Take Care of Your Brakes

Shortly after landing, the flight crew of an A330 aircraft heard a strong and unusual noise during taxi-in. When the aircraft reached the parking stand, ground crew observed smoke coming from the area of the left Main Landing Gear (MLG) and informed the flight crew. The fire brigade arrived but did not see any fire. The flight crew noticed a 400 °C temperature on wheel no. 6. Maintenance personnel performed a quick inspection of the landing gear, which revealed that one of the brake pistons of wheel no. 6 had twisted and dislodged from its housing with evidence of a hydraulic fluid leak (fig.1). There was no sign of fire on the landing gear structure and components.

The investigation showed the most probable cause was a rupture of the brake pressure plate during brake application. The brake piston pushed through the pressure plate and came into contact with the first rotor (fig.2). This applied a lateral force to the piston causing it to be dislodged from its housing and causing the hydraulic fluid leak. The hydraulic fluid that leaked onto the hot parts of the brake created the smoke. The pressure plate was found to be significantly oxidized, which was the reason it ruptured when the brake piston pressed against it.

Brakes are subject to two different phenomena: Wear and oxidation.

Brake wear is the progressive loss of width on the brake disks due to friction. Brake wear on carbon brakes depends on the number of brake applications and on the brake temperature. Each carbon brake type has its own temperature range for optimum operation and its temperature range for maximum wear. The temperature range varies from one brake manufacturer to another.

Guaranteed braking efficiency until the wear limit

Brakes are guaranteed to provide sufficient braking until the brake wear indicator is flushed with the reference surface. If the indicator is below the reference surface, the brake disks are too worn, and the braking performance can be significantly reduced. If the brake disks are too worn, their width is reduced. As a result, the pistons do not have enough extension to push the disks and create sufficient braking friction to slow down the aircraft (fig.3). In addition, if the brake is too worn, the amount of heat sink mass that is available to absorb braking energy is reduced. In the event of a high speed RTO this can lead to increased risk of runway overrun or brake fire.

Carbon from the brakes naturally combines with oxygen from the ambient air to become carbon dioxide (CO2). Under normal circumstances the oxidation occurs at a very slow rate. However the rate of oxidation can be accelerated by external factors such as high temperature and catalytic (chemical) pollution (table 1). This results in a loss of carbon mass from the brake disks, carbon softening, and delamination. It can ultimately lead to brake rupture if an affected brake is not changed in due time (fig.4). Carbon oxidation due to exposure to high temperatures is referred to as thermal oxidation. When carbon oxidation is due to the presence of catalysts, it is usually referred to as catalytic oxidation.

Thermal oxidation

Thermal oxidation is the main cause of accelerated degradation of carbon brakes. It can occur if high brake temperatures are reached after landing and during taxi. Thermal oxidation affects all brake disks, but the middle disks are most affected because they reach a higher temperature and take longer to cool down. Worn brakes tend to reach higher temperatures making them more prone to the effects of thermal oxidation.

Catalytic oxidation

Catalytic oxidation of the brakes is generally caused by contact with deicing or cleaning fluids. The potassium or sodium coming from some aircraft and runway deicing fluids acts as a catalyst and further accelerates the oxidation (table 1). The presence of the catalyst also reduces the temperature at which significant oxidation occurs. When the potassium or sodium is absorbed by the carbon it remains within the material causing catalytic oxidation to continue well after the end of the winter season. The outer disks, including the pressure plate, are most exposed to external pollution and are usually more susceptible to catalytic oxidation.

In addition to high maintenance costs, brake oxidation can lead to brake rupture and a loss of braking for the affected wheel. If maximum braking is necessary, such as in the case of a rejected takeoff at or close to the maximum takeoff weight, it may result in a runway overrun.

Brake rupture can also damage brake pistons and lead to leakage of hydraulic fluid. The fluid may vaporize and create smoke if it comes into contact with hot components. This could result in fire. The hydraulic fuses will limit the amount of hydraulic fluid lost and the fire should remain contained to the brake, but damage may be caused to nearby components. Maintenance personnel and flight crews both have a role to play to prevent brake rupture.

There are a number of ways to identify worn brakes and prevent brake rupture including visual checks, inspection, and taking precautions when using deicing or cleaning fluids.

The Maintenance Planning Document (MPD) requires a regular visual inspection of the brake wear indicator to assess the level of thickness loss of the brake disks (table 2). The check must be done with the braking applied (parking brake ON or pedal pressed or BITE activated). If the brake wear indicator is flushed with the reference surface, the brake unit must be changed.

Visually inspect the brake assembly at every wheel removal, in accordance with the corresponding AMM/MP procedure, to check that there is no damage or crack on the disks and to check the condition of the brake components. Pay particular attention to any signs of oxidation marks, and if the oxidation is beyond acceptable limits, replace the brake (fig.6).

(fig.6) Examples of non-oxidized and oxidized brakes

Report any brake damage/rupture

Airbus encourages operators to report any brake damage or rupture through the Tech Request tool using the Brake Disk Failure Reporting Sheet available in the AMM/MP procedure for brake inspection.

When cleaning the aircraft or performing deicing, particular care should be taken to prevent fluids coming into contact with the wheels and brakes. Always follow the AMM/MP/AMP procedures for cleaning and deicing and protect wheels and brakes to prevent them from becoming contaminated with chemicals that will accelerate oxidation.

The flight crew can detect worn brakes before the flight during the exterior walkaround. They can reduce wear and oxidation by using the brakes in an optimal manner during taxi and landing.

A quick check of the brake wear indicator (fig.9) during the exterior walkaround will determine if the brakes are worn. If there are only a few millimeters remaining before the indicator is flush with the reference plate, inform maintenance personnel to anticipate and plan for a brake replacement before the wear limit is reached. On A220 aircraft, the flight crew can also check the brake wear status on the EICAS STATUS synoptic page.

Flight crews should reduce the number of brake applications during taxi to limit brake wear. The FCTM and A220 FCOM recommend that on long, straight taxiways, and with no ATC or other ground traffic constraints, the PF should allow the aircraft to accelerate to 30 kt of ground speed, and then use one smooth brake application to decelerate to 10 kt.

Keep thrust at idle

Maintaining idle thrust during taxi enables a reduced number of brake applications to keep the aircraft below the 30 kt maximum taxi speed.

Single engine taxi

Single engine taxi is a fuel saving initiative that also reduces brake wear, because it further reduces the idle thrust during taxi.

The number of thermal oxidation reports is increasing, especially on the A320 family fleet. This phenomenon may be linked with efforts by many operators to save fuel. It was observed that a majority of operators reporting high thermal oxidation were using CONF 3 and thrust reversers on IDLE at landing. There is a trade-off between fuel savings, engine maintenance costs, and increased brake replacement due to higher rates of oxidation. This will depend on the flight conditions, aircraft condition, and the operator’s policy.

Use of Flaps FULL (or FLAP 5 on A220)

The use of flaps FULL (FLAP 5 on A220) at landing reduces the approach speed, and therefore, the aircraft energy to be absorbed by the brakes.

Use of autobrake or Brake-to-vacate (BTV) at landing

Use of autobrake or BTV (if installed) enables a single brake application with an optimized braking intensity. When autobrake is used and if conditions permit, the use of autobrake LOW reduces the heat of the brakes, and therefore, reduces the likelihood of oxidation.

Timely thrust reduction during flare

A timely thrust reduction during the landing flare prevents extra thrust provided by the autothrust trying to maintain Vapp after the flare. The flight crew should retard the thrust levers at 20 ft (A320/A330/A340/A350/A380) or 30 ft (A220/A300/A310) as per the SOP, and at the latest, at landing gear touchdown to enable spoiler extension.

Use of thrust reversers at landing

The use of thrust reversers reduces the energy to be absorbed by the brakes. It is therefore a good option to use thrust reversers to limit brake oxidation, especially on short runways.

Use most appropriate runway exit

Taking over the autobrake to use full or strong manual braking to quickly slow down the aircraft in order to reach a specific runway exit may save some taxi time. However, this will also significantly increase brake wear. Using the next exit may slightly increase taxi time, but will also reduce brake wear and temperature.

The use of brake cooling fans, when available, reduces the exposure time of the brake units to high temperature after landing. This reduces the effects of carbon thermal oxidation.