On 18 June 1998, the crew of a Swearingen SA226-TC (C-GQAL) being operated by Propair on a scheduled passenger flight from Montreal to Peterborough requested and received approval for a daylight air turnback because of a hydraulic problem. During descent, they advised that they had shut down the left engine because it was on fire and had now decided to divert to Mirabel. During final approach at Mirabel during which Visual Meteorological Conditions (VMC) was regained, they advised ATC that the left engine was again on fire. As the aircraft came over the runway, the left wing broke upwards and the fuselage pivoted left and struck the ground where the effects of the impact and a cabin fire destroyed the aircraft and fatally injured all 11 occupants.
An Investigation was carried out by the Canadian TSB. The 30 minute Cockpit Voice Recorder (CVR) was recovered; the aircraft was not fitted with a Flight Data Recorder (FDR) nor was it required to be.
It was noted that the 35 year-old Captain was Propair's Chief Pilot and that 4200 hours of his 6515 hours total flying time had been accumulated on the accident aircraft type. The First Officer, also 35 years old, had just joined the airline and was still undergoing line training having accumulated 93 hours on type out of a total flying time of 2730 hours.
It was established that very light winds had prevailed for the take off but the aircraft had pulled to the left with right rudder needed to keep the aircraft straight and the extent of this input did not reduce as speed increased which would normally be expected. It was also estimated that the take-off roll had been about twice as long as normal - around 1250 metres - and the time to rotation had been "about 6 seconds longer than the calculated time of 21 seconds".
It was evident that "these cues were not strong enough to elicit a reject response from the crew" and that they "apparently did not suspect that the left brakes had dragged and overheated during the take-off" and so retracted the landing gear without delay after take-off. The overheated brake and wheel assembly was retracted into the enclosed wheel well where the heat was dissipated into the tyre and the surrounding structure and eventually caused a fire. No definite cause for the loss of hydraulic pressure which the crew reported 12 minutes after take off could be determined but it was concluded that the intensity of fire or intense heat in the wheel well would have been enough to "have ruptured hydraulic hoses or damaged seals in the hydraulic components". Almost immediately after the report of hydraulic pressure loss, it became obvious to the crew that "something was wrong with the controls" and this was quickly followed by "the first perceived indication that engine trouble was developing" and the illumination for 30 seconds only of the left wing overheat light.
Seventeen minutes after take off, "the left engine appeared to be on fire and it was shut down" by the Captain less than a minute later. An abnormal amount of right aileron pressure was required to keep the aircraft on the desired heading and ATC, having been advised that the left engine had been shut down, suggested a diversion to Mirabel which was accepted. Soon after this, the crew advised ATC that flames were coming out of the "engine nozzle". They made preparations for an emergency landing and reviewed the procedure for manual landing gear extension. Five minutes after reporting that the left engine had been shut down, "the crew informed ATC that the left engine was no longer on fire, but three and a half minutes later, they advised ATC that the fire had started again". By this time, "the aircraft was getting harder to control in roll, and the aileron trim was set at the maximum". The Investigation subsequently determined that "aileron control became abnormal because the heat from the fire reduced the stiffness of the left wing rear spar, allowing abnormal bending of the wing" which had changed the wing's lifting ability and affected the amount of aileron deflection required to maintain control.
Approximately 26 minutes after take off by which time the aircraft was on short final for Runway 24 at Mirabel, the landing gear lever was selected, but only two gear down indicator lights followed. Then near the runway threshold, the left wing failed upwards, the aircraft "rotated more than 90º to the left around its longitudinal axis and crashed, inverted, on the runway". It immediately caught fire and, after sliding approximately 760 metres, came to rest 75 metres from the left hand edge of the runway. AFS were already on standby near the runway threshold and responded promptly but although the cabin fire which instantly erupted was quickly brought under control, there were no survivors.
No definite cause for the dragging brakes was identified but the brake system was examined to establish how hydraulic fluid leakage would have occurred and developed into an ignition source. It was demonstrated that "residual pressure as low as 50 psi in the brake system - too little for the indication to be readily noticed by the crew - would be capable of causing the dragging experienced. The dragging would have led to overheating of the brake components during the take-off run and such overheating, transferred to the fluid in the brake lines, causes the fluid to expand, thereby increasing pressure in the brake system". Such an increase in system pressure then further aggravates the dragging and the overheating to the point where normal operating temperatures are exceeded. It was further noted that "discolouration observed on the brake discs indicated that they reached at least 600ºC for an extended period" with "the uniformity of the discolouration and the dragging marks on both sides of both left discs indicating that there was brake pressure at the wheels, sufficient to cause overheating during taxi and the take-off roll". Thereafter, it was concluded that:
- Following gear retraction, the heated brakes were in an enclosed space close to hydraulic lines, air and fuel lines, and several other lines and electrical wires. With limited cooling, the temperature of the brake assembly components continued to rise. Dynamometer tests demonstrated that over a distance similar to that of the take off and prior taxi out, with a brake pressure of 50 psi, the carrier, lining and button assembly and the brake disc could generate temperatures exceeding 450ºC.
- Since Nitrile brake cylinder piston seals start to degrade above 135ºC, the likelihood increased of a brake fluid leak in one of the brake assemblies. Under these conditions, the fluid, which was slightly contaminated, would have self-ignited at temperatures exceeding 425ºC, as shown in laboratory tests. The tyres then ignited when exposed to flame and these conditions were sufficient to perpetuate the cycle and continue to raise the temperature in the wheel well.
- This caused the aircraft hydraulic system to fail and damaged electrical wiring and all other systems in the wheel well. This partial destruction of the aircraft systems in the wheel well links the pieces of apparently unrelated information that the crew had to deal with in this occurrence.
It was noted that the indications of progressively more serious damage to the left wing were initially subtle and had included abnormal response to aileron control input, the illumination of the wing overheat light and the disappearance of a panel which would normally cover the engine exhaust nozzle from the upper wing surface.
Eventually, fire within the left engine nacelle had become visible after engine shut down action had not prevented it continuing. It was found that "intense heat" had led to the failure of the front wing spar with the twisting motion of the rear spar suggesting that "its failure was subsequent to the upwards failure of the front spar". Once the wing had failed, asymmetrical lift had caused the aircraft to "rotate left on the longitudinal axis through the left wing wheel well" which had released the left landing gear pivot pin and allowed the landing gear assembly to fall onto the runway followed by the aircraft then striking the runway on its left side before sliding left and leaving the runway at which point "the engine, then the left wing, separated from the rest of the fuselage, which continued its course and came to rest inverted".
It was found that there was a history of similar brake system problems on SA226 and SA227 aircraft beginning in 1983 with a large number of incidents and a few accidents involving landing gear failures, tyre failures, flat tyres, wheel fires and loss of control on the ground. These were found to have included "62 incidents and 3 accidents involving circumstances similar to those found in this accident", some of which "had the potential to result in a catastrophe similar to this accident" but in these cases, the flights had culminated in successful emergency landings.
The formally documented Findings of the Investigation were as follows:
Causes and Contributing Factors:
- The crew did not realise that the pull to the left and the extended take-off run were due to the left brakes dragging, which led to overheating of the brake components.
- Dragging of the left brakes was most probably caused by an unidentified pressure locking factor upstream of the brakes on take-off. The dragging caused overheating and leakage, probably at one of the piston seals that retain the brake hydraulic fluid.
- When hydraulic fluid leaked onto the hot brake components, the fluid caught fire and initiated an intense fire in the left nacelle, leading to failure of the main hydraulic system.
- When the L WING OVHT light went out, the overheating problem appeared corrected; however, the fire continued to burn.
- The crew never realised that all of the problems were associated with a fire in the wheel well, and they did not realise how serious the situation was.
- The left wing was weakened by the wing/engine fire and failed, rendering the aircraft uncontrollable.
- Numerous previous instances of brake overheating or fire on SA226 and SA227 aircraft had the potential for equally tragic consequences. Not all crews flying this type of aircraft are aware of its history of numerous brake overheating or fire problems.
- The Aircraft Flight Manual (AFM) and the emergency procedures checklist provide no information on the possibility of brake overheating, precautions to prevent brake overheating, the symptoms that could indicate brake problems, or actions to take if overheated brakes are suspected.
- More stringent fire-blocking requirements would have retarded combustion of the seats, reducing the fire risk to the aircraft occupants.
- A mixture of the two types of hydraulic fluid lowered the temperature at which the fluid would ignite, that is, below the flashpoint of pure MIL-H-83282 fluid.
- The Aircraft Maintenance Manual indicated that the two hydraulic fluids were compatible but did not mention that mixing them would reduce the fire resistance of the fluid.
- The master cylinders were not all of the same part number, resulting in complex linkage and master cylinder adjustments, complicated overall brake system functioning, and difficult troubleshooting of the braking system. However, there was no indication that this circumstance caused residual brake pressure.
- The latest recommended master cylinders are required to be used only with specific brake assembly part numbers, thereby simplifying adjustments, functioning, and troubleshooting.
- Although the emergency checklist for overheating in the wing required extending the landing gear, the crew did not do this because the wing overheat light went out before the crew initiated the checklist.
- The effect of the fire in the wheel well made it difficult to move the ailerons, but the exact cause of the difficulty was not determined.
Six Safety Recommendations were issued by the TSB as a result of the Investigation as follows:
On 26 October 1998, the following five Recommendations were issued. Three (as indicated) were identical in substance to ones issued on the same date by the National Transportation Safety Board (USA) (NTSB) to the Federal Aviation Administration (FAA). The NTSB also issued three other Safety Recommendations arising from this event to the FAA on the same date.
- that Transport Canada consult with the Federal Aviation Administration regarding a timely amendment of the Aircraft Flight Manual for the Fairchild/Swearingen SA226 and SA227 Metroliner to have the Manual specify the risk of wheel well fires caused by overheated brakes, and include procedures both to mitigate this risk and address emergency situations of actual and potential wheel well fires. [A98-02] (the same as NTSB A98-115)
- that Transport Canada, in consultation with the Federal Aviation Administration and the aircraft manufacturer, explore options for the installation of a brake temperature or overheat detection system on Fairchild/Swearingen SA226 and SA227 aircraft; [A98-03] (the same as NTSB A98-116)
- that Transport Canada, in consultation with the Federal Aviation Administration and the aircraft manufacturer, explore means to protect or otherwise harden the hydraulic and fuel lines in wheel wells to minimize the damage to these lines in the event of bursting tires or wheel well fires. [A98-04] (the same as NTSB A98-118)
- that Transport Canada, as a matter of urgency, notify all Canadian operators of Fairchild/Swearingen SA226 and SA227 aircraft of the importance of, and requirement for, using only MIL-H-83282 hydraulic fluid in the main and brake hydraulic systems of these aircraft; [A98-05]
- that Transport Canada, in consultation with the Federal Aviation Administration and the aircraft manufacturer, review the adequacy of existing aircraft standards, procedures, manuals and maintenance practices for the Fairchild/Swearingen SA226 and SA227 aircraft with an aim to ensuring that only MIL-H-83282 hydraulic fluid is used in the main and brake hydraulic systems of these aircraft. [A98-06]
At the conclusion of the Investigation, the TSB issued one further Recommendation:
- that Transport Canada, the United States Federal Aviation Administration, and Fairchild explore options for SA226 and SA227 aircraft to be equipped with a brake pressure warning indicator for each main wheel brake system. [A02-03]
The Final Report of the Investigation was authorised for release on 2 April 2002.