How to properly operate the carbon brakes?

How to properly operate the carbon brakes?
How to properly operate the carbon brakes?
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A/C Type:
A300
A300-600
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First Issue Date: 08-AUG-2014
00.00.00099
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03-NOV-2015
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Documentation:
03-NOV-2015
Flight Operations and Training
First Issue Date:
08-AUG-2014
Last Publication Date:
03-NOV-2015
Last Internal Publication
Date:
03-NOV-2015
1. INTRODUCTION
Operators often ask how to increase the life of carbon brakes, because the brake life obtained
may be less than expected. Two different factors affect the life of carbon brakes:
- The wear of the discs (refer to Chapter 2),
- The oxidation of the discs (refer to Chapter 3).
This article also develops best practices for operating the carbon brakes (refer to chapter 4).
2. FACTORS THAT AFFECT BRAKE WEAR
Carbon wear is a very complex process. There are also different types of carbon, and each
type wears in a different manner.
The two main factors that affect the wear of carbon brakes are
- The number of brake applications,
- The temperature of the carbon discs at which the brakes are operated.
2.1
Number of Brake Applications
The number of brake applications, even at low speeds, is the primary factor that affects
brake life. Aircraft that fly to/from congested airports, that favor multiple brake applications
during taxi, usually have a 20 % to 30 % reduced brake life. Approximately 75 % of brake
wear occurs during taxi operations.
2.2
Carbon Disc Temperature
All brake manufacturers highlight the fact that brake temperature is a significant factor in
carbon wear.
The diagram below illustrates the typical spectrums of the rate of carbon wear at different
indicated temperatures, as provided by three brake manufacturers (Messier-Bugatti,
Honeywell-ALS, and UTAS – previously BF Goodrich). The relationship between the disc
temperature and the temperature indicated in the cockpit is usually not linear, and is also
different for different brake manufacturers.
Figure 1 – Typical Spectrums of Brake Wear vs. Brake Temperature
Note: The wear spectrum from UTAS displays the combined results from dynamometer
simulation (above 80-100 °C) and theory (below 80-100 °C). Wear rates below 100 °C are
reduced, due to lubrication by the atmospheric moisture that adheres to the carbon particles.
Above 100 °C, wear rates reduce as the temperature increases.
The energy absorbed by the brakes (e.g. at landing) does not significantly affect the wear of
the carbon brakes (unlike it is the case on the steel brakes).
However, the application of more energy on the brake has an effect on carbon brake wear, as
it causes an increase in brake temperature.
2.3 Other Factors
Some other, less important factors that may affect carbon brake life:
2.3.1 Maintenance Procedures
In order to maximize brake life, apply all life-extension procedures recommended by the brake
manufacturer.
Airbus also recommends that Operators use brakes until the wear pin is flushed. However,
Operators must replace the brake as soon as the wear pin is flushed as the wear of the heat
pack is complete.
It is also important that Operators define their criteria for brake removal at the maintenance
center. (For example: The length of the remaining wear pin is 3 mm. If the Operator has an
average of 2 000 LPOs, and a 65 mm wear-pin length, the wear rate is 2000:65 = 30.7
landings per mm. Therefore, the remaining 3 mm wear pin should enable the aircraft to
complete 92 landings before its next visit to the maintenance center).
2.3.2 Area of Operation and Weather Conditions
Some brake manufacturers (e.g. Messier-Bugatti) report that the rate of carbon brake wear
decreases as humidity increases. Hot and humid weather results in longer carbon brake life.
3. FACTORS THAT AFFECT BRAKE OXIDATION
Brakes may degrade rapidly due to carbon oxidation. Oxidation may also cause the rupture of
a disc.
Two types of oxidation may affect carbon:
- Catalytic Oxidation: Mainly due to runway and aircraft de/anti-icing fluids and other
cleaning agents
- Thermal Oxidation: That is accelerated at high temperatures. Therefore, if the brakes
absorb too much heat, carbon oxidation increases.
However, there is no difference between the effects of Thermal or Catalytic oxidation. The
potassium or sodium present in de/anti-icing fluids acting as a catalyst further accelerates the
oxidation. It should be understood that once the potassium or sodium is absorbed by the
carbon it does not simply go away. Catalytic oxidation can continue after the winter has
finished based on the damage caused by absorbed alkalis.
The table below indicates the time required at different temperatures, to lose 5 % of the
carbon mass (or 25 % of the mechanical strength).
Time to reach 5% weight loss = 25% loss of strength
Temperature
Thermal oxidation
Catalytic oxidation
25°C
7.5 1018 years
3.6 1018 years
400°C
3 years
33 days
500°C
14 days
15 hours
600°C
12 hours
45 minutes
700°C
49 minutes
4 minutes
Figure 2 – Carbon oxidation vs. brake temperature
4. PREFERRED CARBON BRAKING TECHNIQUES
All recommended braking techniques should mainly aim at reducing the number of brake
applications and at optimizing the carbon brake temperature.
4.1
To Reduce the Number of Brake Applications
The main factors that affect the number of brake applications are:
 Airport layout and traffic volume: Long runways and taxiways, number of turns,
congestion
 Taxi speed and engine-idle settings
 Aircraft weight
 Use of the autobrake at landing.
In order to reduce the number of brake applications, consider the following techniques:
4.1.1 Do Not "Ride" the Brakes
The Airbus FCOM SOP emphasises that for the taxi phase:
 The normal, maximum taxi speed should be 30 knots for a straight line and 10 knots for a
sharp turn.
As it is difficult to estimate the ground speed, monitor it via the Navigation Display. Do not
"ride" the brakes. If the aircraft exceeds 30 knots with idle thrust, apply the brakes
smoothly and decelerate to 10 knots, then release the brakes to enable the aircraft to
accelerate again.
Figure 3 – Riding the brakes during taxi
4.1.2 Single-Engine Taxi (Two-Engine Taxi on A340 and A380 aircraft)
The single-engine taxi procedure (or the two-engine taxi procedure on A340 and A380
aircraft) saves fuel in congested airports. It also decreases the total engine thrust when the
aircraft accelerates at idle (e.g. at low aircraft weights), and therefore prevents excessive use
of the brakes during taxi.
However, this procedure is not recommended for short taxi times, in order to comply with the
required engine-warming or engine-cooling times.
4.1.3 Use of Autobrake/BTV at Landing
The purpose of the autobrake system is to maintain a constant deceleration rate during
landing roll, or to apply maximum braking as soon as the throttles are reduced during a
rejected takeoff.
In order to achieve this, the autobrake system modulates the brake pressure within a single
brake application. Therefore, use of the autobrake reduces the number of brake
applications to one and reduces brake wear. Use of the autobrake system also optimizes
the brake temperature (easy and accurate management in daily cycles), as described in next
paragraph.
Use of the Brake To Vacate (BTV) function, if available, reduces brake wear in the same way
as the autobrake system and also decreases the energy absorbed by the brakes as the brake
pressure will be applied at the end of the landing roll taking full advantage of the other means
of braking (reversers, rolling, or aerodynamic drag).
4.2
To Optimize Brake Temperature
As demonstrated in Figure 1, the typical spectrum of carbon wear, versus carbon-disc
temperature, illustrates how low-wear and high-wear vary with the disc temperatures.
Therefore, operational recommendations to increase carbon brake life should aim to keep the
carbon temperature outside the high-wear range.
In order to increase carbon-brake life, the brakes should be operated in general when either
cold or hot, but not at intermediate, warm temperatures.
Pilots should receive simple instructions on brake operation, based on the average optimum
temperature ranges for low carbon wear and in accordance with operational constraints.
For example, based on data provided in Figure 1:
-
For Messier-Bugatti brakes the pilot should receive instructions to taxi with an
indicated brake temperature below 80 °C, or above 250 °C.
- For Honeywell-ALS brakes, the indicated temperature should be below 150 °C, or
above 315 °C.
- For UTAS brakes, the indicated temperature should be significantly below 80 °C, or
above 250 °C.
These instructions must correspond to the specific brake type of the aircraft, and to the
specific operating conditions.
This is because, as demonstrated in Figure 1, the relationship between carbon wear and
approximate brake temperature varies significantly between different brake types.
4.2.1 Brake Temperature Management During Taxi
Based on Figure 1, the brake temperature at taxi should be on one side or the other of the
peak of the appropriate wear curve.
Figure 4, below, typically illustrates changes in brake temperature on a series of short legs, in
specific conditions (high landing energy, no brake fans). The diagram displays both disc and
indicated brake temperatures.
 For most brake types, the taxi-in temperature is to the right of the wear-curve peak given in
Figure 1, due to heat build-up during landing. Therefore, it is not recommended to cool
the brakes after landing, as this would make it necessary to operate in the high-wear
range, in order to move to the left of the wear curve.
700
Taxi in
Temperature (°C)
600
Disk
temperature
500
landing
Indicated
temperature
Taxi out
400
300
200
parking
100
0
1st flight
0
50
2nd flight
100
150
3rd flight
200
250
300
350
400
Figure 4 - Example of brake temperatures achieved with specific route data
(Aircraft not equipped with brake cooling-fans)
 For taxi out, Figure 4 illustrates a scenario where the short turnaround time means that the
brakes are not sufficiently cooled to achieve a temperature to the left of the wear-curve
peak. Some brake manufacturers (e.g. UTAS and ABS) therefore recommend that the taxiout temperature remains to the right of the peak. Note that the wear curve of UTAS is to the
left of the Messier-Bugatti and Honeywell-ALS wear curves. This explains why:
- UTAS, based on the data provided in Figure 1, "believe that based on typical operating
scenarios, it is usually more practical to reduce wear rates by generally operating with
warm or hot taxi stops than to taxi out below 100 °C".
- ABS "suggest the optimum BTMU (indicated) temperature for taxi out is 150 °C".
Based on Figure 1, it is obvious that, for Messier-Bugatti and Honeywell-ALS, an indicated
temperature of approximately 200 °C (achieved during taxi out, as illustrated in Figure 4), is
not the optimum temperature for brake life. That is why both manufacturers recommend the
following low brake temperatures for taxi out:
- Messier-Bugatti: "below 80 °C".
- Honeywell-ALS: "below 150 °C".
However, it is not easy to manage brake temperature. The best solution is to use the brake
cooling fans, if available.
Note: Brake temperature may be further difficult to manage because, in normal operations,
there may be a significant difference between the temperatures of the different wheel
brakes. The brake temperature of some wheels may be on one side of the peak of the
wear curve, some may be on the other side, and some may be on the peak.
The FCOM and the TSM list the brake temperature limitations that require maintenance
actions.
For example:
- The temperature difference between the two brakes on the same gear is more than
150 °C, and the temperature of either one of the brakes is more than or equal to 600
°C, or
- The temperature difference between the two brakes on the same gear is more than
150 °C, and the temperature of one brake is less than or equal to 60 °C, or
- The difference between the average temperatures of the LH and RH brakes is more
than or equal to 200 °C.
4.2.2 Hot Brakes: Setting the Limits
While taxi with hot brakes may be advantageous in terms of carbon brake life, it is important
to remember the following:
 Taxi Out
For safety reasons, Airbus does not recommend any procedure that would systematically
and intentionally increase brake temperature before takeoff. The aircraft may not be able
to sustain the certified maximum brake energy in some cases of high-energy rejected
takeoff with hot brakes (e.g. with worn brakes, without the use of reversers...). Certification
does not include this type of situation judged not highly probable. However, if the aircraft
does not have brake fans, repeated short flight legs and short turnaround times (as
described in Figure 4), may result in a taxi out with hot brakes. This is acceptable only if
the BRAKE HOT warning does not come on before takeoff. In all cases, do not start a taxi
out with the BRAKE HOT warning on.
 Landing and Taxi In
In-service experience has demonstrated a rapid degradation of some brakes due to
carbon thermal oxidation because of repetitive high brake temperatures achieved at
landing. There are also reports of some disc ruptures due to oxidation.
 Parking
If brake temperatures remain high for a long time, the temperature of the critical
surrounding structures (e.g. the brake housing, wheel rim and axle) may reach levels that
are not acceptable. In order to avoid this problem, avoid parking brake application at high
brake temperatures.
4.3
General Airbus Recommendations
It is difficult to provide accurate and detailed recommendations on the operation of carbon
brakes, that take into account both safety and economy and are applicable to all customers
and all brake types.
All brake manufacturers also highlight the importance of customized brake-saving
recommendations for each Operator. The environment, the conditions, and the fact that the
aircraft may or may not have brake fans, significantly affect how the flight crew should operate
the brakes.
The following are general Airbus recommendations on the correct operation of carbon
brakes. These recommendations list all the systems involved in brake operation:
4.3.1 Autobrake/BTV at Landing
 The selection of autobrake (or BTV if available) is at the pilot’s discretion. However, the
use of autobrake has many advantages, for example reduced pilot actions, and smoother
and optimized deceleration. The pilot may use the autobrake to reduce the number of
brake applications to one, and to increase the brake temperature to a more acceptable
level for taxi in. The autobrake may also provide constant brake-function conditions for
landing and taxi in.
The selection of autobrake is particularly recommended when a significant brake
application is foreseen. It is recommended to select the mode that corresponds to the
available distance to the targeted runway exit.
If manual braking is used, modulate the brake pressure (avoid repeated applications).
In order to prevent thermal oxidation of the carbon, avoid landing techniques that result in
repeated, excessive brake temperatures (400-450 °C). For example, when MED or any
other high deceleration rate mode is selected, perform the landing in FULL configuration,
with the use of Reverse MAX, in order to minimize the amount of energy absorbed by the
brakes.
 Use of the autobrake is recommended when landing on short or evenly-contaminated
runways and for operations in low visibility. (Use of the autobrake on runways with both
dry and contaminated areas may result in directional deviations.)
4.3.2 Engine Thrust Reversers
For safety reasons, always select (at least idle) reverse thrust, immediately after the main
landing gear touches down. Airbus recommends the selection of full reverse, particularly
on short runways and on contaminated runways. Select full reverse thrust at the beginning of
the landing roll to maximize its effect.
If airport regulations restrict the use of reversers, or to save fuel on long runways, select and
maintain idle reverse until taxi speed is reached.
After the initiation of reverse thrust, make a full stop landing (no touch-and-go).
4.3.3 Brake Cooling Fans (if installed)
 General Remarks
- The brake cooling fans are an option to increase heat dissipation and brake cooling. They
reduce the cooling time, and therefore prevent takeoff delays on short turnarounds.
In fact, Airbus designs its aircraft so that the brake fans are necessary for short
turnarounds. This avoids the extra weight of oversized brakes. Brake cooling fans also
help to control brake temperature and therefore increase brake life bringing the brake
temperature in the areas where the wear is minimum.
- For many aircraft/brake types, the brake temperature indicated in the cockpit when the
brake fans are running is significantly lower than the temperature indicated when the brake
fans are OFF. This is because the brake fans cool the brake temperature sensor located
outside the heat pack. As soon as the brake fans are running, the indicated brake
temperature decreases almost immediately. Similarly, when the brake fans are switched
off, it takes several minutes for the indicated temperature to increase to the real brake
temperature.
As explained in the relevant FCOM, with the fans running, the difference between the
indicated brake temperature and the real brake temperature varies from approximately 50
°C (at a real brake temperature of 100 °C) to approximately 150 °C (at a real brake
temperature of 300 °C). Therefore, if the BRAKE HOT warning is activated just before
takeoff, the ECAM will request the flight crew to select the brake fans, if installed, and to
delay the takeoff. When the fans are selected, the warning disappears immediately, but
takeoff must be delayed until the indicated temperature is consistently below 150 °C (for a
warning set at 300 °C). Then, the brake fans must be switched OFF for takeoff. Refer to
the FCOM for more details.
- None of the indicated brake temperatures (listed in the above Figures 1 to 4) take into
account the effect of brake cooling fans. One must therefore consider the effect of the
brake fans on the indicated temperature, in order to achieve a specific recommended
brake temperature for taxi.
- Do not use the brake fans during takeoff and landing roll, in order to avoid damage by
objects to the fans or the brakes at high speeds.
- If not already set OFF, the brake fans automatically switch OFF when the landing gear is
retracted.
- If the BRAKE HOT warning appears after landing gear retraction, the landing gear should
be extended (if performance permits) to enable in-flight cooling, as requested by the
ECAM and published procedures. This situation should not occur, provided that the FCOM
published Standard Operating Procedures are applied. Do not select the brake fans in
flight, as their cooling efficiency is low.
 Taxi Out
If an arc appears on the ECAM WHEEL page, above the brake temperature (with a brake
temperature > 100 °C), select brake fans ON. This will ensure:
- A maximum energy rejected takeoff will be sustained
- An acceptable brake temperature for taxi, in terms of brake wear.
 Before Takeoff
If the brake fans are running, check that the brake temperature is below 150 °C (except for
some brake types - see General Remarks above). If the brake temperature is above 150 °C,
delay takeoff until the temperature decreases to 150 °C with the fans running, then switch the
fans OFF.
 Taxi In
Airbus recommendations are based on the advantages (described by all brake
manufacturers) of a taxi with hot brakes, but also take into account the possibility of oxidation.
Delay selection of the brake fans for at least 5 minutes, or until arrival at the gate (whichever
occurs first), to enable thermal stabilization, and avoid the oxidation of hot spots on the brake
surfaces. (Select the brake fans just before arrival at the gate, in order to avoid blowing
carbon dust on the ground crew.)
However, when turnaround times are short, or brake temperatures will probably exceed 500
°C, use the brake fans (disregard oxidation) to avoid a possible melting of the wheel fuse
plugs.
For taxi in, disregard the arc on the ECAM WHEEL page. Also, the display of the BRAKE
HOT warning and/or light on the ECAM (on some aircraft) should not result in the immediate
selection of the brake fans; provided that the brakes will probably not reach 500 °C (wait 5
minutes or until arrival at the gate).
 Parking
Select fans OFF if no arc appears on the ECAM WHEEL page (temperature < 100 °C). If the
turnaround is short, keep the fans running until the brake temperature is almost the same as
the ambient temperature.
4.3.4 Parking Brake
Release the parking brake at the gate, when the chocks are in position and when brake
temperatures are above 300 °C with the fans OFF (150 °C with the fans ON, if applicable).
Unless operationally necessary, avoid use of the parking brake at temperatures above 500 °C
with the fans OFF (350 °C with the fans ON, if applicable) in order to prevent damage to the
brakes.
5. CONCLUSION
Airbus liaises constantly with brake manufacturers, in order to provide the best
recommendations on the correct operation of carbon brakes. All parties agree that, in terms of
brake life, it is important to customize the recommendations, to take into account the specifics
of each Operator. Remember also that not all brake manufacturer recommendations are
exactly the same, due to the different types of carbon used. However it is vital that any airline
policy does not deviate from Airbus published procedures and limitations.
The information below is for INTERNAL USE ONLY. Do not
distribute outside Airbus.
INTERNAL INFORMATION
Applicability:
References:
General Overview
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Question
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Answer
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Additional Note
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© Airbus SAS, 2015. All rights reserved. Confidential and proprietary document.The technical
information provided in this article is for convenience and information purposes only. It shall in
no case replace the official Airbus technical or Flight Operations data which shall remain the sole
basis for aircraft maintenance and operation. These recommendations and information do not
constitute a contractual commitment, warranty or guarantee whatsoever. They shall not
supersede the Airbus technical nor operational documentation; should any deviation appear
between this article and the Airbus or airline's official manuals, the operator must ensure to
cover it with the appropriate document such as TA, FCD, RAS. Access to WISE is supplied by
Airbus under the express condition that Airbus hereby disclaims any and all liability for the use
made by the user of the information contained in WISE. It shall be used for the user's own
purposes only and shall not be reproduced or disclosed to any third party without the prior
consent of Airbus.
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