The brakes of a large aircraft are required to absorb a huge amount of energy during landing and in the event of a rejected takeoff. Due to the amount of energy involved, aircraft brakes need to be strong and reliable.
How does the braking system of an aircraft work
In terms of functionality, the brakes of a heavy aircraft are not that different from the brakes of a car. In the braking system of a typical car, the brake pads or friction pads are pressed against a rotor which rotates at the speed of the wheels. When the driver hits the brakes, brake fluid which is under pressure is sent to a piston, which moves the pads against the rotor and stops the wheel from spinning. Because of its simplicity, light aircraft also use a similar system. This type of brake is most known as single-plate disc brakes.
A single disc brake system. Picture: Oxford ATPL
There are a few reasons why such a braking system might not be the best idea for a larger aircraft. One is that such a system cannot simply dissipate the required energy. We can do a simple calculation for a Cessna 172 and an Airbus A320 for comparison. As brakes convert kinetic energy into heat, we could calculate the energies involved when both aircraft come into land. We would assume that the Cessna 172 has a mass of 700 kg and a landing speed of 32 m/s. And for the A320, a mass of 64,000 kg and a speed of 70 m/s.
KE = ½ x m x v^2
= ½ x 700 x 30^2
= 315,000 J
KE = ½ x m x v^2
= ½ x 64000 x 70^2
= 156,800,000 J
According to the calculations, the brakes of an Airbus A320 must absorb almost 500 times more energy than a Cessna 172 on landing. So, it becomes quite clear that the former requires a better braking system.
To improve the braking capability and efficiency in most large aircraft, a braking system called a multi-disc brake setup is used. In this system, there are stacks of rotors with something called stators sandwiched in between them. The stators are sort of like friction pads. The rotors move with the wheels, while the stators remain static.
At the start point of the stators, there is a pressure plate that controls the movement of the rotors and stators. Unlike the single piston found in an average car, in this type of braking system, multiple pistons are used. Because these brakes tend to be heavy, hydraulic pressure from the main hydraulic system is used to actuate the pistons.
A detailed diagram of a multi disc braking system. Picture: Oxford ATPL.
When the brakes are released, the pressure plate is moved away by a brake adjustor assembly. This is a spring-loaded device that is fixed to the plate. When the brakes are pushed, the spring is compressed, and when released, the spring extends, moving the pressure plate. This releases the stators from the rotors, and the braking no longer exists.
In many braking systems, the adjustor has a pin attached to it which can be used to gauge brake wear. As the brakes wear, the pin, which is normally seen from outside observation, keeps going in. Once the pin cannot be seen to the naked eye, and if it cannot be felt when touched, the brakes need to be replaced.
The adjustor assembley helps to release the brake rotors and stators. Picture: Oxford ATPL.
For redundancy purposes, airplanes use two or more hydraulic systems to power the brakes. If one system fails, the other can take over. There are also emergency braking systems which will be looked at later.
In the olden days, steel was used to make the brakes. However, these days, carbon brakes are used. Carbon brakes are lighter, they are able to absorb more energy, and are more durable. Due to these reasons, carbon brakes are preferred by aircraft manufacturers.
Brake pistons and wear indicator pin. Picture: Anas Maaz.
How do pilots operate the brakes?
The brakes are normally operated by feet by pressing on the top of the rudder pedals. The brakes can also be used differentially and can be used to make a sharp turn on the ground. For example, if the pilot wants to go to the right while the aircraft is on the move, he/she could press on the right brakes. This will lock up the right wheel while the left wheel keeps moving, making the aircraft turn.
Pilots use their feet to control the brakes. Picture: Airbus.
Anti-skid and Automatic braking system
On a short, wet, or contaminated runway, a brake lock-up can be quite dangerous. The locking up of brakes can cause the aircraft to skid off the runway or cause excessive heat generation, which could lead to a fire. Due to these reasons, a system is required to prevent the wheels from locking up and skidding.
The anti-skid system prevents skidding by releasing the brakes when it senses a potential wheel lock-up. It works by comparing the speed of the aircraft, called the reference speed, to that of the speed of each wheel. The wheel speed is measured by a tachometer, and the aircraft speed is determined by calculating the acceleration of the aircraft using inertial data. When the speed of a wheel drops by a threshold, the brake release command is given by the anti-skid system.
When landing, the anti-skid helps to prevent a wheel lock up. Picture: Tomás Del Coro via Flickr
The automatic braking or the autobrakes automatically applies the brakes. It is armed by the pilot when required. When armed, certain conditions are required for it to come on, and this highly depends on the aircraft. For instance, in Airbus aircraft autobrakes are only actuated with the extension of ground spoilers. They can be deactivated by the pilot when manual braking is applied.
There are many levels of automatic braking. On takeoff, the autobrakes are usually set to maximum. This will ensure maximum braking is applied if the pilot were to reject the takeoff. On landing, the autobrakes can be set to a pilot’s preferred level. On a wet runway, pilots usually go for a higher level of braking to stop the aircraft as quickly as possible.
The parking brake and the Emergency braking system
Airplanes also have a parking brake, which is used by pilots to keep the aircraft from moving when parked for long periods of time. For example, when holding at some place at the airport with the engines running.
When parked at the gate, the parking brake is usually set to off. This is because the wheels have chocks placed on them. With chocks properly in place, the brakes are no longer required because the chocks prevent the wheels from inadvertent rolling. This is quite beneficial in short turnarounds, where high brake temperatures become a problem. By keeping the parking brakes off, the rotors and stators are separated from each other, allowing proper airflow. This keeps the brake cool.
Airplanes are usually parked with chocks on. Picture: American Airlines
The emergency braking system is available to the pilot in the event of a dual hydraulic failure which renders the main wheel braking system useless. In most cases, a brake accumulator which is pre-charged before the flight is used for this purpose. The pilots can use this accumulator pressure to actuate the brakes if the main braking system fails. As the accumulator can only contain a limited amount of pressure, the number of brake applications is limited. Most of the time, about seven brake applications are available to the pilot.
In some aircraft, like in the Dash 8, the parking brake also acts as the emergency braking system. Picture: ATSB report.
Controlling brake temperature and brake wear
As highlighted in the previous paragraph, keeping the brakes cool is a priority for pilots. Cooler brakes perform better than hot brakes especially when they are required for a rejected takeoff. There is a temperature beyond which the brakes can no longer absorb the required energy. Hot brakes are also a fire hazard.
As the landing gear retracts into the wheel well of the aircraft where hydraulic lines are present, if a leak occurs and if it were to fall onto hot brakes, a wheel well fire is a highly likely scenario. Due to this reason, there is a maximum brake temperature that must never be exceeded before commencing the takeoff.
In almost all airliners, pilots are shown brake temperatures in the cockpit displays.
There are many ways by which the brakes can be cooled if they get too hot. Some airplanes are equipped with brake fans which can be switched on by the pilots. This system blows air over hot brakes, cooling them in the process. There are also portable cooling fans that can be hooked up to the wheels to cool the brakes.
An internal brake cooling fan of an Airbus aircraft. Picture: Safran .
The pilot flying technique also plays a major role. Too many brake applications during taxi could lead to overheated brakes. Consequently, pilots should try to minimize brake usage as much as possible. Also, the use of reverse thrust can help to keep the brakes cool as it minimizes the requirement for heavy wheel braking. It is also recommended to go for a longer rollout when landing on a long runway. This will cause the aircraft to use more runway but in effect reduce the brake usage.
When it comes to carbon brake wear, the science is a little complex. The carbon brakes tend to wear less when the temperatures are significantly lower or when the temperatures are significantly higher. They wear more in mid-temperatures. Below is a graph produced by Airbus, which is plotted with brake wear against the temperature. It shows the brake wear trend of three brake manufacturers. Messier-Bugatti, Honeywell-ALS, and BF Goodrich. As can be seen in the graph, all brakes have a peak wear rate at middle temperatures.
However, it is not recommended to operate brakes at high temperatures to reduce wear as it reduces the efficiency of the brakes. When it is possible, particularly in long turnarounds, the pilots should try their best to lower the brake temperatures as operating them in very low temperatures helps to both save wear and increase their efficiency. Moreover, high temperatures lead to the oxidation of carbon. Carbon naturally combines with oxygen in the air to form carbon dioxide. Heat accentuates this process, and this leads to loss of carbon mass from the brakes, increasing brake wear.
Carbon oxidation can lead to brake wear. Picture: Boeing.
As like before, proper piloting techniques can be used to reduce brake wear as well. The carbon brakes are very sensitive to the number of brake applications. So, during taxi, pilots should try their best to minimize brake applications. This can be done by applying brakes one single time until the speed is reduced to a lower level. Usually, airplanes take time to accelerate back to a higher speed with engines at idle. So, pilots can use this as an advantage if they want to slow down the aircraft during taxiing.
Further, when conditions are right, a single-engine taxi can be performed. This reduces the amount of thrust generated by the aircraft, which in turn reduces the need for intermittent braking. One other way is to use autobrakes for landing. Autobrakes tend to use one brake application when they are active, and it simply modulates the brake pressure with that single application. As this reduces the number of brake applications, it reduces brake wear.
For both break wear and brake cooling, nothing beats proper piloting techniques. Picture: Bryan Shirota