What happened
On 12 August 1999, a Boeing 737-50 and registration G-MSKB, operated by Maersk Air UK, was descending from 37,000 feet toward Geneva-Airport when the cabin crew reported a strong, acrid smell. While the flight crew initially attempted to troubleshoot the issue by checking galley power and adjusting air pack settings, the situation escalated during the descent. As the pilots reduced power to manage the descent profile, a thick smoke began to spread through the mid and rear sections of the cabin, significantly reducing visibility for the cabin attendants.
The flight crew declared a "pan-pan" call and executed emergency smoke procedures. The aircraft completed a rapid descent and landed safely on runway 23 in Geneva. Although the crew initially planned to evacuate via the aircraft's airstairs, the accidental shutdown of the APU during post-landing checklists prevented the stairs from extending. Consequently, the 40 passengers and crew evacuated using the inflatable slides. There were no injuries and no damage to the aircraft.
The investigation
Investigators focused on identifying the source of the smoke-like aerosol, as no actual fire was detected within the airframe. Because the smell was difficult to categorize, the investigation involved a systematic examination of all aircraft systems, specifically targeting the air conditioning system.
Technical analysis revealed that the contamination was linked to the right-hand engine. A borescope inspection of the 9th stage high-pressure bleed air manifold found small traces of burned oil in the tubing. The investigation noted that the high-pressure bleed valve on the right-hand engine had been replaced the night before the incident to address previous pressure fluctuations.
Findings
- The primary cause of the incident was engine oil penetrating the air conditioning system through the right-hand high-pressure bleed air manifold.
- As the power levers were retarded during descent, the high-pressure bleed supply engaged to maintain pressure, injecting the oil into the cabin air stream where it evaporated into a smoke-like aerosol.
- The flight crew and cabin crew followed established emergency procedures effectively.
- Air traffic control and airport rescue services provided timely and appropriate assistance.
Safety action
To prevent similar occurrences, it is recommended that aircraft utilizing engine or APU bleed air for air conditioning and pressurization be equipped with a system capable of detecting smoke or contaminants within the air conditioning system.