It’s amazing enough that airplanes can fly. But beating gravity and getting planes to glide through the skies is not enough. How do we get these huge bulks of metal moving at such high velocity to return to a state of rest? This was the exact question early designers of the airplane were faced with. Though most of the earliest aircraft didn’t have braking systems, the approach to accomplishing this task has changed significantly over the years.
When compared with the brakes on your car, airplane brakes are more complex and come in a much heavier form. Today, airplanes are fitted with different braking systems including disc brakes, air brakes, thrust reversers, and other types of brake system enhancements. However, in general, these brakes can be classified into two groups: air brakes and landing brakes.
In essence, air brakes are used to reduce the speed of the aircraft mid-air. This is why they are called speed brakes as well. Air brakes decelerate the plane by opening up in a manner that increases drag without significantly affecting lift. They can also be sometimes be used to increase the plane’s angle of approach in the process of landing.
On the contrary, landing brakes include all the different brakes involved in reducing the speed of the plane when it’s approaching the ground and on the ground. Because landing brakes need to bring the aircraft to rest, they have to both reduce lift and increase drag. Disc brakes are the major components of the landing brake system. They will, therefore, be the main focus of many sections of this article.
Table of Contents
History of Airplane Brakes
The first airplanes that were designed did not include any brake systems. The evident question then pops up. If these aircraft didn’t have brakes, how did they land?
Interestingly, pilots then had to rely on some other factors. In the place of brakes, they depended on the low speed of the aircraft, soft airfield surfaces, and friction. But these could no longer suffice as airplane weight and size increased and aviation technologies improved over the years.
After World War I, the first brake systems were designed for usage in aircraft. The first type of brakes used in an aircraft is the drogue parachute. A drogue parachute is a parachute attached to the rear of a plane that is deployed right before landing to slow the aircraft. The drogue parachute was invented in 1912 by a Russian Gleb Kotelnikov. But they didn’t come into full use in aviation until 1937.
Another type of aircraft brakes that received early admission into common planes are air brake systems. However, at the time, they were mostly in the form of simple flaps manually controlled by a lever in the cockpit. Following closely were the disc brakes.
Disc brakes were first developed in England in the 1890s but were earlier used in automobiles and railroad passenger trains only. It wasn’t until just before the Second World War that disc brakes were first used in aviation. Since then, aircraft brake systems have developed from the multi-disk steel brakes to more advanced electric braking systems.
Types of Aircraft Brakes
In aviation today, most aircraft primarily use disc brakes. Normally, in a disc brake system, a disc rotates together with the turning wheel assembly. When the brakes are applied, a stationary caliper resists the rotational movement of this disc by causing friction against the disc. A disc brake system’s complexity and design often depends on the weight, size, and landing speed of the aircraft. The most common types of disc brakes used in aircraft are single, dual, and multiple disc brakes.
Single Disc Brakes
A single disc is typically sufficient to brake a small, light aircraft effectively. This disc is keyed or bolted to each of the aircraft’s wheels. To brake the plane, friction is applied to both sides of the disc using a non-rotating caliper attached to the landing gear flange. The movement of the caliper is initiated by pistons inside it. These pistons, under hydraulic pressure, force the brake pads or linings against the rotating disc when the brake is applied.
Single disc brakes can either be floating disc brakes or fixed disc brakes. The major difference between floating and fixed disc brakes is that when a brake pad is pushed in a floating disc brake, the caliper moves in such a way that the opposite pad touches the disk. However, in fixed disc brakes, the pistons in both sides of the disc move simultaneously to push the pads against the disc.
Dual Disc Brakes
In bigger aircraft, single disc brakes cannot produce a sufficient amount of braking friction that is needed to stop or slow down the plane. Dual disc brakes are often used in such aircraft. In dual disc brakes, two discs are keyed to the wheel instead of one. There’s a center carrier with linings on each side located between the two discs. Each time the brakes are applied, these linings contact each of the discs.
Multiple Disc Brakes
Multiple Disc Brake
The biggest and heaviest aircraft require the use of multiple disc brakes. These types of brakes are built for heavy-duty purposes. They are used with machine parts like power brake control valves or power boost master cylinders.
Multiple-disc brakes make use of an extended bearing carrier that resembles a torque tube-type unit. This carrier is bolted to the axle flange and provides support for the different brake parts. These parts include the annular cylinder and piston, an alternation of steel and copper or bronze-plated discs, a backplate, and a blackplate retainer.
The bearing carrier connects to the stators, which are made of steel while the rotating wheel has either copper or bronze-plated discs that are keyed to it. The whole assemblage of stators and rotors are compressed when hydraulic pressure is applied to the piston. The end result is the production of a large amount of heat and friction which in turn reduces the wheel’s speed of rotation.
Air Brakes and Thrust Reversers
Apart from disc brakes, other common types of aircraft brakes include air brakes and thrust reversers. As earlier mentioned, air brakes are used to increase drag acting on a plane mid-air. By increasing drag, air brakes are used to reduce the plane’s airspeed. The most common types of air brakes are lift dumpers and flaps.
Lift Dumpers
Thrust reversers decelerate the plane by temporarily diverting the thrust generated by the aircraft’s engine so that it opposes the aircraft’s forward travel. Thrust reversers are often used when the plane is already on the ground. They help to reduce the wear on the brakes and make the landing distance shorter.
Thrust Reverser
How Do Aircraft Brakes Work?
In this section, we’ll be focusing mainly on the working principles of disc brakes—the most common type of brakes in modern aircraft. As we now know, disc brakes depend on the friction between rotating and stationary discs inside the brakes to function. Disk brake systems are initiated through an auto brake system or by the pilot depressing a foot pedal.
Once the brake receives the initiation signal, actuators within the brake move a piston which squeezes the disc together. A frictional force is hereby generated in the process which in turn reduces the speed of the wheel’s rotation. During this process, the friction between the discs converts the aircraft’s kinetic energy to heat energy.
Aircraft brakes absorb a huge amount of heat that can often exceed 1800°C. Every time the brakes are applied, the disc material experiences a lot of wear and tear due to the excess frictional forces involved. After several applications (hundreds typically), the discs begin to become thinner. This is why they often require replacement after periodic maintenance intervals.
What Materials Are Airplane Brakes Made of?
For a very long while, the majority of aircraft brakes were made of steel. It wasn’t until 1963 that beryllium was introduced as an aircraft brake material. The use of beryllium, however, came at its own cost. While beryllium provided highly improved thermal properties—which is an important consideration in aircraft brake design—there were also difficulties handling the material due to the toxic nature of beryllium oxide.
Today, modern commercial airplanes make use of carbon brakes. Carbon brakes became widely accepted in the 1980s. And they generally perform well by many indexes. For example, carbon brakes made from carbon fibers in a graphite matrix are lighter, more stable thermally, cool faster and can absorb energy better.
Thanks to carbon’s higher specific heat, carbon brakes always weigh less than steel brakes. Carbon also has a lower thermal expansion, higher thermal shock resistance and a higher temperature limit that steel. Unlike both steel and beryllium, carbon has more constant specific strength over a wide range of temperatures. Steel and beryllium also typically exhibit a steep decline in specific strength at high temperatures exceeding 650°C.
Recently, Safran Landing Systems boasted that their Boeing 787s’ Sepcarb III oxidation-resistant carbon brakes are 4 times lighter than steel brakes. They also claimed the brakes have 3 times more endurance and 2 to 3 times higher absorption capacity. Other manufacturers also employ other materials when constructing brakes. For example, Honeywell’s Cerametalix is a sintered combination of powdered metals and ceramics.
Factors Considered When Constructing Brakes
Put simply, the major factor that decides the type of braking system that is employed in an aircraft is the size of the aircraft. This factor then defines certain parameters that must be taken into account when designing the brakes. These primary design parameters include the number of discs, the diameter of the discs and the material of the discs.
Another important concept that pops up in aircraft brake design is a worst-case scenario called rejected takeoff (RTO). RTO occurs at a maximum rolling speed commonly called the decision speed. At speeds beyond this decision speed, a takeoff cannot be safely aborted without putting the aircraft at significant risk of being unable to stop before the end of the runway. Aircraft brakes are designed to absorb more energy in such circumstances.
Typically, before designing a plane’s brake system, its kinetic energy during RTO is calculated. The amount of frictional force required to conquer this energy is then determined too. To generate the necessary frictional forces, large commercial transport aircraft typically require multiple discs per brake assembly and brakes on most, if not all, of their wheels.
A380
For example, an A380 has 22 wheels distributed on five landing gear legs to support its massive weight. These wheels are distributed in this manner:
- 2 nose wheels on a leg beneath the nose of the aircraft;
- 8 wing wheels split between two legs that fold out from under the fuselage to support the left and right wings and;
- 12 body wheels split between two inboard landing gear legs under the fuselage.
Sixteen of these wheels have brakes (four of them are body wheels and the nose wheels are not braked).
Where Are Brakes Located in a Plane?
Different types of airplane brake systems are placed in different parts of the plane. Today, aircraft disc brakes can always be found in the landing gear, air brakes—on the wings and thrust reversers—on the engine. But these are mechanical parts that are not seen or controlled by the pilot during flight.
Most modern aircraft brakes are activated from the top section of the rudder pedals. This type of brakes is called toe brakes. In toe brakes, the top of the rudder pedals is connected directly to the brake system. However, it is highly necessary to apply toe brakes at the right time. If they are applied when the plane is moving at a high speed on the runway, this can result in a violent change of direction.
But not all planes have toe brakes. Some older aircraft are fitted with heel brakes. Pilots find it more difficult to apply this type of brakes. An even rarer type of aircraft brakes is the handbrake. In some other aircraft like Cessna and Mooney, the pilot is required to first apply the toe brakes and then pull out a knob to lock the brakes.
How Do Pilots Control Airplane Brakes?
Aircraft brake systems are mere mechanical parts, in some cases a combination of mechanical and electronic parts. These parts must be deployed and controlled by the pilot. Brakes can be activated either manually by the pilot or by using auto brakes. Auto brakes, just as the name implies, are electronic systems that are automatically activated as the plane approaches the ground just before touchdown.
Most of the wheels of any modern airplane are equipped with a brake unit. The nose and the tail wheel, however, do not have brakes. In any typical airplane, pilots can control the brakes using the mechanical or hydraulic linkages to the rudder pedal.
The brake on the right main wheel(s) is activated when the pilot depresses the top of the right pedal. In the same manner, when the pilot pushes the top of the left rudder pedal, it activates the brake on the left main wheel/wheels.
However, some new aircraft do away with the use of the hydraulic system and employ electricity instead to power the brakes. One prime example of this approach is the 787 Dreamliner. Going with an electric brake system allows the designers to cut down on the weight of the airplane considerably.
In this system, when the pilots press on the brake pedals, an electrical signal is sent to the brake unit on the wheel. The electrically powered actuators are then used to press the carbon brake disc against the wheel. This consequently slows the aircraft down.
How Often Are Airplane Brakes Replaced?
Due to the high levels of temperature changes aircraft brakes undergo, they must be replaced frequently. In general, after approximately 1000 to 2000 landings, aircraft brakes are taken for a maintenance check. Every braking system has a pin located inside the brake. This pin is essentially an indicator that helps to detect the level of wear the brake has experienced.
The frequency of brake replacement in aircraft also depends largely on the type of brake material. On average, steel brakes have a lifespan of 1,100 cycles between repairs and replacement. One can, however, expect between 1,500 and 2,000 landing cycles from carbon brakes for the same reasons discussed earlier.
During repairs, the common parts of the braking system that are replaced are the linings and the discs. Maintenance engineers can often consult the manufacturer’s manual for proper break-in procedures when working on new brakes.
Cost of Replacing and Repairing Aircraft Brakes
Purchasing, replacing and repairing aircraft brakes can be an enervating process. Asides the monetary cost, finding the right parts to make a great buy can be time-consuming too. The cost of a brand new unit of aircraft brake can vary over a wide range of figures. One good example, however, is the Boeing 777. A complete 12-piece brake set of a Boeing 777 costs approximately $100,000. On the other hand, brake sets of smaller aircraft cost significantly less.
In 2019, it was estimated that the total MRO (Maintenance, Repair, and Operations) demand for aircraft wheels and brakes was $2.5 billion. It goes to show that this is a market in high demand. The cost of repairing your aircraft brake can be very unpredictable. It depends mainly on the component of the brake system that needs to be replaced.
The price of a typical Cleveland standard organic or metallic brake lining can easily range from $12.25 to $469. Brake discs from the same manufacturer will set you back as much as $149.75 to $1769. Some other components like rivets, valves, and reline kits may also be needing replacements. So, it’s hard to tell what one should expect beforehand.
How Do Water and Ice Impact Braking Performance?
When a plane lands on a wet or icy runway, it is constantly squeezing the water from the tread. This squeezing action generates water pressures which can not only lift portions of the tire off the runway but also reduce the amount of friction the tire can develop. This action is called hydroplaning.
Hydroplaning causes tire-to-ground friction which can be low at high speeds and improve as speed reduces. There are three types of hydroplaning namely viscous, dynamic and reverted rubber hydroplaning.
Viscous hydroplaning is the most common effect wet runways have on aircraft braking performance. It occurs on all wet runways and is a technical term used to describe the usual slipperiness or lubricating action of the water. While viscous hydroplaning does reduce the friction, it is not to such a low level that the wheel cannot be spun up shortly after touchdown to initiate the anti-skid system.
In the event of a highly rare dynamic hydroplaning, the tire lifts off the runway completely causing a very substantial loss of tire friction that may prevent a wheel spin-up. Reverted rubber hydroplaning, on the other hand, can occur whenever a locked tire is skidded along a very wet or icy runway for a time long enough to generate frictional heat in the footprint area.
Brake System Enhancements
Aircraft brakes are no longer as simple as they used to be. Apart from the basic types discussed earlier, airplanes also pack some enhancements that help to improve the performance of aircraft brakes. The most common ones available include anti-skid protection, auto brake, and brake temperature indicators.
Anti-skid Protection
When aircraft brakes are applied, there’s a high probability that the wheels of the planes may begin to skid. To stop this from happening and to maintain maximum effective braking, each wheel is equipped with anti-skid protection.
An anti-skid protection system uses various mechanisms to compare the speed of the aircraft with the rotational speed of each main wheel. In a case whereby a wheel’s speed is too slow in comparison with the speed of the aircraft, the brake on that wheel is released for a while in order to prevent skidding.
Anti-skid systems are designed to minimize hydroplaning and the potential tire damage which can occur when a wheel is locked or rotating at a speed that does not correspond to the speed of the aircraft. Anti-skid also removes the possibility of reverted rubber skids caused by locked wheels.
Auto Brake
Auto-brake systems can be used on takeoff where they provide maximum braking in the event of a rejected takeoff. They can also be used during landing where they provide a scheduled rate of deceleration depending on the auto-brake level selected in a single brake application. These features combine to optimize the brake usage with respect to the requirement and also to minimize brake wear.
Brake Temperature Indicators
It is very important to monitor the high heat levels that are generated as a result of the friction in the brake system. Therefore, the flight deck has a wheel synoptic page where the temperature of each brake unit is displayed. On this synoptic page, numerical values of the temperature of the brake are shown next to each wheel. A value of 0 – 4.9 is in the normal range. When a temperature reading exceeds 5.0, a cautionary message is sent to the pilots.
In the event of the brakes becoming too hot, there’s a chance that the heat transferred to the wheels could cause the tires to explode. To stop this from happening, when a certain temperature is reached, fuse plugs in the tires melt. This allows the air to be released safely and slowly deflates the tires.
Aircraft Brakes Certification Requirements
Many certification requirements govern the approval, replacement, and modification of aircraft brakes. In general, it is required that the braking system of an aircraft must have the ability to stop the aircraft at maximum certified takeoff weight with the rejected takeoff initiated at decision speed.
The certification process must be done with all brakes worn to near their service limit (nominally 10% left on the lifespan). Also, the brake and wheel heat sink must be robust enough that no intervention in terms of fire fighting or artificial cooling is required for 5 minutes after the aircraft has been stopped.
Other certification requirements demand that the components of the wheels, brakes, and braking systems should be designed to:
- Withstand all pressures and loads, applied separately and in conjunction, to which they may be subjected in all operating conditions for which the airplane is certificated.
- Accommodate simultaneous applications of normal and emergency braking functions, except other appropriate design measures have been taken to prevent such a contingency.
- Satisfy all the requirements pertaining to energy absorption requirements without making use of secondary cooling devices (e.g. cooling fans, etc).
Braking-Related Accidents
The two major factors associated with braking that can cause aircraft accidents or crashes are overheated brakes and brake failure. Overheated brakes can, in turn, cause a loss of braking performance, fire, and tire deflation.
One brake-related accident was the crash of a 19-seat turboprop Swearingen Aircraft airliner in 1998. There was a fire in the wheel well caused by overheating of the brakes. The overheating continued until the left wing of the plane failed rendering the aircraft uncontrollable.
Nowadays, temperature indicators of brakes are checked frequently to ensure that there’s no overheating. In the case of overheating, the pilot sometimes leaves the gear down for an extended period provided this will not have an impact on the climb performance.
Summary
Brake systems are a very vital part of an aircraft. From the days of drogue parachutes, brakes have now evolved into more complex multi-disc and electronically controlled systems. And thanks to material innovation, they are now more durable and reliable than ever before.
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