The importance of aircraft wheels and tires cannot be overstated. They are critical in ensuring safety and are consistently subject to the punishment of takeoff and landing. Additionally, aircraft tires must endure extreme temperatures. While airborne, they experience temperatures below -40 degrees celsius, and at touchdown, tires briefly withstand temperatures in excess of 200 degrees celsius. This blog will provide a look into the details of aircraft wheels and how they are able to withstand the forces resulting from their operation.
Unlike common automotive tires, aircraft tires are too rigid to be forced onto a rim. Because of this, an aircraft wheel construction comes in two parts: an inboard and outboard rim. The two rims are bolted together with the tire between them and then pressurized with nitrogen. Aircraft tires are filled with nitrogen rather than air for a variety of reasons. First, large airline tires must be filled with an inert, dry gas. Second, tires filled with nitrogen have a lower chance of fire or explosion. Tire rubber is highly flammable and wheel brakes reach intense temperatures. A large tire full of pressurized air would significantly feed a fire. As nitrogen does not support combustion, it greatly reduces the risk of a tire fire or explosion.
Not only are aircraft tires subject to extreme temperatures, they must work under the extreme weight of a large airliner. For instance, a Boeing 767 has a maximum take off weight of more than 400,000 pounds, and a fully loaded 747-8 weighs nearly a million pounds. All of that weight rests on the tires. Automobile tires are pressurized between 30 and 40 psi. In order to support the weight of a large airliner, aircraft tires are inflated to more than 200 psi. Aircraft are equipped with a number of tire safety devices to protect the wheels and tires. Among these are fusible plugs, over pressure relief valve, tire pressure monitoring system (TPMS), and the brake temperature monitoring system.
Fusible plugs, also known as thermal plugs, are used to protect tires and wheels from exploding if the brakes get too hot. A fusible plug is a small hollow bolt filled with a low melting-point metal (like solder). If the wheel becomes too hot, the soft metal within the plug melts at a controlled temperature, allowing the tires to safely and slowly deflate. Fusible plugs are often called upon after heavy braking, such as during a rejected high-speed take off. After the aircraft stops, the sweltering brake assemblies continue to heat the wheels until the fuse cores reach their melting temperature and deflate the tires. Fusible plugs are mounted inside the wheel hub. As the plugs begin to deflate the tires, the nitrogen is directed over the brakes to aid in cooling.
The over pressure relief valve (OPRV) is a hollow bolt with a rupture disk inside. The disk ruptures once the nitrogen pressure exceeds the design limit. OPRVs are installed on most wheel rims to protect the tires from high pressures or explosions that can occur during nitrogen servicing. On a Boeing 767, the OPRVs release pressure at 375-450 psi. OPRVs are a very important safety tool. Over-pressurization accidents have caused serious injury and even resulted in fatalities. Aircraft tires are so strong that the wheel rim and bolts fail before the tire does, often launching shrapnel outward. OPRVs are there to reduce the risk of this happening. Additionally, maintenance technicians must receive special training before they can service aircraft wheels.
Many aircraft models feature tire pressure monitoring system (TPMS) sensors in their wheels. The TPMS in aircraft is very similar to that in automobiles. Cockpit displays show tire pressures for all tires equipped with the sensor. The TPMS triggers an alert in the cockpit if a tire has low pressure.
The final safety system aircraft wheels have is the brake temperature monitoring system. A common example of a brake temperature monitoring system is that of the Boeing 767-300F. In this configuration, a screen displays eight boxes (each representing a main gear wheel) with a number from 0-9 within the box. These numbers represent a temperature range. Temperatures 0-2 denote cool to warm temperatures. The normal temperature range is 3-4. After a normal landing, the system will typically display twos and threes, but a four is nothing to be concerned about. Temperatures from 5-9 are considered high. When brake temperatures reach this level, the brake temperature indicator light will illuminate in the cockpit.
At 5-6, (where brake temperatures exceed 700 degrees Fahrenheit), the wheel fuse plugs may begin to melt and deflate the tires. If the system reaches 7-9, the crew will stop the aircraft immediately and leave the runway. Airport firefighters will be called to monitor the landing gear in case of fire. Once brakes reach these temperatures, tire, wheel, or brake replacement may be required. Temperatures in this range are rare, and are generally only caused by emergency landings or rejected take offs.
Apart from pressure and temperature limits, aircraft wheels also have speed ratings. Most airliners have tires rated for speeds around 220-235 miles per hour, but this is much faster than an aircraft is typically moving on the runway. Take off landing speeds range between 140 and 200 miles per hour, so there is a significant margin of error in the event that an aircraft needs to land at high speeds. Aircraft tires are inspected by maintenance technicians after every landing. Tire life cycles vary based on factors such as runway conditions, weather, and aircraft operating weights, but a general estimate is about 100 take off/landing cycles for a main tire.
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