How do aircraft brakes work?
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As the old saying goes, “what goes up must come down” and it’s no use coming down if you can’t stop safely.
Bringing at aircraft down to the runway from 37,000 feet is only part of the challenge. It’s not over until the aircraft is safely parked at its gate. In order to do this, pilots have a number of systems available to slow the aircraft down, even in a blizzard on a snow-covered runway.
This is how we do it.
The 787 braking system
Stopping a 200-tonne aircraft landing at 180 mph requires a lot of braking force. To do this, the 787 has one brake unit on each of the eight wheels on the main gear assembly. On other aircraft types, the brake units are powered by the hydraulics system. An electrical signal is sent from the flight deck to hydraulic actuators near the main landing gear. Here, hydraulic fluid at 3,000 pound per square inch is used to force the brake unit against the wheel, thus slowing it down.
This system works fine, but the pipes and actuators that form this part of the hydraulic system come at a considerable weight cost. Extra weight means more fuel burn, which in turn increases costs and carbon emissions. What if the brakes could be powered a different way?
Read more: How and why pilots dump fuel during a flight
That’s the case on the 787 Dreamliner. Designers did away with the use of the hydraulic system and all its associated architecture and instead used electricity to power the brakes. When the pilots press on the brake pedals, an electrical signal is sent to the brake unit on the wheel. Here, electrically powered actuators are used to press the carbon brake disc against the wheel, slowing it down.
By changing to electric brakes, a 787-8 saves 141 pounds per aircraft and a 787-9 saves 245 pounds. The brakes are also known as “plug and play” because electrical wiring replaces the traditional hydraulics and it’s much easier and quicker to change the brake units when needed. Smart features also allow engineers to monitor the brake performance more closely, giving a real-time measurement of wear on the carbon disks.
The brake system on the 787 Dreamliner is controlled by the pilots pressing the tops of the rudder pedals under their feet, demanding the rate of braking which they require. This sends an electronic signal to the Left and Right Brake System Control Units (BSCUs).
These then send signals to the four Electronic Brake Actuator Controllers (EBAC) which control the rate of braking on the wheels. Each wheel has four Electric Brake Actuators (EBA), a kind of piston which presses against the carbon brake discs.
The brake disks themselves are made up of two parts. Firstly, there are the rotors. These are connected to the wheel by drive tabs (in the video below they are the black rectangles on the rotor disks). As these drive tabs are in contact with the inside of the wheel, they spin at the same speed. Depending on the brake manufacturer, there are either four or five of these rotors on each brake assembly.
The second part of the disks are the stators. These sit around each rotor and are fixed in place and thus don’t move. As the wheel turns, the rotors spin round inside the stators.
When the brakes are applied, the four EBAs apply pressure to the first stator. This in turn squeezes the stationary stators up against the spinning rotors and it’s this friction which slows the wheel down. https://www.youtube.com/watch?v=n0nXiQPwHE8
When landing on slippery runways, there’s a chance that the wheels may start to skid as the brakes are applied. To stop this from happening and to maintain maximum effective braking, each wheel has anti-skid protection.
Using a variety of sources to determine the aircraft speed, the brake units know how fast the wheels should be spinning. If that speed drops significantly, it’s because the current brake pressure on that wheel is too great and the wheel is just skidding over the surface.
In this situation, the anti-skid system automatically reduces the braking on that wheel to a point where the skid stops before reapplying the pressure. All this is done in a fraction of a second.
Pressing on the brakes is pretty straight forward when taxiing in a straight line at low speed. However, when we’re landing in strong winds, it can be a little tricky. We need to use our feet on the rudder pedals to line the nose of the aircraft up with the runway centerline at the last moment. Then, whilst holding that position, slide our feet up to press the toe brakes. Really not very easy when moving at 160 mph.
To help us get the braking underway as soon as we touch down, we have the autobrake system.This provides automatic braking at a preselected rate as soon as the aircraft senses that it is on the ground. It also provides full braking pressure in the case of a rejected takeoff if the speed is above 85 knots (98 mph).
Brake temperature indication
With friction comes heat. As a result, each brake unit displays its temperature on the wheel synoptic page in the flight deck. Here, numerical values relating to brake temperature are shown next to each wheel. A value of 0-4.9 is in the normal range. When a temperature becomes 5.0 or above, an advisory message is displayed to the pilots.
Should the brakes become 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 deflate the tires.
Every so often we need to engage the brakes and keep them on. This is particularly useful on long taxis to the runway and obviously when parked at the gate. The park brake is set by fully pressing down both toe brakes and pulling the parking brake lever up. With this set, we can then release the pressure from the pedals. To release, we just press the brake pedals again.
A helping hand
In addition to the brakes, there are two other systems which help slow the aircraft down on landing. The spoilers and the reverse thrust.
Have you noticed the large panels on the top of the wings raise up on touch down? These are the called the spoilers as they literally spoil the airflow over the wing. This dumps any remaining lift the wings are generating, allowing the wheels to take all the weight and achieve maximum efficiency from the brakes.
The final part of the braking process comes from reverse thrust. Just after we have touched down, we pull two levers on top of the thrust levers to engage the reverse thrust, a sort of “reverse gear” for jet engines.
This causes blockers inside the engine to deploy and a door in the side of the engine to slide backward. The air which normally leaves the engine out the back is deflected forward by the blockers and out through the door.
Read more: 10 years of the Dreamliner — why pilots love flying the 787 https://www.youtube.com/watch?v=NBFM8yCBqa4
There are two stages of reverse thrust — idle reverse and max reverse. Idle reverse is used on most landings and max reverse is used when the landing performance requires it, normally when the aircraft is landing at hot or high elevation airfields.
This forward directed airflow helps slow the aircraft down but is most efficient at high speeds. If using max reverse, we drop this down to idle reverse at 60 knots (70 mph) and then back to normal thrust as we vacate the runway at around 20 knots (25 mph).
Takeoff and landing performance
For every takeoff and landing which we carry out, we run a performance calculation to ensure that it is safe to do so. A large part of this is based on the braking effectiveness.
On takeoff, we have to calculate the point at which it is no longer safe to stop. This speed, V1, marks the point after which we must continue to get airborne, no matter what the problem is.
The calculation of this speed is based mainly on the ability of the aircraft to stop. If the runway is slippery or a brake unit is inoperative, it will take a greater distance to stop. As a result, the V1 speed will be much slower than in normal dry conditions. This means that we will reach that speed sooner and there will be more runway remaining on which to stop.
The performance on landing isn’t normally as restrictive. The aircraft is a lot lighter having used most of its fuel in flight and we are touching down at the start of the runway. This gives the brakes much more distance to bring the lighter weight aircraft to a safe stop.
All this is great at the design stage, but how did Boeing know for sure that the systems would perform as expected for real? There’s only one way to find out and that’s to do tests on the actual aircraft.
Before the first 787 was approved for flight, Boeing had to prove that vital systems, such as the brakes, would work in conditions far worse than would ever be expected in normal operation.
The greatest strain on the brakes comes not on landing as you might expect, but in the event of a rejected takeoff. In these situations, the aircraft is much heavier and there is far less runway remaining on which to stop. https://www.youtube.com/watch?v=u6DLlFrk-6c
To make things even tougher, each of the eight brakes were worn down to just 1% of their full value and one of the brake units was deactivated. The aircraft was then loaded up to its maximum takeoff weight and accelerated to its maximum takeoff speed.
The pilots then slammed the thrust levers closed and allowed the rejected takeoff autobrake to do its thing.
With the aircraft stopped safely on the runway, the brakes glowed red. They became so hot that the fuse plugs melted and the tires deflated. Yet still there was no danger to the aircraft.
The firefighters even waited five minutes before dousing the brakes with water. This replicated the time it may take emergency services to reach an aircraft in a real world scenario.
With all things aviation, there’s always redundancies built into the system. Part of the reason behind testing the aircraft so rigorously is to determine how well it performs when the systems are degraded.
It may come as a surprise to you but almost every aircraft you’ve been on was carrying some kind of technical fault. This may have just been something minor such as a rivet missing up to something more important such as a brake unit not working.
In these situations, pilots consult the Dispatch Deviations Guide to see what is legally acceptable to fly without and what implications this may have.
On the 787, we are permitted to depart with two brake units not working.
As a result of the degraded stopping ability, there are extra operational procedures which we must apply. This includes calculating the takeoff performance with two brakes inoperative and also leaving the gear down after takeoff.
When we select the gear up after takeoff, the wheels will be spinning at around 180 mph. Bringing these into the aircraft would cause serious vibration and discomfort in the passenger cabin.
To stop this from happening, when the gear is selected up, the brakes automatically engage, stopping the wheels from spinning.
However, if we dispatch with brakes inoperative, those wheels will keep spinning. Therefore we leave the gear down for two minutes to allow the wheels to spin down naturally.
If you’ve ever seen a Dreamliner leaves its wheels down after takeoff, this is most likely the reason why.
Bringing an aircraft to a safe stop is the most important part of a flight. In order to do this, pilots have a number of facilities available to them.
The brakes bear the brunt of the hard work with the spoilers and reverse thrust lending a helping hand to ensure maximum efficiency. The autobrake system helps keep the deceleration nice and smooth whilst the anti-skid system reduces the distances needed to stop by modulating the brakes on individual wheels and stopping them from skidding.
Aircraft can also depart with part of the braking system not working. By carrying out a performance calculation before every takeoff and landing, we ensure that the aircraft will stop safely in the given conditions, even at maximum takeoff weight.
Featured photo by Nicky Kelvin/The Points Guy
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