What happened
On 17 December 2010, an Airbus A3/A319-131, registration G-EUPO, was conducting a commercial passenger flight from Geneva to London Heathrow. During the descent, the aircraft encountered icing conditions. While the crew was reviewing procedures, several ECAM caution messages appeared regarding anti-icing systems, prompting the commander to switch the Air Data selector to the Captain 3 position.
As the aircraft progressed on final approach to Runway 27L, the commander's indicated airspeed dropped rapidly to approximately 50-60 KIAS, while the standby airspeed indicator simultaneously fell to 0 KIAS. The commander declared a MAYDAY and initiated a go-around. The crew followed the 'Unreliable Speed Indication' procedure from the Quick Reference Handbook, eventually determining that ADR 2 was the only reliable data source. The aircraft diverted to Luton Airport, where it landed safely in Direct Law.
The investigation
The AAIB examined the flight data, cockpit voice recordings, and the aircraft's maintenance history. Investigators analyzed the Electronic Centralized Aircraft Monitoring (ECAM) messages and the communication between the Probe Heat Computers (PHC) and the Centralised Fault Display System (CFDS). The investigation also looked into the possibility of electrical interference, as abnormal sound pulses were recorded on the cockpit area microphone. Extensive post-flight testing was conducted on the PHC units, the CFDS, and the air data sensors, including a pressure leak test on the standby pitot probe.
Findings
- The loss of displayed airspeed on both the commander's and standby instruments was caused by the simultaneous icing of the standby pitot probe and a loss of communication between the PHC and the CFDS.
- The loss of communication between the PHC and the CFDS was an intermittent fault that occurred during the flight.
- The crew's decision to switch to the Captain 3 position, intended to address anti-icing warnings, inadvertently removed the serviceability of ADR 1 from the crew's view, making them unable to verify the status of the primary data source.
- Electrical interference, likely internal to the aircraft, was identified as the source of the 'popping' sounds recorded on the cockpit voice recorder.
Safety action
Following the investigation, the aircraft manufacturer committed to clarifying the go-around procedure in the Quick Reference Handbook for instances where unreliable airspeed occurs during final approach. A Safety Recommendation was issued to ensure the Airbus A320 family manuals meet all phase-of-flight requirements.