How the landing gear and brakes of a passenger aircraft work

Hello everyone. Continuing the topic of describing aviation systems “for dummies” (here and here), I have prepared a new text about the landing gear and wheel brakes of aircraft.

The system of wheels on which the plane rests when moving on the ground is called the landing gear. Modern airliners use a three-pillar landing gear system with two main pillars located under the wing behind the center of gravity and one A-pillar located in the nose of the aircraft. The main landing gear is equipped with brakes, and the front landing gear is pivotable so that the aircraft can maneuver when moving on the ground.

On large aircraft such as Airbus 380 or Boeing 747 in addition to the main struts, auxiliary struts are made to distribute the significant weight of the giant aircraft. All landing gear struts are equipped with shock absorbers. The principle of operation and purpose of which are similar to automobile ones, but the main task is to mitigate overloads at landing so that the loads on the aircraft components do not exceed the permissible ones. On small planes, there are no carts, but there are racks on which, as a rule, one hangs wheel

1. Swivel nose stand

In addition to distributing the weight of the aircraft, the nose strut rotates left and right so that the aircraft can maneuver when moving on the ground.

The rotation of the nose pillar can be controlled in two ways:

  • With the rudder pedals,
  • With the help of a special control knob for turning the bow strut.

The control of the rotation of the nose strut using the pedals is carried out during the takeoff run and the landing run, when the aircraft speed is high enough. At the same time, with the help of the same pedals, the pilot controls the deflection of the rudder.


clickable picture

The limit of deflection of the nose strut when controlling from the pedals is specially limited, as a rule it is 10 degrees. It will not work to turn onto taxiways when it is necessary to deflect the nose strut at angles of the order of 50-70 degrees. At low speeds, the nose leg control stick is used for steering.

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


clickable picture

2. Main landing gear and wheel brakes

The main landing gear is a bogie on which wheels equipped with brakes are suspended.

Airplane brakes are similar to car brakes, only much more powerful, which is not surprising, because they have to brake a car weighing 30-600 tons from speeds of about 250 km / h to zero on a runway (runway) limited in length.

Aircraft brakes are made up of a sandwich of brake discs and pads.

The wheel brakes can be applied in two different ways: manually and automatically.

“Manually” the pilot brakes with the pedals. The question may arise, how does the pilot manage to control and brake with the pedals and the nose strut? The fact is that the pedals of an airplane are not designed in the same way as in a car. Directional control is performed by moving the pedals back and forth. In this case, two pedals move synchronously: left forward-right backward and vice versa. The brakes are controlled by pressing the pedal. Each pedal can be pressed separately, so-called differential braking is another way to control the direction of travel on the ground. If the left brake is used more intensively than the right one, then the plane will turn to the left and vice versa.

The automatic braking mode is activated by itself when a certain event occurs. There can be two such events:

  • During landing: Simultaneous touching the lane (triggering of the landing gear sensors) and finding the engine control sticks in the “idle” position,
  • During takeoff: Moving the throttle control sticks from the takeoff position to the idle position. This braking mode is called “rejected takeoff” (RTO)

The pilot can activate / deactivate the autobraking mode using one of the four buttons under the landing gear retraction-release knob. Three buttons (LOW, MED, MAX) correspond to different intensities of braking during landing, and the fourth (RTO) activates the rejected takeoff mode.

With auto-braking when landing, everything is obvious. Let’s take a look at rejected takeoff mode.

A rejected takeoff is a mode where the crew decides to terminate takeoff due to a significant failure. Takeoff can only be interrupted before reaching the “decision speed”. The decision speed depends on the length and condition of the runway surface and is calculated based on the ability to brake without rolling off the runway. If, during the climb, a malfunction occurs after reaching the decision-making speed, the crew will continue takeoff, no matter what happens. If before, it will slow down.

Before each takeoff, the crew must activate autobraking. The speed of start and the intensity of braking during a rejected takeoff directly affects whether the plane rolls out of the strip or not. Activated autobrake ensures that braking begins immediately after the engines are removed from takeoff mode.

If the takeoff has to be interrupted at the maximum takeoff weight and at the maximum speed, then despite the fact that, in addition to the wheel brakes, the crew engages the reverse and air brakes, the energy that the brakes must absorb heats them up so that they start to glow no worse than a light bulb. After the aircraft has come to a complete stop, the work of the brakes does not end. They must wait at least another 90 seconds before setting fire to the landing gear. According to the standards, a fire brigade will arrive in time for the plane in 90 seconds, which is always on duty at the airports (and has time!).

3. Cleaning – landing gear

In addition to brakes and control of the nose strut, another important function is associated with the landing gear – landing / retracting the landing gear. The landing gear retraction control in normal mode is carried out using the corresponding knob on the dashboard.

Up – remove, down – release. By the way, you can not be afraid to accidentally “fold” the landing gear when the plane is on the ground – modern airliners provide for blocking from such actions when the landing gear is “compressed” – the shock absorbers are compressed under the influence of weight LA

To improve the aerodynamic properties of the aircraft, the niches in which the retracted landing gear are located are closed with flaps, so the procedure for normal landing gear retraction looks like this:

  1. The calculator removes the locks of the closed position of the sashes and gives the command to open the sash
  2. The leaves are fully open and locked in the open position. The corresponding sensors report this to the calculator
  3. The calculator opens the locks of the extended position of the landing gear and begins to clean them.
  4. The legs are completely retracted and locked in the closed position. The corresponding sensors report this to the calculator.
  5. The calculator opens the locks of the open position of the leaves and begins to close them
  6. The leaves are fully closed and locked in the closed position. The calculator fixes the sign of the end of the landing gear retraction

The whole process takes 20-40 seconds. If something goes wrong in the process, then the system interrupts the process, because there is a chance of breaking something. Normal landing gear extension is reversed.

Video from testing the landing gear retract system

In case of malfunctions in the retraction-release system, a special landing gear release procedure is provided – emergency release. Emergency release is activated by the emergency release button located under the cap next to the landing gear retraction-release handle. In case of emergency release by means that do not depend on the computer of the landing gear retraction system, the locks of the retracted position of the landing gear legs and flaps are removed. The chassis falls out under its own weight. The mass of each of the racks is enough to break the sash, even if it does not open itself. On the locks of the lower position, the racks also stand up under their own weight.

4. Sensor of compression of the landing gear

The information about the landing gear compression that I mentioned above is very useful information for many systems. Perhaps it is worth listing some functions that depend on this signal:

When the landing gear squeeze signal appears:

  1. Landing: The control system, if the air brake auto-release is activated, releases the air brakes. Air brakes spoil the picture of the flow around the wing, lift drops sharply, weight appears on the struts and the wheel brakes can start working efficiently
  2. When landing: the automatic wheel braking system is activated (see above)
  3. The blocking of turning on the engine reverse is removed
  4. Some of the emitting radio devices are turned off (so as not to irradiate ground personnel)
  5. After stopping the aircraft, maintenance messages appear that do not affect the pilot’s actions in flight.
  6. The pressure regulation system equalizes the pressure inside and outside the aircraft
  7. The blocking of maintenance systems is disabled, in particular, it becomes possible to update the software of the on-board computers

When the landing gear down signal is removed:

  1. The blocking of the landing gear is removed.
  2. The brakes are briefly activated in order to brake the wheels rotating by inertia after the aircraft is lifted off the ground
  3. The possibility of turning on the reverse of the engine is blocked
  4. Part of the messages of the crew warning system is blocked, which does not require the pilot’s reaction directly in flight (To be precise, the blocking starts from the moment the engine control sticks are moved to the “takeoff” position, but it is the landing gear compression sensor that is a direct indicator that the aircraft is in the air )

Bonus

While I was preparing this text, I decided to figure out for myself why on some aircraft, for example the Boeing 757, the main landing gear bogie in flight is tilted so that the front wheels are higher than the rear ones:

And on the Boeing 767, on the contrary, the front wheels are lower than the rear ones:

As it turned out, the whole point is how the niche is designed, where the landing gear struts are removed, thanks to the video:

And, most curiously, in the military transport C5 Galaxy, the main landing gear is extended in a position across the movement of the aircraft and only then rotated 90 degrees to the desired position.

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