Modern jets go fast. Typical cruise speeds are close to the speed of sound. Did you realize you were moving that fast? As inevitable as a Black Friday WalMart riot, at the end of every flight we must bring this speeding vehicle to a safe stop on a short length of concrete. Sometimes, that runway is wet, snowy, or even icy. It takes some serious engineering to pull this off routinely. How do pilots stop a jet and steer it to the gate? We use several systems to make it happen.
Spoilers And Speed Brakes – Get rid of the lift
Landing speeds for airliners are around 160-170 miles per hour. When an aircraft is moving this fast, it still wants to fly after the wheels touch the pavement. The first thing we want to do at touchdown is stop the wings from generating lift (so we stay on the ground). We do this with spoilers; panels on top of the wings that we can raise up upon landing. They do a couple of things for us:
- Spoilers disrupt (spoil) the airflow over the wing to destroy any remaining lift being generated. This puts the full weight of the aircraft on the landing gear so our wheel brakes can do their job.
- Spoilers are often called “speed brakes.” At high speeds, spoilers are very effective in slowing the airplane (imagine holding a big piece of plywood out your car window at highway speed).
Quick deployment of spoilers after touchdown is important, especially on short runways. In order to get the maximum benefit from the spoilers, we arm them for auto-deployment before landing so when the wheels touch down, the spoilers deploy. Watch the wing during landing on your next flight. When you see the spoilers come up, say in a loud voice: “Hey, they deployed the spoilers on this baby!” Your fellow passengers will be impressed with your AeroSavvy.
Watch the spoilers extend in slow motion as this 737 touches down:
Brakes – NOT the brakes on your dad’s old Plymouth
The most important system we use to stop the aircraft are the wheel brakes. Modern aircraft use disk brakes, similar to your car; but instead of having just one rotor disk, heavy airliner wheels have stacks of rotors, often 4 or more, hiding inside each wheel. One reason big jets have a lot of wheels is so they can have a lot of brakes. During a landing on a short runway or an emergency stop on takeoff, the brakes work hard and can get very hot. Having more wheels (and more brakes) spreads out the workload to keep the brake temperatures under control. The more brakes, the better.
To make the brakes work more efficiently, they include an anti-skid system. It’s like the anti-lock brakes on your car, but on steroids. The anti-skid system monitors all the wheel speeds after touchdown. If it senses one wheel starting to slow more than the others (indicating a skid), it reduces braking on that wheel. The system is very effective for slowing down on runways contaminated with rain, snow, slush, or ice.
Aircraft anti-skid systems have been in widespread use since the 1950’s. All modern airliners (even those considered “old” by passengers) are equipped with computer-controlled anti-skid.
Ever wonder how pilots apply the brakes?
The controls are a bit different than in your car. The brakes in an airplane are activated by pressing down the top portion of the left and right rudder pedals. The left rudder pedal operates the left landing gear wheel brakes; the right pedal operates the right brakes. Having separate brake controls allows us to apply different amounts of braking to each main gear (it’s called “differential braking”). This helps us in tricky braking situations and when making really tight turns while taxiing.
To further improve braking, most airliners have an auto brake system that applies the brakes for us upon landing. In the 757 & 767, I can choose auto brake settings based on landing conditions. A setting of 1 will provide a slow deceleration; good for a light airplane and long runway. “Max Auto” tells the system that I need serious braking action. A situation that would call for Max Auto is a snow or slush covered runway or a short, wet runway. For most landings, a setting of 2 or 3 provides a smooth deceleration that ensures passengers (who shouldn’t be drinking coffee during landing) won’t spill their coffee.
Just how good are the brakes? Darn good
The absolute worst-case scenario for maximum brake punishment is a “rejected takeoff.” For certification, aircraft manufacturers must demonstrate that their fully loaded airplane can accelerate to takeoff speed then stop safely on the remaining runway. And it has to do it using old, worn out brakes! In the real world, this maneuver is very rare. Even if you’re a frequent flyer, you aren’t likely to experience one. Check out this video of Boeing’s 747 undergoing brake certification. The nearly one million pound machine accelerates to 200 mph before full braking is applied. The brakes do their job exactly as designed while heating to a red hot 2500ºF. After the aircraft comes to a stop, the tires deflate by design so they don’t burst. Perhaps more amazing; after a tire and brake change, this aircraft is once again ready for action.
Thrust Reversers (Bonus braking!)
Wouldn’t it be great if we could take some engine thrust and divert it forward to get a little extra braking? Absolutely! Reverse thrust systems are very effective at slowing down airplanes while saving wear and tear on brakes. You’ll find thrust reverse on most, if not all, large airliners. Some smaller regional jets don’t have reversers due to their lighter weight. “Reverse Thrust” doesn’t mean the engine runs backwards (that wouldn’t work). To generate reverse thrust, a series of doors and deflector panels on the engine move into position to divert some of the engine exhaust in a forward direction. When you hear the engines start to roar after touchdown, you’re hearing the reversers doing their job.
There are a few types of thrust reversers out there. The bucket-type reverser shown below can be found on the old Fokker 70 & 100, DC-9, and MD80 variants. Engine thrust coming out the rear of the engine is diverted forward at about a 45º angle.
Clamshell reverser doors can be seen on a lot of Airbus aircraft like the A319, A320 and A340.
Many Boeing airplanes, as well as the Airbus A380, use cascading thrust reversers. The best way to show these is with a video. Here’s a Boeing 747 deploying its cascading reversers after landing on a wet runway. Watch the rear part of the engines slide open just after touchdown. The moisture makes it easy to see the thrust being diverted forward.
How do pilots deploy thrust reversers?
We use special levers that are attached to the back of the thrust levers (throttles). At touchdown, the pilot reaches behind the thrust levers, grabs the reverser levers, and raises them up. This signals the reverser doors and deflectors to deploy and the engines to spool up (causing the racket passengers hear after touchdown).
Reverse thrust is most effective at high speed. Deploying the reversers at touchdown greatly reduces landing distance. Very important on a wet, slippery runway.
Steering on the ground
While we’re on the subject of wheels and brakes, I’ll hit on a related, often asked question. How do we steer a large aircraft while taxiing? I already mentioned that we can use the tops of the rudder pedals for differential braking. This gives us a little steering control, but it wouldn’t be good for the brakes or tires to always steer this way.
The rudder pedals give us limited ability to turn the nose wheel at high speeds on the runway. The steering tiller is used for the majority of ground steering. The tiller uses hydraulics (like the power steering in your car) to turn the nose gear. Tillers are located on the left side of the flight deck near the captain’s left hand. Some jets have tillers on both sides so either pilot can taxi. Taxiing to the gate is a fairly easy task that requires a little thrust from the engines and using the tiller to steer the plane like a car – a really, really long car. It takes a little practice to get the hang of it (thank heavens for simulators).
There you have it – spoilers, brakes, and thrust reverse. The big three airplane stoppers pilots (and passengers) count on at the end of every flight!