Description

On 24 February 2020, a Fairchild SA-227-DC Metro 23 (C-GIVB) being operated by Perimeter Aviation on a scheduled domestic passenger flight marketed under the wholly-owned ‘Bearskin Airlines’ brand from Dryden to Sioux Lookout as BLS344 and being used for First Officer Line Training began its takeoff on runway 12 at Dryden in normal day visibility but almost immediately directional control was lost and the aircraft departed the side of the runway and hit a frozen snow bank. The aircraft sustained substantial impact damage and one of the six passengers was seriously injured when propeller blade debris penetrated the fuselage.

The aircraft in its final stopping position showing the damaged propellers. [Reproduced from the Official Report]

Investigation

An Investigation was carried out by the Canadian Transportation Safety Board (TSB) using comprehensive data successfully downloaded from the removed FDR and 2 hour CVR of the aircraft involved. 

It was noted that the Captain, who was acting as PM for the flight involved, had a total of approximately 20,000 hours flying experience which included 19,000 hours on type. He had been working for the operator for over 20 years, had gained his SA-227 type rating in 2000 and was supervising the first day of the First Officer’s Line Training. The First Officer, who had joined the operator the month prior to the accident and had since gained his SA-227 type rating, had approximately 270 hours flying experience which included 15 hours on type. The accident flight was the sixth of the day for the both pilots.

What Happened

Since there was no cabin attendant on board, the First Officer provided a safety briefing to the passengers prior to engine start. The crew then started both engines in turn and followed this with the ‘After Start’ and ‘Before Taxi’ checks. Since the aircraft was not fitted with a steering tiller at the right hand position, the Captain had to taxi the aircraft before handing control to the First Officer on reaching the departure runway. 

The ‘Before Taxi’ Checks included the ‘Start Locks’ task, which required three preliminary subtasks to be confirmed complete by each pilot. ‘Their’ prop area must be checked clear ahead and behind, ‘their’ wing must be checked free of contaminants and ‘their’ fuel cap must be checked in place. Having done this, with the First Officer going first, each pilot is required to verbalise “Clear, Clean Wing, Cap On” and only when this has been done are the Power Levers pulled over the Flight Idle gate toward reverse to release the start locks and complete the checklist ‘Start Locks’  task.

On this departure, when the First Officer initiated the required preliminary ‘Start Locks‘ checks, the Captain instructed him standby. The First Officer then verbalised the correct subtask responses but there was no response from the Captain. A few seconds later, the Captain began assessing runway conditions and spoke to the Sioux Lookout radio operator about flight plan and departure details before calling for the ‘Before Takeoff’ checklist, which was then completed. He then began to taxi the aircraft to Runway 12 for the departure with directional control achieved using only the nose wheel steering with no differential thrust required to assist in turns.

After the ‘Line Up’ checks had been completed and the aircraft was on the runway and ready for takeoff, the Captain transferred control of the aircraft to the First Officer. Takeoff power was set but whilst the aircraft was accelerating, directional control was lost and the aircraft veered off the right hand side of the runway after about 150 metres and struck a frozen snow bank before coming to a stop about 18 metres beyond the edge of the runway in snow which was about 0.5 metres deep. The crew shut down the engines and all occupants then left the aircraft through the main cabin door and were met by the airport emergency response team.

Impact with the frozen snow bank led to the blades of both propellers shattering and the resulting high energy release of the nickel-cobalt erosion strips and splintered wood core debris. Some of this penetrated the fuselage where it was reinforced to prevent damage by ice shed from the propellers in normal operations and then continued into the passenger cabin causing a consequent serious injury to the hand of one of the passengers.

The snow bank impact also led to both engine mounts being fractured and bent and both nacelles being substantially distorted and both inboard upper wing and forward fuselage skins being “slightly wrinkled” and the fuselage also sustained smaller punctures to the belly skin.

After one of the crew had exited the aircraft to check for the presence of any potential hazards, the passengers were advised to remain on board until the emergency services arrived and the First Officer provided interim first aid to the seriously injured passenger using the on board kit. An ambulance and fire truck arrived at the site between 20 and 25 minutes after the accident and the seriously injured passenger was taken to a local hospital.  

Fuselage damage by propeller debris and the in-cabin consequences. [Reproduced from the Official Report]

Why it happened

No evidence of any relevant airworthiness fault relating to the propeller control system was found and the Investigation of cause therefore focused on the operational factors that contributed to the aircraft departing the runway. The fact that the reinforcement panels and fuselage skins on either side of the fuselage were penetrated by propeller blade debris was a separate issue of considerable concern that was also separately investigated.

The available evidence confirmed that the ‘Start Locks’ item of the ‘Before Taxi’ Checklist was unintentionally not completed and thereafter, the Captain failed to release the start locks prior to beginning to taxi the aircraft. It was considered that the chances of this omission occurring were increased by the lack of a specific action to release them in the ‘Before Taxi’ Checks. The Checklist item referring to this necessary pre flight action merely required a response confirming that both pilots had completed three sub tasks unrelated to the release of the start locks themselves.    

Taxiing with the start locks still engaged but without a need to use differential power in turns meant that it was possible for the slight or rapid transient movements of the engine power levers required to taxi the aircraft to result in the release of the left propeller start locks whilst the right propeller start locks remained engaged. The situation would still have been recoverable if the SOP was followed which required that when the power is advanced for takeoff and the torque indication has increased through 20%, the Captain must verify the torque indication for both engines and call “positive torque”. The recorded CVR data for the accident takeoff showed that this call was not made and both pilots remained unaware of the status of the start locks. FDR data showed that once takeoff power was applied, the rpm of both propellers increased but the corresponding engine torque only increased on the left engine, whilst the right engine torque remained close to zero. The resulting almost immediate asymmetry compromised directional control and as the increase to takeoff power was completed, the aircraft veered to the right and off the side of the runway approximately 150 metres from the beginning of the runway. Having struck the frozen snow bank, it eventually stopped about 18 metres from the runway edge in snow about half a metre deep.

Discussion

The Investigation sought to examine the context for the accident and concluded that it was primarily founded in the failure of the crew, and in the circumstances essentially the Captain, to follow established normal procedures for no apparent reason. The Investigation also came to the conclusion that the presentation of the start locks check in the ‘Before Taxi’ Checklist was conducive to its inadvertent omission in that there was no requirement for a crosschecked confirmation that the start locks had been removed. The presence and potential consequences of an extremely steep authority gradient on the flight deck were also noted as was the risk of checklist items not being fully completed if the execution of a checklist is interrupted.   

The fact that propeller debris was able to penetrate the passenger cabin and cause a serious injury to be sustained by one of the passengers was of obvious concern. It was noted that although the purpose of the reinforced panels on the sides of the fuselage was to prevent damage to the fuselage by ice shed from the propellers during normal operations, they were not intended or required to prevent penetration of the passenger cabin by propeller debris.  

The eight formally-stated Findings as to Causes and Contributing Factors were as follows:

1. While the crew was carrying out the ‘Before Taxi’ checklist, the ‘Start Locks’ task was initiated but it was interrupted and not completed.
2. After the Captain told the First Officer to stand by, the crew’s focus shifted to other tasks. It is likely that this slip of attention resulted in the power levers not being pulled over the flight idle gate to release the start locks. 
3. The ‘Before Taxi’ checklist did not contain a task to ensure that the start locks were removed and, as a result, the crew began taxiing unaware that the propellers were still on the locks.
4. After the engine was started or while the occurrence aircraft commenced taxiing for departure, it is likely that slight or rapid transient movements of the engine power levers, which were needed to taxi the aircraft, resulted in the release of the left propeller start locks while the right propeller start locks remained engaged. 
5. As the power was advanced through 20%, the “positive torque” call required by standard operating procedures was not made, and the engine torque differential was not noticed by the crew. As a result, power lever advancement continued although the right engine torque remained near zero. 
6. The engaged start locks on the right propeller prevented right engine forward power being set, which resulted in a significant power differential. This differential power during the takeoff roll resulted in a loss of directional control of the aircraft and, ultimately, a lateral runway excursion.
7. Following the runway excursion, the propellers, which were operating at a high rpm, shattered and splintered when they struck a frozen snow bank.
8. High-energy release of the nickel-cobalt erosion strips and splintered wood core debris from the propeller blades penetrated the reinforcement panel, fuselage skin and cabin wall, and resulted in serious injuries to a passenger sitting next to the penetrated cabin wall. 

A formally-stated Finding as to Risk was also made as follows: 

• If procedures for challenge-and-response checklists do not include guidance on task interruptions, pauses, or non-standard responses, there is a risk that checklist tasks will be incomplete or omitted, which may result in the aircraft operating in an unsafe or undesirable configuration. 

Safety Action taken by the aircraft operator Perimeter Airlines as a result of the investigated event prior to the completion of the Investigation was noted as having included the following:

• The SA-227 Standard Operating Procedures Manual has been amended to highlight the importance of confirming that the start locks are disengaged. 
• In the SA-227 ‘Before Taxi’ Checklist, ‘removal of start locks’ is now a stand-alone checklist item and the last checklist item to complete before taxi commences. 
• A scheduled semi-annual simulator training at the 6 month mark has been introduced for all CARs subpart 703 and 704 pilots who have less than two years’ experience in the seat they are occupying. This training includes effective directional control techniques during the takeoff roll and a review of company policies, procedures and techniques related to engine propeller lock engagement and disengagement on the SA-226 and SA-227 aircraft operated by the company.

The Final Report of the Investigation was authorised for release on 24 March 2021 and it was officially released on 14 April 2021. No Safety Recommendations were made.

Related Articles

• Runway Excursion
• Standard Operating Procedures (SOPs)
• Propeller