Alabama CDL Combination Vehicles flashcards

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

Offtracking definition

The difference between the path of the tractor's front wheels and the trailer's rear wheels; it increases with vehicle length and turn sharpness.

Trailer hand valve use

Used only to test trailer brakes; never use it to stop a combination because it can cause a jackknife.

Static leak test limit

With the engine off and system charged, pressure loss must not exceed 3 psi in one minute for a single vehicle or 4 psi for a combination.

Applied leak test limit

Fully apply the foot brake for one minute; leakage must not exceed 4 psi per minute for a combination vehicle.

Low-air warning threshold

The low-air warning must activate before pressure drops below 60 psi, alerting the driver to stop safely.

Spring brake deployment

Spring (emergency) brakes must fully deploy between 20 and 45 psi as pressure drops, and the tractor protection valve then closes.

Tractor protection valve function

Keeps air in the tractor when the trailer breaks away or the trailer air supply is pulled out; it closes at about 20-45 psi.

Compressor build-up test

At normal governed engine speed, pressure must rise from 85 to 100 psi within 45 seconds.

Safety valve release pressure

The safety valve on the air compressor releases at about 150 psi to prevent over-pressurization of the air system.

Fifth wheel inspection

Check the jaws and kingpin for cracks, bends, or wear before coupling; a worn or open jaw is the most common cause of trailer separation.

Coupling pull test

After coupling, pull forward gently twice; the trailer should not move, confirming the locking jaws are closed around the kingpin.

Air line glad hands

The emergency (red) line connects to the trailer emergency glad hand and the service (blue) line to the service glad hand.

Uncoupling sequence

Park on level ground, set parking brakes, lower landing gear, disconnect air and electrical lines, pull the release handle, then pull forward slowly.

Landing gear check

After coupling, raise the landing gear fully and confirm the trailer is supported by the fifth wheel, not the landing gear.

Walk-around inspection items

Covers coupling system, air lines, electrical cord, landing gear, mud flaps, lights, tires, and the trailer frame.

Air line chafing check

Look for chafing or leaks at the glad hands; a leaking line can prevent trailer brakes from releasing or applying fully.

Electrical cord check

Confirm the cord is plugged in, not dragging, and that trailer turn, brake, and tail lights work with the tractor.

Trailer tire inspection

Inspect for tread depth, cuts, bulges, and proper inflation; dual tires must not touch and should be matched in size.

Trailer frame check

Verify the trailer frame is not cracked, crossmembers are intact, and mud flaps are present and not dragging.

Backing steering rule

Steer the tractor opposite the desired trailer movement: turn the wheel right to make the trailer go left.

Back from driver's side

Back from the driver's side whenever possible; blind-side backing is more dangerous because the trailer is not visible in the mirrors.

Parking on a grade

Set the parking brakes, use wheel chocks, and turn the front wheels toward the curb when parking on a grade.

Parking brake requirement

Before parking, set the trailer spring brakes and the tractor parking brake; do not rely on air pressure alone to hold the vehicle.

Rollover threshold

The lateral acceleration at which the vehicle tips; it decreases as the center of gravity rises and as the load shifts.

Ramp speed caution

Posted ramp speeds are for cars; a loaded combination may need to travel well below the posted speed to avoid rollover.

Brake fade cause

Brake fade occurs when brakes overheat; on long downgrades, select a lower gear and use brakes intermittently to control speed.

Trailer swing correction

If the trailer starts to swing, release the brakes and steer in the direction of the skid; do not use the trailer hand valve.

Total stopping distance

Perception distance plus reaction distance plus brake lag distance plus braking distance; at 55 mph this is roughly a football field for a loaded rig.

Air brake lag distance

At 55 mph, brake lag distance is about 32 feet before the brakes even begin to apply.

Following distance rule

Use one second for each 10 feet of vehicle length at speeds under 40 mph, plus one extra second above 40 mph.

Trailer breakaway action

When the trailer breaks away or air pressure is lost, the emergency relay valve applies the trailer spring brakes automatically.

Right turn position

Make right turns from the rightmost lane and position so the trailer's rear wheels clear the inside curb without entering an adjacent lane.