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On 11 July, 2022, a Spirits Airlines A320 touched down after a short flight from Tampa at Hartsfield-Jackson airport in Atlanta, Georgia. As the plane began taxiing to the gate the crew were alerted to billows of smoke emanating from their wheels.
The aircraft was brought to a stop by the crew to assess the situation during which they requested that the emergency services be sent out to the aircraft to help deal with the problem.
In the cabin, understandably concerned passengers were asked to remain seated by the cabin crew and wait for more information from the pilots. Within a couple of minutes, the airport fire services arrived at the scene where they immediately dealt with a small fire that had broken out around the aircraft’s wheels. A tug was then used to tow the aircraft to the gate where the passengers disembarked safely.
Heat from brakes has caused minor emergencies for a number of aircraft over the last few months. Is it just a coincidence that these have also been some of the hottest months on record? Or can hot weather really cause an aircraft’s brakes to overheat?
Before we dive into the details of how and if the weather affects the brakes, we need to know how the brakes on an aircraft work.
To stop a wheel from turning, be it on a bicycle or an aircraft, you need to apply a frictional force on the wheel. On a bike, this is done by using a cable to push the brake pads onto the rim of the wheel. On a car, the system is similar, pressing brake pads against the rotating disc rotor inside the wheel.
On an aircraft, the principle is much the same but on a much larger scale and differs slightly between aircraft types.
The Boeing 787 Dreamliner has one brake unit on each of the eight wheels on the main gear assembly. These are 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).
The BSCUs 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 is the stators. These sit around each rotor, fixed in place and don’t move. As the wheel turns, the rotors spin around 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.
With friction comes heat and 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 tyres to explode. To stop this from happening, when a certain temperature is reached, fuse plugs in the tyres melt. This allows the air to be released safely and slowly deflates the tyres.
On touchdown, an aircraft like the 787 can be travelling at nearly 180mph and weigh 192tons. That is a lot of energy that needs to be dissipated as the aircraft decelerates on the landing roll, most of it going into the brakes.
As a result, before we get anywhere near the ground, we perform a landing data calculation in the calm of the cruise. This has several functions.
Firstly, and most importantly, we need to know that we are able to stop safely on the available runway length with the expected weight and weather conditions at the time of landing. If it turns out that the runway is too slippery to stop before the end, a decision must be made whether to wait for the runway conditions to improve, or to divert to another airport.
When driving a car and approaching a junction, we don’t go as fast as we can and then slam on maximum braking. Not only does this increase the chances of sailing through the red light, but it also causes unnecessary wear on the brake pads. It’s also particularly uncomfortable for any passengers that we may be carrying.
Instead, we either apply a braking force that brings us to a safe, comfortable stop at the red light or we start braking further away from the junction to reduce the braking pressure required.
The same principles apply to an aircraft. However as we only have a defined length of runway available to us, the only way of changing how quickly we stop is by varying the braking pressure. Yet, there is a trade-off to be made.
Use too little braking and we run the risk of going off the end of the runway (bad). Use too much braking and we run the risk of the brakes overheating and catching fire (also bad). As a result, we need to find a happy medium that has us stopping safely on the paved surface, without allowing the brakes to get too hot.
We also need to ensure that we don’t spend too long on the runway. A busy airport like London Heathrow has an aircraft landing every 50 seconds or so. One of the most common causes of go-arounds is the aircraft ahead taking too long to vacate the runway. As a result, we must decide which runway exit we will take and plan the braking accordingly.
To help us with this, we have the autobrake system which provides us with automatic braking at a pre-selected rate as soon as the aircraft senses that it is on the ground.
The landing data calculation takes the runway length, aircraft weight, landing speed and weather information into consideration to calculate how much runway we use up when using a particular autobrake setting. On the 787, we use the Onboard Performance Tool (OPT) to do this and it conveniently gives us the calculated distances for each autobrake setting in an easy-to-see format.
Having looked at our charts to see which runway exit we plan to use and calculated how far down the runway this is, we can then select the autobrake setting that best fits in with this number. However, there is one final check to be made and that’s the brake cooling schedule.
The Quick Reference Handbook (QRH) on the 787 is a thick spiralbound book containing a wealth of technical data about the aircraft. Within its pages is a table that allows us to calculate how hot the brakes will get depending on the same data used in the OPT.
By working our way through the table with the settings that we have selected, we can calculate how hot the brakes can be expected to get when landing as planned. At the cooler end of the spectrum, this details how long it will take for the brakes to cool before the aircraft can depart. At the hotter end, there is a chance that the fuse plugs may melt – a situation we want to avoid at all costs.
If we find that our current plan has us in the melt zone, or even in the caution zone, we must go back to our landing data calculation and find a way to reduce the amount of energy going into the brakes.
This could be done by using a higher flap setting, using maximum reverse thrust instead of idle reverse, or quite simply using a lower autobrake setting and using up more of the runway in which to stop.
Most of the time the weight of the aircraft, runway length and environmental conditions are no issue when it comes to selecting the autobrake. However, there are certain situations where, as an experienced pilot, you know that there may be a problem with the brake temperatures and one of these is when the outside air temperature is high.
In these scenarios, the brakes are unable to lose the heat as effectively as they would if the air temperature was cooler. As a result, there is a higher risk of the brakes overheating if they are not managed properly.
Having flown both the A320 family and the 787, I found that the brakes on the A320 tend to get warmer than those on the 787. Why this is, I can not say for sure but in my personal experience, we would end up using the brakes more often on the taxi between the gate and the runway and the runway and gate on the A320. This was due to the fact that the high idle engine power would result in the aircraft accelerating away quite quickly, needing to be slowed down more often.
To help keep the brake temperatures down, most A320 family aircraft now have brake fans. This enables the pilots to circulate air around the brake unit, increasing the rate of cooling which helps stop them from getting too hot.
However, like with all things in aviation, incidents never happen for a single reason.
Should the crew get airborne with the brakes already warm from a long taxi out, they would have little chance to cool on a short flight. If they were to then land at a high weight on a hot day using a high braking force to get off the runway quickly, it’s easy to see how the brake temperatures could rocket.
There’s a lot more thought that goes into landing than just throwing the aircraft on the runway and slamming on the brakes. Pilots must consider the effects their braking has not only on the comfort of their passengers but also on the brakes and the aircraft itself. A failure to consider all the factors that go into the landing, deceleration and taxi to the gate can easily result in the brakes getting too hot.
However, if the brakes do get too hot, the tyres are designed to automatically deflate once they reach a certain temperature to avoid them exploding. In addition, the way in which the landing gear assembly is designed means that even if a fire does break out, it will not spread to other parts of the aircraft.
In these situations, more often than not it’s safer for the pilots to keep everyone on board the aircraft instead of evacuating. As airports have specific requirements that the fire services must be able to reach any part of the airfield in a matter of minutes, the fire will be extinguished before it causes any real harm to the aircraft and its occupants.
Photo by Robert Alexander/Getty Images.
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