Long Descent with Falling Air Pressure
You are driving a 40-tonne tractor-semitrailer down a continuous 8 km descent with an average gradient of 6%. The speed limit is 60 km/h and you have been holding the service brake on lightly to maintain speed. After 3 km the air pressure gauge has fallen from 120 psi to 75 psi. The low-air warning is set at 60 psi. A retarder and an engine brake are available and working. The trailer brakes were adjusted at the start of the shift. You notice a hot-brake smell and the steering feels normal. Traffic is light. You must decide what is happening, how to complete the descent, and what checks to make after stopping.
1. What is happening and what immediate action reduces risk?
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Continuous service-brake use on a 6% grade is overheating the brakes and causing fade, while repeated applications are consuming air faster than the compressor can replace it. The pressure has dropped from 120 to 75 psi; the 60 psi warning is the last clear alert before spring brakes may apply. Release the service brake, select a lower gear and bring in the retarder and engine brake progressively. Reduce speed before the next steeper section and watch the gauge. Do not fan or pump the brakes. If pressure continues to fall or the warning sounds, make a controlled stop in a safe place rather than waiting for automatic intervention.
2. How should you descend the remaining grade?
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Use a low gear that holds a safe speed without continual service braking. Let the retarder and engine brake do most of the work, and use short, firm service-brake applications only to trim speed, then release. Keep the gauge above the warning threshold and check it frequently. If the brakes fade, the pressure keeps dropping or the smell worsens, stop at the first safe place; use an escape ramp if one is available and stopping distance is increasing. Do not coast in neutral, and do not try to save time by letting the speed build up. The descent is complete only when the vehicle is safely stopped or back on level ground.
3. What checks and actions should you take after stopping?
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Park on level ground, secure the vehicle and let the brakes cool. Do not touch hot drums or discs, and keep clear of any smoke or glowing metal. Check for air leaks, compressor build-up and brake adjustment, and record the low-pressure event and hot-brake smell. If pressure will not build correctly, if a leak is found or if the brakes have overheated badly, do not continue; report the defect and arrange inspection. A short cooling stop is not a repair. The vehicle should not re-enter the descent until the air system and brakes are confirmed serviceable.