What happened
On 11 May 2019, an Airbus A319-111, registration G-EZNM, was performing a scheduled passenger flight from Newcastle International Airport to Bristol Airport. Approximately 30 minutes into the flight at 29,000 feet, the co-pilot detected an acrid burning smell. Shortly after, both crew members observed smoke rising behind their seats and in front of the flight deck door.
In response to the smoke, the crew donned their oxygen masks and initiated an emergency descent. However, within 90 seconds of donning the equipment, the lenses of both masks misted up, significantly obscuring the crew's vision. The co-pilot could no longer read the flight instruments, and the commander, unable to monitor the aircraft's path, made the decision to remove his mask to regain visibility.
After removing the mask, the commander noted the smoke had dissipated. The crew subsequently decided to divert to Birmingham, which was a closer landing option. During the descent, the smoke became perceptible in the cockpit again, and the commander reported feeling slightly unsteady. The aircraft landed safely at Birmingham with emergency services in attendance, and no injuries were reported among the 107 passengers or 6 crew members.
The investigation
The AAIB investigation focused on the source of the smoke and the failure of the oxygen masks to remain clear. Investigators examined the aircraft's transformer rectifier units (TRU) and found light dust accumulation on the ventilation grids and exhaust ducts.
Regarding the oxygen masks, testing revealed that the co-pilot's mask lenses were in poor condition with scratches, and the inner surfaces of both masks were sensitive to fogging. While the masks were functioning correctly in terms of oxygen delivery, the emergency pressure selector—which is designed to provide overpressure to prevent condensation—had not been utilized by either pilot. The investigation also noted that the commander's increased breathing rate due to the stress of the event likely contributed to the condensation.
Findings
- The smoke and fumes were likely caused by dust accumulation in the transformer rectifier units (TRU).
- The oxygen masks misted up due to a combination of the cold temperature of the stowage compartment, the warmer cockpit environment, the condition of the anti-fog coating, and the increased breathing rate of the crew.
- The crew did not utilize the emergency pressure setting on the oxygen regulators, which could have cleared the condensation and allowed the commander to keep his mask on.
Safety action
- The aircraft manufacturer updated the Aircraft Maintenance Manual (AMM) to include a specific cleaning procedure for the TRUs.
- The manufacturer also updated the AMM to include a prominent caution regarding the risk of damaging the mask's anti-mist layer during cleaning.
- The operator has implemented practical demonstrations of oxygen mask selector use during recurrent pilot training.