What happened
On 2 November 2003, a Boeing 777-200B, registration N795UA, was operating a passenger flight from Frankfurt to Washington. Prior to departure, the crew identified a fault in the left air conditioning pack, which resulted in the flight being delayed by four hours while engineers deactivated the defective air cycle machine (ACM) under the operator's Minimum Equipment List.
Approximately 90 minutes into the flight, while cruising at FL330 approximately 100 nm northwest of Glasgow, the cabin crew reported heavy smoke in the cabin. Although no flames were visible, the smoke was increasing in density. The commander declared a MAYDAY and initiated an immediate diversion to Glasgow. During the descent, the crew inspected the rear galley area by removing ceiling panels, but found no evidence of fire, and the smoke began to dissipate.
During the descent, a second fault appeared in the right air conditioning pack. Despite the smoke, the crew did not find it necessary to don oxygen masks. The aircraft landed at Glasgow approximately 28,00 and lbs overweight because the crew had to terminate fuel jettisoning to prepare for approach. No injuries were reported to the 261 passengers or 15 crew members, and no damage was found to the aircraft interior.
The investigation
Investigators examined the aircraft's environmental systems and found that both the left and right ACMs were defective. Further engineering analysis revealed that the right ACM's 2nd stage turbine bypass valve had likely seized in the closed position. The investigation also identified that the condenser/reheater units in both packs were blocked.
Findings
- The smoke in the cabin was caused by the failure of the air cycle machines' air bearings.
- These failures were triggered by an imbalance in the rotating machinery caused by ice formation on the turbine blades.
- Moisture had entered the first turbine stage because blockages in the condenser/reheater units prevented efficient water separation.
- The blockages were caused by the accumulation of particulates, moisture, or oil on the heat exchanger inlet surfaces.
- Both units failed nearly simultaneously as they had been in service for the same duration.