SW4, Thompson MB Canada, 2017
SW4, Thompson MB Canada, 2017
On 2 November 2017, a Fairchild SA 227-AC Metro III landing at Thompson after a ferry-permit flight issued to facilitate a hydraulic fluid leak rectification left the runway when reverse pitch was selected and sustained substantial damage. The Investigation found that the flight had been continued without shutting down the left engine when its oil pressure dropped below the level requiring this in the QRH. The oil loss was found to be attributable to the same cause as a similar loss identified two days previously for which rectification had supposedly been achieved. The hydraulic leak did not affect the flight.
On 2 November 2017, a Fairchild SA 227-AC Metro III, (C-FLRY) being operated by Perimeter Aviation on a non-revenue positioning flight as PAG959 from Gods River to Thompson in accordance with a ferry flight permit in order to facilitate required left engine maintenance departed the left side of the runway during the destination landing in night VMC. The only occupants, the two pilots, sustained minor injuries whilst evacuating the significantly damaged aircraft.
An Investigation was carried out by the Transportation Safety Board (TSB) of Canada. The aircraft involved was not fitted with a FDR nor was it required to be but its 30 minute CVR was removed and its data were downloaded. The aircraft had been retrofitted with a Garmin 950 ‘Integrated Flight Instrument System’ which was capable of recording navigation data and flight control data on two SD data cards. Only navigation data SD card was installed in the accident aircraft.
In respect of the lack of any regulatory requirement for the aircraft to be fitted with an FDR, the Investigation noted that “numerous TSB aviation investigation reports have referred to investigators being unable to determine the reasons for an accident because of the lack of on-board recording devices” and observed that “the benefits of recorded flight data in aircraft accident investigations are well known and documented”. The recurrence of this issue in this Investigation after it also attracted attention during the TSB Investigation into a fatal accident in 2016 together with the lack of progress on an outstanding Safety Recommendation issued to Transport Canada in 2013 resulted in the release of a new Recommendation during the course of this Investigation to seek action and it is reproduced toward the end of this summary article.
It was noted that the Captain had a total of 1,400 flying hours including 1,000 on type. She had completed her First Officer Training with the operator in January 2016 and completed line training on the type involved the following month. A little over four months later, she had commenced Captain upgrade training and just 8 days prior to the accident, she had been released for unsupervised command flying after completing 82 hours of line training (the minimum is 20 hours). The First Officer had a total of 950 flying hours of which 700 hours were on type. He had completed his First Officer line training in August 2016.
It was established that after landing at Gods River after a scheduled flight from Winnipeg two days prior to the accident flight, an alert for low left engine oil pressure had been annunciated when the speed levers had been selected to low for taxi. The taxi had been completed and a normal shutdown followed. On subsequent inspection, the pilot found the left engine oil quantity as low and when the left engine oil pressure only reached 35psi, after the engines had been restarted in order to taxi to the refuelling pad, the pilot decided to shut the engine down and use only the right engine to complete the repositioning. After shutting down the aircraft on the fuel pad, the pilot contacted company maintenance control to report the situation and was advised that a maintenance crew would be sent to the aircraft location to investigate. On arrival, the maintenance crew added 3 quarts of oil and an inspection of the left engine found oil leaking from the left engine oil pressure snubber (a component which “eliminates erratic oil pressure indications”).
The aircraft remained grounded awaiting the parts needed for the repair and the following day, a different maintenance crew arrived with the required parts and replaced the left engine oil pressure snubber. After a ground run showed no sign of any oil leak, the aircraft was released to service. However, during a pre-flight inspection, an engineer then noticed a hydraulic fluid leak and on checking the hydraulic fluid reservoir found it to be low but on inspection could find no source for the leak so the aircraft was again parked overnight. The next day, another maintenance crew were sent to Gods River to investigate and rectify the hydraulic leak and another flight crew were sent there to fly the aircraft back to base. However, a lengthy inspection and engine ground run also failed to identify the source of the hydraulic leak and it was decided to obtain a ferry flight permit to allow the aircraft to be positioned to Thompson for further inspection and repair - a fifty minute flight.
Hydraulic fluid was added as was an additional 1 quart of oil to the left engine. It was noted by the Investigation that the Engine Maintenance Manual (EMM) specified that oil consumption should not exceed 1 quart per 12½ hours of engine operation and that the same oil reservoir had been replenished the previous day ready for flight. The ferry flight permit was obtained for a gear down flight crew only positioning sector as requested and the pilots then contacted the company Assistant Chief Pilot for a briefing on the safety issues associated with the flight which included that the landing gear should be retracted only in an emergency.
The flight departure was normal but approximately 15 minutes after takeoff, the left engine oil pressure was observed to have dropped into the ‘yellow’ range (40-70 psi). After talking to maintenance control, the crew “decided that if the oil pressure dropped below the yellow band and the oil temperature increased beyond the green band, then the engine was to be shut down”. The crew began to run the “Pre-planned Engine Shutdown Checklist” in the QRH “so that they would be prepared if the left engine indications changed (and) retracted the landing gear to reduce drag” but stopped at the point where both generators are switched off as the oil temperature indication had remained normal.
When approximately 40 nm southeast of their intended destination, the crew advised ATC that they had an indication of low oil pressure on one engine that it “might need to be shut down”. The ARFF at Thompson were advised and put on standby. There were no further developments and the aircraft commenced final approach to Runway 24 at Thompson with normal gear extension but as the runway threshold was approached, the left engine rpm dropped slightly to 96% and the Captain responded by making a corresponding adjustment to the right engine rpm. By this time, the left engine oil pressure indication had dropped to 16 psi - well into the red range - but as the indicated oil temperature had not risen above 88°C, it had been decided to keep the engine running.
In weather conditions which were “not considered a factor” in the accident, the initial touchdown on the 1,768 metre long 45 metre wide runway was on the right main gear. Once touchdown was complete approximately 365 metres past the runway threshold, the Captain selected reverse pitch on both engines which was immediately followed by a loss of control with the aircraft pitching up and rolling to the right. The right wing touched the runway and the aircraft then began to pivot around the right wingtip and veered off the runway. As the aircraft completed a 180° turn to the right to face towards the approach just made, the nose dropped and it came to a stop with the engines still running. After shutting the engines down, the crew exited the aircraft via the left overwing emergency exit having been unable to open the main passenger door which was subsequently found to have been pierced by a piece of propeller blade from the left propeller.
All three landing gear legs were detached during the excursion, the nose section of the aircraft was “severely distorted”, both wings were damaged and both sets of propeller blades were severed at the blade roots. The left propeller blades were found in the feathered position and the right propeller blades were found at a low angle (start lock) position.
An initial inspection found that no oil level was visible in the left engine oil tank sight gauge and the oil tank was empty. There was a pool of oil in the exhaust duct but no evidence of any oil leak inside the engine compartment. When the engine oil system was drained, ¾ litre of oil was collected and inspection of the magnetic chip detector showed no evidence of any contamination. The left engine was then removed and shipped to manufacturer for teardown.
It was noted that according to the AFM, the minimum oil pressure limitation for operation of the Honeywell TPE331-11U-612G engines is 50 psi. The left engine had been on the wing for 1,900 hours since the last overhaul at which point the rear turbine oil seal assembly had been replaced. Teardown of this seal assembly disclosed that excessive soot and solid carbon residue of the type created when oil has reached high enough temperatures (200-230°C) to cause its thermal breakdown.
It was found that the AFM Limitations included a requirement that the engine must be allowed to cool down by running the engine below 20% torque for three minutes before shutdown. If this is not done, “oil that remains trapped within hot locations of the engine will heat up to a point where the oil decomposes into a carbon deposit”. The Investigation was unable to determine whether the engines of the accident aircraft had been routinely shut down correctly but “a random sampling of engine shutdowns was gathered using flight data monitoring information downloaded from a similar aircraft operated by Perimeter Aviation (and it was found that ) of 20 samples taken, 50% of engine shutdowns occurred before the full 3-minute cool down period had elapsed”.
It was determined that the diagnosis and consequent rectification action by maintenance after the low oil pressure observed on the ground two days prior to the accident flight had not identified the real cause of the lack of oil pressure, which had been “a steady oil leak past the rear turbine air-oil seal assembly” with the depleted oil supply resulting in cavitation of the engine driven oil pump and a consequent loss of oil pressure which accounted for the loss of propeller control authority - and thereby loss of control of the aircraft - when reverse was selected after touchdown.
It was also noted that since there had been no evidence of any issues with the hydraulic system on the accident flight and given the substantial accident damage to the aircraft, “further inspection and repair of the hydraulic leak were not carried out”.
Flight Operations Issues
It was noted that regulatory requirements for flight by the aircraft required that the AFM must be carried on board. The Investigation “determined that the crew did not refer to the AFM during the occurrence”. It was noted that the QRH carried and referred to by the crew during flight had been developed by Perimeter Aviation and that a QRH, which is not subject to regulatory approval, must reflect the procedures contained in the AFM although they “may be abbreviated for ease of reference”. The relevant QRH procedure for low oil pressure was found to require that “in the event of a low oil pressure light illuminating, check the corresponding oil pressure gauge (and) if oil pressure indication is below 40 psi or abnormal, the engine is to be shut down”. It was noted that after their consultation with company Maintenance Control, the crew had failed to follow this required procedure.
A discrepancy was found between the AFM-specified response to a low oil pressure indication which it was considered “may lead to continued operation” in such circumstances and the equivalent Perimeter QRH response with only the latter requiring engine shutdown if oil pressure drops below 40 psi. It was therefore observed that “if operators of the SA227-AC Metro III aircraft rely solely on the emergency procedures listed in the AFM, continued engine operation with low oil pressure may result in loss of control of the aircraft”.
It was noted also that, although there was no regulatory requirement for operations under Canadian Aviation Regulations (CARs) for Subpart 703 (air taxi) or Subpart 704 (commuter) flight operations to have an approved SMS in place, Perimeter Aviation, which operated its SA226 and SA227 aircraft under Subparts 703 and 704 and its DHC8 aircraft under Subpart 705, had an approved SMS which covered all three types of operation. This included providing a formal CRM course for its DHC8 pilots at the time of the accident but only a more limited CRM course for its SA226 and SA227 pilots. It was noted that by 28 January 2019, it would be necessary for all pilots working in CARs Subpart 702, 703, 704 and 705 operations to have received comprehensive CRM training, including training in ‘Pilot Decision Making’ (PDM) which it was considered may have better prepared the accident aircraft crew for the issues which arose during their flight.
The Findings of the Investigation were formally documented as follows:
Causes and Contributing Factors
- The left engine low oil pressure indications during the previous and the occurrence flights were likely the result of a steady oil leak past the rear turbine air-oil seal assembly.
- The loss of engine oil pressure resulted in a loss of propeller control authority on landing and the upset of the aircraft.
- After consultation with maintenance, the crew considered the risks associated with a single engine landing and without hydraulic pressure for the nose-wheel steering and decided to continue the flight with both engines running, even though this was not consistent with the QRH procedures for low oil pressure indications.
- Carbon deposits that accumulated within the inside diameter of the bellows convolutions interfered with the bellows’ ability to expand and to provide a positive seal against the rotor seal.
- If Canadian Aviation Regulations (CARs) subparts 703 and 704 operators do not provide initial or recurrent crew resource management training to pilots, these pilots may not be prepared to avoid, trap, or mitigate crew errors encountered during flight.
- If operators of the SA227-AC Metro III aircraft rely solely on the emergency procedures listed in the aircraft flight manual, continued engine operation with low oil pressure may result in loss of control of the aircraft.
- If an engine is not allowed to sufficiently cool down prior to shutdown, oil that remains trapped within hot areas of the engine may heat up to a point where the oil decomposes, creating a carbon deposit.
- If flight data, voice, and video recordings are not available to an investigation, the identification and communication of safety deficiencies to advance transportation safety may be precluded.
- The Investigation was unable to determine the length of cool-down periods for the occurrence aircraft. However, a random sampling of engine shutdowns for similar company aircraft showed that 50% had not completed the full 3-minute cool-down period.
- Despite having received limited crew resource management (CRM) training, the crew demonstrated many skills and a knowledge of CRM principles.
It was noted that a TSB Safety Recommendation issued whilst Investigation was in progress on 26 April 2018 sought to address one of the concerns raised during the Investigation although not explicitly in response to it and was as follows:
- that the Department of Transport require the mandatory installation of lightweight flight recording systems by commercial operators and private operators not currently required to carry these systems. [A18-01]
This Recommendation was issued concurrently with the cancellation of a similar previous Recommendation [A13-01].
The Final Report of the Investigation was authorised for release on 17 October 2018 and it was officially released on 22 November 2018 and contained no new Safety Recommendations.