What happened
On 2 December 2012, a scheduled passenger flight operated by a commercial carrier, involving a Boeing 777-236 with registration G-VIIK, was traversing North Atlantic oceanic airspace. The aircraft was traveling from Philadelphia to London Heathrow at a cruising altitude of FL 380 when the flight crew encountered a series of three smoke events within the flight deck.
Following the third and most severe instance of smoke, the commander declared a MAYDAY and initiated an emergency descent to FL 1150. To mitigate the risk of smoke inhalation, the First Officer donned an oxygen mask. The crew subsequently requested a diversion to Shannon Airport, Ireland. During the descent, the crew performed smoke checklists as prescribed by the Quick Reference Handbook (QRH), which resulted in the smoke clearing. The aircraft landed at Shannon at 06:04 hrs without further incident. There were no injuries and no fatalities among the 174 passengers or 13 crew members on board.
The investigation
Investigators examined the aircraft's equipment cooling system, which utilizes fans located in the forward cargo compartment to circulate cabin air over electronic components. The investigation focused on the right-hand (RH) primary equipment cooling supply fan.
Technical analysis revealed that the RH fan had experienced a front-end bearing failure. Specifically, the front bearing race had collapsed, leading to friction between the rotating and stationary internal components. This mechanical breakdown caused local overheating, which generated the smoke and burning odor detected by the crew. While the failure caused the smoke, the aircraft's protection logic functioned correctly, automatically deactivating the RH fan and activating the left-hand (LH) backup fan to maintain cooling to the flight deck electronics.
Findings
- The primary cause of the smoke was a bearing failure within the right-hand equipment cooling supply fan.
- The collapse of the bearing race caused internal components to make contact, resulting in overheating.
- The aircraft's automated systems successfully transitioned to the backup cooling fan, preventing a total loss of electronic equipment cooling.
- The flight crew managed the emergency effectively by declaring an emergency and initiating a timely diversion.
Safety action
To prevent similar occurrences, the operator is working toward installing vibration monitors on the primary equipment cooling fan position. These monitors are intended to provide early detection of abnormal vibrations, allowing the fan to be shut down before a bearing failure can lead to an in-flight smoke event. Additionally, it was noted that newer aircraft configurations from the manufacturer now include a bearing brake to stop fans immediately upon detecting a failure.