Safety Assumptions

Combustor Can Weld Repair

It was assumed that the weld repair of the Number 9 combustor can was appropriate. British Airtours had performed a fusion weld on the combustor can. Pratt & Whitney knew that this type of weld was not appropriate for the type of crack that had been repaired. A critical miscommunication by the engine manufacturer and a faulty assumption by the airline led to British Airtours believing that the repair made was in fact appropriate, stating "The manufacturer had advised operators that direct fusion weld repaired cans have lower fatigue lives than ones repaired using material replacement techniques but had not quantified this reduction. British Airtours interpreted this as applicable to cans with a much greater time-in-service than any they operated at the time."

Fuel Tank Access Panel Design

There were no design provisions for impact resistance of the tank access covers, nor was there a regulatory requirement for such.

Escape Slide Jam

It was assumed by manufacturers that the escape slide would deploy when the doors were opened, regardless of the time taken to open the door. However, a design fault with the slidebox lid-release lanyard caused the door to jam during rapid door opening; the door unlocked normally, but as it was moving out through the aperture, the slide container lid jammed on the doorframe, preventing further movement of the door.

Cabin Address System Volume

It was assumed that cabin address system volume would be adequate in all emergency situations. The cabin address system volume was linked to the left engine operation during normal revenue service. The volume needed to be reduced when engines weren't operating. However, during the accident when the left engine failed, the decibel level of the cabin address system automatically dropped to a lower level. As a result, the captain's call for evacuation was initially not heard.

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