What happened
On 31 March 2017, an Airbus A319-111, registration G-EZIM, was performing a commercial passenger flight from Bristol to the Isle of Man. Approximately 90 seconds after departing Runway 26 at Isle of Man Airport, the commander observed a light grey mist emerging from a cockpit vent, accompanied by a sharp, burning smell. Simultaneously, cabin crew reported that the cabin was filling with a hazy, grey smoke and a similar acrid odor.
The flight crew declared a MAYDAY and requested a level-off at 5,000 feet. While the co-pilot maintained control of the aircraft, the commander communicated with the cabin manager to brief the crew on the potential for an emergency evacuation. The crew decided to return to the airfield for an immediate landing. During the approach, the visible smoke began to dissipate, though the unpleasant smell persisted. The aircraft landed safely and without further incident, with no injuries reported among the 124 passengers or 6 crew members.
The investigation
The AAIB investigation focused on the condition of the aircraft's air conditioning system. It was discovered that the aircraft had been dispatched with a known defect in Air Conditioning Pack 1. Following maintenance performed on the morning of the incident, it was determined that the Air Cycle Machine (ACM) within Pack 1 had seized. Under the aircraft's Minimum Equipment List (MEL), the pack had been configured to operate in 'heat exchanger mode' for the return leg.
Examination of the faulty ACM during its overhaul revealed significant damage. The turbine wheel had suffered friction damage from the turbine scroll, and the rotary tie rod also showed signs of frictional wear. Testing confirmed the rotary body was indeed seized. Subsequent testing of Pack 2 showed no similar issues, and the replacement of the Pack 1 ACM resolved the problem, as no further smoke or smells were reported in subsequent flights.