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| Date: | Saturday 13 April 1996 |
| Time: | c. 12:00 LT |
| Type: |  Boeing 747-436 |
| Owner/operator: | British Airways |
| Registration: | G-BNLL |
| MSN: | 24054/794 |
| Year of manufacture: | 1990 |
| Fatalities: | Fatalities: 0 / Occupants: |
| Other fatalities: | 0 |
| Aircraft damage: | None |
| Category: | Incident |
| Location: | over Atlantic Ocean -
 Atlantic Ocean
|
| Phase: | En route |
| Nature: | Passenger - Scheduled |
| Departure airport: | London-Heathrow Airport (LHR/EGLL) |
| San Francisco International Airport, CA (SFO/KSFO) | |
| Investigating agency: | AAIB |
| Confidence Rating: |  Accident investigation report completed and information captured |
Approximately 2.5 hours into a flight from London to San Francisco, the flight deck filled with
"hazy, acrid fumes". The 'Smoke Air Conditioning Drill' was carried out, which included the
donning of flight deck oxygen masks and switching off the flight deck humidifier. However,
although the fumes did not intensify, the smell remained. Accordingly, the decision was made to
return to Heathrow where an uneventful landing was made.
Subsequent investigation confirmed that the flight deck humidifier unit smelt strongly of burned
insulation. This unit is connected to the aircraft potable water system, and generates an
atomized water spray which is introduced into the flight deck air conditioning ducts. The spray is
produced by a rotating, vaned disc, which is driven by a three-phase, 115 volts AC motor. Water is
drawn, via an aspiration tube, from a chamber at the base of the unit.
Before disassembly, the unit was run on a test bench, where it was found to be very noisy in
operation. After stripping, it was found that the motor bearings were dry and badly worn,
although they had not in fact failed. A small amount of rubbing had occurred on the rotor as a result
of the worn bearings, and there was evidence of intense heat around the motor, with areas of charred
insulation. The vaned disc was encrusted with heavy limescale deposits, which conceivably could
have caused out-of-balance forces that contributed to bearing wear. It was also noted that additional
limescale deposits had almost completely blocked the aspiration tube, thereby significantly reducing
the effectiveness of the humidifier.
The records indicated that the unit had been fitted to the aircraft about 11 months earlier, and had
achieved more than 4,700 flying hours.
Accident investigation:
 |
|
Sources:
https://assets.publishing.service.gov.uk/media/5422ec6fed915d13710000ef/dft_avsafety_pdf_502361.pdf
https://www.jetphotos.com/photo/9985991 (Photo)
History of this aircraft
Other occurrences involving this aircraft
| 28 March 1995 | G-BNLL | British Airways | 0 | Seoul-Gimpo International Airport (GMP/RKSS) |  |
min |
| 27 March 1999 | G-BNLL | British Airways | 0 | London-Heathrow Airport (LHR/EGLL) |  |
min |
| 15 October 2007 | G-BNLL | British Airways | 0 | London Heathrow Airport (LHR/EGLL) | |
min |
| Collision with other aircraft on apron | ||||||
| 22 December 2013 | G-BNLL | British Airways | 0 | Johannesburg-O.R. Tambo International Airport (JNB) |  |
sub |
Revision history:
| Date/time | Contributor | Updates |
|---|---|---|
| 10-Aug-2025 11:55 | Justanormalperson | Added |
| 10-Aug-2025 11:55 | Justanormalperson | Updated [Accident report, ] |
Corrections or additions? ... Edit this accident description
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