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This story appears in the July 1 print edition of Transport Topics.

Crashes such as the recent one in upstate New York where a trailer apparently separated from a tractor and struck a minivan with deadly consequences are rare, according to federal crash experts and investigators working on the case.

“It’s one of those that everybody says: ‘This doesn’t happen,’ ” said Capt. Mark Helms of the Cortland County Sheriff’s Department. “That’s what the true question is — why with this one?”

The commercial vehicle industry takes “extra precautions” to ensure against all crashes, Helms said, and “this one is one of those . . . out of the norm.”

The Sheriff’s Department and New York State Police investigators have determined a “mechanical failure” in the fifth wheel caused the trailer separation in the May 29 accident in Truxton, N.Y. Helms declined to elaborate because, he said, the accident still is under investigation.

Data collected by the federal government over five years show that only 1 in 1,000 fatal large truck crashes involves a coupling, hitch or chains, Ralph Craft said. Until his retirement May 31, Craft was senior economist in the Federal Motor Carrier Safety Administration’s Office of Analysis, Research and Technology.

Whether the rig has a fifth wheel or a hitch, responsibility for safety “basically comes down to well-maintained vehicles and well-trained drivers,” said Michael Irwin, director of driver and training programs for the Commercial Vehicle Safety Alliance.

Correctly coupling a trailer is so critical that FMCSA warns doctors in its medical examiner handbook to check drivers for musculoskeletal diseases such as arthritis that can affect the necessary “muscle strength and agility” for the task.

The handbook also lists coupling among the tasks that make “commercial driving a stressful occupation.”

There are “adequate” rules and equipment to help prevent trailer separation, said Gary Gaussoin, president of Silver Eagle, a Portland, Ore.-based family firm that manufactures truck and trailer equipment, including fifth wheels.

“However, things could fail, equipment can fail,” he said, stressing the importance of proper maintenance and training.

“Fifth wheels can be coupled improperly, and so the driver gets out of the yard not realizing it is not coupled correctly and there’s a thing called high pinning,” Gaussoin said. The kingpin can be stuck in the upper part of the fifth wheel, “especially, if the trailer was left high and the driver backs in and gets a false coupling,” he said.

In those cases, there’s usually a “big air gap between the fifth wheel and the trailer that can be seen, but off they go down the road and circumstances become right where it dislodges,” he said.

That’s why federal regulations require drivers, after a coupling, to inspect the locking mechanism, said Robert Braswell, technical director for American Trucking Associations’ Technology & Maintenance Council.

“When you back the tractor into the trailer, the kingpin goes into the fifth wheel,” Braswell said. “You’re supposed to go out and visually look to see if those locks have engaged.”

However, merely eyeing a fifth wheel or other coupling device is not sufficient, Braswell said. Locked devices do not all look alike; brands look different from one another. “You’ve got to know what you’re looking at,” he said.

Nearly all fifth wheel makers provide installation, operation, and maintenance manuals because they aren’t all alike, said Gregory Laarman, vice president of engineering for Jost International, Grand Haven, Mich.

In guidelines published by the American Association of Motor Vehicle Administrators, drivers are warned against performing couplings on uneven ground, lest the coupling be insecure and against inadequate lighting that limits ability to spot an insecure connection.

Drivers must also do a “tug” or “pull” test after a coupling. FMCSA regulations say, “Before driving away, apply the trailer brakes; and pull gently against them to check coupling.”

A tug test is no substitute, however, for a visual inspection of a fifth wheel coupling, Gaussoin said. “You can do a tug test, and it will still pull,” he said, but the kingpin “may be caught in the upper part of the jaws.”

Trailer brakes are a critical maintenance item, experts said. Brake systems include air brakes and backup spring brakes.

“If there’s a catastrophic loss of air pressure, the spring brakes should apply; they’re designed to sense that,” Braswell said. “There’s a bunch of stuff that can go wrong if [brakes are] not maintained properly.”

Helms, of the sheriff’s department, told Transport Topics that, in the New York crash, the trailer brakes worked properly.

“It is a freakish type of accident,” said Jim Clark, director of engineering for North America at TMD Friction Inc., which makes truck and trailer brake linings. “How you prevent that — that will come out in the details of the investigation.”