Air pressure falls on a mountain descent
At 14:20 a 32 t loaded tipper begins an 8 km descent on a 6% grade in northern Iraq. Ambient temperature is 42°C. The truck has full air brakes, an engine retarder that is isolated because of a coolant sensor fault, and a six-speed manual gearbox. The driver selects 7th gear and uses the foot brake almost continuously to hold 50 km/h. After 2.5 km the primary air gauge has fallen from 8.2 bar to 5.4 bar; the low-air warning sounds at 5.5 bar and the spring brakes are designed to apply at 4.5 bar. The driver smells hot brake lining and hears a rhythmic squeal. The next safe pull-off is 600 m ahead; beyond that the grade steepens. The driver's phone has no signal. The load is 19 t of gravel, and the tarpaulin is flapping.
1. Interpret the gauge fall, the smell and the squeal. What is happening, and why is continuing at 50 km/h unsafe?
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The gauge has fallen 2.8 bar in 2.5 km because continuous service-brake use is consuming air faster than the compressor can recharge it, especially at low engine rpm in 7th gear. The low-air warning is already active at 5.4 bar, so the margin before spring-brake application at 4.5 bar is only 0.9 bar. The hot-lining smell and rhythmic squeal indicate overheated drums and probable brake fade; stopping distance is increasing even though the driver is pressing harder. Continuing at 50 km/h is unsafe because the grade steepens in 600 m, the retarder is unavailable, and the next safe pull-off is the last chance to stop without blocking the carriageway. If pressure reaches 4.5 bar the spring brakes will apply automatically, possibly at a bend or on a narrow section, creating a secondary collision risk.
2. State the immediate actions for the next 600 m and after stopping.
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Use the 600 m to reach the pull-off, not to 'make time'. Downshift progressively into a lower gear to use engine braking, apply the brakes in short, firm applications rather than riding them, and keep the hazard lights on. Do not switch off the engine or coast. At the pull-off, stop fully, apply the parking brake, chock the wheels if possible, and let the drums cool naturally. Do not pour water on hot drums. Inspect for air leaks, dragging brakes, damaged hoses and adjustment problems, and check that pressure builds normally. Phone for recovery/fitter assistance; with no signal, send a passing driver or walk to the nearest safe point only if it is safe to leave the vehicle secured. Do not continue until the air system and brakes are repaired and the retarder fault is assessed.
3. What should have been done before starting the descent?
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Before the descent the driver should have completed an air-brake check: build pressure to governor cut-out, check for leaks with the engine off, confirm low-pressure warning and spring-brake application, and verify that the compressor recharges. He should have known the retarder was isolated and treated the truck as having no auxiliary braking. At the crest he should have selected a low gear before the grade, kept speed low, used snub braking - firm applications with cooling intervals - and checked the air gauge regularly. He should also have considered the 42°C ambient temperature, the 32 t weight and the 6% grade, and identified the first safe pull-off. Continuing in 7th gear with continuous foot braking turned a known equipment limitation into a brake-fade and low-air emergency.