Description

On 26 November 2020, an ATR 42-300 (C-FAFS) operated by Calm Air International on a cargo flight from Rankin Inlet to Naujaat as CAV464, was descending to destination when the pilots observed the left engine at an abnormally low propeller rpm. Shortly after touchdown at Naujaat in day VMC, directional control was lost when the crew selected asymmetric reverse pitch without first checking both systems were functional. The aircraft veered off the runway into snow before stopping 33 metres from the runway. The captain was seriously injured, and the other pilot and a non-operating cabin crew occupying a passenger seat in the cabin suffered minor injury. The aircraft sustained substantial damage.

The significantly damaged aircraft where it stopped after the veer off. [Reproduced from the Official Report] 

Investigation

An Investigation was carried out by the Canadian Transportation Safety Board. The FDR and CVR were removed from the aircraft and their data were successfully downloaded.

The captain had a total of 21,000 hours flying experience including 1,477 hours on type. He had been employed as a pilot by the operator for almost 22 years, having joined as a first officer on the Saab 340, subsequently gaining a command on that type and in 2014 on the ATR42. The first officer had a total of 16,500 hours flying experience, including 1,749 hours on type, and had also been employed as a pilot by the operator for almost 22 years. He had been flying the ATR42 since 2017, having previously served as first officer on the operator’s former Saab 340 and Fairchild Dornier 328JET aircraft. 

What Happened

The accident flight was the crew’s third sector of the day, and it involved a different aircraft from their previous two flights. When they boarded this aircraft (which was in combined passenger/cargo configuration) and commenced their pre-start checks, they found that the left and right propeller feather solenoid CBs were tripped (but not collared). They contacted the maintenance supervisor, who told them the CBs had been tripped as part of routine maintenance carried out overnight, and the supervisor instructed them to reset the breakers..

The flight departed with the captain as PF and was uneventful until the descent into Naujaat, when the pilots noticed that the left propeller was operating at a lower rpm than usual, with other engine parameters remaining normal. They “briefly discussed the situation and made various attempts to troubleshoot and identify the problem but did not consult the QRH” and, having not identified a specific malfunction, took no action. The captain reported having (silently, without involving the first officer) considered his options, which he assessed were returning to Rankin Inlet or shutting down the left engine or both of these. He stated that as there was a crosswind at Rankin Inlet and “he had not wanted to land on a runway with compacted snow in a crosswind or initiate a long return flight there in arctic conditions, particularly with only one engine operating” and also stated that he had presumed that the split in propeller indications was “related to the overnight maintenance task”.  

During the approach, the captain “advanced the condition levers to see if he could achieve 100% propeller rpm” but as engine torque was reduced, the left propeller rpm started to decay, and by short final “the aircraft was becoming noticeably more difficult to control." Initial touchdown on the 1,036 metre-long 30 metre-wide gravel surface runway 34 at Naujaat occurred 230 metres beyond the threshold. The captain immediately selected reverse thrust on both power levers without either pilot first checking that the ‘LO PITCH’ lights were both illuminated. The left one was not illuminated and so almost immediately, the aircraft veered to the right. The captain’s attempt to use asymmetric thrust and nosewheel steering to regain directional control failed. As a result, the aircraft left the runway to the right at speed and then travelled about 150 metres through a runway-edge windrow of compacted snow. The aircraft then crossed over a shallow ditch before continuing over rough terrain and coming to stop a little over 30 metres from the runway edge and 760 metres from the runway 34 threshold. The engine fire handles were then pulled to shut down the engines, because the condition levers were jammed.

The aircraft's final resting position. [Reproduced from the Official Report]

Some of the cargo was released from the restraints, partially blocking access to the flight deck, but this did not prevent the pilots from exiting the aircraft. At some point during the excursion, the captain’s safety harness released, and he was thrown forward by deceleration forces. The captain struck his head on the forward upper flight deck structure. The aircraft sustained damage to the lower forward fuselage, the nose landing gear collapsed, and both main landing gear assemblies were damaged, the left being “almost completely severed from its attachment points”.

Why It Happened

Analysis of FDR and CVR data showed that the left propeller had entered a pitch-lock condition at the same time the pilots had observed the difference in propeller rpm with the left propeller blades locking at an approximate 22.5° blade angle. A tear-down inspection of the left propeller control unit (PCU) confirmed that it no longer met certification criteria and was slow to function at normal operating temperatures. More detailed testing found that the propeller’s high-pressure oil pump pressure regulating valve (PRV) had failed and this was preventing sufficient oil pressure building up in the PCU. The source of a metal fragment discovered stuck to the sealing surface of the PRV and which was preventing it from sealing, could not be determined. It was considered that given the size of this fragment, “it should have been trapped by the engine oil filter”.  

Inspection and testing of the captain’s safety harness did not identify any pre-existing defects or anomalies that could have contributed to its uncommanded release, and the cause of this could not be established.

The absence of the prescribed pilot response to abnormal propeller rpm in flight, the captain’s failure to check for both reverse pitch lights, and the absence of a PM call to confirm this before selecting reverse pitch during the landing roll were considered against relevant procedures. The QRH was found to contain an abbreviated version of the FCOM abnormal and emergency procedures in the form of a checklist allowing pilots “quick and easy access to critical aircraft information so that they are able to respond appropriately to an emergency or abnormal situation”. The LO PITCH IN FLIGHT’ procedure required that the affected engine must be shut down. 

Applicable Normal Procedures required the PM to call ‘TWO LOW PITCH’ if both LO PITCH lights illuminate on touchdown and stated that reverse thrust must not be used if only one LO PITCH light illuminates. The FCOM is unequivocal on the matter and states that “the use of any reverser is prohibited” if asymmetry is indicated or if one LO PITCH light is not ON since in this situation, “the propeller pitch change mechanism is probably locked at a positive blade angle” which would mean a positive effect at any power lever position.

The captain’s failure to involve the first officer in diagnosing the abnormal propeller indication in flight when both rpm levers were set equally was contrary to normal practice on a two-pilot flight deck and explicitly contrary to the CRM principles in which both pilots had received training in accordance with regulatory requirements.

The following Findings were formally documented based upon completion of the investigative work:

Causes and Contributing Factors 

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 without discussing any options.
3. Immediately on touchdown, reverse (power) 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 power 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 released during the runway excursion, and the captain’s head struck the forward upper area of the flight deck, resulting in serious head injuries.

Risk Factors (Safety deficiencies which were assessed not to have been a factor in this occurrence but could be in future ones)

1. If flight crews do not assess abnormal situations as a team, there is a risk that they will not identify the nature of the abnormal situation and determine the most appropriate action to take.
2. If the layout and design of a Quick Reference Handbook make it difficult for flight crews to find a procedure to address a malfunction, they may not take the appropriate actions quickly or efficiently, which may lead to an unsafe aircraft state.

Safety Action taken by Calm Air International as a result of the accident was noted at its completion as having included the issue of a Flight Operations Bulletin on pitch-lock and the introduction of pitch-lock scenarios in its ATR 42 recurrent simulator training.

The Final Report of the Investigation was authorised for release on 19 October 2022 and officially released on 1 November 2022. No Safety Recommendations were made.

Related Articles

• Runway Excursion
• Standard Operating Procedures
• Abnormal and Emergency Procedures
• Emergency and Abnormal Checklist
• Crew Resource Management (CRM)
• Propeller
• Blade Pitch