Malaysia Airlines flight MH 134 to Kuala Lumpur has been forced to return to Brisbane shortly after takeoff and make an overweight landing because vital external instruments under the nose of the aircraft were inadvertently left covered.
The instruments are pitot probes, sometimes called pitot tubes. They measure the plane’s airspeed and transmit the information to the flight deck. They are crucial.
At Brisbane Airport, pitot probes are routinely covered with special sock-like sleeves while planes are parked, to help keep mud-dauber wasps from nesting in the probes and blocking them – as has happened before.
In the Malaysia Airlines case, soon after the plane (an Airbus A330-300 with the registration 9M-MTK) arrived at the airport on 18 July 2018, covers were placed on the plane’s three pitot probes, which sit under the nose of the aircraft.
The covers should have been removed before takeoff later that evening but they weren’t, forcing the plane to return to Brisbane and make an overweight landing, triggering an investigation by the Australian Transport Safety Bureau (ATSB), which continues.
An aircraft’s pitot-static system, of which the pitot probe is an essential part, is used to determine the plane’s airspeed, altitude and altitude trend. Several commercial airline incidents and accidents have been traced to a failure of the pitot-static system.
The most notorious was the crash of Air France flight 447, an A330 which plunged into the Atlantic Ocean in 2009 with the loss of all 228 people aboard. French air safety authority BEA concluded that pitot probe obstruction was a contributing factor. Inconsistencies between airspeed measurements – likely due to the aircraft’s pitot probes being obstructed by ice crystals – caused the autopilot to disconnect, after which the crew reacted incorrectly and ultimately caused Air France flight 447 to enter an aerodynamic stall, from which it did not recover.
In contrast, the Malaysia Airlines crew picked up the problem very early, as the plane commenced takeoff from Brisbane, and took quick action.
An ASTB preliminary report, published yesterday, said the Malaysia Airlines flight on 18 July 2018 (aircraft 9M-MTK) arrived in Brisbane that evening.
Pitot probe covers removed from 9M-MTK after the incident flight
“The captain, first officer and certifying maintenance engineer from the previous night’s flight, who had been resting at a Brisbane hotel, arrived at the airport to commence their duties for the 18 July outbound flight.
“Soon after the aircraft had landed, covers were placed on the aircraft’s three pitot probes. Subsequent inspections during the turnaround did not identify the presence of the pitot probe covers and they were not removed prior to the aircraft’s departure.”
The flight crew detected an airspeed anomaly as the plane commenced takeoff from Brisbane. After takeoff they took action as soon as it was safe to do so.
The ASTB continues: “After take-off the flight crew carried out actions for unreliable airspeed indications and made a PAN call to air traffic control (ATC), advising they had unreliable airspeed indications.”
A PAN call is an internationally recognised radio call announcing an urgent condition which concerns the safety of an aircraft or its occupants but where the flight crew does not require immediate assistance.
“The flight crew continued to climb above 10,000 ft and manoeuvred the aircraft to the north-east of Brisbane Airport where they carried out several checklists, troubleshooting and preparation for an approach and landing on runway 01,” the ATSB continues.
Reconstruction of pitot probe covers on the Malaysia Airlines plane, showing pitot cover damage and rub marks on aircraft skin from the streamer. ATSB
In accordance with published procedures, the Malaysia Airlines flight crew turned off the three air data reference systems (ADRs). Normal landing gear extension was not possible with all three ADRs off so the flight crew performed a landing gear gravity extension (a manual system used in such circumstances) before conducting an overweight landing. (An ‘overweight’ landing is when an aircraft weighs more than certified maximum landing weight.)
“After landing the flight crew stopped the aircraft on the runway as nose wheel steering was unavailable following a landing gear gravity extension,” the ATSB preliminary report says.
“The main landing gear doors, which remain open following a gravity extension, had minor damage where they contacted the runway surface. The aircraft was towed to the gate where the passengers and crew disembarked. There were no reported injuries during the flight.
“A subsequent inspection identified that the pitot probe covers were still fitted to the aircraft’s three pitot probes after it landed.”
Aircraft about to be pushed back with pitot covers in place (two of three visible). Image by Brisbane Airport Corporation and modified by ATSB.
The ATSB says the maintenance engineer, captain or ground handlers did not detect the presence of the pitot covers during separate external aircraft inspections.
“The operator’s maintenance engineer boarded the aircraft during turnaround, and the engineering support personnel left the bay to attend to other aircraft. The pitot covers were not detected by ground handlers during pushback.
“Examination of the three covers fitted to the aircraft following the occurrence found that they were partially burned by the heated pitot probes. They each had a hole burned through where the cover folded around the probe in the airstream. The streamers were damaged by contact with the aircraft skin during the flight.”
At Brisbane Airport the pitot probe covers are there mainly to stop wasps entering the probes to build nests.
The ATSB noted that there have been “multiple reports of insect activity disrupting aircraft systems at Brisbane Airport. These included blocked pitot probes, mainly from nests built by mud-dauber and other wasps, resulting in airspeed discrepancies and other effects.
“A preliminary review of the ATSB database indicated that, from 2008 to 2018, there were at least 15 incidents involving high-capacity regular public transport aircraft departing from Brisbane Airport where one of the pitot probes had a partial or total blockage, at least four of which were identified as insect nests. These resulted in three rejected take-offs, four aircraft returning to Brisbane Airport after continuing the take-off and one aircraft that continued to its destination.”
“The Australian Transport Safety Bureau advises all operators that conduct flights to Brisbane Airport to consider the use of pitot probe covers and, if covers are used, ensure there are rigorous procedures for confirming that covers are removed before flight.”
The ATSB preliminary report can be read on the ATSB website here.
Edited by Peter Needham
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