Airbus is evaluating ways to prevent avionics fan cooling system failures by detecting anomalies before they lead to inflight problems that often require diversions, a U.K. Air Accidents Investigation Branch (AAIB) report reveals.
The report examines a July 27, 2015, incident in which a British Airways (BA) Airbus A320 experienced an emergency near the end of a scheduled flight between Paris and London.
“Whilst cruising at FL240 and approaching the descent point for London, the flight crew became aware of an unusual noise and an electrical burning smell,” the AAIB states. “The noise quickly developed into a high-pitched squeal with some associated vibration and the smell became stronger, although there was no visible smoke.”
The fumes led the crew to don oxygen masks and declare a Mayday. The crew received clearance from London controllers for an “expeditious descent,” the report says. The aircraft landed without further incident.
The aircraft’s fault-detection system reported two problems with the avionics ventilation system: a “blower fault” and an “extract fault.” Visual examination of the system’s blower fan “revealed no anomalies,” the AAIB found. When the fan was powered up, it made a “rumbling noise,” causing the cabin floor to vibrate, the report explains.
The symptoms pointed to worn bearings—a known problem with that system. Removal and analysis of the fan confirmed the worn-bearings theory.
The system, manufactured by Hamilton Sundstrand, now part of UTC Aerospace Systems, uses two types of bearings. The original systems used steel bearings, and a ceramic bearing was introduced to improve reliability. The BA aircraft had ceramic bearings.
The ceramic bearings were introduced more than a decade ago. In 2005, Hamilton Sundstrand issued a service bulletin recommending installation of the ceramic bearings; Airbus followed up with a similar recommendation.
“The introduction of ceramic bearings has reduced the in-service arising rate, but the aircraft manufacturer reported that fan failure still causes between five and 10 aircraft diversions per year,” the AAIB notes.
In 2013, UTC Aerospace Systems recommended replacing the bearings “and other components subject to wear” every 10,000 flying hours or, at the very least, overhauling the system every 12,000 hr.
The BA fan that failed had about 16,000 hr. and was slated for overhaul at the aircraft’s next scheduled C Check, about two months after the incident. But BA was in the process of lowering the interval to 12,000 hr. based on “a number of recent events,” the AAIB states.
Airbus told the AAIB that “fan vibration monitoring will be the subject of an in-service evaluation aimed at reducing similar events in the future,” the report says.