Maryland CDL Combination common mistakes
Maryland CDL Combination · Understand the traps and how to avoid them.
During the walk-around inspection, the candidate checks the fifth-wheel jaws and the kingpin but never verifies that the release lever is fully latched and secured with its safety latch, and does not do a firm pull test with the tractor against the trailer brakes.
Candidates memorize the visual checklist (jaws closed, no gap, apron greased) and treat the tug test as optional or perform it with only a light bump, so a high-hitch or unlatched jaw condition goes undetected. The lever is often overlooked because it is small and in shadow under the trailer nose.
What to do instead
Build a fixed order into the coupling routine: after backing under, check the fifth-wheel jaws closed around the kingpin, the release lever is in the locked position with the safety latch engaged, and the trailer apron rests flat on the fifth wheel. Then set the trailer parking brakes, put the tractor in a low gear, and pull firmly against them while watching the tractor/trailer gap; a secure coupling holds and the tractor does not move forward. Repeat the visual check after the pull.
The candidate connects the service and emergency (supply) gladhands backwards, or fails to check that the trailer air lines are not crossed, chafed, or leaking at the seals.
Both couplers look similar and the color coding (blue service, red emergency/supply) is easy to reverse under time pressure. Candidates also skip the leak and cross-check because they think the tractor protection valve will sort it out.
What to do instead
State the color convention aloud during the inspection: blue is the service line, red is the emergency/supply line, and they must not be crossed. Push each coupler on until it clicks and tug it to confirm the seal. After charging the system, listen at both gladhands and the trailer relay valve for leaks, and confirm the trailer brakes release and reapply correctly with the tractor protection valve.
The candidate stops the engine or releases the parking brake before the trailer spring brakes have applied, or skips the trailer brake test entirely, so an uncoupled trailer could roll.
The uncoupling sequence is taught as a list of steps but candidates rehearse it without the logic behind each step, so they release the tractor parking brake to 'settle' the trailer before the trailer brakes have set, or they pull the tractor out with the trailer brakes still released.
What to do instead
Fix the order: park, chock the trailer wheels, lower the landing gear to firm ground contact, disconnect the gladhands and electrical cord, release the tractor parking brake only after confirming the trailer spring brakes are applied (the trailer will not move when the tractor pulls away), then pull forward slowly and stop as soon as the tractor clears the kingpin. Verify the trailer stayed put before walking away.
When turning, the candidate swings wide only at the start of the turn and then cuts the corner, letting the trailer's rear wheels track over the curb, a sign, or a pedestrian area.
Drivers underestimate off-tracking because the tractor appears to clear the corner; the trailer's rear axle follows a shorter path and the driver is watching the front of the tractor, not the trailer in the mirrors. Anxiety about oncoming traffic encourages an early, tight turn.
What to do instead
Turn later and wider than in a car: pull past the corner before starting the turn, keep the tractor's front wheels well into the far lane, and check both mirrors for the trailer's rear wheels throughout the turn. If space is tight, stop and re-evaluate rather than completing a turn that will drag the trailer over the curb. Remember the rear wheels off-track toward the inside of the turn, and the longer the wheelbase and trailer, the greater the cut.
The candidate brakes hard while the combination is still traveling straight through a curve, or enters the curve too fast and brakes mid-curve, causing the trailer to swing out or the rig to jackknife.
Drivers carry car habits into the rig: brake when the corner 'feels fast' rather than slowing before it. Hard braking on a curve unloads the rear and lets the trailer push the tractor, and the candidate does not connect the rollover/jackknife risk to entry speed.
What to do instead
Slow to a safe speed before entering the curve, then maintain a steady speed or gentle acceleration through it; never brake hard in a curve. If speed must be reduced, do it in a straight line first. On downgrades, select a low gear before the descent and use engine braking plus light, steady brake applications rather than a single hard application.
The candidate performs the air brake check on the tractor only, or skips the trailer portion of the test, and reports a static pressure reading without watching the gauge for a full minute or checking the low-pressure warning and spring brake pop-out.
The pre-trip air brake test is long and candidates rush it, treating 'gauge held at 100 psi' as the whole test. They forget the trailer tanks, the tractor protection valve, and the low-air warning and emergency spring brake application, which are separate checkpoints.
What to do instead
Run the test as a sequence and say each step: build pressure to governor cut-out, shut off the engine, confirm pressure does not drop more than the allowed amount in one minute with the brakes released and again with the brakes applied, check the low-pressure warning activates before pressure falls too far, and confirm the spring brakes apply automatically when pressure drops to the pop-out range. Repeat the leak check on the trailer system through the gladhands and verify the tractor protection valve closes.
The candidate relies on the foot brake only when stopping a loaded combination and does not know the stopping distance grows with weight and speed, or fails to use a lower gear on long downgrades.
The rig stops well when empty during practice, so the candidate never builds the habit of planning stops for a loaded trailer. On the test, following distance and speed judgment are not corrected, and the candidate brakes late.
What to do instead
Plan stops using the increased stopping distance: leave at least one second per 10 feet of vehicle length plus one second at speeds under 40 mph, and more when loaded or on wet pavement. On downgrades, downshift before the grade and use the engine brake with short, firm brake applications to keep speed controlled; never ride the brakes or let speed build until the brakes are needed hard.
The candidate responds to a skid by braking harder and steering sharply away from the skid, or by locking the steering while the drive tires are already sliding.
Panic response mirrors car instincts, and the candidate has not practiced the counterintuitive rule that you must release the brake and steer in the direction the rear is sliding. In a combination, the trailer's mass makes the wrong input worse quickly.
What to do instead
State and practice the rule: if the drive tires lock, release the brake to restore rolling and steer in the direction of the skid; if the trailer swings out, steer to follow the trailer and avoid hard braking. Recognize the causes (overbraking, too much speed on a curve, sudden steering) and prevent the skid by slowing before the hazard and braking in a straight line.
The candidate takes a curve or off-ramp at a speed that feels fine in the tractor cab but is above the trailer's rollover threshold, or loads cargo high and off-center without checking the center of gravity.
The cab sits low and the driver feels little lean, so the rollover risk is invisible from the seat; candidates judge curve speed by comfort rather than by the trailer's high center of gravity. Loading decisions are made without considering that a high, off-center load raises rollover risk sharply.
What to do instead
Set curve speed by the posted advisory speed and slow further when the trailer is loaded high or the load is off-center; the posted limit is not the safe limit for a tall combination. Keep the load low and centered, secure it, and remember that the trailer rolls before the tractor, so the driver must watch the trailer in the mirror and feel for the lean rather than trusting cab comfort.
The candidate drives at the same speed as surrounding traffic in rain, fog, or on a downgrade, and does not adjust following distance or use the engine brake, treating speed control as a limit to reach rather than a margin to keep.
Speed control is tested as a judgment item, not a number, and candidates equate 'at the limit' with 'safe'. They also fail to reduce speed before entering a hazard such as a work zone, sharp curve, or wet pavement, and they do not use the engine brake to hold speed on a grade.
What to do instead
Treat speed as a margin: reduce speed before the hazard, not in it, and increase following distance when traction is reduced or visibility is poor. On grades, select a gear that holds the desired speed with the engine brake and light brake applications, and keep a cushion of speed so a sudden stop does not require hard braking. If the rig starts to feel unstable, slow down immediately rather than correcting with steering.