What happened
On 20 April 2012, an Airbus A321-231, registration G-EUXM, was operating a passenger flight from Stockholm Arlanda to London Heathrow. While descending through 14,000 ft near London, the aircraft entered a region of thunderstorm activity. Following a visible lightning flash, the crew observed the airspeed indications on the primary flight displays fluctuating wildly. The aircraft's flight control laws degraded from Normal Law to Alternate Law, and a TCAS Resolution Advisory (RA) was issued, though the crew did not immediately hear the associated audio alert. The crew successfully managed the situation, established the aircraft in a holding pattern, and diverted to London Stansted for a safe landing.
A second, similar event occurred on 16 June 2012, while the same aircraft was climbing through 26,500 ft. During this flight, the airspeed indications again became unreliable, and the autopilot disconnected. This second event triggered a TCAS RA due to spurious vertical speed fluctuations. The crew followed established procedures to maintain control and safely complete the flight.
The investigation
The AAIB investigation examined the aircraft's flight data, cockpit voice recordings, and maintenance history. Investigators found that the first incident coincided with visible St Elmo’s fire and a lightning flash, which also caused significant electrical pulsing on the cockpit voice recorder's ambient audio channel. Physical inspections of the fuselage revealed damage consistent with lightning strikes occurring a few days prior to the first incident.
Technical examinations of the pitot probes and air data systems showed no mechanical defects or failures in the heating elements. The investigation also reviewed the operator's training and procedures, noting that while the crew's response was effective, the complexity of the Quick Reference Handbook (QRH) procedures could be improved.
Findings
- The primary cause of the airspeed fluctuations was likely the obstruction of the pitot probes by ice crystals encountered in cold cloud tops.
- The first event occurred within current icing certification standards, while the second occurred in conditions involving temperatures and altitudes that fall within proposed, more stringent new envelopes.
- The TCAS RA during the second event was triggered by erroneous altitude data caused by the same air data disruption.
- Electrical pulsing on the cockpit voice recorder was likely caused by electrostatic discharge, potentially linked to the aircraft's recent lightning strike history.