A 9% descent in the rain with the pedal going long
Kiri is driving a six-axle semi-trailer rated at 44,000 kg, loaded with bagged fertiliser to about 41,500 kg, on a state highway. At the top of a hill a sign reads: 'Steep descent, 9% for 5 km, no escape lane'. It has been raining for an hour and the chipseal is wet and greasy. Her truck has a compression brake and a driveline retarder, and the air pressure gauge reads normal as she crosses the crest at 88 km/h. A light car is sitting close behind her. She selects a gear and starts down. About 2 km in, holding the same gear, she notices the speed creeping from 55 to 68 km/h, the service brake pedal feels long and spongy, and there is a faint burning smell. Ahead, the road narrows at a one-lane bridge with a moderate curve just before it.
1. What should Kiri have done before crossing the crest? Set out the descent plan she should have had.
Read the worked answer
The descent is won or lost at the top, not halfway down.
- Check the system first. Air pressure up and the low-pressure warning off; a low-speed brake test at the top to confirm the service brake is working.
- Gear before speed. Select the lowest gear that will hold the load - usually the gear she would climb the hill in, sometimes one lower. Selecting it at 88 km/h on the way in is already too late; she should have slowed before the crest.
- Engine braking first. Retarder and compression brake on maximum. The service brakes trim speed; they do not hold a 41 tonne load down 5 km of 9%.
- Aim low. A steady 40 to 50 km/h, well under the 90 km/h heavy vehicle limit, leaves time to correct a mistake.
- Brake technique. Firm, short applications rather than light continuous pressure, which overheats and glazes the linings.
- Read the road. Wet chipseal, a one-lane bridge and a curve, and no escape lane - so she should be noting shoulders, any uphill run-off, and where a stop would be possible. And she should let the car behind past if it wants to go.
2. She notices the speed creeping, a long spongy pedal and a burning smell. What is happening, and what should she do now?
Read the worked answer
That cluster - speed creeping in a fixed gear, a long spongy pedal and a burning smell - is brake fade. The drums and linings are overheated; the drum has expanded away from the lining and friction has dropped, and the retarder is no longer holding the load on its own.
In order:
- Do not pump the pedal. That burns air and adds heat without giving braking back.
- Drop a gear now, while the speed is still manageable, and put the retarder to maximum.
- One firm, sustained application to get the speed down, then release and let the engine and retarder hold it. Progressive, deliberate braking beats a series of panicked stabs.
- Pick a stopping place early - a wide shoulder or slow vehicle bay, hazards on, and not the narrow bridge approach. Let the brakes cool, then check drums, linings and slack adjusters before moving.
- If the speed is genuinely getting away, choose the least-bad option while there is still a choice: a gravel shoulder, an uphill bank, a fence line. That is a far better outcome than the bridge or the curve at speed.
3. What should she do about the car sitting close behind her on the descent?
Read the worked answer
The tailgater is a hazard to manage, not a reason to speed up. Accelerating on a wet 9% descent to open a gap is exactly how descents end badly, and it makes the brake fade worse.
Her approach:
- Hold the planned descent speed. The car can see a large vehicle and can pass when there is a straight, and she should not compromise her own braking margin for it.
- Signal and move as far left as is safe wherever there is a passing opportunity, so it can get past cleanly.
- Show the brake light early with a light touch before any real application, so the driver behind reads the speed change rather than reacting late.
- Do not swerve if the car overtakes on a curve or sits in a blind spot. Hold the lane and ease off slightly to increase the gap ahead.
- At the bottom, if the car is still glued to her, pull over where it is safe and let it go. A tailgater behind a heavy vehicle on a wet road is a rear-end collision waiting to happen, and she does not want to be part of it.