On 21 November 2013, the crew of an Airbus A330-200 (A6-EYJ) being operated by Etihad on a scheduled passenger flight from Brisbane to Singapore successfully rejected a daylight take off in normal ground visibility due to an airspeed indication failure. The aircraft returned to the gate. After maintenance intervention and release to service, the flight departed again almost 2 hours later. This time an airspeed discrepancy again occurred but was reported not to have been detected in time to reject the take off. Further system problems followed and a MAYDAY was declared and an uneventful air turnback was completed.
An Investigation was carried out by the Australian Transport Safety Bureau (ATSB). FDR data was successfully downloaded and used in the Investigation.
It was established that the initial take off from Runway 01 had been rejected after the Captain had observed an airspeed indication failure flag on his PFD. The recorded maximum airspeed recorded was 88 KCAS. After a return to the gate, maintenance attended and troubleshooting was carried out. ADIRU 1 and ADIRU 2 were transposed and the aircraft was released to service with the ADR part of ADIRU 2 inoperative in accordance with MEL provisions. Prior to the release, the First Officer’s ADR source was switched to ADIRU 3. The Captain’s ADR source remained selected to the normal ADIRU 1 position.
After an interval of almost two hours, a second take off was commenced from the same runway with the Captain as PF. The crew subsequently advised that "they became aware of an airspeed discrepancy after V1 and (so) the tak eoff was continued".
As a result of the airspeed discrepancy, the A/T and both FDs disengaged automatically. Once airborne, the flight control system reverted from Normal law to Alternate law. The Captain reported having handed over control to the First Officer. Soon afterwards, the slat/flap lever was selected to the 'up' position but only the flaps subsequently retracted and as the slat limit speed was exceeded, the corresponding alert was annunciated. A MAYDAY was declared to ATC and a return to Brisbane was made with the Captain's instrument display selected to the ADIRU 3 position. An overweight landing on the departure runway was uneventful. A visual inspection of the pitot heads found that "there was an internal obstruction of the Captain’s probe while the FO and standby probes were clear".
The maintenance intervention following the initial rejected take off was reviewed. It had been based on a combination of crew observations and messages downloaded from the CMC (Central Maintenance Computer) using an MCDU connected to the aircraft on the flight deck. This information was then used as the means to find the appropriate response in the Trouble Shooting Manual (TSM). The prescribed function tests were then carried out but "no faults had been positively identified". On a precautionary basis, and in order to maintain ETOPS functionality, the engineer released the aircraft to service as described above. It was noted that "the TSM procedure did not specifically identify the pitot probe as a possible cause". However, it was also found that on 15 January 2013, Airbus had published a new edition of SIL (Service Information Letter) 34-084 which did discuss this possibility and explicitly recommended additional "specific tasks related to pitot probes" in the event of reports of airspeed discrepancy. This SIL was not referred to by the maintenance personnel involved in the initial intervention.
Following the air turnback and discovery of the blocked pitot tube which had been installed at build 7.5 years earlier, the pitot tube was removed from the aircraft for examination. The inlet condition was found to be in accordance with CMM requirements and there was no evidence of drain hole obstruction. However, when a borescope examination was performed through both the inlet and through the pneumatic port, it was found that "the interior of the probe was occluded by an incomplete insect nest and the nest material was consistent with that of the mud-dauber wasp" (see illustration below). After application of compressed air to the component had failed to dislodge any of the material, "the base of the nest was broken away with a sharp instrument and (then) fully removed by flushing with hot water". After functional testing, it was possible according to the CMM to re-certify it for return to service.
View looking into the Pitot Tube Inlet [reproduced from the Official Report]
It was noted that the (unintended) effect of the maintenance intervention after the rejected take off was to disable two of the aircraft's three airspeed display systems which had resulted in "consequential serious degradation of other aircraft systems" as experienced. It was also noted that after the second speed display failure, the crew had reported that the airspeed fault flag had "appeared after V1" but that downloaded flight data "again showed that CAS sourced from ADR1 (i.e. the CAS that was displayed on the Captain’s PFD) was zero when ground speed increased through 50 knots", at which point there had been plenty of time to reject the take off again.
The Investigation noted that during the course of this Investigation an occurrence of similar nature and origin had occurred at Brisbane. On 3 April 2014, during the take off of a Boeing 737-800, the crew detected incorrect airspeed indications just prior to becoming airborne and intermittent stick shaker activation followed. The aircraft returned to Brisbane where subsequent investigation found that the Captain's pitot head "was partly obstructed by material consistent with a mud-dauber wasp nest". There had also been another similar occurrence at Brisbane on 19 March 2006, involving an Airbus A330-300 rejected take off. After "a post-flight engineering inspection of the aircraft found what appeared to be wasp-related debris in the Captain's pitot head", the aircraft operator had concluded that this contamination was the likely cause of the airspeed display fault and both "the (aircraft) operator and (the) airport owner undertook a number of safety actions to minimise the risk of future wasp activity at Brisbane Airport".
After the 2013 event, the Brisbane Airport Corporation "reviewed their Wildlife Hazard Management Plan (which includes wasp activity)" as detailed below. The airline's maintenance contractor advised that "the use of pitot covers was dependent on customer requirements and was not a standard practice" and they had not been installed during the 2 hour period that the aircraft was at the gate. The Investigation considered that aircraft operators could minimise the risk of pitot tube obstruction by consistently using pitot covers, even during short transit periods.
The Findings of the Investigation in the form of Contributing Factors were formally documented as follows:
- Pitot probe covers were not installed by maintenance staff during the period in which the aircraft was at the gate.
- The Captain’s pitot probe was almost completely obstructed by an insect nest, consistent with mud wasp 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.
Airbus' response to the identified Safety Issue - which included modification of the A330/A340 TSM to include the possibility that pitot probe malfunction could be a possible "root cause" of airspeed display malfunction - was considered adequate by the ATSB.
Other relevant Safety Action taken by the Brisbane Airport Corporation in respect of additional risk management of the already-known wasp nest hazard at the airport was noted as including the following:
- Implementation of a weekly inspection and eradication program for the International Terminal Building to replace the monthly inspections which had been undertaken since 2006;
- Implementation of a weekly inspection and eradication program for the Domestic and Terminal Services Buildings;
- Engagement of an entomologist to provide the airport and its stakeholders with a better understanding of wasp activity, habits and behaviour;
- Issue of a NOTAM to communicate wasp activity;
- Extension of the pest management program to include removal of spider webs on the grounds that spiders are a wasp food source;
- Acquisition of pitot probes from Qantas and Virgin Australia to undertake research as to what aircraft type pitot tube is likely to be at greatest risk of wasp nest use;
- Identified amendments to be made to the Airport Wildlife Hazard Management Plan in relation to wasp activity.
Aircraft Operator Ethihad also advised that it had been decided that in future pitot and TAT probe covers would be installed during all Brisbane turnrounds irrespective of ground time.
Finally, State Safety Regulator CASA took various actions aimed at raising awareness of the mud wasp hazard including the issue of an Airworthiness Bulletin on the various ways in which a wasp nest infestation can compromise airworthiness.
The Final Report was published on 6 May 2016. No Safety Recommendations were made.
- Airworthiness Bulletin 02-052: Wasp Nest Infestation – Alert, CASA (Australia), March 2016.
- ACRP Report 145: Applying an SMS Approach to Wildlife Hazard Management, R. DeFusco et al. (Transportation Research Board, US), 2015.
- ACRP Report 32: Guidebook for Addressing Aircraft/Wildlife Hazards at General Aviation Airports, E. C. Cleary & A. Dickey (Transportation Research Board, US, 2010).
- ICAO Doc 9137: Airport Services Manual Part 3 - Wildlife Control and Reduction, 4th edition, 2012.