How to properly operate the carbon brakes? ATA: 00-00 FIN: Ref: A/C Type: A300 A300-600 A310 A318 A319 A320 A321 A330 A340 A340-500 A340-600 A350 A380 AST Topic: First Issue Date: 08-AUG-2014 00.00.00099 Part Number: Last Publication Date: Supplier: Last Internal 03-NOV-2015 Publication Date: Linked Articles: Linked 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 . Question . Answer . Additional Note . © 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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