Deadly Wheel Well Fire Prompts Call for Better Cockpit Warning

The pilots and passengers of a regional airliner paid the price of a known fire hazard that had gone uncorrected for years, according to the Transportation Safety Board (TSB) of Canada.

The accident involving a Fairchild/Swearingen Metroliner II twin-turboprop occurred June 18, 1998, but the TSB's final report was just released. Briefly, on the 4,000- ft. take off run from Dorval, Quebec, the brakes were dragging on the left main gear. The crew noticed that the airplane was pulling to the left, and applied right rudder to maintain alignment with the centerline in a take off roll that was twice as long as calculated. The crew experienced control problems, a hydraulic failure warning, problems with the left engine, and a left wing overheat light, all within two minutes.

Unbeknownst to the pilots, a fire was raging in the left wheel well. According to the TSB report, "During the take-off roll, the heat generated by the two dragging brakes increased exponentially. As a result, a brake seal failed, allowing brake fluid to leak onto the hot brake components and ignite, creating a fire in the wheel well."

As the crew was desperately attempting an emergency landing at nearby Mirabel airport, the fire literally burned off the left wing just as the airplane was over the runway. The fuselage and remaining wing careened 90?, struck the ground, and the two pilots and nine passengers aboard were killed.

As a measure of the fire's intensity, the discoloration of the brake discs recovered at the scene of the crash showed they had been exposed to temperatures exceeding 600?C (1,112?F).

The case of the Metro II crash, operating as Propair Flight 420, further highlights the danger of fires in inaccessible areas of the aircraft. The TSB is now investigating a May 13 belly hold fire in an Air Canada B767-300 (see ASW, June 10). The B767 fire is the third to occur this year in belly holds of large jets, in which flammable thermal acoustic insulation blankets contributed to the fire's spread.

The Propair crash occurred about two and a half months before the fatal Sept. 2, 1998, crash of a Swissair MD-11 at Halifax. This case, too, involved a spreading in- flight fire, presenting the TSB with two fatal accidents involving in-flight fire in the same year.

In the cases of the larger jets, first the MD-11 crash and most recently the B767 fire, the TSB is focusing not only on the ignition source, but on the question of why flammable materials are installed in the first place. The TSB has made its position clear: If there were no combustible materials in an airplane, fires would not occur (see ASW, Sept. 10, 2001).

In the case of the Propair crash, the TSB has raised a related set of issues - design deficiencies, inadequate fire warnings, inadequate crew training, poor understanding of the flammability properties of hydraulic fluid, and a history of events that showed the persistence of the hazard and the failure to correct it. In the Propair crash, the litany of precursor incidents finally combined to kill. We deal briefly with each of these areas:

Incident history
To cite from the TSB report:

"The TSB did extensive research and found that, since 1983, landing gear failures, tire failures, flat tires, wheel fires, and loss of control on [the] ground were frequent for SA226s and SA227s [two models of the Metroliner]. Sixty-five of these occurrences involved circumstances similar to this accident and had the potential to result in tragic events."

Parking brakes
How did the Flight 420 crew come to attempt take off with dragging brakes? The short answer is that "OFF" does not necessarily mean fully off. The TSB highlighted this caution from a 1993 issue of the manufacturer's newsletter:

"Before taxiing or beginning the take off roll, ensure that parking brake is fully released. Just moving the parking brake control knob to the 'OFF' position is not enough to ensure release of the brakes ... since some residual pressure can remain even with the knob in the 'OFF' position."

The TSB noted with mild understatement, "For the above reasons, Metro II crews tend to not trust the parking brake." Another source was more acerbic, describing the parking brake situation as a "glaring design deficiency."

Dangerous cocktail
The aircraft originally was certified to use MIL-H-5606 hydraulic fluid. It has a flashpoint of 82?C (180?F). After two cockpit fires involving this fluid, the U.S. Federal Aviation Administration (FAA) issued an airworthiness directive (AD No. 83-19-02) enjoining operators to use a fluid with a higher flashpoint. Designated MIL-H-83282, it has a flashpoint of 205?C (400?F). Operators were ordered to purge the lower-flashpoint fluid from the hydraulic system and refill with the higher-flashpoint fluid. However, the two fluids look very much alike (appearance, smell, viscosity, etc.), and TSB investigators believe that some operators might be using the type 5606 fluid as a substitute for the 83282 fluid.

A mix of the two fluids was found in the brake system of the accident aircraft. The TSB conducted a number of fire experiments and found that 100 percent MIL-H-83282 ignited into a "powerful flame about 10 inches high" when poured onto a surface heated to 450?C (842?F). When a blend containing 34 percent MIL-H-5606 fluid was poured onto a surface heated to 424?C (795?F), a flame was produced as powerful as that in the first test.

"The aircraft maintenance manual [AFM] indicated that the two hydraulic fluids were compatible but did not mention that mixing them would reduce the fire resistance of the fluid," the TSB found.

Aircraft maintenance manuals have since been modified to prohibit the mixing of fluids because of the increased vulnerability to fire. Placards have been placed on aircraft saying only MIL-H-83232 fluid is to be used.

Cockpit warnings
The TSB reserved its greatest concern for the cockpit warnings in the event of a fire in the main landing gear wheel well. At the same time the crew was dealing with engine and flight control problems, the left engine overheat warning (L WING OVHT) illuminated. But what did it mean? Within some 30 seconds, the light went out. "Not because the overheating problem was corrected but because the fire in the wheel well destroyed the warning system electrical circuit," the TSB surmised.

According to the emergency checklist:

A continuous L WING OVHT light can mean:

A temperature greater than 177?C (350?F) in the engine nacelle, or
A temperature exceeding 232?C (450?F) in the associated air conditioning ducts.
A flashing L WING OVHT light can mean:

Temperature in the leading edge wiring harness exceeding 121?C (250?F), or
An overheat condition in the air intake duct, or
Overheating of the associated generator.
The TSB added:

"The instructions in the AFM [airplane flight manual] offered no other significant indications, such as the possibility of a fire in the wheel well. The pilots receive no ground training for fires in the engine nacelle, since the AFM does not mention this possibility. Operators of this aircraft type ... were generally not aware that several overheating incidents and nacelle fires have occurred."

Absolutely necessary
As the expression goes, ignorance can kill. In the face of this apparent confusion about what a wing overheat light might really mean, the TSB concluded that a brake pressure warning system indicator is "absolutely necessary" for each main-wheel brake system. The TSB noted that a brake overheat detection system was not required when the airplane, built in 1977, was certified. "They still are not required for this class of aircraft," the TSB added. Nor were the airplane's tires outfitted with fuse plugs. These devices melt when hot, allowing for a controlled release of excess tire pressure. Bursting tires can add rubber shards to a conflagration, making it worse. Without fuse plugs, the TSB said, "Flammable fluids flowing from melted or damaged aluminum hydraulic and fuel lines, and [from] the rubber fuel crossover line, would have further fueled a fire."

The manufacturer indicated to the TSB that a brake pressure cockpit indicator for each wheel system is feasible. However, sources advise that instead of a brake fluid pressure sensor, a brake disk temperature sensor would be better. This capability would allow a direct comparison between the left and right main landing gear wheel- sets and would disclose a dragging brake much sooner than a brake fluid pressure warning. To be sure, it might be easier to rig a warning trip threshold to existing pressure gauges than to plumb in a temperature sensing system. Indeed, the TSB noted in its report that this very argument was raised, that a brake temperature sensing system would be "too costly to implement in view of the expected remaining life of the aircraft." The accident aircraft had a book value of about $230,000, according to sister publication Aircraft Value News. However, when the $2.7 million statistical value of a life is included, the Propair crash involved a total loss of about $30 million in lives and property.

In the case of the recent Air Canada belly hold fire, an infrared closed circuit television (CCTV) system was suggested for giving the crew more information than a simple warning light as to the fire situation in an inaccessible area (see ASW, June 10). Such a system also would be applicable to wheel wells in aircraft of all sizes. Any warm glows in a darkened wheel well would show up on camera.

In any event, the Propair crash may illustrate a second level of safety for smaller aircraft, despite declaratory rhetoric about "one level of safety" for the airline system. Consider warning systems for brake pressure, brake temperature and tire pressure. "We have all three," said a captain who flies the B777 widebody twinjet. >> The full TSB investigation report may be viewed at http://www.tsb.gc.ca/ENG/REPORTS/AIR/1998/a98q0087/ea98q0087.htm <<

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