Long Downgrade with Load
A 44,000 kg five-axle tractor-semi is descending a 9% grade that runs for 7 km. The speed limit is 60 km/h. The driver has selected 9th gear in a 13-speed transmission and switched the engine brake to high. To hold 50 km/h, the driver has been making repeated light brake applications. After 3 km the speed has crept to 65 km/h with the same pedal effort, and there is a hot brake smell with a slight pull to the right. The pedal remains firm, and the air gauge holds steady at 110 psi.
1. What is the most likely explanation for the speed increase, smell, and pull, and why is an air leak unlikely?
Read the worked answer
The pattern fits brake fade from heat, not an air leak. Repeated light applications at 44,000 kg on a 9% grade generate a lot of heat without giving the brakes time to cool. As drum and lining temperatures rise, friction drops, so the same pedal effort produces less braking and the speed creeps up. The hot smell supports overheated linings. A pull to the right can mean the right side is hotter, contaminated, or has a dragging brake. The pedal is still firm and the air gauge is steady at 110 psi, which argues against a major air loss or a failed chamber. The driver should treat this as a thermal capacity problem and act before the brakes fade further.
2. What should the driver do immediately? Explain why continuing to the bottom of the grade is unsafe.
Read the worked answer
The driver should stop braking repeatedly, shift to a lower gear so the engine brake can hold speed, and look for the next safe pullout. If speed is still climbing, use one steady, moderate brake application rather than a series of light ones, and slow enough to get off the road. Continuing is unsafe because fade is progressive: stopping power can fall to the point where the service brakes cannot control the load, and the driver may then have to use the spring brakes at speed, which can lock wheels or cause a jackknife. The pull also suggests uneven braking. At a pullout, the driver should stop, let the brakes cool, and check for smoke, dragging, oil or coolant contamination, and cracked drums before proceeding.
3. Describe the correct braking technique for the rest of the descent and the role of air pressure monitoring.
Read the worked answer
Before starting down, select a gear low enough that engine braking alone holds a safe speed; do not rely on the foot brake to control speed. Use the engine brake and a steady, moderate service brake application only when needed, and avoid fanning or riding the brakes. Keep the air gauge in view, but recognize that a steady gauge does not mean the brakes are cool; temperature is the limiting factor. If pressure begins to fall, treat it as a separate emergency and stop. At the bottom, check brake adjustment and drum temperature. If the grade is too steep for the load, the safe choice is not to descend until the brake system and technique are suitable.