Tractor-trailers are required to have "fifth wheel" coupling mechanisms to ensure that the trailer is locked onto the tractor, according to Rick Gunn, a licensed tractor-trailer inspector in Virginia. The trailer has a steel pin, called a pintle, that's about 3 inches thick and 6 inches long. The tractor has a large spring-loaded lock, shaped like a horseshoe, which locks into place around the pintle when the driver backs the tractor under the trailer, he said.
Gunn said he's occasionally seen the spring in the lock break so the mechanism can't lock into place over the pintle. But if that happens, the driver should detect it during a safety check the driver is supposed to do, Gunn said. Before a driver begins driving a load, he's supposed to pull a 2-foot handle on the fifth wheel to make sure the lock is set. If the handle pulls loose, the driver would know the trailer wasn't locked in place, Gunn said.
"Once he pulls that handle and it comes loose, he should say, 'I want a new truck,'" Gunn said.
