Inspecting Combination Vehicles: Practice Questions & Explanations

The fifth wheel inspection covers: (1) Mounting — bolts tight, brackets not cracked or bent, mounting bracket plates intact; (2) Locking jaws — closed around the kingpin shank, not the kingpin head, no visible space between upper and lower fifth wheel; (3) Release arm — properly seated and safety latch in place; (4) Skid plate — adequately greased, no excessive wear; (5) Kingpin engagement — fully seated in the throat, no high hitch; (6) Clearance — adequate space between the tractor frame and the front of the trailer so they will not contact during turns. Defects in any of these can lead to trailer separation while driving, one of the most catastrophic possible failures.

If the trailer air supply line breaks (commonly called a 'glad hand' separation), air pressure in the trailer brake system drops rapidly and the trailer's emergency (spring) brakes apply automatically. This is the fail-safe design: a broken line cannot allow a runaway trailer. The driver should expect a sudden, strong pull and possibly a jackknife if the failure happens during braking. Get off the road as safely as possible — do not try to drive with the trailer emergency brakes applied; this can damage the brakes and may not allow normal driving anyway. After stopping, inspect the air line; commonly a glad hand has popped off the trailer connector, which can be reconnected, but a torn or cut line requires repair before continuing.

When uncoupling a trailer on soft surfaces — wet ground, asphalt softened by heat, gravel, or unpaved lots — the landing gear can sink into the surface under the trailer's weight, causing the trailer to drop after the tractor pulls out. This can damage the trailer, cargo, and surrounding property, and can injure people. Prevention: park trailers on hard, level surfaces whenever possible. On softer surfaces, place landing gear feet on wide boards or steel plates to spread the load. Avoid parking on grades; if you must, chock the wheels in both directions. After lowering landing gear, look to confirm it is firmly contacting the ground before disengaging the fifth wheel.

SLIDING FIFTH WHEELS allow the fifth wheel position to be adjusted forward and backward on the tractor frame. This is used for axle weight distribution — moving the fifth wheel forward puts more weight on the steer axle, moving it back puts more on the drives.

Inspection requirements:

Locking mechanism: The slide must be fully locked before driving — inspect the locking pins or teeth to verify they are fully engaged with the slide rail; pull on the fifth wheel assembly to test for any movement; if the fifth wheel can slide, it is NOT properly locked.

Position marking: Note the position indicator markings; the position should match the weight distribution requirements for the load.

Bracket and rail condition: Check for cracking, deformation, or damage to the slide rail and mounting brackets; damaged slides can fail under load.

Air-operated slides: Some sliding fifth wheels are air-operated; check that the air supply is connected and the air-operated release valve is in the locked/closed position.

Consequences of unlocked slide: If the fifth wheel slides forward while loaded at highway speed, it can cause: sudden steer axle overload; loss of steering control; potential collision; if it slides back, it can reduce steer axle weight to the point of steering instability; an unlocked slide can allow the combination to fold or jackkife if the fifth wheel moves suddenly during braking or turning.

THE CONVERTER DOLLY is the critical connecting device between trailers in a doubles or triples combination. It converts a semitrailer (which has a kingpin) into a 'full trailer' that can be pulled by another trailer.

Inspection items:

Pintle hook: The hook that connects the dolly drawbar to the first trailer's pintle ring; must be latched and the safety clasp engaged; a failed pintle hook separates the rear of the combination.

Fifth wheel on dolly: Connects the second trailer's kingpin to the dolly; inspect the same as any fifth wheel (locking jaws, mounting bolts, plate condition, kingpin engagement); DRAWBAR (TONGUE): The A-frame structure connecting the pintle to the fifth wheel; check for cracks, bending, or damage.

Safety chains: Required as backup if the pintle connection fails; must be crossed under the drawbar and have proper length (enough slack for turns but not dragging).

Landing legs: Some dollies have small landing legs for support when parked; verify they are retracted and secured.

Air and electrical: Verify glad hands (air connections) are properly coupled and sealed; check electrical connector to confirm lights will function; verify both service and emergency lines connected correctly.

Brake function: Apply trailer brakes and observe for pressure drop indication on gauges; check for air leaks around the dolly connections.

Landing gear inspection is critical pre-trip for semitrailers. Partially lowered landing gear that contacts the road during transport destroys the gear and can cause the trailer to swerve. Check: fully cranked up (no gap between pad and frame); handle secured (loose handles fall and become road hazards); structural integrity of the legs; for air-operated gear, release valve in the locked/transport position.

TRAILER REAR INSPECTION covers:

Doors: Sealed, latched, and in a configuration that allows them to open safely when needed — doors left unlatched can swing open at speed; UNDERRIDE GUARD (ICC bumper): Must be structurally sound — protects other vehicles from going under the trailer in a rear collision.

Lights: All rear lights required — brake lights, turn signals, clearance lights, marker lights, license plate light.

Reflective tape: Required conspicuity tape on trailers — check it's present and undamaged.

Damage: Any visible structural damage, cracks, or missing components get documented on the DVIR.

Landing gear inspection: Check that the landing gear (the support legs that hold up the front of a parked trailer) is: FULLY RAISED when traveling (landing gear left partially down can strike the road, railroad crossings, or other obstacles, causing damage or accidents); not bent, damaged, or missing parts; the CRANK HANDLE is secured in its holder (a loose crank handle can swing out); legs and support structure intact.

Landing gear function: Supports the trailer when uncoupled; must be raised before traveling.

Other combination inspection items: fifth wheel (secure mounting, proper lubrication, locking jaws, no gaps or cracks); kingpin and apron (not bent/cracked); air and electrical lines (not damaged, properly connected, not dragging or tangled); the gap between tractor and trailer; sliding fifth wheel locked; trailer frame; combination vehicle inspection includes all the standard items PLUS the coupling system and trailer-specific components; landing gear position is a commonly tested and commonly missed item.

Fifth wheel gap inspection: After coupling, there should be NO GAP between the upper fifth wheel (the trailer's apron plate) and the lower fifth wheel (the plate on the tractor). A GAP indicates: the trailer is not fully and properly seated; possibly a HIGH HITCH (where the kingpin rests on top of the locking mechanism rather than locked in the jaws) — extremely dangerous, can release at speed; the coupling is incomplete.

Proper coupling: The two plates should be flush against each other with the kingpin locked in the jaws; INSPECT from the side with a flashlight.

Lubrication: The fifth wheel plate should be properly greased (reduces friction and wear, allows the plates to pivot) — but grease is not a substitute for proper seating.

Other fifth wheel checks: secure mounting to the frame (no missing/loose bolts); no cracks or damage; locking jaws closed around the kingpin shank; release handle in locked position; sliding fifth wheel locked in place; the no-gap check is a key indicator of proper coupling and is essential to verify before every trip.

Air and electrical line inspection: Check the lines connecting tractor and trailer for: AIR LINES (glad hands/hoses): not cut, cracked, worn through, or damaged; not tangled, pinched, or rubbing against moving parts; not dragging on the ground; securely connected to the correct couplers; no audible air leaks; ELECTRICAL LINE (pigtail/cord): firmly seated and locked in the socket; not cut, cracked, or with exposed wires; not dragging or tangled.

Proper routing: lines should have enough slack to allow turning without stretching or disconnecting, but not so much that they drag or tangle; SECURED so they don't catch on the fifth wheel or moving parts.

Function: air lines operate the trailer brakes (critical safety); the electrical line powers trailer lights, signals, and ABS; DAMAGED LINES can cause: brake failure (air leak), loss of trailer lights/signals (electrical), or a disconnection while driving; these lines are a key combination vehicle inspection item — damaged or improperly routed lines are a common violation and safety hazard; inspect every trip and watch that they don't drag or tangle during turns.

When inspecting the FIFTH WHEEL, check the MOUNTING: it must be SECURELY mounted to the tractor frame with NO missing, loose, or broken mounting bolts or brackets; and there should be NO CRACKS in the fifth wheel itself or its mounting. CDL combination.

Also check: the locking jaws (closed around the kingpin shank after coupling); no gap between the fifth wheel and trailer apron; proper lubrication of the fifth wheel plate; the release arm/handle in the locked position; for a sliding fifth wheel, that it is locked in position. The fifth wheel bears the weight of the trailer front and holds the coupling — a cracked or loosely mounted fifth wheel could fail catastrophically. Knowing to inspect the fifth wheel mounting (secure, no missing/loose bolts, no cracks) is commonly tested.

To test the TRAILER EMERGENCY (spring) BRAKES on a combination: charge the air system fully; then (with the vehicle stopped) PULL OUT the trailer air supply valve (or the system simulates air loss) — this should cause the trailer SPRING BRAKES to APPLY; gently try to pull forward with the tractor to confirm the trailer brakes HOLD. CDL combination. This verifies the fail-safe works (loss of air to the trailer applies its emergency brakes). Also test the trailer SERVICE BRAKES: push the supply valve in (charge), move forward slowly (~5 mph), and apply the trailer hand valve (trolley valve) — the trailer should brake. The combination air brake check includes testing both trailer service and emergency brakes plus the tractor protection valve. Knowing how to test the trailer emergency brakes is commonly tested.

A SLIDING FIFTH WHEEL (one that can be moved forward/backward to adjust weight distribution over the axles) must be: properly POSITIONED for the load (to keep axle weights legal and balance the load); and securely LOCKED in place with the LOCKING PINS fully engaged. CDL combination. If a sliding fifth wheel is NOT locked, it could move while driving — shifting weight, affecting handling, and potentially causing loss of control or the trailer separating. POSITIONING affects axle weights (sliding forward puts more weight on the steer/drive axles; back puts more on the trailer axles) — adjusted to stay within legal weight limits. Always verify the locking pins are fully engaged and the fifth wheel won't slide. Knowing to check that a sliding fifth wheel is positioned correctly and locked is commonly tested.