Aviation Safety Letter 1/2005
Brakes: What's Stopping You?
by Rob Laporte, Civil Aviation Safety Inspector, System Safety, Ontario Region
Have you ever taken a moment to stop and think about what's stopping you? Whether the aircraft is an Airbus A-340 or an amateur-built aircraft, brakes are relied on for ground manoeuvring, parking and stopping the aircraft after-landing or an aborted take-off attempt.
Brakes work by applying a friction force to the main wheel assemblies to slow and stop the wheels' rotation. This friction absorbs the aircraft's kinetic energy (KE) and converts it into thermal energy. Aircraft brakes must have a minimum energy rating equivalent to the KE generated by the aircraft at its maximum landing weight and speed.
In large airplanes, the thermal energy potential is so great that the wheels are manufactured with thermal relief plugs that melt in an over temperature occurrence to deflate the tire, preventing the tire from bursting due to over pressurization. Many large airplane types have temperature sensors on the wheels to monitor wheel temperature, and some even have cooling fans to cool the brakes on the ground. Pilots and constructors of amateur-built aircraft are not required to be as cognisant of the hazards related to brake thermal energy, as are their large airplane counterparts. Yet, even on a light or very light aircraft, the brake thermal energy can be significant and should be considered. A light aircraft weighing 1 100 lbs (500 kg) and touching down at 60 kt has a KE equivalent to the amount of heat energy required to bring 2 cans of light beer to a boil on a 15°C day at sea level.
When constructing amateur built aircraft, Canadian Aviation Regulations (CARs) Subchapter A, 549.7 states: "Materials shall be appropriate and should conform to aviation quality specifications." Brake line assemblies in certified light aircraft use: Aluminium alloy 5052 tubing; rubber hose that meets Mil-H-8794 specification with an operating range of -54°C to 121°C; or, stainless steel braided Teflon hose meeting Mil-H-25579 specification with an operating range of -54°C to 232°C.
Are your brake lines up to the thermal challenge? Nylon tubing with a recommended operating range of -51°C to 100°C, are frequently used in amateur-built aircraft braking systems. This tubing is used because it is lightweight, flexible, inexpensive, able to withstand brake system pressure, and is compatible with Mil-5606 hydraulic fluid.
There is a significant hazard when brake lines fabricated from nylon tubing are attached directly to a wheel brake assembly. Heavy or repetitive brake applications can cause the temperature at the brake assembly and the brake line interface to exceed the temperature rating of the nylon tubing, resulting in brake line failure. In addition, nylon tubing loses some of its properties and becomes brittle when exposed to ultraviolet light from direct sunlight. The risks associated with brake line failure include brake failure and/or brake fire when the brake fluid, usually Mil-5606, is pumped onto the hot brake assembly and ignites.
The following case study is a factual account of a brake line failure resulting in a brake fire. The cause is attributed to brake temperatures exceeding the thermal capability of the nylon brake line. It is probable that the fire would not have occurred had the brake lines been manufactured with material conforming to aviation quality specifications.
In September 2003, a pilot took delivery of a recently purchased Van's RV-3. The RV-3 is a single-seat amateur-built aircraft with a maximum take-off weight of 1151 lbs (522 kg), and was built by the previous owner. The pilot had significant flying experience in light airplanes and had elected to assess the aircraft's handling characteristics in a series of high-speed taxi runs. The pilot requested a runway from the control tower and was assigned a 5 300-ft runway for the taxi runs. During a taxi run, the control tower reported to the pilot that flames were visible, emanating from the right main wheel. The pilot then reported "no break pressure on the right wheel," and exited the runway, parking the aircraft on the grass. After the aircraft stopped, the pilot exited the aircraft and discharged the aircraft's fire extinguisher on the right wheel, extinguishing the fire. The fire destroyed the right wheel fairing (pant), tire, and wheel assembly.
The fire was probably caused when the brake assembly temperatures exceeded the operating temperature of the nylon brake line, which caused the brake line to burst. When the nylon brake line burst, hydraulic fluid (Mil-5606) was sprayed on to the hot brake assembly and ignited. Though the damage was restricted to the wheel areas, the owner felt that had he not been able to extinguish the wheel fire, the aircraft would have been destroyed. To mitigate the risk of brake line failure the pilot-owner has replaced the nylon tubing with steel brake line at the interface between the brake assembly and brake line.
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