1、AEROSPACE INFORMATION REPORTAIR4762Issued 1994-02Reaffirmed 2010-01Compilation of Freezing Brake Experience and Suggested Designsand Operating Procedures Required to Prevent Its Occurrence1. SCOPE:This AIR describes conditions under which freezing brakes can occur and offers suggested design feature
2、s to minimize occurrence. It also suggests operating procedures which have been shown to prevent or lessen the severity of brake freezing.2. REFERENCES:There are no referenced publications specified herein.3. PROBLEM:Encountering frozen brakes is not an uncommon winter weather problem for some aircr
3、aft operators. During cold weather operation when snow, slush or wet runway departure conditions are present it is possible for brake freeze-up to be experience. Freezing can occur with minimal water present. It can result in tire skid damage on landing at subsequent destinations. The freezing occur
4、s because runway roll operations produce moisture contamination of the brake stack.Freezing can occur on the ramp, during taxi operation and prior to landing. The first two situations result in an inability to move the aircraft, while the latter results in a much more serious condition which can cau
5、se a locked wheel landing with attendant damage, tire blow outs and possible control problems.The normal or average shear strength of ice which forms between the rubbed surfaces of the brake is 30 psi. This strength will vary depending upon the thickness of the ice and the level of brake lining wear
6、 debris contamination of the surface. Very thin ice thickness results in even higher shear strengths. Wear debris can result in three times this normal strength level. The ice bond is strong enough to produce drag levels capable of skidding tires on dry runways.SAE Technical Standards Board Rules pr
7、ovide that: “This report is published by SAE to advance the state of technical and engineering sciences. The use of this report is entirely voluntary, and its applicability and suitability for any particular use, including any patent infringement arising therefrom, is the sole responsibility of the
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10、esae.org SAE WEB ADDRESS: http:/www.sae.org SAE values your input. To provide feedbackon this Technical Report, please visit http:/www.sae.org/technical/standards/AIR4762Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without li
11、cense from IHS-,-,-SAE AIR4762- 2 -3.1 Contributing Factors:Factors which influence the potential for freezing brakes include weather, runway contamination (snow, slush and water), brake rubbed area, number of disk pair, brake stack location outside versus inside the wheel (wheel flange driven rotor
12、s versus wheel well driven), multiple wheel landing gear, aircraft design (specifically wheel well) and aircraft operating procedures.It is relatively independent of brake heat sink material since freezing occurs with both carbon and steel heat sink brakes. Although carbon brakes absorb moisture (ma
13、ximum of 4% by weight), their susceptibility to freezing is no greater than steel brakes.The problem is most prevalent on aircraft which have unheated or open wheel wells where the landing gear doors seal around the tire OD (wheel and tire exposed to the slip stream). It is exacerbated by low aircra
14、ft sink rates where wheel load is gradually applied during landing, making tire skids more likely.4. OPERATIONAL PROCEDURES FOR PREVENTING BRAKE FREEZE-UP:There are two basic types of corrective operational procedures. One depends on maximizing weight on wheels at touchdown to weaken the ice bond. T
15、he other relies on elevating brake stack temperatures during taxi operations to prevent ice formation.The greater mass and running clearance of the steel brake, as compared with the carbon brake, is an advantage when attempting to shear the disk ice bond by firm ground contact. The carbon brake is c
16、onsiderably more tolerant of heat than the steel brake, making prevention of ice bond formation the more practical approach for carbon brakes. The ideal temperature would evaporate all brake moisture accumulated at departure.It should be noted that brake heating by any method without monitoring the
17、brake temperature may increase the risk of blown wheel fuse plugs. Therefore, it is recommended that aircraft, which require heating of the brakes to prevent freeze up, be equipped with brake thermal monitoring systems (BTMS).4.1 Recommendations for Operating in Freezing Conditions:4.1.1 Parking: Wh
18、en icing or excessive moisture with freezing weather conditions exist, parked aircraft should have their brakes in “OFF condition“ (not pressurized) with wheels chocked.When washing the aircraft, be careful not to saturate the wheel or brake. Carbon brakes will achieve 98% of their water absorption
19、capacity within 15 min (over 50% within 3 min). Any water between the disks of both carbon and steel brakes can result in brake drag if it freezes.Application of hot air sources may be used on both steel and carbon brakes.Copyright SAE International Provided by IHS under license with SAENot for Resa
20、leNo reproduction or networking permitted without license from IHS-,-,-SAE AIR4762- 3 -4.1.1 (Continued):Suitable chemical anti-icing or defrosting agents (such as ethylene glycol) which do not have a detrimental effect on brake operation may be applied to STEEL BRAKES ONLY. Taxi stops made after ap
21、plication of chemical anti-icing agents will degrade their effectiveness in preventing freeze up. NEVER use chemicals on carbon brakes since this can have an adverse effect on vibration, oxidation and wear of the carbon.If the brakes are frozen, apply maximum brake pressure several times statically.
22、 Applying sufficient and repeated brake pressure will cause the brake frame to deflect and may be helpful in breaking up the ice bond. This procedure can be ineffective for some brakes and depends on brake frame stiffness and number of disks in the heat sink.4.1.2 Taxi Operation: When the aircraft i
23、s stopped on the taxiway or runway and brake freeze-up occurs, pressurize the brake several times using maximum pressure. To reduce the possibility of brake freeze-up during taxi operations in severe weather conditions, use firm brake pressure permitting the brake stack to reach a warm condition, ev
24、aporating any moisture buildup within the disk stack.4.1.3 At Departure: At departure, unless weather conditions prohibit, delay retraction of the landing gear until excess water, snow or slush is thrown off by wheel rotation and/or slip stream force. If appropriate, consider cycling the gear severa
25、l times to dislodge slush or snow.4.1.4 Prior to Landing: Prior to landing and with gear up or down, maximum brake pressure should be applied several times to alleviate any brake freeze-up caused by icing due to flying conditions.Again, this can be ineffective depending on the brake design. Also, so
26、me antiskid and brake control systems may preclude cycling the brake with the gear up.Ensure antiskid is “on“ for the landing.The use of landing lights to warm the wheel well has been effective for some aircraft.4.1.5 At Touch Down: When landing, make a positive contact with the runway and achieve w
27、heel load as quickly as possible. Avoid soft or flared landings. Use brakes during landing rollout.During dynamometer testing it has been shown that dynamic impact, representing a hard landing, reduces the strength of the ice bond. During subsequent brake application, this results in less torque req
28、uired to break the disks free. This break out torque can be reduced by a factor greater than two due to the hard landing.Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SAE AIR4762- 4 -4.2 Procedures
29、 for Heating Brakes:Carbon brakes should be exercised during ground roll at arrival and departure to assure elevated disk temperature (200 to 260 C) at lift-off on departure. This temperature can be reached by one landing stop or taxi snubs or a combination of both. Use firm brake pressure for landi
30、ng stop and taxi snubs to assure near constant temperature on all disks. Note: Disk temperature can be estimated on the ramp by pyrometer measurement at the pressure plate. The preferred approach is to equip the aircraft with a BTMS to monitor temperatures. Steel brakes should be exercised during gr
31、ound roll to assure elevated temperature (93 to 149 C).This can be achieved by making taxi snubs during ground roll at arrival and departure. Use firm brake pressure for taxi snubs to assure near constant temperatures on all disks.5. DESIGN OPTIONS WHICH PREVENT FREEZING BRAKES:5.1 Slush Shields:Wat
32、er or slush shields and spray deflectors can be very effective in eliminating or greatly reducing the incidence of frozen brakes. These features can be an integral part of the brake assembly and are very effective in keeping contamination from reaching the brake stack/wheel well area of the landing
33、gear.These shields are not recommended for summer operation since they do cause heat to build up in the wheel/fuse plug area during repetitive braking applications.5.2 Heaters:Original work in this area was done with resistance (electrical wire) heating coils located on the inside diameter of the br
34、ake torque tube. Design complexity and low reliability make this unattractive as a solution to the freezing problem. Since these devices were rather inefficient, preheating of the brake stack during taxi operation (121 C) was required with a 100 watt heater to prevent freezing of the 9 kg carbon hea
35、t sink in flight. With more power applied, 400 watts, good results are obtained without the need for heating the brakes during taxi.More efficient heaters using etched foil heating elements are now being investigated for this application. They are attached directly to the brake pressure plate. Heat
36、is transferred from the pressure plate to the other brake disks to maintain the stack temperature above freezing in flight.They have 1000 watt capacity and require 400 watts input to raise a 9 kg carbon heat sink from -57 to 54 C in still air at -57 C ambient temperature. A proportional heater contr
37、ol would maintain a desired temperature range by monitoring the pressure plate temperature.Bleed air manifolds are used as heaters on some aircraft and are very effective in preventing/thawingfrozen brakes. A typical design is a sheet metal manifold attached to the brake by the brake tie bolts.Copyr
38、ight SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SAE AIR4762- 5 -5.2 (Continued):Hot air injection has been applied thru BTMS ports and was found to require 0.9 kg/min per brake at 66 C without an enclosed
39、 wheel well and 0.4 kg/min at 66 C with an enclosed wheel well to prevent the freezing of a 9 kg carbon heat sink in flight at 12 200 m and -57 kg C ambient temperature.Although heaters provide protection against freezing with the gear up, this protection quickly erodes when the gear is lowered into
40、 the airstream. A 9 kg carbon brake heated to 30 C drops to below freezing in about 3 min with the gear lowered. However, the ice bond which forms with the brake discs vertical is not as strong as with the brake discs horizontal and compressed (gear up). Testing has shown this difference to exceed a
41、 factor of 3 with vertical requiring less than one third the break away torque.5.3 Aircraft Design:Although shields, heaters and operating procedures are very effective for preventing freezing brakes,the best solution is a heated and enclosed wheel well. Simply locating the wheel wells close to the
42、aircraft engines, such as with many military aircraft, will maintain a nonfreezing environment for the brakes.PREPARED BY SAE SUBCOMMITTEE A-5A,WHEELS, BRAKES AND SKID CONTROLS OFCOMMITTEE A-5, AEROSPACE LANDING GEAR SYSTEMSCopyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
