Iowa CDL Combination Vehicles Test common mistakes
Iowa CDL Combination Vehicles Test · Understand the traps and how to avoid them.
During the walk-around, the candidate looks under the trailer, sees the fifth wheel is 'under there,' and never confirms the jaws are closed around the kingpin or that the locking lever safety latch is engaged.
The fifth wheel is partly hidden, and candidates treat 'trailer is sitting on the tractor' as proof of coupling; the release handle can look latched even when the jaws are not fully closed.
What to do instead
Use a flashlight and check from the side and rear: jaws closed around the kingpin, no gap between the trailer apron and fifth wheel, locking lever seated with the safety latch over it, and the release handle spring-loaded. Then do the tug test.
After backing under the trailer, the candidate immediately connects the air lines and drives off without a tug test or a second visual check of the fifth wheel.
Backing under feels and sounds like coupling, and the candidate is rushed by the test clock; the fifth wheel may have hit the kingpin and not latched.
What to do instead
Set the trailer brakes, put the tractor in a low gear, and pull forward gently; the tractor should stop or the trailer should not move. Then get out and recheck the jaws and locking lever before connecting the lines.
When uncoupling, the candidate pulls the fifth wheel release handle before chocking the trailer wheels and lowering the landing gear to the ground.
The sequence is memorized as 'pull handle, pull out,' and candidates skip chocking and landing gear because the trailer appears level or the dock is flat.
What to do instead
Chock the trailer wheels, lower the landing gear until it touches the ground and bears weight, disconnect the air lines and electrical cord, then release the fifth wheel. Never rely on the tractor to hold the trailer.
The candidate performs the air brake test only on the tractor, never checking that the trailer spring brakes apply when pressure drops or that the tractor protection valve cuts off the trailer.
The pre-trip air brake test is often practiced on a single vehicle, and the trailer hand valve or red trailer air supply knob is mistaken for a parking brake.
What to do instead
With full pressure and the engine off, fan the brakes and watch for the red trailer air supply knob to pop out around 20-45 psi; verify the trailer brakes apply and stay applied. Rebuild pressure and confirm they release. Check for air line leaks at the gladhands.
At a railroad crossing, the candidate rolls through without stopping when the vehicle is placarded for hazardous materials or is a passenger-carrying bus, and does not check that the trailer will clear the tracks.
The candidate applies the general 'look and listen' rule and forgets the special stop requirements for certain combination vehicles; they also misjudge trailer length.
What to do instead
Know the stop requirements: stop 15-50 feet before the tracks when required, open the window, look and listen, and do not start across until you can see that the entire combination, including the rear trailer, will clear the crossing without stopping.
On a slippery downgrade or in a curve, the candidate applies the brakes hard and late, locking the tractor drive wheels or trailer wheels and causing a jackknife.
Following too closely and treating the combination like a car; panic braking is instinctive when the trailer starts to push.
What to do instead
Brake early and gently, use engine braking on downgrades, and keep a longer following distance. If wheels lock or the trailer starts to swing, release the brake pedal to regain rolling traction and steer straight; do not keep pressing harder.
When the trailer skids, the candidate steers sharply away from the skid and stays on the brakes, or overcorrects the steering wheel.
Car-skid instincts say 'brake and countersteer,' but a heavily loaded trailer needs the brakes released and gentle steering to straighten.
What to do instead
Release the brakes, steer in the direction you want the trailer to go, and make small corrections. For a drive-wheel skid, release the brakes and steer in the direction of the skid. Avoid hard braking and abrupt steering until the combination is straight.
The candidate enters a ramp, curve, or lane change at the posted speed or faster, without slowing before the turn, and feels the trailer lean.
The rollover threshold is lower for a loaded combination with a high center of gravity; candidates use car speed and brake while turning.
What to do instead
Slow down before the curve, not in it. Keep cargo low and centered, avoid sudden lane changes, and use a speed that leaves margin for the trailer's lean. If the trailer leans hard, ease off the accelerator and steer smoothly.
With doubles or triples, the candidate changes lanes abruptly or maintains high speed, ignoring that the rear trailer swings wider and faster than the tractor.
Rearward amplification is not visible from the cab, and the candidate drives the lead unit as if the rear trailer follows exactly.
What to do instead
Reduce speed, especially in curves and lane changes; make gradual steering inputs; allow extra space and time; and watch the rear trailer in the mirrors. Do not make sudden lane changes with multiple trailers.
In a tight right turn, the candidate steers as if the rear trailer wheels follow the tractor's front wheels, cutting the corner and striking a curb or object with the trailer tires.
Off-tracking is hard to see from the cab; candidates misjudge the trailer's path and start the turn too early.
What to do instead
Turn wide and start the turn later; watch the trailer tires in the mirrors; allow for the rear wheels tracking inside the front wheels. In a lane change, leave extra room because the trailer cuts inside the path of the tractor.