One of the most obvious sights an onlooker notices when a plane lands on the runway is the amount of smoke that is generated as the plane hits the ground. This rather interesting sight leads most people to contemplate and ask a couple of questions such as: “Why don’t airplane tires explode when they land?”, “How are those relatively small cylinders able to support the weight of the plane”, etc. Another commonly asked question is, “Why don’t airplane wheels spin before landing?”
While this question has been raised quite frequently in aviation, there are a couple of important reasons pre-spun wheels are not common practice yet. Some of them include development and certification costs, the additional weight of the batteries powering the wheel’s motors, and safety and handling problems.
Each of these problems bears significant importance in deciding how safe and economical flying is.
The major argument for spinning an airplane’s wheels before landing is that this will help to minimize the amount of wear the tires undergo during landing. Tires rotating at the same speed as the aircraft will definitely experience less friction than non-rotating ones. A reduction in wear will also result in less frequent retreading and tire replacement in general. In other words, airlines will be spending much less on tires.
Another advantage of having independently rotating wheels in aircraft is that this can be an efficient way of moving the aircraft on the runway. On average, a typical plane like the Airbus A320 taxies for about 3.5 hours every day. And the majority of this motion is powered by combustion engines which expend about 600 liters of fuel in a day — merely on taxiing. A motor-powered rotation, on the other hand, can be significantly more cost-efficient.
However, in spite of these two apparent advantages of using pre-spun wheels in aircraft, the concept is still yet to be accepted. This is mainly because on a closer look, the cons may actually outweigh the pros.
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How Much of Tire Wear Occurs During Landing?
Immediately when an aircraft touches down, the tires are actually skidding, not rotating. In the seconds that follow, the velocity of the plane is transmitted gradually to its tires until the tire’s rotational speed matches the plane’s velocity.
In the process of skidding, a lot of heat is generated by the frictional force between the surface of the tire and the runway surface. This heat melts a significant portion of the rubber on the tire. The melted rubber is then eroded by the same frictional force acting between the two surfaces. The visible landing smoke is a result of the erosion of this heated aircraft tire rubber.
Aircraft tire wear is pretty consequential as a typical Boeing 747 tire loses about 1kg of rubber each time the plane lands.
This has a big impact on both aircraft running costs and the effect of air travel on the environment. Rubber lost in this process is the reason why tires are changed and retreaded quite frequently. It is, therefore, logical to try to limit or even eliminate tire wear during landing if possible.
The majority of the rubber loss during landing occurs at the particular moment of impact with the ground. Consequently, one of the apparent ways of reducing tire wear is to use a wheel that is pre-rotated before impact in order to reduce the friction experienced due to skidding.
However, in the 1960s and 1970s, the US Air Force conducted tests investigating how much difference pre-spun wheels can make as regards aircraft tire wear. By observing the test models, it was discovered that only about 10% of initial tire wear can be conserved by making use of pre-rotating wheels.
This may seem like a huge contrast from what we see when planes land. But it is necessary to note that the excess rubber on the ends of runways is always from a thin layer on the surface of the tire.
Reasons Why Airplane Wheels Are Not Pre-Rotated
Compared to the 10% rubber conservation that airlines can achieve by using wheels that rotate before landing, the practice actually raises a lot more concerns and problems. As earlier mentioned, some of the most significant problems include development and running costs, certification costs, handling and safety problems, and weight-related problems among a host of others.
Development and Running Costs
Developing new and improved parts of aircraft to the point of adoption in new plane models is a very expensive process. Aside from the cost of the human labor expended on developing new technology — which isn’t cheap in the case of aircraft design — testing and simulating new models can easily ramp up costs. The major development concern here is how to create suitable motors powerful enough to rotate all the wheels of the aircraft. They can be as many as 22 in some aircraft.
The motors must also be efficient enough to drive the wheels to the required speed — preferably equivalent to the landing speed of the aircraft. This is often about 300km/hr (at least 3 times faster than your average car speed on a highway).
Battery problems
If the motors are powered electrically, then they must be connected to batteries which will likewise be fitted into the aircraft. Batteries connected to the landing gear can also cause a design problem as placing them too close to fuel is far from ideal.
A very good example of how battery problems can affect aircraft design is the Boeing 787 Dreamliner. Merely in its first year of service, the aircraft suffered a series of notorious electrical systems issues that were tied to its lithium-ion batteries. In January 2013, a battery in one of the aircraft units owned by Japan Airlines overheated, and a fire broke out as a result.
Other similar incidents were recorded in aircraft units operated by other airlines. This, therefore, led the manufacturer to carry out other expensive troubleshooting and modification processes. As is obvious from this example, adding extra batteries to an aircraft isn’t as simple as it seems.
Certification Costs
Development costs are not the only cost-related issues associated with the inclusion of pre-spun wheels in aircraft. After successfully developing this type of wheels, getting them certified by the relevant authorities is another cost-consuming process.
Getting new aircraft designs certified by bodies like the American Federal Aviation Authority (FAA) and the European Union Aviation Safety Agency (EASA) cost aircraft manufacturers a lot of money and time. In general, the costs pre-spun wheels conserve do not justify these development and certification costs.
Handling and Safety Problems
Using pre-pun wheels on an aircraft will demand that the wheels rotate at a speed equal to that of the aircraft. And this is where a couple of technical problems arise.
Landing conditions can often vary at various times and in various locations. So, the effect of different environmental conditions such as windy conditions can actually result in different unprecedented situations.
In these types of conditions, the landing speed of the aircraft will change very drastically. And the wheel’s motors must be built to accommodate these possible changes. Otherwise, a difference in tire speeds or a tire speed that doesn’t correspond with the aircraft speed will result in maneuverability and steering issues.
Another possibility is a situation whereby one of the motors fails. In other words, one tire is spinning at 300km/hr and the other is completely static. In such a case, the aircraft will definitely experience serious difficulty balancing itself. And considering the narrow nature of runways, this is most certainly undesirable.
Weight-Related Problems
It is a common fact that weight is one of the most influential factors in aircraft design. For every kilogram added to an airplane’s mass, a corresponding amount of thrust needs to be generated to keep it in the air. While most engines are powerful enough to carry the additional weight of wheel motors and their batteries, they will need to consume more fuel to do so.
Considering all the costs involved in successfully executing this wheel design, it becomes evident that pre-spun wheels actually cause more problems than they solve.
How Much Does It Cost to Replace Airplane Tires?
On average, it can cost between $15 and $5500 to procure a new aircraft tire. This wide range is largely due to the fact that the cost of a tire depends largely on the size of the aircraft it is made for.
As expected, most large commercial aircraft tires are more costly than those on their private counterparts. For example, a single tire unit on the Airbus A380 costs approximately $5500.
How Frequently Are Aircraft Tires Replaced?
Due to the wear aircraft tires experience during landing, they have to be replaced pretty often. A typical commercial airliner’s tires can withstand about 1500 landings — approximately 3-6 months of usage. They are also retreaded about 6 times within that time frame.
Nevertheless, these figures are merely estimates based on average usage data. Some aircraft tires can be replaced even after 1 landing due to various factors. Aircraft tire maintenance practices also vary for different aircraft categories. Military aircraft tires often experience the most wear — requiring replacement after as few as 10 landings.
Summary
Aircraft wheels that spin before landing are definitely a great idea that has been under debate for several years now. However, as is evident from the content of this article, there are many problems associated with their development and implementation. Hopefully, we can see future technologies that will provide real solutions to these problems.
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