April 17, 2023 | Susan Napier-Sewell

Unreliable Altitude Indications, 737 Freighter

737 freighter

A 737 freighter had contaminated pitot tubes before unreliable airspeed indications.

The flight crew of a 737 freighter was presented with unreliable flight data information two days after the aircraft’s pitot probes were contaminated during maintenance, an ATSB (Australian Transport Safety Bureau) investigation details.

The Airwork-operated Boeing 737 freighter, with four crew on board had departed Perth on June 10, 2022, for a freight service to Christmas Island, via Port Hedland.

As the aircraft was leveling off at 33,000 ft, the flight crew observed a 340 ft. discrepancy between the altitude displayed on the captain’s altimeter and the altitude on the first officer’s altimeter, which was connected to the autopilot.

What happened to uncover the discrepancy of the 737 freighter?

On June 10, 2022, the flight crew of a Boeing 737-476SF freight aircraft, registered ZK-TLJ, noted a 340 ft. discrepancy between the captain’s and first officer’s altitude when operating in reduced vertical separation minimum airspace after departing Perth Airport, Western Australia. They had also observed an airspeed and Mach number difference, but this was within the manufacturer’s stipulated limits. The aircraft descended and the flight crew completed the Quick Reference Handbook – Airspeed Unreliable procedure. It was determined that the first officer’s instruments were reliable for a return to Perth. After landing, ground crews found foreign residue adhered to the lower surfaces of all 4 pitot-static probes.

Read the rest of the ATSB report here, including what the ATSB found and recommendations made.

Content and photo source/credit: ATSB, “Unreliable altitude indications, Boeing 737-476SF, ZK-TLJ 167 km west of Meekatharra Airport, Western Australia on 10 June 2022,” occurrence date: 10/6/22, investigation number: AO-2022-033. Final report.

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Accident, Investigations
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