Steep Descent with Fading Service Brakes
You descend a 9% mountain road in a loaded Category C truck, gross 16 t, in third gear. The descent is 6 km. After about 2.5 km you notice the brake pedal travelling further before biting, and a faint hot smell. Your speed has crept from 40 km/h to 55 km/h despite the same pedal pressure. There is a runaway gravel escape lane signposted 800 m ahead on the right. A car is 40 m behind you. The road curves left beyond the escape lane.
1. What is happening, and what must you do in the next few seconds?
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The symptoms are brake fade from heat: longer pedal travel, hot smell, and speed rising at constant pedal effort mean the linings and drums are beyond their working temperature and the friction coefficient is dropping. On a 9% grade a 16 t vehicle's brakes absorb enormous energy, and fade progresses quickly once it starts.
Immediate actions: shift to a lower gear now (second) to use engine braking; apply firm, sustained pedal pressure rather than pumping, since pumping adds heat cycles without shedding speed; keep the wheel straight; and treat the escape lane as the planned option, not the last resort. Do not switch off the engine, which would cost you power steering and possibly brake assistance.
2. The car behind is 40 m back and closing. How does that shape your choice between the escape lane and continuing?
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At 55 km/h you cover 15.3 m per second, so 40 m is about 2.6 s of gap. If you brake hard for the escape lane, the car may not stop and could hit your rear, pushing you past the gravel bed's entrance. If you continue and fade worsens, you enter the left curve with uncertain brakes and a following car.
The better judgement is to commit to the escape lane, but warn first: brake lights on early, hazard lights on, and a brief horn or indicator right before turning in. The gravel bed is designed to stop a heavy vehicle and is the lower-consequence outcome. Continuing on fading brakes into a curve risks a runaway that takes the car with you. Choose the controlled stop over the uncontrolled one.
3. After stopping in the gravel bed, what sequence gets you back on the road safely?
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First secure: parking brake, hazard lights, engine off if the smell is strong, and check for fire risk around the wheels and drums. Let the brakes cool naturally; do not pour water on hot drums, which can crack them.
Then inspect: lining thickness, drum condition, brake fluid level and any leaks, and tyre condition after the gravel. Do not simply restart and continue the descent. Use a much lower gear for the remaining 3.5 km, stop every 1-2 km to let heat dissipate, and verify pedal feel at low speed before rejoining. If pedal travel remains long or the smell returns, call for assistance rather than finishing the descent, because the second fade will come faster than the first.