There were several reports of high brake temperature and spongy brakes in ATR 72 Aircraft.
Here I’m going to share my experience which I have observed during troubleshooting of high brake temperature in ATR aircraft. Not doing proper Brake Low Pressure Line Bleeding is one of the main reason behind it.
Reason for High Brake Temperature
During rectification, it was observed that sufficient importance is not given to Brake Low Pressure Bleeding, resulting in unequal brake metered pressure to the pair of brake. Unequal brake meter pressure may result in high brake temperatures.
Normal Braking System in ATR 72 Aircraft
The hydraulic oil contained in the forward reservoir is used, by master cylinders through rudder pedals, to generate a pressure which acts on the metering valve.
The metering valve meters the pressure supplied by the Green Hydraulic System according to the pedals rotation. The hydraulic pressure is sent to the brakes via anti skid module.
It receives the pressure metered by the pedals and performs a control to each brake, according to the electrical signals from anti skid control box.
The fuse on each brake line works with the low pressure relief valve, to stop any fluid leakage due to an incidental break of the exposed brake pipes.
Significance of Low Pressure Bleeding
Low Pressure Bleeding maintains strong fluid coupling between master cylinders and brake metering valves. The air in the system will create spongy pedal feeling and resulting into inefficient brake applications.
GOOD MAINTENANCE PRACTICES
- During the bleeding process, ensure that the fluid dispenser is appropriate for the type of fluid being used – Phosphate Ester Based.
- Ensure that bleeding container is clean and free of moisture.
- Ensure that the initial deliveries are checked for cleanliness and free of air bubbles.
- Ensure to remove minimum 2 quarts of hydraulic fluid from each bleed screw (master cylinder – brake metering port).
- Ensure that 20 – 30 minutes of settling time is given after completion of bleeding cycle 1 on each bleed screw port before you carry out bleeding of cycle 2 which will completely remove the air trapped in the system before you disconnect the fluid dispenser.