2 December 2008
Locked carbon disc brake due to moisture absorption and freezing
Reference: SAE Aerospace Information Report AIR4762; Compilation of Freezing Brake Experience and Suggested Designs and Operating Procedures Required to Prevent Its Occurrence
The purpose of this Airworthiness Advisory is to inform Canadian operators and flight crews operating aeroplanes equipped with carbon disc brakes of the possibility of moisture absorption and subsequent freezing during flight, resulting in tire failure and damage to the aeroplane on landing due to a locked wheel brake.
Transport Canada Civil Aviation was advised of an occurrence involving an aeroplane equipped with carbon disc brakes that landed with a locked wheel brake. During the landing rollout on a dry runway, the locked brake caused the associated tire to rupture. As the rollout continued, the wheel became free to rotate, causing the tire to shed and flail large portions which caused significant damage. The tire rupture resulted in the loss of two of three hydraulic systems, damage to the flap structure, and damage to electrical wiring, which controlled multi-function wing spoilers.
Prior to departure, the aeroplane was exposed to a significant amount of rainfall and the carbon disc brakes were soaked by water. The aeroplane taxied with minimal use of braking and took-off under dry conditions about 12 hours after the rain had stopped. Automatic brake application occurred after landing gear retraction. Moisture on the contact faces of the brake froze as the aeroplane climbed to high altitude in sub-freezing conditions.
The braking materials in the stators and rotors of carbon disc brakes are porous which allows them to absorb or retain moisture. After extensive water soaking the brakes can be dried by a prolonged period of exposure to dry warm conditions or deliberate braking action during taxi to heat the brakes. If a wet brake is not heated sufficiently to evaporate moisture from the disk surfaces, there is a possibility that after in-flight cold soak or parking in freezing conditions, the brake surfaces may freeze together. In addition to exposure to environmental moisture, brakes may become soaked with water during washing if the correct procedures to protect the brakes are not followed.
In this occurrence the failed tire was of cross-ply construction. Tires of radial-ply construction do not possess the same failure mode and detached debris is likely to be significantly smaller and lighter. However, it is still possible that debris from a radial-ply construction tire failure may damage the aircraft.
- Flight crews and maintenance personnel are reminded that carbon disc brakes can absorb or retain moisture. If a wet brake is not heated sufficiently to evaporate moisture from the disk surfaces, there is a possibility after in-flight cold soak or parking in freezing conditions, the brake disk surfaces may freeze together. Should this occur, taxiing might produce a flat spot on the tire or the tire may burst on landing.
- Maintenance personnel are reminded to protect aircraft wheels and brakes from direct washing spray and inform the flight crew if the aircraft or landing gear has been washed recently.
- In accordance with the AFM and any other manufacturer’s documents, if carbon disc brakes have been exposed to moisture, flight crews are reminded to:
- During taxi, use light brake applications to warm the brakes before take off. If equipped, monitor brake temperatures during taxi.
- When landing, carry out a positive landing to ensure initial wheel spin- up and breakout of frozen brakes if frozen brakes are suspected.
- Avoid touch-and-go landings if frozen brakes are suspected.
- During the landing roll and subsequent taxi, use brakes to prevent progressive build-up of ice on the wheels and brakes. If equipped, monitor brake temperatures during taxi.
Following take-off or landing on wet, snow or slush covered runways and taxiways; tires should be inspected for flat spots prior to the next flight.
Caution – The freezing of Carbon disc brakes may occur prior to or following take-off even though conditions prior to take-off are not considered to be cold nor adverse weather operations (Adverse weather conditions include rain, snow or slush or operations on taxiways and runways covered with these contaminants).
For further information contact a Transport Canada Centre, or call Roman Marushko, Certification and Operational Standards 613-993-4692 or firstname.lastname@example.org
For Director, Aircraft Certification
A/Chief, Continuing Airworthiness
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