Mountain descent with fading brakes and a heavy load
You drive a loaded category C lorry, gross combination weight 24 t, down a long mountain pass in Bulgaria. The descent is 9 km with gradients up to 8%, air temperature 30 °C. The vehicle has a retarder and an engine brake, but the retarder was switched off by the previous driver and you have not used it before. Your service brakes are air-operated with a pressure gauge showing 7.5 bar. After 4 km of continuous braking in a low gear you notice the pedal feels spongy and the lorry needs more pedal travel to slow down; a faint burning smell is present. There is a lay-by 400 m ahead and a tunnel 1.2 km ahead. The tachograph shows you have driven 3 hours 40 minutes since your last 45-minute break.
1. What do the spongy pedal and burning smell indicate, and what should you do in the next 400 m?
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Indication:
- A spongy, long-travel pedal with a burning smell after prolonged braking points to brake fade: overheating of the brake drums/discs and, in an air system, possible overheating of components or air loss. Fade means braking performance is falling and the brakes may fail completely if heating continues.
Action within 400 m:
- Do not continue to the tunnel; stopping in or before a tunnel with overheating brakes is dangerous.
- Use the lay-by: signal early, reduce speed using engine braking and the retarder if you can engage it safely (test it gently first), and stop.
- Apply the parking brake, chock the wheels if on a slope, switch off the engine and let the brakes cool. Do not pour water on hot drums or discs; thermal shock can crack them.
- Check for air leaks, brake drag and any smoke. Do not resume until the brakes have cooled and the pedal feels normal; if performance is still poor, call for assistance.
Reasoning: the priority is to stop before the heat becomes a fire or total brake loss, and a lay-by is the only suitable place in the next 400 m.
2. How should the descent have been driven to avoid this, and what is the role of the retarder and gear selection?
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Correct descent technique:
- Select a low gear before the descent begins and stay in it; the engine brake and retarder should carry most of the braking, with the service brakes used only for short applications to trim speed.
- Use the retarder as designed (it is most effective at higher engine speeds and in the correct gear); switching it off and never testing it was a mistake. It should have been checked before the descent, not during it.
- Brake in short, firm applications rather than riding the pedal continuously; continuous light braking builds heat without giving cooling intervals.
- Keep speed low enough that a stop is possible; on an 8% gradient with 24 t, momentum builds quickly.
- Monitor the air pressure gauge; 7.5 bar is normal, but a falling reading means stop immediately.
- Plan for high ambient temperature: at 30 °C cooling is slower, so lower descent speed and more use of retardation are needed.
Reasoning: service brakes convert motion into heat; on a long descent they cannot dissipate it fast enough if used continuously, so the engine brake and retarder must do the work.
3. What are the tachograph and documentation implications of stopping in the lay-by?
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Tachograph and records:
- You have driven 3 h 40 min since the last qualifying break, so you have 50 minutes of driving left before the 4.5-hour limit. The stop in the lay-by is other work (checking the vehicle) and does not count as a break unless you actually take one. If the vehicle needs repair or cooling time, use the waiting time as a break if it qualifies, and record the mode correctly.
- A 45-minute break can be split into 15 minutes then 30 minutes; a single 45-minute break is simpler here.
- Switch the tachograph to the correct mode for each activity: other work for the check, rest/break for a genuine break. Do not leave it on driving while stationary.
- Record the defect and the reason for the stop in the daily defect report and, if the brakes are found faulty, do not move until repaired; the operator must be informed.
- If a repair is needed, note the time and place in the records so the delay is documented for the transport manager and any enforcement check.