Contaminated pitot probes likely cause of 737 turnback

The crew on board the Airwork-operated flight to Christmas Island noticed discrepancies in altitude, airspeed and Mach numbers between the captain’s and first officer’s altimeters. After the plane made a safe landing in Perth, ground crews identified foreign residue on all four probes.

“The ATSB established that, during an engine ground run 2 days prior to the incident, the pitot‑static probe covers were not removed, and the automatic pitot heat was not isolated as required by the ground run procedures checklist,” said the ATSB’s director of transport safety, Kerri Hughes.

“As a result, the covers melted onto the probes. Although cleaned, residue remained on the probe surfaces, which had the potential to compromise the accuracy of the pitot-static instruments in-flight.”

As the discrepancies had not been seen on seven flights between the ground run and the incident flight, the ATSB was unable to establish a conclusive link, but noted that pitot probes are “extremely sensitive to damage and disruption”.

“Data received from an aircraft’s barometric air data sensing instrumentation components is critical for safe flight, and this incident highlights the importance of maintaining a high level of attention to damage and contamination when working on and inspecting these components,” said Hughes.

The incident came four years after a Malaysia Airlines Airbus A330 carrying 215 passengers to Kuala Lumpur was forced to turn back to Brisbane due to pitot probe covers – which are necessary at Brisbane Airport to prevent mud wasps from flying into the tubes – being left on prior to takeoff, causing inaccurate airspeed readings.

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The ATSB said in a statement last year that its investigation into the July 2018 incident was “one of its most substantive and complex” in recent years, and highlighted poor decisions made by Malaysia Airlines pilots and crew, alongside those from other organisations.

In particular, the ATSB’s report found: