Empty Trailer, Icy Bridge, and a Trailer Hand Valve
A driver is pulling an empty 53-foot van across a concrete bridge at 45 mph. The temperature is 30°F, and the bridge deck has patchy ice; the approach is wet but not icy. The tractor has ABS, and the trailer ABS lamp is on. The trailer brakes were last adjusted 8,000 miles ago. The driver is in the right lane of a two-lane road with no shoulder and a guardrail on the right. He sees a line of stopped cars 300 feet ahead and applies the foot brake firmly. The drive axles lock; the tractor begins to rotate clockwise, and the trailer's rear swings to the left. The driver reaches for the trailer hand valve to straighten the rig and keep the trailer behind him.
1. Explain the dynamics of the skid and why the trailer hand valve will make it worse.
Read the worked answer
The drive axles locked because the foot brake application exceeded the available traction on the icy bridge. A locked tire has no lateral grip, so the tractor's rear began to slide and the tractor rotated clockwise. The trailer, still rolling, pushed the tractor from behind; that is the start of a jackknife.
The trailer hand valve is the wrong tool. It applies only the trailer brakes, which can lock the trailer wheels and make the trailer swing harder. It also does nothing to restore steering traction at the tractor's drive axles. The trailer ABS lamp being on means the trailer ABS may be disabled, so the trailer wheels are more likely to lock. The driver should not add trailer brake pressure; he needs to release the foot brake and let the drive wheels roll so steering can return.
2. What immediate steering and braking inputs should the driver use?
Read the worked answer
Immediate action: release the brake pedal fully and smoothly. Do not stab the brakes or use the trailer hand valve. Steer in a controlled way in the direction the driver wants the tractor to go. Because the tractor is rotating clockwise, that means a smooth left steering input to stop the rotation and bring the tractor back ahead of the trailer; hard right steering would worsen the rotation. Keep looking and steering toward the open path, not at the stopped cars.
Once the tires are rolling and the rig is straight, use very light, steady braking if more speed must be removed, or let the vehicle roll to a safe stop. If the trailer's rear continues to swing left, the driver should steer left to follow the trailer and bring it back in line, but without overcorrecting. Any abrupt input on ice can start a second skid.
3. What prevention steps should have been taken before the bridge?
Read the worked answer
Before the bridge, the driver should have slowed well below 45 mph, increased the following distance, and avoided cruise control. He should have checked the trailer ABS lamp during the pre-trip; a lamp that stays on means a fault that should be repaired. He should have confirmed trailer brake adjustment and operation, because an empty trailer with weak or uneven brakes puts more work on the tractor.
On the icy approach, braking should have been early, light, and progressive, before the bridge deck. The trailer hand valve should not be used for normal stops or to straighten a rig. If conditions were already icy, the driver should have considered stopping or using a lower gear at low speed, not a firm foot-brake application at 45 mph.