After inserting the trailer electrical plug, what should you do with its retaining safety catch?
Correct answer: Fasten it and confirm the cord is secure.
A loose plug can disconnect as the rig moves. Secure the connection and route the cord away from damaging contact.
Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.1, Trailer electrical connection; printed 6-9; physical PDF page 105; reviewed September 21, 2026. ↗
After the coupling is verified, how far should the landing gear be raised before travel?
Correct answer: All the way into its proper travel position.
Partly raised supports can strike a crossing or other obstruction. Fully raise them and secure the operating handle before moving off.
Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.1, Landing gear travel position; printed 6-9; physical PDF page 105; reviewed September 21, 2026. ↗
What should happen to the landing-gear crank after the supports are raised?
Correct answer: Secure the handle in its proper stowed position.
A properly stowed crank cannot swing into hazards or come loose during the trip. Check the manufacturer’s retention arrangement.
Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.1, Landing-gear crank; printed 6-9; physical PDF page 105; reviewed September 21, 2026. ↗
After the trailer’s full weight rests on the tractor, which clearance check matters?
Correct answer: Space between tractor components, landing gear and the trailer during turns.
Check frame-to-landing-gear and tire-to-trailer clearance with the trailer’s weight on the tractor. Correct coupling must also leave adequate clearance for turning.
Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.1, Clearance after load transfer; printed 6-9; physical PDF page 105; reviewed September 21, 2026. ↗
Before uncoupling a loaded semitrailer, what must be true of the parking surface?
Correct answer: It can support the trailer’s weight on its landing gear.
Once uncoupled, the supports transfer weight to the ground. An unsuitable surface can let the landing gear sink or the trailer fall.
Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.2, Uncoupling surface; printed 6-9; physical PDF page 105; reviewed September 21, 2026. ↗
Why should the tractor and trailer be lined up straight before pulling apart?
Correct answer: Pulling out at an angle can damage the landing gear.
The handbook calls for straight alignment to avoid side loads on the supports. Keep the rig secured while preparing to release the coupling.
Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.2, Straight uncoupling alignment; printed 6-9; physical PDF page 105; reviewed September 21, 2026. ↗
During the properly secured uncoupling sequence, why gently back against the kingpin before releasing the fifth wheel?
Correct answer: To ease pressure on the locking jaws.
Relieving tension can make the locking lever easier to release. Follow the complete sequence, including trailer security and tractor parking brakes.
Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.2, Relieving jaw pressure; printed 6-9; physical PDF page 105; reviewed September 21, 2026. ↗
When lowering landing gear on a loaded trailer before uncoupling, what does the handbook advise after firm ground contact?
Correct answer: Ease tractor load without lifting the trailer off the fifth wheel.
After firm ground contact, the handbook uses a few additional low-gear turns to take some weight off the tractor and ease release. Do not jack the trailer completely off the fifth wheel.
Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.2, Loaded-trailer support setting; printed 6-9; physical PDF page 105; reviewed September 21, 2026. ↗
How does the handbook advise stowing the electrical cable after uncoupling?
Correct answer: Hang it securely with the plug facing down to keep moisture out.
The downward-facing plug helps prevent moisture entry. Support the cable so it cannot be damaged during bobtail travel.
Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.2, Disconnected electrical cord; printed 6-9; physical PDF page 105; reviewed September 21, 2026. ↗
Why stop with the tractor frame still under the trailer after the fifth wheel clears it during uncoupling?
Correct answer: The frame offers protection if the landing gear collapses or sinks.
The intermediate stop provides a safeguard before fully leaving the trailer. Secure the tractor and inspect the supports before pulling completely clear.
Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.2, Partial pull-out safeguard; printed 6-9; physical PDF page 105; reviewed September 21, 2026. ↗
Before the tractor moves completely clear of an uncoupled trailer, what should be checked?
Correct answer: That the ground supports the trailer and the landing gear is not damaged.
Confirm the supports are sound and bearing the trailer safely. A failed support can let the trailer fall after the tractor moves away.
Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.2, Support verification before clearing; printed 6-10; physical PDF page 106; reviewed September 21, 2026. ↗
A pintle-hook mount has a missing fastener and visible movement. What is the proper response?
Correct answer: Have the unsafe mounting corrected before towing.
The pintle hook and its mounting must be secure and undamaged. A locked latch cannot make an insecure mounting safe.
Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.3, Pintle-hook mounting; printed 6-10; physical PDF page 106; reviewed September 21, 2026. ↗
Before towing with a pintle coupling, what must be verified after the drawbar eye is seated?
Correct answer: The latch and any required locking pin are properly secured.
Correct seating must be followed by the equipment’s locking procedure. An unsecured latch can permit separation; do not generalize one manufacturer’s handle geometry to every hook.
Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.3 Step 4, Lock Pintle Hook (continued on next page); printed 6-10–6-11; physical PDF pages 106–107; reviewed September 21, 2026. ↗
Under the cited federal rule for air-braked towing units, in what pressure range must the tractor protection valve or similar device operate automatically?
Correct answer: Between 20 and 45 psi.
49 CFR 393.43(a) specifies 20–45 psi. The exact set point and inspection procedure come from the equipment manufacturer; this is distinct from the low-air warning threshold.
Source: 49 CFR 393.43(a); eCFR title 49 current through September 17, 2026; reviewed September 21, 2026. ↗
For an ordinary two-axle trailer required to have brakes, outside driveaway-towaway operations, how long must its automatically applied breakaway brakes remain applied under the cited federal rule?
Correct answer: At least 15 minutes.
49 CFR 393.43(d) requires immediate automatic breakaway application and at least 15 minutes of holding. The question excludes the driveaway-towaway exception and the separate three-or-more-axle application qualification.
Source: 49 CFR 393.43(d), (f); eCFR title 49 current through September 17, 2026; reviewed September 21, 2026. ↗
During the secured check for service air at the back of the last trailer, what must be applied before opening that service-line shut-off briefly?
Correct answer: The trailer hand brake or service brake pedal.
The service line needs an application signal for this test. Close the valve after checking; all intermediate valves must pass air, and the complete procedure requires rollaway protection.
Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.5.2, Service air through the complete rig; printed 6-14; physical PDF page 110; reviewed September 21, 2026. ↗
