What happened
On 6 September 2007, a BAe 146-300, registration G-JEBC, was conducting a non-revenue positioning flight from Belfast. During the climb, the commander noticed an unusual odour. Shortly after selecting the engine air bleeds, the co-pilot also detected a strange smell. The commander subsequently instructed the flight crew to use oxygen and declared an emergency, requesting a descent and diverting the aircraft back to Belfast City Airport.
As the flight progressed, the commander experienced symptoms including difficulty concentrating and a sensation of being inebried. The co-pilot reported feeling a reduced capacity to fly, while cabin crew members experienced light-headedness, nausea, and distress. Upon landing, the crew were met by fire services and taken to the hospital for testing for carbon monoxide poisoning, which returned negative.
The investigation
Investigators examined the aircraft's engines, APU, and air-conditioning packs, finding no evidence of oil leaks, seal degradation, or contamination in the ducting. The investigation focused on the aircraft's recent maintenance history, noting that a major 'C' check had been completed at Exeter on 4 September 2007. During this check, the toilet modules had been deep-cleaned and refilled with a chemical mixture.
Analysis of the toilet compartments revealed a strong odour in the forward toilet. Samples taken from the aircraft's toilets and the maintenance chemicals were sent to a laboratory. While the tests did not find high concentrations of formulated formaldehyde, they did identify oxidising agents consistent with the 'Aqua Kem Blue' chemical used during servicing.
Findings
- The fumes likely originated from the forward toilet compartment.
- The odour was likely caused by the degradation of 'Bronopol', an active ingredient in the 'Aqua Kem Blue' toilet chemical, which can release low concentrations of formaldehyde.
- As the aircraft climbed and cabin pressure dropped, the chemical became more volatile, increasing the release of fumes.
- The activation of the air-conditioning packs likely dispersed the accumulated gases through the ventilation system into the cockpit.
- The crew's physical symptoms were consistent with exposure to low-level formaldehyde, potentially exacerbated by the stress of the in-flight emergency.