Maryland CDL Combination flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Tug test

After coupling, set the trailer brakes and gently pull forward in a low gear. If the tractor does not move, the fifth wheel jaws are locked around the kingpin.

Fifth wheel jaws

The locking mechanism that grips the kingpin. Inspect that jaws are fully closed, the locking lever is secured, and no gap exists between apron and fifth wheel plate.

Kingpin

The vertical pin on the trailer apron that the fifth wheel jaws lock onto. It carries the trailer's pulling and braking loads and must be fully seated.

High hook

Coupling with the trailer too high, so the kingpin rides over the fifth wheel instead of locking. The trailer can separate from the tractor once moved.

Landing gear

The retractable legs supporting an uncoupled trailer. Lower them to the ground before uncoupling and raise them fully, with the handle secured, after coupling.

Gladhands

Couplers joining tractor and trailer air lines. The emergency (supply) line is usually on the driver side and the service line on the passenger side; never cross them.

Trailer air supply valve

The dash valve that charges or shuts off air to the trailer. Closing it should apply the trailer spring brakes and hold tractor air pressure.

Seventh pre-trip step

After coupling, check that air lines and electrical cord are connected and clear of the frame, landing gear is raised, and trailer lights work with the tractor.

Off-tracking

The tendency of a combination's rear wheels to follow a shorter path than the front wheels. Longer wheelbases and extra trailers increase the amount of cut.

Swing-out

The rear of the tractor and front of the trailer swing outward opposite the turn direction. Allow extra clearance at curbs, poles, and parked vehicles.

Right turn setup

Approach wide enough that the trailer clears the curb, then complete the turn without hugging the right edge, which causes trailer off-tracking over the curb.

Following distance rule

Allow at least one second per 10 feet of vehicle length below 40 mph, and add one second when faster. A 60-foot combination needs six seconds minimum.

Downgrade gear choice

Before descending, shift into a gear low enough to hold speed without heavy braking. Use engine braking and stab braking; never coast in neutral.

Stab braking

Apply the brakes firmly until the vehicle slows, then release fully to let the brakes cool. Repeat as needed on long downgrades instead of holding constant pressure.

Curve entry speed

Posted curve speeds are set for cars. A loaded combination with a high center of gravity must enter well below the posted speed and accelerate gently.

Blind spots

Areas along both sides and directly behind the trailer not visible in mirrors. Signal early and check mirrors before changing lanes or completing a pass.

Static leak test

With engine off and system charged, release parking brakes and hold the service brake. Loss should not exceed 3 psi per minute single or 4 psi per minute combination.

Applied leak test

With the system charged and engine off, apply and hold the service brake for one minute. Air loss should not exceed 4 psi per minute.

Low-air warning

A buzzer or light must warn the driver before air pressure falls below 60 psi, giving time to stop safely before spring brakes apply.

Spring brake pop-out test

With engine off, fan off air pressure. The tractor parking valve and trailer spring brakes should pop out between 20 and 45 psi.

Compressor build rate

With the engine at operating rpm, system pressure should rise from 85 to 100 psi within 45 seconds in a typical system.

Governor cut-out

The pressure at which the compressor stops pumping, typically about 120 to 125 psi. Cut-in, when pumping resumes, is usually around 100 psi.

Slack adjuster angle

With brakes released, the slack adjuster should sit about 90 degrees to the push rod. A pull of more than about one inch indicates the brake is out of adjustment.

Brake lag

The roughly half-second delay for air pressure to travel from the treadle valve to the brakes. Add the distance traveled during lag to stopping distance.

Rollover threshold

A loaded combination can tip at about half the lateral acceleration a car tolerates. Ramp and curve speeds posted for cars may be too fast.

Rear-wheel skid recovery

Release the brakes to restore wheel rotation, steer in the direction the rear is sliding, and reapply brakes gently after traction returns.

Front-wheel skid

Caused by steering too sharply or braking too hard. Release the brakes and reduce steering input so the front tires can regain grip.

Jackknife cause

Locked tractor drive wheels lose lateral grip, letting the trailer push the tractor sideways. Release the brake pedal and steer to straighten the combination.

Escape ramp use

When brakes fade or fail on a downgrade, steer straight into the ramp and keep the vehicle upright. Do not choose a ditch, guardrail, or embankment.

Tire blowout response

Hold the wheel firmly, avoid hard braking, ease off the accelerator, and let the vehicle slow gradually before steering off the road.

Visibility speed rule

Drive at a speed that lets you stop within the distance you can see. In rain, fog, or snow, reduce speed until stopping distance fits visibility.

Hazard response order

Identify the hazard early, check mirrors, choose the least risky option among slowing, stopping, or steering, and signal your intentions to other drivers.