What happened
On April 12, 2015, a Boeing 737-500, registration D-ABJB, departed Geneva for Frankfurt as a ferry flight. Shortly after takeoff during the initial climb, the flight crew detected a heavy odor of evaporating engine oil within the cockpit. Because the right engine had been replaced the previous day, the pilots initially suspected residual oil, but as the smell persisted, they donned oxygen masks at approximately 0 as 09:31 UTC.
Upon reaching cruise altitude, the crew initiated the Smoke, Fire, and Fumes checklist, which involves deactivating certain electrical components and diverting to the nearest suitable airport. The crew declared an emergency and requested a diversion to Zurich. The aircraft landed at Zurich Airport at 09:58 UTC, where emergency services were on standby. Although the crew taxied the aircraft to the stand under their own power, the oil odor remained detectable even after the engines were shut down.
The investigation
An inspection of the aircraft in Zurich initially revealed fresh oil traces on the ground and the underside of the right engine cowling. However, subsequent ground runs and pneumatic system tests performed in Zurich—including high-power tests—failed to reproduce the odor or find new leaks.
Later, following clearance from the SUST, the aircraft was moved to Frankfurt. There, Lufthansa Technik engineers successfully reproduced the odor during testing. The investigation identified damage to the bearings and seals of air cycle machine 1 as the source of the oil leak. The investigation noted that such wear is not uncommon in components with high operating hours, as this specific unit had reached 18,135 flight hours since its last overhaul.
Findings
- The cockpit odor was caused by oil escaping from the damaged air cycle machine 1.
- The oil traces observed on the right engine cowling in Zurich were unrelated to the cockpit incident and were likely remnants from the engine change performed the previous day.
- Damaged seals and bearings in the air conditioning system allowed oil to enter the air supply.
Safety action
- The investigation highlighted that inhaling oil vapors poses health risks and can impair pilot visibility and communication via oxygen masks.
- It is recommended that inspection intervals for components within the air conditioning system be reviewed to ensure that developing malfunctions in older equipment are detected before they result in in-flight oil emissions.