Another Airline Braking Question

Flying from North Carolina to Denver, and landing on a snow covered runway, I wondered how braking works for a plane the size and weight of a 737-900.

  1. Do these planes have brakes like a vehicle does, and if they do do they have ABS systems to keep them from locking up?

  2. Do these planes also, or instead, throw the engines into reverse to slow down the plane once it on the runway?

  3. Why do so few planes each year skid off snowy or rain soaked runways?

  1. Yes to both questions. The brakes are significantly beefier than normal automotive brakes, usually consisting of multiple rotors/pistons stacked together, but functionally equivalent. The ABS-equivalent is usually called an “anti-skid” system.

  2. Yes as well. The engine cannot actually run in reverse, but instead a component called a “thrust reverser” deploys that deflects the engine thrust towards the front of the plane. You can usually hear this happening if you pay attention while a plane is landing, and usually see it too if you have a view of the rear of the engine.

Interestingly, the Airbus A380 (the largest/heaviest passenger plane in existence) was originally designed without thrust reversers and intended to depend solely on the brakes, but was redesigned with thrust reversers on the inboard two engines only sometime during certification.

Thrust reverse poses an increased risk of foreign object damage (FOD) to the engine and wings. Different airlines (and different airplanes) will have different rules for when thrust reverse is used, and for how long. Obviously, depending solely on the brakes will cause them to wear out faster. Most commonly, thrust reverse is used immediately after landing, and then brakes only once the plane has slowed below some threshold (e.g 70 knots).

Here is an interesting video of the brakes on an Airbus A380 being tested during certification. This test is for an overweight landing with (I assume) inoperative thrust reversers. The fact that the brakes caught fire is acceptable - this is an emergency situation and the fire does not pose a threat to the plane.

  1. Because of careful planning. Airlines know exactly how much their airplane weighs when it comes in to land, and how much runway length they need to stop within that length in various conditions (dry, slightly slippery, very slippery, etc.) Commercial airport keep track of the runway conditions - some have special vehicles that can drive out on the runway and quantitatively evaluate the available traction - and keep the pilots advised. Also, the airlines operate with significant safety margins in case the runway conditions are worse than estimated, in case the pilot lands long, etc.

Even so, it happens at least several times per year, but is usually not widely reported unless someone gets hurt, which is rare.

Yes, yes, and because of data.

A 737 has an anti-skid system on the brakes, which are installed on the main landing gear.

It also has thrust reversers on the engines.

These are both normally used on landing. I say normally because if you land and are exiting at the end of a long runway they might not use reverse thrust and only brake at the very end.

As to why more airplanes don’t go off the end of runways - we calculate landing distance before landing on a runway, and if it is longer than the available runway we either go somewhere else or wait for conditions to improve.

The data is based on how heavy the airplane is, the flap and brake settings to be used and the environmental conditions at the airport. This includes temperature, wind, and runway condition. The runway condition is the trickiest one. There are many ways to measure it, and getting it right can be very important.

Large commercial airports are VERY good at clearing runways of snow and debris as quickly as possible, giving us a smooth dry surface to land on. But sometimes conditions outpace the airport and things just pile up. In those cases we rely either on airport or pilot reports of “braking action.”

At smaller airports and at ones that get a lot of snow but few arrivals they sometimes send out an airport operations vehicle to go barreling down the runway, slam on the brakes and see how they work. The vehicle is calibrated, so they can come up with a number that corresponds to braking action. We can plug this number into our performance computer and calculate how much runway it will take for us to stop.

At larger airports with a lot of arrivals we rely on other pilots to report braking action. It can be reported as Good, Fair, Poor or Nil. Every airline is different, but normally Good and Fair reports are fine. When you get into Poor or Nil reports, it is time to start looking at the data carefully.

So after gathering all of the data, we calculate our landing distance versus the distance available. There are some assumptions made - you will touchdown 1500 feet down the runway, reverse thrust is normally a “bonus” and not included in calculations, etc. If the numbers are close (ie landing distance is close to runway available) we start to look for other options -a higher flap setting, a longer runway or one more aligned with the wind. Sometimes these are not available, so you have to trust the numbers and make sure you touch down in the zone and on speed.

The reason you don’t see these things happen often is because it is one of the most preventable mishaps. No one can predict an engine failure on takeoff or sucking up some birds on climbout, but on landing you have all the data in front of you and you pretty much know the outcome ahead of time. If you do things right, you will be OK. If you land long, or fast, or are late getting on the brakes…yeesh, a bad day for you.

And if you’re plane does a quick turn on a multi-leg stop then they may keep the gear down longer after takeoff to cool things down.

Here’s a similar video of a brake test during certification of the 777. It simulates a worst-case aborted takeoff; plane at maximum weight, brakes at maximum wear limit. It has to stop with brakes only (no thrust reversers) and not catch fire in the time it would take for emergency vehicles to reach it.

Another device that’s important for braking are the wing spoilers. The small flaps that raise up upon touchdown and ‘spoil’ the airflow over the wings killing their lift. Without them the plane could briefly remain ‘flying’ at zero altitude (i.e. down the runway), and then the landing gear’s brakes wouldn’t be effective because they’d have no load on them.

Yes, more importantly they create a lot of drag. The memory items for a brake failure in the BAe146 is to first confirm the lift spoilers are deployed because in the past pilots have interpreted a lack of ground spoilers as a brake failure. The difference in deceleration with zero brake application and with spoilers out or in is quite noticeable.

Not only at touchdown. They are also used to lower the speed of the plane if it has to make a steep descent, like if it’s going to land shortly after passing a mountain ridge.

This is not wrong, and I know what you meant, but saying it thus might be less confusing to non-pilots:

Deploying the airbrakes in level flight, without adding power, would slow the airplane.

The spoilers/airbrakes are often used to prevent the speed from building up as much in a steep decent as it otherwise would. So it can be the case that the airplane doesn’t slow, or even may speed up when it deploys the airbrakes and begins a steep decent.

As a former aircraft mechanic, I can attest that aircraft brakes are very big. A car has a couple of pads squeezed against a rotor. It’ a very similar setup to bicycle brakes as the pads only cover a part of the braking surface.

On larger aircraft brakes the rotors and ‘pads’ called stators are the same size. Think of rubbing two large plates together. And them might be a dozen rotors and stators stacked together on each brake. One brake on one wheel could weigh a couple hundred pounds.

These brakes can produce a lot of heat. So much that when you walk up to the aircraft after landing, you could feel the heat from a few feet away. I once saw a brake glowing red after a heavy landing. There is a condition called hot brakes that’s dangerous as it could cause the tire to explode or even start a fire. Most aircraft wheels have plugs made with a soft metal like lead which are designed to melt and deflate the tire if things get too hot.

Aircraft were one of the first adopter of anti-lock brakes. I worked on aircraft built in the 1960s which had them. I worked on another aircraft (Sabreliner) built in the 50’s and early 60’s were we retrofitted anti-lock brakes on them.

As an aside, aircraft tires usually last only 100 to 200 landings before needing to be replaced. Also most aircraft tires are retreads. I’ve never seen an aircraft throw off the tread like semi-truck tires do.

ORD folks got to see someone drive a stretch DC8 onto the runway and blow all the tires. It was a bad landing of epic proportions.