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CDL · Emergencies

If the trailer wheels begin to skid and the trailer starts to swing out (trailer jackknife), what should the driver do?

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

Release the brakes to allow the trailer wheels to regain traction and roll, which helps the trailer fall back into line behind the tractor

  1. A Release the brakes to allow the trailer wheels to regain traction and roll, which helps the trailer fall back into line behind the tractor
  2. B Apply the trailer hand valve fully
  3. C Brake harder
  4. D Accelerate hard and steer sharply

Why this is the answer

TRAILER JACKKNIFE (trailer swings out): Occurs when the trailer wheels lock up and lose traction, causing the trailer to slide and swing out to the side.

Correction: RELEASE THE BRAKES — this allows the locked trailer wheels to start rolling again and regain traction; rolling wheels track straight, helping the trailer come back in line behind the tractor; locked (skidding) wheels have no directional control.

Prevention: Brake smoothly and gradually; avoid hard braking, especially on slippery surfaces or in curves; brake before curves, not in them; keep the vehicle's speed appropriate for conditions; empty/lightly loaded trailers are MORE prone to wheel lockup and jackknife (less weight on the wheels); TRACTOR JACKKNIFE (drive wheels skid) is corrected similarly — release the brakes and steer to regain control; the counterintuitive but correct action for skid-induced jackknife is to STOP braking so the wheels can roll and regain traction; this is a critical safety concept for combination vehicle drivers.

Source: FMCSA CDL Manual — Trailer Jackknife Recovery

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About this topic: Emergencies

Emergencies in a combination vehicle escalate faster than in a straight truck: a trailer can jackknife, roll, or lose air to its brakes. These questions cover trailer skids, emergency braking with a trailer, air-line failures and how the tractor protection valve behaves when the trailer loses pressure. This is one of 3 Combination Vehicles questions on emergencies, 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.

Related questions

Emergencies

To prevent a trailer skid (and possible jackknife), what is the most important driving practice?

 Emergencies

If your drive wheels (tractor) begin to skid and the tractor starts to jackknife, what should you do?

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