20/08/2011
It might seem obvious, but when you're designing an aircraft capable of sustained flight at high speeds, you'd logically build in some form of means to stop that aircraft when it returns to the ground, wouldn't you? Well, not necessarily. While the need for aircraft brakes seems simple and self-explanatory now, back in the early days of aviation, aeroplanes often went without any braking system whatsoever (the Sopwith Camel, for example, is one such aircraft). Designers, builders, and pilots alike relied on a combination of the relatively low speeds of aircraft at the time, along with the friction generated by smooth airfields and tail skids, to naturally slow and stop their machines. It wasn't until after World War I, when aircraft speeds increased and more paved runways emerged, that these braking systems began to gain popularity. Much like other aircraft systems, aircraft brakes have continued to evolve and improve over the last century. Let's explore the history of brakes together. This is a fascinating journey spanning over a century of technological advancements. From their rudimentary beginnings with a basic braking system to the sophisticated hydraulic brake system used in modern aeroplanes, the evolution of aircraft brakes has been instrumental in enhancing aviation safety and performance. Let's get started!
History of Aircraft Brakes
Many early aircraft brakes consisted of a single lever that mechanically transmitted brake control input via cables to drum brakes located on the main wheels and, at times, the nose wheel as well. All brakes were linked to the same controller, so they were all applied simultaneously and with equal force. Because the input was manually transmitted, the system was inefficient and only worked well on smaller aircraft. The brake drums also wore out and needed to be replaced relatively quickly. Another variation of brakes, popular between the 1930s and 1950s, were expander tube brakes. This type of brake system owes its name to the flat, fabric-reinforced neoprene tube whose surface was covered with a type of brake lining material called brake shoes. The tube was installed inside an iron brake drum and contained a metal nozzle through which pressurised hydraulic fluid entered when the brakes were activated. As the fluid expanded the tube, the brake shoes pressed against the drum, creating friction and slowing the wheel. Overall, expander tube brakes performed reasonably well; however, their performance in cold climates was less than optimal, and in high temperatures, they were prone to swelling and leaking and then dragging inside the drum after the brakes were released. These original options were better than nothing, but an improved aircraft braking system was clearly needed.
Modern Aircraft Braking Systems
Today, aircraft braking input is primarily transmitted hydraulically, with hydraulic pumps providing the necessary fluid pressure to support the expansive braking systems of large aircraft. Instead of a single-lever control that applies equal pressure to both brakes simultaneously, pilots use individual, finger-operated brake controls located on top of the rudder pedals. One control operates the left main wheel brake, and the other operates the right main wheel brake. The nose and tail wheels have no brakes. This individual brake control setup offers more functionality as the left and right main wheel brakes can be applied separately and to varying degrees. Today's pilots can use this differential braking capability to their advantage while steering the aircraft on the ground. While hydraulic brake activation may be the most common now, as we look to the future, more aircraft are expected to transition to lighter, electrically actuated braking systems. This will eliminate the need for bulky hydraulic fluid pumps and systems. The drum brakes that prevailed through the 1950s have been replaced by more efficient and effective disc brakes. In a disc brake, the rotating wheel assembly and the brake disc, or rotor, spin together. When the brakes are applied, a stationary brake caliper resists rotation, creating friction against the disc, thereby slowing the aircraft. The material composition, configuration, and type of disc brakes vary depending on the aircraft's weight, size, and landing speed.
Composition
The rotors for disc brakes are typically made from iron or steel. The issue, especially in larger and heavier aircraft, is that when activated, the brakes are subjected to high levels of kinetic energy and the resulting heat. Steel and iron brakes will begin to "fade" and lose efficiency as their temperature increases. They are also relatively heavy. To reduce weight and improve the efficiency of brakes on large, high-performance aircraft, designers have been transitioning to carbon fibre. This new and improved construction offers enhanced performance in high-temperature conditions while also reducing the overall weight of the aircraft's braking system. In fact, carbon fibre brakes are approximately 40% lighter than standard iron or steel disc brakes. This translates to a potential weight saving of several hundred pounds on large aircraft. The thermal benefits are impressive. Carbon fibre can withstand 2 to 3 times more heat than steel. A carbon fibre rotor can also dissipate residual heat more quickly than its steel counterpart. Finally, carbon fibre composition is stronger and tougher. It can retain its dimensions even at very high temperatures. From a longevity and maintenance perspective, another key benefit of carbon fibre over steel is that carbon fibre brakes last between 20% and 50% longer. As technology improves and the price of carbon fibre decreases, this type of brake is expected to become even more widespread.
Type
Modern brake systems come in 4 main types depending on the size, weight, and speed of your aircraft:
| Type | Description |
|---|---|
| Single Disc | These smaller disc brake types and lighter aircraft will have single disc brakes in either a fixed or floating configuration. With a single disc brake, hydraulic pressure pushes against pistons located in a non-rotating caliper. These pistons press brake shoes against both sides of the disc, creating even levels of friction and slowing the aircraft. In a floating single disc brake configuration, friction is generated by brake shoes or discs. Half are located inside the brake caliper, and the other half are on the outside. The inner discs are stationary, while the outer discs move with the pistons. The brake disc then floats laterally between these sets of discs. As the brakes are applied, the outer discs make contact with the disc, the disc slides inward, and the inner discs also make contact to brake the wheel. In a fixed single disc configuration, the disc is bolted to the wheel, and only the caliper and brake shoes float laterally depending on the pressure being applied. When the brakes are applied, the brake shoes move toward the disc and make contact to brake the wheel. |
| Dual Disc | If the friction generated by a single brake disc is not enough to stop the aircraft, a second disc is added to the configuration. This is called a dual disc configuration. In a dual disc brake system, each wheel has two discs instead of one. The discs are connected by a centre support with linings on each side. When the brakes are applied, the linings on the centre support make contact with both discs and slow the wheel. |
| Multi Disc | Large, heavy aircraft need something even more powerful than a dual disc setup. These aircraft use a multi-disc brake system built on an extended bearing support. The support holds a series of alternating steel stators and copper- or bronze-plated discs. Stators are a type of flat plate covered with a wear-resistant brake lining material that is designed to press against the rotors. Hydraulic pressure on the piston compresses the entire stack, generating high levels of friction and heat to slow wheel rotation. This system is combined with power-boosted master cylinders or brake control valves. The problem with the multi-disc system is that the rotors and stators are thin and heat up quickly. Since they are also inefficient at dissipating heat, residual heat gives them a tendency to warp. |
| Segmented Rotor | The next-generation option for braking systems on large, heavy, and high-performance aircraft are segmented rotor disc brakes. This is a variation of the multi-disc system but with some significant modifications and improvements to control heat build-up and aid in its dissipation. The brake rotors in a segmented rotor disc brake system have fixed, high-friction brake linings that make contact with the rotors to brake the wheel. Instead of being a flat, solid disc, the rotors in a segmented system have segmented grooves or spaces cut into them to allow heat an escape path. The cutouts also help prevent or reduce the possibility of warping in high-heat conditions. This type of aircraft braking system has become standard on airlines and high-performance aircraft. |
Most Common Issues with Aircraft Brakes
Modern brakes are becoming increasingly reliable and require little maintenance. Of course, even under normal flight conditions, brakes are subjected to extreme stress. This can lead to damage, wear, and potential malfunctions. The most common brake issues you might encounter include:
- Chatter: Your brakes may produce a squeal or even a screech if the brake shoes are not pressing smoothly and evenly against the brake disc. This uneven pressure can be the result of a warped disc or misalignment.
- Dragging: When you release the brake pedal, you should feel the wheel begin to spin freely as the brake shoes retract completely from the brake disc. If there is a residual dragging sensation, this could be due to a malfunctioning return mechanism, a weak return spring, a warped disc, or air in the brake fluid line. If you experience brake drag and the technician cannot find any other issues but the temperature is high, they will likely recommend bleeding the brakes to remove any additional pressure caused by hot air. This should resolve the dragging issue.
- Overheating: The last common issue with aircraft brakes is overheating. Although you are taught as a pilot to use the brakes in a manner that prevents excessive heat generation, situations like aborted takeoffs or other heavy braking manoeuvres can put significant strain on the brakes and cause them to overheat. If your brakes have been subjected to an overheating situation, an authorised technician should carefully inspect them for damage.
Wear
While not considered necessary for early aircraft, brakes are a vital component for modern aircraft. The aircraft braking system has continued to evolve over the last century with new materials and designs helping to slow even the largest and fastest aeroplanes. Key among these are toe brakes, which allow for precise control during taxiing and landing manoeuvres. Furthermore, the anti-skid system, integrated into the braking system, plays a crucial role in preventing skids and maintaining optimal traction on the runway, especially in adverse weather conditions. While most of the intricacies are left to the mechanics, as with all key systems in our aircraft, it is advisable to have a basic understanding of what type of brake system we have, how it works, and the potential issues we might encounter while in the cockpit. Understanding the function of toe brakes and the anti-skid system allows pilots to make informed decisions and ensures safer landings in various challenging situations.
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