What happened
On 23 June 2000, a Boeing 747-436, registration G-BNLT, was performing a scheduled public transport flight from New York to London Heathrow. While the aircraft was descending through approximately 2,000 feet, the crew noticed a fine white smoke and a distinct smell of equipment overheating within the flight deck.
In response to the smoke, the crew immediately deactivated the flight deck humidifier. To ensure safety, the pilots engaged the autopilot and donned their oxygen masks. The smoke levels did not noticeably increase as the descent continued. The crew notified Air Traffic Control of the situation as the aircraft descended through 500 feet. Upon landing, the aircraft was met by the Airport Fire Service and escorted to its stand. There were no fatalities and no injuries to the 16 crew members or 295 passengers, and the aircraft sustained no damage.
The investigation
The AAIB investigation focused on the flight deck humidifier system, which uses a rotating disc driven by a 115V AC motor to atomize water into the air conditioning ducts. This system is designed to operate only during the cruise phase of flight and should automatically disable itself prior to descent based on signals from the Flight Management System.
Following the event, the operator's engineering department examined the specific humidifier unit. While the aircraft had a history of humidifier-related issues, the workshop examination found no physical evidence of overheating in the unit. This led to a theory that the perceived smoke might have been caused by the unit continuing to operate during the approach, creating excessive moisture in the cockpit. However, this theory failed to account for the specific odor of overheating reported by the commander.
Findings
- The flight deck humidifier system was the focus of the smoke event.
- The operator'2s examination of the unit did not reveal any evidence of overheating.
- The source of the reported overheating odor remained unexplained by the physical examination of the hardware.
- The aircraft had a documented history of humidifier removals and moisture-related issues.
- Following a subsequent smoke incident on 5 July 2000, the operator permanently locked out the humidifier system.