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CDL · Coupling and Uncoupling

Before uncoupling a trailer, what must you do first?

Reviewed by Jordan Morley, Editorial Lead at 247SimpleTests. Written by the 247SimpleTests Editorial Team from the official source material cited on every question.

Correct answer

Lower the landing gear and chock the trailer wheels (if no spring brakes engaged)

  1. A Move the trailer forward
  2. B Disconnect the air lines
  3. C Open the fifth wheel release
  4. D Lower the landing gear and chock the trailer wheels (if no spring brakes engaged)

Why this is the answer

Before opening the fifth wheel release: position the trailer where it will be parked (on stable, level ground if possible), lower the landing gear until firmly on the ground (then crank a few more turns to take weight off the tractor), and make sure the trailer cannot roll. If the trailer has spring brakes that engage when air is released, those will hold it. If not, chock the wheels. Only then disconnect the air lines and electrical cord and release the fifth wheel locking jaws. Pulling out from under a trailer whose landing gear is not lowered will cause the trailer nose to drop several feet, possibly damaging the trailer, cargo, the fifth wheel, and anyone nearby. The order matters.
Source: FMCSA CDL Manual 6.2.7 Uncoupling Procedure

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About this topic: Coupling and Uncoupling

A bad coupling can drop the trailer or separate it from the tractor at speed. The tug test, visual inspection of the kingpin, and proper line-up are the defenses. This is one of 19 Combination Vehicles questions on coupling and uncoupling, each with a full explanation on a single study page.

New to this exam? Our CDL exam guide explains the format, scoring, and how to prepare.

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