History of Flight
The aircraft had completed a major 'C' check at Exeter on 4 September 2007. It then flew to Belfast City Airport; the flight was normal except for a yaw damper failure and a brief smell of 'sweaty socks' when engine bleeds were selected on. On 6 September, the aircraft was prepared for a positioning flight to Southampton to replace an unserviceable aircraft. The crew included two flight crew and two cabin crew. Pre-flight checks were normal, and the toilets appeared and smelled normal. The cabin had not been prepared for a revenue flight, and the toilets had not been serviced since the 'C' check.
The co-pilot, as pilot flying, noticed an unusual smell shortly after selecting the APU bleed air on during pre-flight. She mentioned it to the commander, who could not smell it. Start-up and taxi were uneventful. The Thrust Management System failed to engage at the runway, but the commander continued after troubleshooting.
Takeoff at 1239 hrs was normal. After passing the minimum sector altitude, engine bleeds were selected on and APU bleed off. Shortly after, the commander became aware of an unusual smell, different from typical 146 smells. He called a cabin crew member forward, who initially could not smell anything. Passing FL100, he turned up the heating and again noticed the smell. He instructed the co-pilot to go on oxygen, then called a cabin crew member forward and told both cabin crew to go on oxygen. The cabin crew member now smelled something. The commander went on oxygen, declared a PAN to Scottish Radar, and requested a descent. He performed the smoke and fumes checklist, noting difficulty communicating with masks. The aircraft diverted back to Belfast City Airport uneventfully.
Crew Experiences
The commander described feeling similar to being inebriated, difficulty concentrating, and feeling 'fuzzy'. He felt faint after getting out of his seat to open the cabin door for the fire service. The next day he had a headache and felt 'woolly-headed'. He had been involved in a serious fumes incident eight years earlier.
The co-pilot initially felt reduced capacity and difficulty concentrating for a few moments after putting on her mask, then felt more normal.
One cabin crew member smelled nothing unusual but felt tired and slightly sick while on oxygen. She had no ill effects the next day. The other cabin crew member did not smell anything until the commander asked her a second time; then she smelled 'sweaty socks'. After going on oxygen, she felt light-headed and sick. She was very anxious and needed assistance from fire crews to get into the ambulance. Her doctor advised two weeks off work. She had been involved in a fumes incident three months earlier.
Examination of the Aircraft
Investigators entered the aircraft and found no distinctive smell. A full examination of the engines and APU, including borescope inspection, showed no signs of oil leakage or seal degradation. The engines had been modified with improved seals. Both air-conditioning packs were examined; heat exchangers and ducting were clean.
A full engine and APU run at various temperatures produced no unusual odour or fumes.
Due to a report of a 'chemical' smell in the technical log, the toilets were inspected. Opening the forward toilet door revealed a very strong odour, similar to the smell in the rear toilet. Surfaces were clean, with no spilled chemical. The suspected source was the toilet water, due to the chemicals used. Samples were taken from both toilets for testing.
The flight crew were invited back to the aircraft. When exposed to the air in the forward toilet, the commander identified the smell as similar to what he experienced in flight. The co-pilot confirmed the similarity.
Toilet System
The toilets were recirculating chemical type with a 45-litre capacity. They were charged with 13 litres of a chemical deodorising fluid (diluted water and chemical). A 4-litre reservoir was used for flushing. The toilet ventilation system vents air overboard, with air taken from around the top of the toilet pan. A test found the ventilation working correctly once the cabin pressurised.
Aircraft Maintenance
During the 'C' check, the toilet modules were removed and deep cleaned using 'Honeybee 76' cleaning agent. When refitted, the toilets were charged manually (since no service vehicle was available) by adding water and a dilute mixture of the deodorising chemical. The origin of the fumes was traced to the forward toilet and was probably due to formaldehyde, released as a degradation product of a toilet chemical added during maintenance at Exeter.
Medical
All crew were taken to hospital and tested for carbon monoxide poisoning; tests were negative. The crew were released.