Make no assumptions when it comes to warnings

The ATSB is urging flight crews to stop and seek further guidance when confronted with any ambiguous situations pre-flight.

The ATSB’s investigation into an incident involving a passenger flight brings an important warning for crews about the potential for anomalies in warning systems.

The incident occurred on 26 May 2018, when a SAAB 340B was taxiing at Esperance Airport, Western Australia. The aircraft was scheduled to conduct a passenger flight to Perth with three crewmembers and 32 passengers on board.

As the aircraft taxied, the first officer (FO) conducted a test of the ice protection system, and the flight crew received a Master Caution. The warnings did not match any of the scenarios in the Quick Reference Handbook (QRH) checklist. The Captain discussed the warnings with the FO, how had not detected any defects in the de-icing system during the daily walk-around check of the aircraft.

The flight crew concluded that the warnings were most likely the result of a faulty sensor and there was no risk to the safety of the flight. They noted that the forecast freezing level was 7,500 ft and if necessary, the flight to Perth could be conducted as low as 4,000 ft. They elected to proceed with the flight as scheduled and the flight was conducted without further incident.

… if confronted with an abnormal or ambiguous situation pre-flight, which have the potential to affect safety of flight, crews should stop and seek further guidance.

After landing, the flight crew found a tear in the right stabiliser boot.

The ATSB investigation found that the crew had likely conducted the flight with a small defect in the boot, which increased in size during the flight. The reason for the ambiguous nature of the warnings could not be determined.

The incident highlights an important warning for flight crews regarding the potential for anomalies to occur in the operation of warning systems. In the event that fault indications are ambiguous and there is the possibility of affecting the safety of the flight, it is important to take the necessary, conservative precautions to resolve the issue prior to conducting the flight.

Damaged de-icing boot
Damaged Saab de-icing boot

Read the final report, AO-2018-045: De-icing boot failure involving SAAB 340, VH-ZLH, Esperance Airport, Western Australia, 26 May 2018

Last update 16 November 2018