Downgrade with a Partially Loaded Tanker
A 2,000 gal baffled tanker is responding to a rural fire. The tank is 65% full (about 1,300 gal) because the driver left the fill site early to cover a reported exposure. The route descends a 7% grade for 1.8 miles; the curve at the bottom is posted 25 mph and is covered with wet leaves. The driver is at 45 mph, automatic transmission in drive, engine brake on, and service brakes applied intermittently. The tank has one transverse baffle but no longitudinal baffles. Outside temperature is 38°F, and the road surface is shaded. The driver reports the truck feels like it is pushing through the curves and the steering feels light. The truck has ABS, new steer tires, drive tires at 60% tread, and a driver with 6 months of tanker experience. Gross weight is about 48,000 lb.
1. Identify the hazards and describe the descent plan you would direct.
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Hazards: a 1,300-gal (about 10,800-lb) partial load, a 7% grade for 1.8 miles, wet leaves, a 25-mph curve at the bottom, and a driver already at 45 mph. The pushing feel and light steering are signs of load surge and reduced steer-axle loading on the downgrade. My descent plan: before the grade, slow to 25 mph or less, select a lower gear so the engine brake and transmission hold the speed, and keep the engine brake on. I would not ride the service brakes; if speed builds, use a firm stab application to bring it down, then release. I would increase following distance, avoid any lane change or sharp steering input on the grade, and treat the wet-leaf curve as a 15-20 mph curve, not 25. If the truck cannot hold 25 mph in the selected gear, I would use a lower gear before the speed builds. I would also consider the longer alternate route if the grade cannot be taken at a safe speed, and I would advise the IC of the delay.
2. Explain how the 65% load affects stability and stopping compared with a full or empty tank.
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A 65% load is the worst case for free-surface effect: the water has room to run the length and width of the tank. A full tank has little free surface, so the mass moves with the truck; an empty tank has no shifting mass. At 65%, braking sends water forward, unloading the rear axle and then surging back, which lengthens stopping distance and can push the truck through a stop. On the downgrade, the forward surge reduces steer-axle weight and makes steering feel light. The single transverse baffle limits fore-aft movement but does nothing for side-to-side surge, so a curve can produce a roll moment. The higher center of gravity also reduces roll stability compared with a passenger vehicle. The practical result is that the tanker must be driven slower than a full or empty tanker, with smoother brake and steering inputs, and the driver should not assume the baffle eliminates the problem.
3. At the bottom, the driver reports a low, spongy pedal and a hot-brake smell. What actions and notifications follow?
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Stop the apparatus in a safe pullout, away from the curve and traffic. Set the parking brake, chock wheels, and do not continue with a low pedal. A spongy pedal plus hot-brake smell indicates brake fade or fluid boil/air in the system; continued use can lead to loss of braking. Let the brakes cool, inspect for fluid leaks, and request a mechanic. Do not pour water on hot drums or discs, because warping or cracking can result. Notify the IC that the tanker is out of service and request a replacement; if the apparatus must be moved a short distance, use the lowest gear and engine brake, not the service brakes. After the incident, document the descent speed, gear, brake use, and load level so the cause can be reviewed. If the situation had been an emergency stop with no pullout, the escape ramp or a controlled off-road stop would be preferable to riding the brakes.