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Analysis

Summary

The engine bearing seals normally protected the bleed air supplied to the airconditioning packs from oil contamination. A technical defect in one or more of the seals resulted in oil entering the flight deck airconditioning system, with the first indication of the defect being an awareness of smoke on the flight deck. The flight crew correctly donned their oxygen masks as the first step in addressing the problem and then proceeded to comply with the emergency checklist by landing at the nearest suitable airport.

The difficulty that the crew faced in correctly determining the source of the smoke resulted in the initiation of a non-normal checklist that was not pertinent to the situation. The aircraft manufacturer's subsequent revision of the QRH, simplifying and clarifying the checklists, has diminished the likelihood of a similar occurrence. As the incident took place during daylight hours, in visual meteorological conditions close to the departure airport, the potential to affect the safety of flight was limited. It is considered that if the smoke event had occurred later in the flight, the crew would have had more time to carry out the checklist procedures and successfully isolate the source of the smoke.

The possibility that the air conditioning system was supplying contaminated air to the flight deck while the crew was carrying out the emergency procedure for electrical smoke, highlights the importance of crews donning oxygen masks at the first sign of fumes or smoke contamination of flight deck air.

 
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