensor on engine number 1 had provided incorrect data to the high-pressure compressor variable stator vanes. This led to a compressor stall and a significant drop in engine speed. Furthermore, the investigation looked into the runway surface conditions, noting that while there were no standing puddles, the concrete surface was wet and slippery following recent rain.
Findings
- The primary cause of the overrun was insufficient deceleration on the wet-damp runway, which resulted from a combination of factors including runway grip, overall braking efficiency, and the pilot's use of braking capacity.
- A critical contributing factor was the lack of manufacturer documentation regarding the specific influence of a wet runway on aborted takeoffs.
- The engine failure was triggered by a defective CIT sensor on engine 1, which caused a compressor stall.
- The aircraft's weight and center of gravity were within legal limits.
- The physical presence of the Glatt River near the runway end presented a significant hazard for any aircraft exiting the runway at high speed.
Safety action
- The investigation recommended that certification authorities require manufacturers to verify takeoff performance on wet runways and include these corrections in technical documentation.
- It was suggested that runway 28 at Zurich be grooved to improve water drainage and surface grip.
- Recommendations were made to improve pilot training for aborted takeoffs using flight simulators and to enhance direct communication links between aircraft crews and airport fire services.