Aircraft brakes are totally different than automobile and truck brakes. Yes, they both work on the rotor/pad principal, much like the principal of bicycles but the application and use of those aircraft brakes take on a whole new difference in application on an aircraft on landing.
First an automobile or truck will be on tires connected to the ground when applied. The speed of the vehicle can, depending on design of the vehicle, have a fairly solid connection to the ground, concrete or asphalt. Aircraft have a varied pressure on the tire to the surface depending on speed and tire size and construction.
To start look at your vehicle tire. They are wider and if they are radial constructed the tire puts a “flatter” load on the surface. We also are seeing tires on newer vehicle that have increased air pressure. This is to put a stronger and flatter profile on the ground.
Aircraft tires are different. First, they have less surface on most of the smaller general aviation aircraft than vehicle tires. They are also a different type of construction, so they are strong in the vertical mode and able to handle a vertical load from direct application, the gear. This type of construction is known as ‘bias’ construction and it uses cord material that each layer is laid ‘bias’ to the previous layer. This is for strength of load and to make the aircraft tire stand up for strength and to maintain a consistent size. A consistent size is critical on gears that retract into the aircraft.
Now we look at the braking effect of an aircraft tire. Remember, in most cases the tire ‘footprint’ of an aircraft tire is smaller. The second major problem we do not consider on landing and brake application is that we are not putting a firm footing on the runway as soon as we land. The aircraft still has some lift component and that unloads the pressure of the tire. The bottom line is the tire force to the runway is lesser then when the tire is sitting on the tarmac without movement.
Although current design practices do not necessarily account for vehicle speed, it does influence pavement loading. In general, slower speeds and stop conditions allow a particular load to be applied to a given pavement area for a longer period of time.
The last sentence is the key to aircraft brake application. The bottom line is if you do not let the aircraft settle and stabilize on the pavement and START A SMOOTH ROLL, you end up with a locked brake as the load on the tire increases its downward force. The load on the tire increases as the aircraft slows. If you are still carrying airspeed, even though the aircraft is not in the air you are still not affectively loading the tires and therefore the brakes are not effective.
Now, if you think you are on the runway yet still carrying high airspeed and you hold down the brakes when the aircraft does lose its lift vector, with that hard brake pressure, you are going to ‘skid” the tire. When that happens the wheel/tire assembly becomes overloaded and possibly could take out the brakes and even strip the caliper/pads out of its housing.
When could this “too early” application of the brakes occurs? When you are in a panic situation from landing too fast and landing long. If you calculate adding landing distance to a landing due to some type of wind, maybe a cross wind, then you land long, LOCK THE BRAKES DOWN TRYING TO STOP and as the plane settles on the tires they lock up. The biggest problem occurs when you are using metal pads to hardened rotors. This is the best braking and stopping power, but it is unforgiving to a rapid application and locked brakes. Again, you are too fast, panic sets in, the tires are on the runway but not settled into the concrete or asphalt with adequate friction and then the brakes lock and possibly reverse hydroplaning occurs. Now you have no braking action and you just continue to roll all the while with pressure on the pedals and you just keep pushing on the brakes.
Solution: What do the airliners have that we do not have? An anti-skid system that loads and unloads and loads and unloads the brakes and therefore the tires. We can do the same thing by doing “brake on, brake off, brake on, brake off” starting with a slight brake pressure and as we slow, adding more pressure. This is the proper way to brake a large aircraft without an anti-skid system. It also works on small aircraft such as a Cessna 172.
Your solution is to allow the aircraft to establish the load on the tires, lightly load the brakes and then off and remember if you have speed and retract the flaps through the 15 degree to “0” degrees retraction, this is a lift of the wing off the tires. Leave your flaps down. Watch the air carriers, they never retract the slats and flaps until they are off the runway. Not just because it is in the after landing check list but because they will go through a release load on the gear in the final end of the flap and slat withdrawal.
Remember, when you push on the master cylinders you are creating a hydraulic pressure at the caliper/wheel assembly that is going to be very high. There is also a chance of a “back pressure” against the line which could cause an overpressure problem. The bottom line requires you to apply the brake pressure very lightly and gradually “on – off, on – off. Once the speed is under control and the tires have settled with the full load and no lift, you can increase the pressure on the pedals but again just enough to bring the plane to a slow stop.
Airplane brakes and airplane tires are totally different creatures compared to the ones we drive on. Learn the procedure for proper aircraft braking and you will not have brake failure.
In another article we will discuss trunnion loading and the problems with the electric gear aircraft you must consider.
Train hard. Fly safe.