How Does Landing Gear and Brakes Works?

Date: 5 августа 2021
Dear readers,
We continue with our talks about aircraft operation in terms of onboard equipment, and today we’d like to discuss the aircraft’s landing gear and wheel brakes.

A wheel system that enables the aircraft to steer on the ground is called the landing gear. Modern airliners have a tricycle-type landing gear system with two main gears under the wing right behind the center of gravity and one nose gear in the aircraft’s nose. The main gears are equipped with brakes, whereas the nose gear has rotary design allowing to steer the aircraft on the ground. In addition to the main landing gears, large aircrafts such as Airbus 380 or Airbus 747 also have several (more than two) main landing gears.
All landing gears are equipped with shock absorbers. Their operation and purpose are similar to those used in cars; however, the main task is to mitigate landing overloads so that the loads on aircraft units do not exceed the allowable tolerances.

Nose steering gear

In addition to distributing the weight of the aircraft, the nose gear rotates left and right making it possible to steer the aircraft on the ground.

The nose gear can be controlled in two ways:
  • With rudder pedals
  • With a special nose gear turn handle
Pedals are used to steer the nose gear during the takeoff run and the landing run when the aircraft speed is high enough. At the same time, the pilot can use these pedals to control rudder travel.

The rudder travel limit for the nose gear when using pedals is intentionally limited, generally, to 10 degrees. It prevents the nose gear travel at angles of 50 to 70 degrees when taxiing. At low speeds, the nose gear is controlled with a steering handle.

This handle is only used for taxiing and is blocked automatically at high speeds.

Main Landing Gears and Wheel Brakes

Airplane brakes are similar to those you might see in cars, except way stronger, which is no surprise since they need to brake a machine of 30 to 600 tons from speeds of about 250 km/h to zero on a limited length runway. A sandwich of brake rotors and pads forms aircraft brakes.

The wheel brakes can be engaged in two different ways – manually and automatically.
For manual braking, a pilot uses pedals. You may ask – how does a pilot use pedals to control a nose gear and to brake at the same time? The thing is, aircraft pedals don’t work the same way as car pedals. For steering, the pilot moves the pedals back and forth. In this case, two pedals are moved synchronously: left pedal forward and right pedal backward and vice versa. To control the brakes, the pilot needs to press the pedal. You can press each pedal separately, that would be so-called differential braking – and that’s another way of steering. If you use the left brake more intensively than the right one, the aircraft will turn to the left and vice versa.

Auto braking mode is activated automatically in case of one of the following two events:
  • During landing: When the wheels touch the runway simultaneously (Weight On Wheel (WOW) sensors activation) and when engine thrust control levers are switched to idle
  • During takeoff: When engine thrust control levers are switched from takeoff to idle. This braking mode is called rejected takeoff (RTO)

The pilot can activate/deactivate the auto-braking mode in Airbus and SSJ-100 by pushing one of the four buttons under the landing gear lever (Boeing aircrafts have a switch for this). Three buttons (LOW, MED, MAX) correspond to different intensities of braking during landing, while the fourth one (RTO) activates the rejected takeoff mode.

As for auto-braking during landing, it’s quite obvious. Let's take a closer look at rejected takeoff mode.

Rejected takeoff is a mode used when the crew decides to reject takeoff due to a significant failure. Takeoff can only be rejected before reaching the decision speed. Decision speed value depends on the length and condition of the runway surface and is calculated based on the ability to brake without rolling out of the runway. If a failure occurs after reaching the decision speed, the crew continues the takeoff no matter what. If it occurs before the climb, the crew will opt for braking.

Before each takeoff, the crew must activate auto-braking. The speed at the start and the intensity of braking during rejected takeoff directly affect whether the plane rolls out of the runway or not. Activated auto-braking guarantees that braking initiates immediately after the engines are switched from takeoff mode.

If takeoff has to be rejected at the maximum takeoff weight and at maximum speed, then – despite the fact that in addition to the wheel brakes the crew also uses the reverser and speed brakes – the energy that the brakes need to absorb heats them up so that they begin to glow as bright as a light bulb. The brakes keep working even after the aircraft has come to a full stop. They should hold for at least another 90 seconds before setting the landing gears on fire. According to standard regulations, in 90 seconds an on-duty fire brigade present at the airports at all times should reach the plane (and they actually make it!).

Airliner brakes also have a critical function, namely the anti-lock braking system (ABS). The main difference between an aircraft ABS and a car ABS is the consequences of wheel locking: in case of a car locking may lead to impeded control and a longer braking path, whereas in case of a landing aircraft locked wheels would simply explode from friction against the pavement. And without main gear tires, braking is neither effective nor safe. This makes aircraft ABS an always-on and rather critical function.

Landing Gear Extension/Retraction

In addition to brakes and nose gear control, the landing gear has another crucial function - landing gear extension/retraction. Normally, landing gear extension/retraction is controlled with the respective dashboard handle.

The handle is switched up for retraction and down for extension. By the way, you don’t need to worry about accidentally retracting landing gears when the aircraft is on the ground: modern airliners have special blocking to avoid retraction when the landing gear is compressed (shock absorbers are compressed under the aircraft weight - WOWl).

  1. To improve aerodynamic properties of the aircraft, the retracted landing gear compartments are closed with access doors, so normal landing gear retraction looks as follows:

  2. The computer unlocks closed-position doors and sends a command for the doors to open.

  3. The doors are fully open and locked in the open position. Relevant sensors report this to the computer.

  4. The computer unlocks extended landing gear and begins to retract it.

  5. Gears are fully retracted and locked in the closed position. Relevant sensors report this to the computer.

  6. The computer unlocks open-position doors and begins to close them.

  7. The doors are fully closed and locked in the closed position. The computer registers completion of the landing gear retraction

The entire procedure takes from 20 to 40 seconds. Should anything go wrong during this process, the system would terminate it because there is a chance something might break. Normal landing gear extension goes in reverse order.

  Watch testing of landing gear retraction system:

A special landing gear emergency extension is introduced specifically for cases of extension/retraction system failure. The emergency extension is activated by pushing the emergency extension button located under the cap next to the landing gear handle. In case of emergency extension by means not related to the landing gear extension/retraction system computer, the landing gear upper position locks and door locks. The landing gear comes out by gravity. The weight of each gear is enough to break the door, even if it does not open automatically. The gears also shift to the lower position locks by gravity

WOW Sensor

WOW (Weight On Wheel) information is crucial for many systems. It would probably make sense to list some of the functions that depend on this signal:

In case of WOW signal activation:

  • For landing: if auto-extension of speed brakes is activated, the control system releases airspeed brakes (spoilers). Speed brakes distort the flow about the wing, the lift force drops dramatically, weight shifts to gears, and the wheel brakes become efficient

  • For landing: the automatic wheel braking system is activated (see above)

  • Engine reverse interlock is switched off

  • Some of transmitting radio devices are switched off (not to expose the ground support personnel to radiation)

  • Once the aircraft has stopped, maintenance system messages pop up that did not require pilot’s actions during the flight

  • Pressure control system equalizes the pressure inside and outside the aircraft

  • Maintenance systems are unlocked, in particular, it becomes possible to update the software of onboard computers

In case of deactivation of WOW signal:

  • Landing gear retraction is unlocked

  • The brakes are briefly activated to brake the wheels that keep mechanically rotating after taking off

  • Engine reverse switching function is blocked

  • Some of the CAS messages are blocked, in particular those that do not call for the pilot's immediate actions during the flight (in fact, this blocking occurs as soon as the engine control sticks are switched to takeoff; however, it’s the landing gear compression sensor that directly indicates that the aircraft is in the air)

Landing gear compression sensors are usually multichannel and are installed on each gear. Data from numerous sensors are collected by special devices – data concentration units. Based on the acquired data, signals for compression of each gear and a signal for compression of all gears are generated. Within the context of the above functionality, different signals are used: compression signals from two main gears are enough to initiate auto-braking, whereas all three racks need to be compressed for maintenance mode to switch on.


As we were working on this article, we decided to find out why the bogie of the main landing gears in some aircrafts (e.g. Boeing 757) is tilted during the flight so that the front wheels are higher than the rear ones:

For Boeing 767 it’s the other way round – the front wheels are lower than the rear ones:

As it turns out (thanks to this video, the trick is in the design of the landing gear compartment. What’s most peculiar, in the military freighter C5 Galaxy, the main landing gears are extended against the aircraft trajectory and only then are they turned through 90 degrees to the intended position.

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