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26th of November 2020, Valve Failure, Blog #627

An ATR 42-300 was scheduled to operate a cargo flight from Rankin Inlet Airport to Naujaat Airport (both in the province of Nunavut, Canada) with a crew of three. During the flight preparation, they noticed two Circuit Breakers (the left and right propeller feather solenoid circuit breakers) were out, and not collared/safetied. The company duty engineer was called and he informed the crew that they had been pulled as part of a scheduled maintenance task the previous night. At his request, the crew pushed the circuit breakers back in and continued with their pre-flight activities. The flight was to carry 3539 pounds of cargo and was refuelled with 1272 litres of Jet A fuel. Bringing the fuel load for the flight to 5150 pounds.

At 12.17 lt (local time) the aircraft departed Rankin Inlet Airport and set course for Nunavut, a flight of around 500 kilometres. After an uneventful flight, the crew observed the left-hand propeller rpm being lower than normal while descending towards its destination airport. This observation was briefly discussed between the crew members, and some attempts to troubleshoot the issue were made (without consulting the Quick Reference Handbook {QRH}). The commander of the flight considered returning to the departure airport, but the weather was unfavourable for a single-engine landing if they had to shut down the left-hand engine. He suspected the indication was the result of the maintenance activities from the previous night. These considerations were not shared with the other crewmembers, and the flight was continued while the situation was being monitored. During the approach the condition levers were advanced and the propellers on both engines achieved the required 100% rpm. A short while later, while the engine torque was reduced the left propeller rpm started to drop, causing control difficulties. At approximately 13.26 the aircraft touched down on runway 34 at Nunavut, 750 feet past the threshold, immediately reverse thrust was selected for both engines, but only the right propeller went into reverse, accompanied by only the "LO PITCH" light for the #2 propeller. The aircraft swerved to the right and attempts to correct this were only effective for a very short moment. Lateral control was lost and the aircraft exited the runway to the right, travelling in a north-easterly direction. After travelling through the snow for ~500 feet the aircraft came to a stop. As the condition levers had become jammed, the crew shut the engines down using the fire handles. The commander sustained serious (head) injuries, as his seat belt released during the runway excursion, and the other two crew members sustained light injuries. The aircraft sustained substantial damage and was later written off as damaged beyond repair.

A pitch-change actuator showing a pitch lock condition (Source & ©: Collins Aerospace)

The Transportation Safety Board of Canada launched an investigation into the accident, and on the 1st of November 2022, they released their extensive investigation report. The following findings as to causes and contributing factors were listed in the report;

1. A contaminant inside the left propeller’s high-pressure pump caused its pressure relief valve to fail. As a result, the propeller entered a pitch-lock condition and remained in that condition until the aircraft landed. 2. Because there is no indication in the cockpit of a pitch-lock condition in flight, the flight crew were not aware that the propeller had entered a pitch-lock condition, and they continued the flight to Naujaat Airport (CYUT), Nunavut, without discussing any options. 3. Immediately on touchdown, reverse thrust was selected by the pilot flying without confirmation that both LO PITCH lights had illuminated. With the left propeller in a pitch-lock condition, the selection of reverse thrust resulted in the aircraft entering an asymmetric thrust state. 4. Due to the asymmetric thrust, directional control of the aircraft could not be maintained. As a result, the aircraft exited the landing surface of the runway, travelled across rough terrain adjacent to the runway, and was substantially damaged. 5. For undetermined reasons, the captain’s safety belt buckle was released during the runway excursion and the captain’s head struck the forward upper area of the cockpit, resulting in serious head injuries.

The location of the metal fragment (left picture) and the fragment, whose composition was foreign to the engine (Source & ©: AeroControlex Group, Inc, with TSB annotations)

The TSB report, which served as the source for this blog can be accessed by clicking on the .pdf file below;

ATR 42, valve failure, 26Nov2020
Download PDF • 1.85MB

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