Propair Accident-Mirabel International Airport - 18 June 1998
Mirabel/Montréal International Airport, Quebec -18 June 1998
Safety Action Taken
(as indicated in the TSB report)
On 26 October 1998, the TSB issued five recommendations covering several aspects of the investigation, as follows.
The Board believes that to help ensure the safety of the crew and the passengers on Fairchild/Swearingen SA226 and SA227 Metroliners, the definitive operating manual__the aircraft flight manual (AFM) for these aircraft__must be amended. Furthermore, both ab initio and recurrent training for aircrew on these aircraft should include instruction on the handling of overheated brakes and wheel well fires. In this vein, the Board believes that the AFM will require amendments to reflect the following, inter alia:
the susceptibility of the brake system on Metroliners to overheating, the precautions to deal with or prevent overheating, the symptoms of potential problematic brake systems, and the fact that overheated brakes can cause wheel well fires
that a L or R WING OVHT warning light may indicate a wheel well fire and that there are other key symptoms associated with the L or R WING OVHT light that are indicative of an on-board fire
- the actions to be followed in conjunction with the emergency procedure for wheel well and wing overheat warning light ON to effectively handle the possibility of a wheel well fire
The US Federal Aviation Administration (FAA), as the regulatory body in the State of the aircraft manufacturer, has primary responsibility for mandating and approving revisions to the AFM. On 26 October 1998, the US National Transportation Safety Board (NTSB), in coordination with the TSB, submitted six recommendations to the FAA in this regard. Transport Canada (TC) has communicated with the FAA concerning amendments to the AFM and to checklists. The Board recommended as a matter of urgency 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 Fairchild/Swearingen SA226/SA227 Metroliners do not have a brake temperature monitoring or overheat detection system. Under current regulations, these systems are not required for certification. Such systems, especially in an aircraft with wheel brake systems that are susceptible to overheating, would allow aircrew to monitor the temperature of the wheel brakes and take appropriate precautions to prevent overheating or to preclude the inadvertent raising, into the wheel well, of main landing gear with overheated brakes.
As noted previously, post-accident examination of the occurrence aircraft determined that a pre-crash fire had occurred in the left main landing gear wheel well. This was shown by burned tires, melted aluminum hydraulic and fuel lines and fittings, and a fire-damaged rubber fuel crossover line. Additional damage to the wheel well, especially to the fuel and hydraulic lines, might have also resulted from bursting tires, since the wheels on the main landing gear of Fairchild/Swearingen SA226 aircraft do not incorporate fuse plugs (which melt when hot, giving a controlled release of tire pressure built up from the heat). If so, flammable fluids flowing from melted or damaged aluminum hydraulic and fuel lines, and the rubber fuel crossover line, would have further fuelled a fire.
TC issued Commercial and Business Aviation Advisory Circular (CBAAC) No. 0146, dated 16 September 1998, to alert Canadian flight crew and operators of these aircraft to the hazard associated with overheating of the brake system. CBAAC No. 0146 outlines the warnings and other possible abnormal aircraft indications associated with known Metroliner brake overheat / wheel well fires, explains the wheel well and wing overheat warning light system, specifies actions to be taken at the first indication of the wing overheat annunciator light, and highlights the susceptibility of the Metroliner brakes to overheating and the conditions for suspecting potential brake system overheating. The CBAAC also states that TC has communicated a recommendation to the aircraft manufacturer; in fact, TC communicated with the FAA regarding amendments to the AFM and to checklists.
The measures outlined in CBAAC No. 0146 and associated with the above recommendation focus on risk reduction through enhanced awareness of the hazard and improved operational practices to deal with possible or actual fires. However, the Board believes that additional preventive actions can be taken in the aircraft's systems to minimize the likelihood and severity of a fire in the wheel well. Therefore, the Board recommended 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; and (A98-03)
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)
Mixing of Hydraulic Fluids
Analysis of fluid from the accident aircraft's main and brake hydraulic systems revealed a mixture of MIL-H-83282 and MIL-H-5606 hydraulic fluids. These hydraulic fluids are nearly identical in colour and consistency. The mixture had a flashpoint of approximately 114ºC (239ºF).
The SA226 and SA227 specification originally called for MIL-H-5606, with a minimum flashpoint of 82ºC, to be used in the aircraft's main and brake hydraulic systems. However, after two Swearingen SA226-TC Metroliner II cockpit fire accidents in which the MIL-H-5606 hydraulic fluid was involved, the FAA issued Airworthiness Directive (AD) 83-19-02, applicable to certain Swearingen SA226 airplanes, including the Mirabel accident airplane. The AD required that operators drain and purge the main hydraulic and brake system reservoirs, refill them with MIL-H-83282 hydraulic fluid with a minimum flashpoint of 205ºC, and change the placards on both reservoirs to specify the MIL-H-83282 fluid. The accident aircraft was placarded in accordance with AD 83-19-02.
Current maintenance instructions state that MIL-H-83282 is to be used in the main and brake hydraulic systems of the aircraft. However, there is no reference to indicate that MIL-H-83282 is used because of the higher temperature at which its vapours will ignite or that a mixture of MIL-H-83282 and MIL-H-5606 can have a significantly lower flashpoint than the 205ºC flashpoint for pure MIL-H-83282. Given that MIL-H-5606 was the original specified fluid for SA226 and SA227 aircraft, that MIL-H-5606 and MIL-H-83282 are similar in appearance and most properties, and that there are no cautions about the consequences of using a mixture of the two fluids, the Board believes that MIL-H-5606 is being mistakenly used by some air operators and aircraft maintenance engineers as an alternative hydraulic fluid in systems requiring MIL-H-83282. Therefore, in view of the increased risk of fire occurring on Fairchild/Swearingen SA226 and SA227 aircraft resulting from the incorrect use of MIL-H-5606 hydraulic fluid, the Board recommended 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; and (A98-05)
- 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)
Responses to Recommendations
(as indicated in the TSB report)
TC concurred with recommendations A98-02, A98-03, and A98-04. TC's Aircraft Certification Branch requested an AFM amendment from the FAA to provide more direction and information on wheel well fires. TC also issued a CBAAC to all Metroliner operators in Canada. The CBAAC effectively provides information and guidance to pilots on wheel well fires.
In addition, the NTSB issued recommendations to the FAA similar to the TSB's recommendations A98-02, A98-03, and A98-04. TC forwarded the TSB recommendations to the FAA for review, comment, and action, in conjunction with review of the NTSB recommendations. The FAA's responses to these TSB recommendations are outlined below.
Response to Recommendation A98-02 (NTSB A-98-115)
Fairchild has released FAA-approved airplane flight manual changes that expand the "Wheel well and Wing overheat light on" material in Section 3, "Emergency Procedures", for the SA226 and SA227 aircraft. Subsequently, TC mandated these revisions for Canadian-registered aircraft. TC considers this recommendation closed.
Response to Recommendation A98-03 (NTSB A-98-116)
The FAA has investigated and evaluated a temperature monitoring or overheat system on the SA226 and SA227 aircraft and concluded that such an installation would be cost prohibitive.
Although this recommendation was not adopted, other action has been taken to reduce the probability of overheated brakes occurring during take-off. FAA AD 2000-17-01 was released on the 22 August 2000, with an effective date of 06 October 2000. This AD mandates compliance with Fairchild service bulletins (SBs) 227-32-017 and 226-32-049 to modify the parking brake system and with BFGoodrich SB 1498 to revise the inspection brake wear and clearance limits. Additionally, the FAA released AD 2001-20-14, which mandates Fairchild SBs 226-26-003 and 227-26-002. Effective after 21 November 2001, this AD is designed to correct potential brake shuttle valve problems, which could cause the brake assembly to drag and overheat. These actions are intended to prevent brake overheating, not to detect brake overheating. TC has mandated these ADs for Canadian-registered aircraft and considers this recommendation closed.
Response to Recommendation A98-04 (NTSB A-98-118)
TC advises that the FAA has responded by issuing ADs 2000-14-01 and 2001-20-14. These ADs incorporate modifications to the parking brake system, establish brake wear and clearance limits, replace the brake shuttle valve, replace a rubber fuel hose with a metal device, and install a shield over the hydraulic lines. TC mandated the ADs for Canadian-registered aircraft.
Response to Recommendation A98-05
TC concurred with recommendation A98-05 and issued an airworthiness notice titled "Hazards of Incorrectly Identifying or Mixing Aircraft Fluids" to the aviation industry.
Response to Recommendation A98-06
TC concurred with recommendation A98-06 and consulted with the FAA regarding the adequacy of existing aircraft standards, procedures, manuals, and maintenance practices for the Fairchild/Swearingen SA226 and SA227 aircraft. The aim of this review was to ensure that only the specified hydraulic fluid (MIL-H-83282), where usage of this fluid type applies, is used in the hydraulic systems of these aircraft.
TC confirms that Fairchild, in coordination with the FAA, has amended the SA226 and SA227 maintenance manuals to include warnings that reinforce the prohibition of fluid mixing.
The FAA had earlier issued AD 83-19-02, which required purging lower flashpoint hydraulic fluid from the aircraft's hydraulic systems and substituting higher flashpoint MIL-H-83282 hydraulic fluid. The AD also required installation of a placard specifying that only MIL-H-83282 fluid be used
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