Alpine descent with brake fade and rising temperature
On a wet August afternoon an 18,000 kg coach with 40 passengers descends a 6 km alpine pass on a 9% grade. The driver selected 4th gear and set the retarder to its middle stage; speed is 65 km/h. After 2 km he smells hot brake lining, the pedal travel has increased, and the brake-temperature warning shows 430°C. The next lay-by is 300 m ahead; traffic behind is close. The road has two hairpins before the lay-by. The driver has 22 years' experience and the schedule is already 15 minutes late. The service brakes are drum brakes; the ABS warning lamp is not on.
1. What is happening, and what should the driver do in the next few hundred metres?
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The symptoms point to brake fade from continuous service-brake use on a long downgrade: hot linings, a long pedal and a temperature warning. Speed is too high for the conditions and for the brake-system state. The driver should immediately reduce speed using engine braking and the retarder, select a lower gear before the hairpins, and avoid riding the service brake. He should make short, firm brake applications only if needed, keep steering and stability, and stop at the lay-by if it can be reached safely.
At the lay-by he should park on level ground, apply the parking brake, chock the wheels, switch on hazard lights and let the brakes cool. He must not pour water on hot drums or crawl underneath. He should check for fluid leaks, damaged lines and unusual wheel temperatures, and report the defect. Continuing at 65 km/h because the schedule is late is not acceptable: a faded brake system may not provide the next emergency stop.
2. If braking performance continues to fall before the lay-by, what sequence should he follow?
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He should treat it as an emergency developing in slow motion. First, use maximum retarder and progressive downshifting, keeping the engine running so retarder, steering assistance and brake assistance remain available. He should warn passengers, switch on hazard lights and use the horn if needed. He should look for an escape route: the lay-by, a wide shoulder, a run-off area or a safe uphill spur. If the coach cannot be stopped before a hairpin, a controlled contact with a barrier or bank at low angle may be less dangerous than a runaway, but that is a last resort.
He should not suddenly yank the parking brake at speed, because it may lock the rear wheels or be ineffective when hot. After stopping, he must keep passengers aboard or evacuate only to a safe place away from traffic, then call dispatch and emergency services. The coach must not continue until the brake system has been inspected and released.
3. How should this descent have been planned from the top, and what does the driver tell the passengers?
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Before the grade he should select a low gear, set the retarder according to the manufacturer's instructions, and choose a safe descent speed, typically well below 65 km/h on a 9% wet grade. The retarder and engine brake should do most of the work; service brakes are for corrections and for stopping. He should keep a large gap, avoid overheating by using intermittent firm applications, monitor the temperature gauge and stop before fade begins. He should also consider the load, wet road and hairpins, not just the schedule.
To the passengers he should give a calm, factual announcement: the coach needs a short safety stop because the brakes are hot, there is no immediate danger, and they should remain seated until told otherwise. He should not blame the vehicle or operator, and he should inform dispatch of the delay and the reason.