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Incident Highlights Difficulties With Fume Investigations

Ryanair replaced two engines and numerous air conditioning system components on a Boeing 737-800, as the airline attempted over nearly a one-month period to troubleshoot the origin of a chemical-type—or “cheesy”—smell reported on the flight deck by multiple crews in September 2014.

WASHINGTON—Ryanair replaced two engines and numerous air conditioning system components on a Boeing 737-800, as the airline attempted over nearly a one-month period to troubleshoot the origin of a chemical-type—or “cheesy”—smell reported on the flight deck by multiple crews in September 2014.

During an incident on Sept. 18, 2014, the aircraft’s captain “became aware of an unusual smell” as the aircraft descended through 20,000 ft. for an approach to London Stansted Airport. The captain and first officer, who did not notice a smell, donned oxygen masks, declared an emergency and landed at London Stansted.

The incident, described in a final report by the Irish Air Accident Investigation Unit (AAIU), highlights some of the difficulties airlines face when troubleshooting subjective reports about odors and difficult-to-access ductwork in air conditioning systems.

Investigators found that the most likely cause of the numerous reports was an internal oil leak in the auxiliary power unit (APU). The leak, which the AAIU found was caused by a faulty bearing repair during APU maintenance, likely contaminated the ductwork in the bleed-air system primarily feeding the flight deck. It caused issues even after the APU was later replaced.

Ryanair, in response, developed a checklist to help its maintenance-control managers troubleshoot reports of fumes or smells. Included is information on which maintenance managers to notify following an initial report, and actions to be taken if further reports are received, including moving the aircraft to a main base for evaluation.

The issues on board the aircraft in question began Sept. 1, 2014, when the aircraft’s APU was replaced due to “hot-section distress.” A flight crew later that day reported an “electrical smell” after landing with the new APU activated. Maintenance crews could not find any problems with the APU, but asked flight crews to monitor the system.

From Sept. 3–18, 2014, there were approximately eight separate reports of odors on the flight deck—with descriptions ranging from “slight smell” to “cheesy smell” to “seriously obnoxious smell”—particularly during descents with the engines idling.

Maintainers investigated the various components of the bleed-air delivery system after every incident, and ultimately replaced numerous components, including both engines and the APU.

Reports from the engine overhaul facility found that: “Oil leakage may have been present, but not to an extent that it would cause significant oil smell in cabin complaints,” according to the AAIU.

The most likely source of the odors was oil in the air conditioning system ductwork from the faulty APU that was installed Sept. 1, 2014. Mechanics replaced the APU on Sept. 10, 2014, but the AAIU said there was no record of a work order for “oil contamination removal” being completed.

“It is likely that the now-contaminated air conditioning system continued to cause unusual smells in the aircraft,” said the AAIU. “Considering the numerous inspections performed by maintenance personnel, during which no contamination was found, it is possible that the nature of the contamination was such that it was not readily visible, and was therefore difficult to detect.”

After the aircraft was taken out of service following the Sept. 18, 2014, incident, maintenance actions included performing the oil-contamination removal task. The aircraft was put back into service on Sept. 25, 2014, and did not experience any further odor events, according to the AAIU.

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