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CDL · Combination Vehicles · Topic Study Guide

Emergencies: Practice Questions & Explanations

3 Combination Vehicles questions on emergencies, each with a worked explanation citing the source handbook.

Source: FMCSA Commercial Driver's License Manual, Section 6 (public domain).

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

Why this topic matters

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.

Below are every emergencies question in our Combination Vehicles bank. Read each question, try to answer before reading the explanation, and use the source citations to look up anything you want to verify in the official handbook.

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

Explanation

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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2. To prevent a trailer skid (and possible jackknife), what is the most important driving practice?
  1. A Never use the brakes
  2. B Accelerate quickly
  3. C Brake as hard as possible
  4. D Drive smoothly and brake gradually — avoid over-braking and hard braking, especially on slippery roads and in curves, which can lock the trailer wheels

Explanation

To PREVENT a trailer skid/jackknife, the most important practice is SMOOTH driving with GRADUAL braking — avoid over-braking and hard braking, which can LOCK the trailer wheels (locked wheels lose traction and the trailer slides out). CDL combination. ESPECIALLY on slippery roads and in curves. Brake BEFORE curves, not in them; slow down for conditions; maintain space to avoid sudden stops. EMPTY/lightly loaded trailers are MORE prone to wheel lockup (less weight on the wheels). If a trailer skid begins, RELEASE the brakes to let the trailer wheels regain traction and roll (which brings the trailer back in line). PREVENTION (smooth, gradual braking) is far better than recovery. Knowing that smooth/gradual braking prevents trailer skids is commonly tested.
Source: FMCSA CDL Manual — Combination, Preventing Trailer Skid

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3. If your drive wheels (tractor) begin to skid and the tractor starts to jackknife, what should you do?
  1. A Brake harder
  2. B Stop braking (release the brakes) to let the drive wheels regain traction and roll, then steer to maintain control
  3. C Accelerate hard
  4. D Turn the wheel sharply

Explanation

If the tractor DRIVE WHEELS skid and the tractor begins to JACKKNIFE (the tractor swings relative to the trailer), you should STOP BRAKING (release the brakes) to let the drive wheels regain traction and roll again — rolling wheels can grip and steer, allowing you to regain control; then steer to keep the vehicle straight. CDL combination.

The cause: locked/skidding drive wheels lose traction and the tractor can swing. RELEASING the brakes lets the wheels roll and regain traction (the counterintuitive but correct action). SAME PRINCIPLE applies to trailer skids (release brakes to let the trailer wheels roll and come back in line).

Prevention: smooth, gradual braking; slow down for curves and conditions; don't over-brake. Knowing to release the brakes to recover from a drive-wheel skid/jackknife is commonly tested and safety-critical.

Source: FMCSA CDL Manual — Combination, Tractor Jackknife Recovery

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