What happened
On 20 December 2011, an Airbus A321-231, registration G-EUXL, was performing a scheduled passenger flight from London Heathrow to Glasgow. During the climb, at approximately 12,000 feet, the commander began feeling light-headed and dizzy. This sensation occurred while the pilot was looking down at the centre console to adjust a radio frequency during a period of aircraft banking and pitch change. Shortly after, the co-pilot also reported feeling light-headed.
In accordance with company procedures regarding potential pilot incapacitation, the crew donned their oxygen masks and declared an emergency. The aircraft returned to Heathrow and landed safely without further incident. No passengers or other crew members were injured, and no aircraft damage occurred. Upon landing, the pilots removed their masks, but experienced a second bout of light-headedness before the symptoms finally subsided.
The investigation
The AAIB investigation focused on two primary areas: the cause of the crew's symptoms and a technical anomaly discovered during the review of the flight recorders.
Regarding the crew, investigators examined the aircraft's pressurisation, bleed air, and air conditioning systems. Extensive ground tests and inspections of the ducting revealed no defects or contamination. The investigation also considered medical factors, such as the effects of a recent cold and the potential for physiological disorientation caused by head movement during a turn.
Regarding the flight recorders, the investigation uncovered a previously unknown fault in the Cockpit Voice Recorder (CVR). During the audio recovery process, investigators found that several channels contained erroneous pulsing sounds and periods of unexpected silence. While the unit's built-in test (BIT) had not flagged any errors, thermal testing and component analysis suggested an intermittent failure within the data packer integrated circuit.