Incident Summary
On 31 March 2017, an Airbus A319-111, registration G-EZIM, experienced smoke and an acrid smell in the cockpit and cabin shortly after takeoff from Isle of Man Airport. The aircraft had been dispatched with a seized Air Cycle Machine (ACM) in Pack 1, which was operated in 'heat exchanger mode' per the Minimum Equipment List (MEL). The commander declared a MAYDAY and returned to land. The landing was uneventful, and no injuries occurred.
Background
Three days earlier, an overheat defect was reported for Air Conditioning Pack 1. Troubleshooting on the morning of the incident determined that the ACM in Pack 1 had seized. The pack was reinstated for in-flight use only in 'heat exchanger mode', which allows operation when the total air temperature (TAT) is 12°C or below.
History of the Flight
The outbound flight from Bristol to Isle of Man was uneventful. About 90 seconds after takeoff from Runway 26 on the return flight, the commander noticed a light grey mist from the vent above his windshield, followed by an acrid burning smell. The cabin manager reported a burning smell about 30 seconds after liftoff, with grey/hazy smoke appearing seconds later. The commander transmitted a MAYDAY, requested a level-off at FL50, and handed control to the co-pilot. The crew decided to return; smoke dissipated near the end of the downwind leg, but the smell remained. An uneventful landing was made, and the aircraft was escorted to the stand.
Aircraft Examination
Post-incident troubleshooting deactivated Pack 1 and tested Pack 2 with no smoke. The Pack 1 ACM was replaced, and no further occurrences were reported in 903 hours of subsequent operation. The removed ACM was sent for overhaul, where it was found to have a seized rotary body. The turbine wheel was damaged by friction on the turbine scroll, and the rotary tie rod was also damaged. The ACM had accumulated 25,000 flying hours.
MEL Procedures
The aircraft manufacturer's MEL 21-52-01G allowed operation of a pack with a failed ACM in heat exchanger mode when certain alerts were absent. The manufacturer stated they had no records of a seized ACM starting to turn in flight from other customers. They could not determine the root cause of the failure based on the available examination.
Analysis
The smells and smoke/mist occurred after Pack 1 was turned on, which contained a seized ACM. Operating the pack in heat exchanger mode causes air to flow through the ACM, possibly turning it, but friction from bearing damage generated heat and smells. The symptoms did not occur on the earlier flight, but replacing the ACM resolved the issue, supporting the conclusion that the ACM was the cause.