Efficiency and Fatigue/Endurance Laboratory Tests of Aviation Friction Brakes

Brake (https://www.merriam-webster.com/dictionary/brake) (Fig. 1) is a device for arresting or preventing the motion of a mechanism usually by means of friction. Braking itself is a process of using the brake. Brakes are used in every machine or vehicle which moves or in which the movement is a part of its operation, e.g., aircraft, cars, lifts, or cranes.

Figure 1.

Friction brakes are most commonly used in vehicles and machines. Basically, a brake uses the friction phenomenon between surfaces of two materials by using a certain amount of force needed to generate and then maintain the desired effect.

Friction materials are designed to withstand high temperatures without losing stability of the process over the time of braking (brake appliance).

Other parts of the brake assembly must be durable/strong enough to work reliably through the high number of braking cycles due to the possible deadly consequences of the failure.

Braking in the aircraft occurs mainly in two scenarios: landing (also during rejected takeoff [RTO]) and ground maneuvers.

Brakes are designed to operate safely and reliable for landing, which is the reference case of normal aircraft brake operation (Fig. 2.). The RTO case is considered as an emergency – brakes must ensure safety but can be destroyed in the process (Fig. 3).

Figure 2.

Figure 3.

Due to the regulation requirements, all brakes must be tested prior to use in order to prove their safe operation. The tests are carried out in stages:

Friction pair materials are tested in order to collect data for ensuring its properties and to give feedback for brake assembly design (laboratory tests).

Usually, friction material tests are conducted in order to prove the properties of the friction pair (Fig. 4) by scaling down the braking process parameters from full-size brake to the model/sample size:

Dynamic tests – in this case, there is a sequence of braking cycles (from 10 to >100). In the tests, the friction pair cannot fail or be destroyed. Parameters for the test are scaled from real brake with assumption of equality of the energy on the elementary area and equality of the sliding velocity between the friction pair surfaces.

Figure 4.

Usually, test models/sample tests are performed using test stands called inertial dynamometers, which are capable of generating high amounts of energies to replicate the braking process. The main principle of the tests is the equality of braking energy dissipated (more precisely kinetic energy of the braking change into the heat) by the unit of area of the friction material used both in the model and in full size brake.

The IL-68 (Fig. 5) friction material testing machine owned by Structure Testing Laboratory (former Landing Gear Testing Laboratory) of Ł–ILot is the example of an inertial dynamometer.

Figure 5.

Test stand parameters:

Maximum drive shaft rotational velocity around 9000 rpm

Scope of tests: friction materials wear, friction pair behavior (parameters: braking torque, braking force, temperature, thermal endurance tests).

An example of results from dynamic tests of aviation brake assembly performed using M3T/6T test stand is shown in Fig. 6:

Figure 6.

Exemplary brake assembly test compliant with ETSO-C26 regulations for aircraft compliant with part 23, 27, or 29 (CS – European, FAR – US, CAR - Canadian) regulations. Similar regulations are ETSO-C135 for part 25 compliant aircraft:

Dynamic test resembling the real-life braking conditions – in this case, it is the energy, inertia, and braking speed. The test sequence contains 100 full braking cycles with one change of brake linings permissible per test sequence. The regulations only state that the brake must not fail in all of the tests, and the deceleration should be no less than 10 ft/s² (3.05 m/s²). In many cases, the regulation requirements are extended by e.g., brake linings wear measurement, specific brake time requirement, or minimum braking moment (torque).

These types of tests are made using test stands capable of delivering the desired inertia – resulting in proper energy delivery – to the brake. There are a number of designs for such test stands. In Structure Testing Laboratory (former Landing Gear Testing Laboratory) of Ł–Ilot, the test stand is a drop test stand expanded with a rotating drum capable of a number of inertia levels.

Below there are exemplary M3T/6T test stand parameters:

Maximum mass of test object including mounting parts 3T (it can be extended to 6.5T for wheel testing)

Scope of tests: drop tests, shimmy and obstacle run tests, brake tests (Fig. 7.), and wheel roll-on tests.

Figure 7.

An example of results from dynamic tests of aviation brake assembly performed using M3T/6T (Fig. 8.) test stand is shown in Fig. 9:

Figure 8.

Figure 9.

According to ETSO-C26 regulations, all of the newly designed hydraulic brake assembles must be endurance/fatigue tested:

The endurance test is a high number of work cycles (100,000 cycles for airplanes) with various levels of brake linings wear (25%, 50%, 75%, and 100%) using average pressure taken from dynamic tests or calculated. Moreover, the brake must be tested against the maximum allowable pressure for 5000 cycles. The brake assembly in this test is free (no dynamic braking) and must not leak (no more than 5cc according to regulations) or malfunction during the tests. The typical time of brake engage is 1 s or can be measured during the dynamic tests in laboratory or on the aircraft.

The mentioned above RTO is an emergency type maneuver that requires aircraft to stop just before taking-off due to onboard malfunctions or control tower command. The velocity of the aircraft would be near the takeoff speed as well as the mass of the aircraft which is full of fuel, which would be consumed during flight. The overall parameter values of the braking are higher than those during normal operational conditions during landing (e.g. CS 23.735, CS 25.735). Also, the aircraft must stop on considerably shorter distance than normally due to the distance already travelled during takeoff. This sets the strain on the brakes on a much higher level than that in usual operations. According to the regulations, aircraft brakes have to be able to dissipate the energy of the RTO even if they are destroyed during process. Usually, brake linings and other structural parts of the brake melt together into one lump, although there are some companies claiming that their brakes can be operational after RTO. Whether all brakes after RTO are considered as destroyed or not, they must be replaced before resuming operation of the aircraft, as well as the structural integrity of the landing gear and mounting nodes must be checked and confirmed to be within allowable limits.

The RTO test is done on the runway using the brakes with 10% left wear (CS-25 AMC 25.101(i)) in order to replicate most unfavorable circumstances during tests. To make the test as close to the reality as possible, it is advisable to hold firefighting for a few minutes, replicating the real airport fire brigade response time. This approach results with ignition (Fig. 10.) of the brakes due to the extremely high temperatures during RTO.

Figure 10.

Figure 11.