What happened
On 18 September 2014, a Boeing 737-8AS, registration EI-EFB, was operating a scheduled passenger service from Almeria, Spain, to Stansted, United Kingdom. While descending through approximately 20,000 feet, the aircraft commander detected a chemical-type odor in the flight deck. The pilot reported a sensation of irritation in the back of his throat.
In response to the unusual smell, the flight crew donned oxygen masks and set their regulators to 100%. A Mayday was declared, and air traffic control facilitated an expedited approach to Stansted. While the flight crew experienced the odor, the cabin crew reported no unusual smells or smoke within the passenger cabin. The aircraft landed safely with no injuries to the 174 passengers or 6 crew members. Airport fire services met the aircraft on taxi, though they were not required for emergency assistance.
The investigation
The AAIU investigation focused on the history of unusual odors reported on the aircraft in the weeks preceding the incident. Records showed that since the replacement of the Auxiliary Power Unit (APU) on 1 September 2014, multiple flight crews had reported various smells, ranging from "chemical" to "cheesy" or "oil-like" odors.
Investigators examined maintenance logs and found that while several troubleshooting steps had been taken—including engine borescope inspections and the replacement of cabin air filters—the issue persisted. The investigation specifically looked into the recent APU replacements and the maintenance of the pneumatic and air conditioning systems. A key finding was that an oil contamination removal task, intended to follow an APU replacement, had no recorded completion prior to the aircraft returning to service.
Findings
- The primary cause of the fumes was residual oil contamination within the aircraft's air conditioning system.
- This contamination originated from an internal oil leak within an APU that had been installed earlier in September 2014.
- The leak was caused by an unsuccessful previous workshop repair on a bearing carrier seat area, which allowed oil to bypass the liner and load compressor housing.
- Despite the APU being declared inoperative and subsequently replaced, the existing oil contamination in the pneumatic manifold remained unaddressed.
- Maintenance records indicated that while various engine inspections were performed, they did not identify the APU's internal leak as the source of the cockpit odors.