An Etihad Airbus A330 suffered an airspeed discrepancy during takeoff from Brisbane, Australia due to obstruction of the pitot tube by an insect nest, an ATSB investigation revealed.
On 21 November 2013, after a flight from Singapore, an Etihad Airways A330, registered A6-EYJ, landed at Brisbane Airport and was taxied to the terminal. It came to a stop at 09:49. Pitot probe covers were not used during the transit. At 11:52, the aircraft was pushed-back for the return flight to Singapore.
The captain rejected the initial take-off attempt on runway 01 after observing that there was an airspeed indication failure on his primary flight display (PFD). The maximum airspeed recorded by the flight data recorder during the rejected take-off was 88 kt.
The aircraft taxied back to the terminal where troubleshooting was carried out. As part of the troubleshooting, air data inertial reference unit (ADIRU) 1 and ADIRU 2 were transposed and the aircraft was dispatched with the air data reference (ADR) part of ADIRU 2 inoperative, which was in accordance with the MEL. The first officer’s (FO’s) air data source was switched to ADIRU 3 and the captain’s air data source remained switched to the normal (ADIRU 1) position.
At 13:45, the crew commenced the second take-off on runway 01, with the captain performing the pilot flying duties and the first officer performing the pilot monitoring duties. During the take-off roll the crew reported that they became aware of an airspeed discrepancy after V1 and the takeoff was continued. As a result of the airspeed discrepancy, the autothrust system and flight directors disengaged automatically.
Once airborne, the auto-flight system reverted from normal law to alternate law for the remainder of the flight. At this time, the captain handed over control of the aircraft to the first officer. While climbing through a pressure altitude of 1,360 feet, the slat/flap lever was moved to the up position and the flaps began to retract, but the slats remained extended. For a 2-minute period, a VFE (maximum speed with the flaps/slats extended) warning occurred as the slat limit speed was exceeded. At 13:47, the captain took over control of the aircraft for the remainder of the flight.
Shortly afterwards, the crew declared a MAYDAY and decided to return to Brisbane. The aircraft was manoeuvred to the east of the airport and maintained an altitude of approximately 2,000 ft. An overweight landing was subsequently carried out on runway 01 and the aircraft taxied clear of the runway with the aviation rescue and fire-fighting (ARFF) services in attendance. The aircraft then taxied back to the terminal.
Subsequent visual inspection of the pitot probes found that there was an internal obstruction of the captain’s probe, while the FO and standby probes were clear.
- Pitot probe covers were not installed by maintenance staff during the period that the aircraft was at the gate.
- The captain’s pitot probe was almost totally obstructed by an insect nest, consistent with mudwasp residue, during the 2 hour and 3 minute period while the aircraft was in transit on the ground at Brisbane.
- The blocked captain’s pitot probe was not detected by engineering staff after the initial rejected take-off. The relevant tasks in the trouble shooting manual did not specifically identify the pitot probe as a potential source of airspeed indication failure. [Safety issue]
- During the second take-off roll, the faulty airspeed indication (displayed on the captain’s PFD) was not detected and acted upon by the crew before V1 and the take-off was continued.
A similar pitot tube obstruction caused a fatal accident involving a Boeing 757 off Puerto Plata, Dominican Republic. In February 1996, all 189 on board a Birgenair flight died when the aircraft stalled following a loss of reliable air speed indication on the captain’s side. It appeared there was a pitot tube obstruction by mud and/or debris from a small insect that was introduced in the pitot tube during the time the aircraft was on the ground (20 days).