Maryland CDL Combination cheat sheet

Pre-trip order for a tractor-trailer: coupling system, air lines and electrical cord, landing gear, lights and reflectors, brakes and air loss, then engine compartment and in-cab checks.

Fifth wheel checks: jaws open and greased before coupling, jaws closed around the kingpin after coupling, locking lever secured with a safety latch, and no gap between apron and fifth wheel plate.

Tug test: after coupling, pull forward gently in a low gear with the trailer brakes set; the tractor should not move. Release trailer brakes and pull forward to confirm the trailer rolls.

Air line placement: supply (emergency) line is typically the gladhand on the driver side of the tractor; service (control) line on the passenger side. Match colors and confirm lines are not crossed or kinked.

Landing gear after coupling: fully raised, crank handle secured, no part of the gear or handle below the frame rail. Before uncoupling, gear must contact the ground and the tractor must be pulled clear.

Uncoupling sequence: park on level ground, chock wheels, set parking brakes, lower landing gear to the ground, disconnect air lines and electrical cord, release the fifth wheel jaws, then pull tractor slowly forward.

Trailer height before coupling: trailer apron should be slightly lower than the fifth wheel so the tractor lifts it; too high causes a high hook, too low causes the tractor to strike the trailer.

After uncoupling, secure the tractor: lower the fifth wheel to horizontal, cap or hang gladhands to keep dirt out, and coil the electrical cord.

Sliding the fifth wheel or trailer tandems changes weight distribution; re-check axle weights and the coupling hardware after any adjustment.

Off-tracking: the rear wheels of a combination follow a shorter path than the front wheels; the longer the wheelbase and the more trailers, the greater the cut.

Swing-out: the rear of the tractor and the front of the trailer swing wide in the opposite direction of a turn; watch mirrors and allow extra room at curbs, poles, and parked vehicles.

Right turns: approach wide enough to keep the trailer clear of the curb, then complete the turn; do not hug the right edge or the trailer will off-track over the curb.

Braking distance: a loaded combination needs substantially more stopping distance than a car or a single-unit truck; total stopping distance includes perception, reaction, and brake lag.

Speed on curves: posted curve speeds are for cars; a combination with a high center of gravity must enter well below the posted speed and accelerate gently through the curve.

Downgrades: select a gear low enough to hold speed without heavy braking before starting down; use engine braking and stab braking, and never coast in neutral.

Following distance: at least one second per 10 feet of vehicle length at speeds under 40 mph, plus one additional second at higher speeds.

Lane changes and passing require longer gaps; signal early, check both mirrors and blind spots, and confirm the trailer has cleared before returning to the lane.

Railroad crossings and rough pavement: slow down before crossing, hold the wheel firmly, and cross tracks without stopping if the path is clear.

Static leak test: with the engine off and system charged, release the parking brakes and hold the service brake; loss should not exceed 3 psi per minute for a single vehicle or 4 psi per minute for a combination.

Applied leak test: with the system charged and the engine off, apply and hold the service brake for one minute; loss should not exceed 4 psi per minute.

Low-air warning: the warning buzzer or light must activate before pressure drops below 60 psi.

Spring brake test: with the engine off, fan the service brake to reduce pressure; the parking brake valves should pop out (tractor protection valve and trailer spring brakes) between 20 and 45 psi.

Compressor build rate: with the engine at operating rpm, pressure should rise from 85 to 100 psi within 45 seconds in a typical system.

Governor cut-out and cut-in: compressor typically stops around 120-125 psi and restarts around 100 psi; pressures outside the manufacturer's range indicate a fault.

Tractor protection valve test: after charging the trailer, close the trailer air supply valve and confirm the trailer brakes apply and the tractor air supply does not leak down.

Slack adjuster check: with the brakes released, the slack adjuster should be about 90 degrees to the push rod; a pull of more than about one inch means the brake is out of adjustment.

Brake lag: air brakes take about half a second to reach full application after the pedal is pressed; add this distance to total stopping distance.

Rollover threshold: a loaded combination can tip at roughly half the lateral acceleration a car can handle; posted ramp and curve speeds for cars can be too fast for a truck.

Rear-wheel skid recovery: release the brakes to restore wheel rotation, steer in the direction the rear of the vehicle is sliding, and reapply brakes gently once traction returns.

Front-wheel skid: caused by steering too sharply or braking too hard; release the brakes and reduce steering input to let the front tires regain grip.

Wheel lock on a tractor-trailer: locked tractor drive wheels cause a jackknife; release the brake pedal and steer to straighten the combination.

Braking in a curve: braking transfers weight forward and reduces rear grip; slow down before the curve and avoid hard braking while turning.

Escape ramps: use them when brakes fade or fail on a downgrade; steer straight into the ramp and keep the vehicle upright rather than choosing a ditch or guardrail.

Tire blowout: hold the wheel firmly, avoid hard braking, ease off the accelerator, and slow gradually before pulling off the road.

Following distance and visibility: increase space in rain, fog, or snow; if you cannot see far enough to stop, reduce speed to match visibility.

Hazard response order: identify the hazard early, check mirrors, choose the least risky option (slow, stop, or steer), and communicate with signals and brake lights.