What happened
On 9 August 2001, an Airbus A319-131, registration G-EUPV, was performing a public transport flight on approach to London Heathrow Airport. During the intermediate approach at 6,000 feet in cloud, the aircraft experienced the uncommanded disengagement of both the autopilot and autothrust systems. This was accompanied by a series of warnings and cautions on the ECAM display, including ADR Disagree, ADR Fault, and Flight Augmenting Computer (FAC) alerts.
The crew noted a discrepancy in airspeed indications, with the commander's display differing from the first officer's and the standby indicator by 15 knots. Following the loss of automated systems, the commander took manual control. The flight control system transitioned to 'Alternate Law' and subsequently to 'Direct Law' once the landing gear was extended. The crew followed the 'Unreliable Airspeed' procedures from the Quick Reference Handbook, increasing approach speeds as recommended. The aircraft landed safely without zero injuries to the 82 passengers or 7 crew members.
The investigation
Following the event, the aircraft was removed from service for a detailed engineering investigation. While initial troubleshooting focused on the Air Data/Inertial Reference Unit (ADIRU) due to previous airspeed discrepancies on recent sectors, no faults were found within the ADIRU itself. To identify the root cause, engineers removed the captain's pitot probe, the Air Data module, and the Total Air Temperature probe for laboratory examination.
Findings
- The investigation established that the blocked pitot probe drain holes were the primary cause of the erroneous pressure sensing.
- Debris and particles were found inside the pitot tube, and both water drain holes were obstructed.
- Internal inspection of the probe revealed burrs on the inside of the drain holes and a thin sliver of material left from the drilling process.
- The inner tube showed evidence of heat damage, including a bulge and carbon-like deposits.
- The aircraft's ability to automatically detect the error via the 'voting' function was unavailable because the No 3 ADIRU had been intentionally rendered inoperative under the Minimum Equipment List (MEL) to facilitate the flight.
- It is believed that moisture from flying through cloud or rain entered the system and could not drain, leading to the intermittent airspeed errors.