Arizona CDL Combination Vehicles key terms

A required walk-around check of lights, tires, brakes, coupling devices, and cargo before driving. It confirms the vehicle is safe and that air pressure builds and holds properly.

The horseshoe-shaped plate on the tractor that locks around the trailer kingpin to connect the two units. Its jaws must be closed and secured with a safety latch before driving.

The thick metal pin mounted under the trailer nose that slides into and locks inside the tractor's fifth wheel, holding the trailer to the tractor and allowing it to pivot.

The flat metal plate under the trailer front that surrounds the kingpin. It must rest flat and firm on the fifth wheel; a gap or damage means the coupling is not secure.

The two curved pieces inside the fifth wheel that close around the kingpin and hold the trailer. They must be fully closed with the release handle pushed in and latched.

After coupling, the driver sets the trailer brakes and gently pulls forward to confirm the trailer does not separate from the tractor, proving the jaws are locked around the kingpin.

The retractable legs under the trailer front that support it when uncoupled. They must be fully raised before driving and firm on the ground when parked.

The orderly sequence of parking on firm level ground, applying brakes, lowering landing gear, disconnecting air lines and electrical cord, then releasing the fifth wheel and pulling clear.

The interlocking couplers that join the tractor's air lines to the trailer's. They must be clean, sealed, and locked, with the supply line (blue) and service line (red) correctly matched.

Two flexible hoses, color-coded red and blue, that carry air between tractor and trailer. They must not drag, chafe, or cross, and must be connected before driving.

The line, usually blue, that carries air pressure from the tractor's brake pedal to apply the trailer brakes during normal stopping. It works together with the emergency line.

The line, usually red, that supplies air to charge the trailer system. If it loses pressure or separates, the trailer brakes automatically apply in a spring-brake emergency application.

A safety feature that applies the trailer brakes automatically if the trailer separates from the tractor, stopping it before it rolls free. The tractor protection valve also seals off lost air.

A valve that closes the tractor's air supply to the trailer if pressure drops too low or the lines separate, keeping enough air in the tractor to stop it.

Parking and emergency brakes held applied by heavy springs and released by air pressure. If air pressure is lost, they apply automatically, which is why low pressure makes a rig hard to move.

With the engine running at normal idle and the system drained, pressure must rise from about 85 to 100 psi within a specified time, showing the compressor and system are working.

With the engine off and brakes released, pressure loss must stay within a small limit; with the brakes applied and held, a slightly larger limit applies. Excess leakage must be repaired before driving.

A buzzer and light that activate before pressure falls below a safe level, warning the driver to stop. The parking brakes must then apply automatically as pressure continues to drop.

The lever arm connecting the brake chamber pushrod to the brake shoe. It should move no more than about one inch when the brakes are applied; excessive travel means brakes need adjustment.

The air cylinder that pushes the slack adjuster to apply the brakes. A cracked housing, loose clamp, or chafed hose is a defect that must be fixed before the vehicle is driven.

With the vehicle secured, the driver applies the parking brakes and gently pulls against them in low gear to confirm they hold, verifying the spring brakes can stop and hold the rig.

The tendency of the rear wheels of a long combination to cut inside the path of the front wheels in a turn. Drivers must swing wide and watch mirrors to avoid hitting curbs or vehicles.

The outward sweep of the tractor's rear or the trailer's front end during a turn. It can strike nearby objects or vehicles, so turns must be started wide and taken slowly.

The hazard created when a combination vehicle turns right: the rear of the rig swings left while the trailer cuts the corner, trapping anyone who tries to pass on the right. Never pass a turning rig on the right.

The way sideways movement at the rear of a long combination grows toward the last trailer, so a small steering input can produce a large tail-end swing or trailer sway.

The total distance traveled from perceiving a hazard to a full stop, combining reaction distance and braking distance. It grows with speed, weight, and poor road or brake conditions.

The gap kept behind the vehicle ahead, measured in seconds. Combination vehicles need at least one second per ten feet of vehicle length plus extra time for speed and conditions.

The safety margin around the vehicle on all sides. In a combination, it must account for longer stopping distances, blind spots, and the extra length needed to change lanes or turn.

Areas around a combination vehicle the driver cannot see in the mirrors, including directly behind the trailer and along both sides. Mirrors must be adjusted and checked frequently.

Loss of braking power from overheating, common on long downgrades when brakes are used continuously. Drivers prevent it by gearing down and using brakes in controlled applications.

Using a lower gear and the engine's compression to slow the vehicle on grades, reducing reliance on the service brakes and helping prevent overheating and brake fade.

An emergency escape lane filled with gravel or sand on steep downgrades for drivers who lose braking. Steer into it early and straight rather than trying to continue down.

A loss of control in which the tractor and trailer fold toward each other at the coupling, often from locked wheels or excessive speed on a slippery surface. Prevention means braking gently and steering smoothly.

Loss of tire grip on the road, caused by hard braking, sudden steering, or slippery surfaces. The remedy is to stop braking, ease off the accelerator, and steer in the direction you want to go.

Side-to-side fishtailing of the trailer, often triggered by speed, wind, or uneven loading. Ease off the accelerator and hold the wheel steady rather than braking hard or steering sharply.

A rollover caused by a rapid sideways whip of the trailer after a sudden steering move, common with high-center-of-gravity loads. Smooth, gradual steering prevents it.

An electronic system that rapidly releases and reapplies brake pressure to prevent wheel lockup during hard stops, helping the driver keep steering control on slippery or emergency stops.

The balance point of the loaded vehicle. A high center of gravity makes rollover more likely in turns, so heavy loads should sit low and be spread evenly across the axles.

Placing and balancing cargo so weight falls properly on the tractor and trailer axles, keeping the center of gravity low and staying within axle, tire, and bridge limits.

Cargo sliding or shifting during driving, which can change axle loads, destabilize the trailer, and cause loss of control. Cargo must be braced and secured against movement.

The maximum weight an axle or axle group may carry, set by law and by tire ratings. Drivers use scales to confirm each axle is within limits before and during a trip.

A federal rule limiting the weight allowed on a group of axles based on the number of axles and the distance between them, preventing concentrated loads from overstressing bridges.

The maximum total weight a vehicle or combination is designed to carry, including cargo, fuel, and occupants. Exceeding it is unsafe and illegal and affects braking and handling.

The system of tiedowns, straps, chains, and blocking that keeps cargo from shifting or falling. It must meet minimum working-load standards and be checked during the trip.

A strap, chain, or other device that anchors cargo to the vehicle. Each must be rated for the load and attached to proper anchor points, with enough of them to meet the rules.

The maximum weight a tire may carry at a given inflation pressure. Overloading or underinflation causes heat, blowouts, and poor handling, especially on a heavily loaded trailer.

A place where tracks cross the road. Combination vehicles must slow, check for trains, avoid stopping on tracks, and never attempt to beat a train or cross when the tail may not clear.

Crossing equipment such as flashing lights, bells, and gates that warns of an approaching train. Drivers must obey it and never drive around lowered gates.

Crossing signs and pavement markings, such as the round yellow advance warning sign and crossbuck, that identify the crossing but do not detect trains. Drivers must look and listen.

Weather and road situations such as rain, ice, fog, and high wind that reduce traction and visibility. In these conditions combination drivers slow down, increase following distance, and avoid sudden moves.