Overheated brakes on a long descent with a 44-tonne combination
A driver is taking a 44,000 kg six-axle semi-trailer down a 6 km descent signed at 8%, carrying a full load of bagged fertiliser. The speed limit is 90 km/h and the truck is sitting on 55 km/h. About 2 km into the descent the driver notices the brake pedal is travelling noticeably further before the truck slows, there is a hot, sharp smell through the window, and the retarder has been switched off since it 'made a noise' last week. Engine rpm is low, around 1,100, because the driver is in a high gear to keep the noise down. Traffic is light, but there is a slow campervan 400 m ahead and a signposted escape lane about 900 m ahead on the left. The next safe place to stop is a gravel pull-off 2.5 km further down.
1. What is happening to the braking system, and why does the truck keep gaining speed even though the driver is pressing harder?
Read the worked answer
The service brakes are doing all the work and are overheating. A 44 t combination on an 8% grade needs a continuous retarding force of roughly 34 kN just to hold speed, and the brakes turn that energy into heat far faster than the drums can shed it. As the linings and drums heat up, friction falls away (fade) and the drums expand, so the pedal has to travel further and the air system uses more air for each application. With the retarder off and the engine at only 1,100 rpm, the engine brake is contributing almost nothing. Pressing harder does not fix fade; it adds heat. Rising speed with a firm, long pedal is the classic fade signature, not a sign that the truck needs more throttle or a higher gear.
2. What should the driver do in the next 900 m?
Read the worked answer
Act now rather than at the pull-off 2.5 km down. Select a much lower gear, one that holds the truck near 30 km/h with the engine at high rpm, put the retarder to maximum, and make firm, short brake applications to bring speed down while there is still brake to use, releasing between applications to let some heat out. Hazard lights on, and hold the gap to the campervan. If speed will not come down, or the pedal keeps sinking, take the escape lane at 900 m rather than committing to two more kilometres of grade; it exists for exactly this. Do not coast in neutral, and if the truck does stop on the grade, do not sit with the brakes cooking.
3. What must change before this truck does the descent again?
Read the worked answer
The truck needs a proper brake inspection by a qualified technician: drum and lining condition, adjustment, air system and leaks, plus a brake performance check. The retarder needs repairing, not ignoring. The descent plan also changes: select the low gear and switch the retarder on before the crest, cross the top at a speed low enough that the brakes only trim speed, keep engine rpm high, and snub rather than ride. Load restraint should be re-checked at the top and again after the descent. The operator should also ask why a driver felt able to enter a signed 8% grade with the retarder disabled and no descent plan.