What happened
On 20 April 2001, a Jetstream 4100, registration G-MAJA, was performing a passenger flight from Glasgow to Cardiff. While climbing through flight level 240, the flight crew observed a 'Toilet Smoke' warning on the Central Annunciator Panel. Although the flight deck remained clear of smoke, the cabin attendant reported a blue haze within the passenger cabin.
In response to the visual evidence of smoke, the pilot declared a 'Mayday' and initiated a return to Glasgow. The crew executed the required emergency checklists. During the descent, the smoke warning disappeared, allowing the pilot to downgrade the distress call to a 'PAN' signal. The aircraft landed at Glasgow without further complications. There were no injuries to the 3 crew members or 13 passengers on board.
The investigation
The investigation examined the aircraft's Environmental Control System (ECS), which utilizes engine bleed air cooled by two Air Cycle Machines (ACM). Investigators reviewed the maintenance history of G-MAJA, noting a similar incident involving a stiff ACM occurred the previous day. Following the April 20 incident, the ACM was replaced.
Further inspections revealed a pattern of intermittent issues, including a report of smoke during landing and taxi on 1 May, and a 'hot' smell accompanied by a toilet smoke warning on 27 July. During the July inspection, investigators discovered clear evidence of oily contamination within the ACM and the heat exchanger. This led to the replacement of the left engine and ACM heat exchanger.
Findings
- The primary cause of the smoke and haze was identified as oil contamination within the ECS components.
- While the aircraft manufacturer and ECS supplier reviewed the system, they could not definitively rule out the ECS as the source, though they noted the system is designed to use dry bearings to prevent oil leaks.
- The investigation focused on the left engine, as the presence of oil in the heat exchanger suggested the leak originated from within the engine itself. The engine was sent for a detailed strip examination to determine the exact source of the intermittent leak.