Preparing for Flight: Pushing Back an Airplane
Aircraft · 7 min read
While pushing back airplane sounds quite straightforward, there are a number of steps involved in the procedure.
Have you ever seen the main landing gear of an airplane? If you have, you obviously must have noticed the big number of wheels an airplane has, right? Well, it totally makes sense given the size and the weight of any aircraft.
The number of aircraft wheels is different from model to model, as it depends on factors such as aircraft weight and landing gear design among others.
For example, the Boeing 777 uses 14 tires, the Airbus A380 22, and the biggest aircraft in the world, the Antonov An-225, requires 32.
As it happens in other vehicles, the wheel of an airplane is composed of a rim and a tire. However, airplane tires are more rigid than those used in cars, so these tires are assembled using bolts and a two-part rim.
The two parts of the rim are called the inboard wheel rim and the outboard wheel rim. The names are given after the position the rims take in the wheel assembly while the tire stays between the sometimes called wheel halves.
While they are very similar to other vehicle tires, the tire assembled for an aircraft wheel is formed by a series of layers and different materials. According to Michelin Aircraft Tires, the components of plane tires are as follows:
Also, it is important to consider that aircraft tires are the ones supporting the entire vessel when moving at high speeds on the runway before take-off and the ones touching the ground at a considerable velocity during landing.
It has been measured that the temperature differential airplane tires experience changes from -40°C while in flight to over 200°C for a short period of time when touching the ground during landing. And they have to sustain that on a regular basis.
Moreover, airplane tires need to be prepared for other extreme situations such as strong stops in a rejected take-off, something that makes each wheel and tire experience high stress and high temperatures.
Obviously, these are extreme conditions for any tire, so it is clear that an aircraft tire cannot be the same as an automobile tire.
To be honest, aircraft tires and car tires are much more similar than you would think. However, aircraft tires are made in a way that makes them stronger and more resistant to temperature changes and stress. Also, the way the tire of an aircraft is mounted on the wheel and pumped is also different.
Let’s take a closer look to see the differences more clearly.
The lead test engineer at Michelin, Lee Bartholomew, mentions that tire pressure is the main difference between car and plane tires. Also, he states that “It’s really pressurized air that’s so strong”, making reference to how durable plane tires can be. In other words, apart from the composition, having the optimal tire pressure is of uttermost importance to guarantee high resistance and reduce tire wear. Having a tire over-inflated or with a pressure lower than ideal can result in take-off and landing failure.
While an airplane tire will wear and require re-treading, with the right pressure levels that is not necessary before the tire experiences landing conditions at least 500 times.
Of course, some of these facts may sound similar to what you hear when speaking about the tires in other vehicles, but there are some differences.
Firstly, instead of simple air, plane tires are typically inflated with inert gas, nitrogen being the most appropriate. The main reason is that using dry nitrogen reduces the possibility of fire or explosion due to the high temperatures reaching high speeds, heavy braking, or when touching down on the ground.
Then, there is the level of pressure. The rated pressure for automobile tires is between 30 to 40 psi. But this does not work for the huge weight of a plane. Rubber that goes up in the air usually reaches 200 psi, with examples like the famous US presidential airplane, the Air Force One, which is a custom Boeing 747-200B that has its main tires inflated at 205 psi.
And speaking of the air force, the F-16, a classic US fighter, requires the pressure of its tires to reach 320 psi, which is definitely very high, to withstand the high speeds during take-off and landing either on a runway or a carrier.
Most aircraft wheels are equipped with an OPRV, which stands for Over Pressure Relief Valve. It is a brilliant protective device to prevent overloading the tire while filling it with nitrogen, thus helping extend the life of the tire.
At this point, it is important to highlight that tire pressures are regulated for safety. Normally, a tire must withstand four times its rated pressure for at least three seconds, which means they are really difficult to break by over-inflating them.
Brakes can become very hot and affect both the wheels and the tires, especially in case of rejected take-offs or emergency landings.
Fortunately, as well as with many other situations in the aviation industry, designers have come up with a clever solution, which is called a thermal or fusible plug.
A thermal plug is a small hollow bolt filled with a metal that has a known low melting point. This melting point is a temperature that is still safe for the wheel and the tire. But, if that temperature is reached, the plug melts and lets the tire deflate safely before the brakes reach an even higher temperature that could pose the risk of a tire explosion.
Cars require tires with tread patterns that make it possible to provide both traction and the possibility to channel water away to avoid the loss of that traction.
However, because the tires of an aircraft rotate freely, the complex pattern for traction is not really necessary. In fact, it would be a problem during landing since the block-like pattern would break when the plane touches the tarmac, due to the fact that tires are dragged until their rotational speed matches the speed of the aircraft.
Any modern runway will have a design prepared to drain water and provide traction, so the tire’s pattern will not need to drain too much water during take-off or landing. However, there are some runways that may still fail to drain enough water for a plain tire to work.
Therefore, there are some features added in case of extreme rain. These include a tread pattern and sometimes a type of deflector often called chines.
The tread pattern you will see on the tires of a plane are simple grooves made in the rubber along the diameter of the tires which help channel some water away.
Chines are a type of water deflectors added to the tires, especially those located at the head of the landing gear near the nose of the planes, with engines mounted on the fuselage. In essence, chines are curved protrusions on the sides of the tires, hence the name and they are used to deflect standing water outward to prevent water from coming into the engines.
While it may sound amazing, and you should not think about it while flying, it only takes 45 inches of rubber between your plane and the tarmac for your flight to land safely.
Given the characteristics described above, airplanes can land over and over again with the same tires without having any issues. It is more common to see shredded rubber or an exploded tire on the highway than have the same happen on the runway.
The different reinforcements added to the rubber of the aircraft tires by manufacturers like Michelin and Goodyear make them extremely resistant to big weights and high speeds. A tire like this is usually filled with steel and aluminum elements before it leaves the factory, ready to be used in many landings. Add the nitrogen pressures we talked about above and you will have a sturdy tire that can basically never explode.
Also, consider that Michelin and Goodyear as well as other manufacturers need to comply with strict regulations and they have a reputation to uphold, so they make sure each tire is fit for work by performing extensive testing.
500 touches on the runway before re-threading, which is usually done up to 7 times, do not end the life of a tire. The good news is that the rubber is recycled and used to make many things including playground mulch and new tires for farming vehicles. In simple words, you can trust each tire will live a long life.