Which combination best reduces rollover risk in a combination vehicle?
Correct answer: Keep cargo low and slow down for turns
A low, centered load and reduced turn speed keep the center of gravity controlled.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.1.1 Rollover Risks, printed 6-1 (physical PDF page 107) ↗
What can the crack-the-whip effect do during a sudden lane change?
Correct answer: Cause the rear trailer to overturn
Rearward amplification makes steering movement more severe toward the back of a multi-unit combination.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.1.2 Steer Gently, printed 6-1 (physical PDF page 107) ↗
Why can an empty combination vehicle be harder to stop smoothly than a fully loaded one?
Correct answer: Light loading can cause wheel lock and poor traction
Light loads can provide poor traction, making wheel lock, trailer swing, and jackknife more likely.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.1.3 Brake Early, printed 6-1 (physical PDF page 107) ↗
What should a driver do first if a low-clearance combination becomes stuck on railroad tracks?
Correct answer: Leave the vehicle and move away from the tracks
Get everyone out of the vehicle and away from the tracks. From safety, call for emergency help and provide the crossing location, including its posted DOT identification when available.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.1.4 Railroad-Highway Crossings, printed 6-2 (physical PDF page 108) ↗
What is the earliest reliable sign that a trailer has started to skid?
Correct answer: Trailer misalignment appears in the mirrors
Frequent mirror checks during hard braking reveal trailer swing before a jackknife develops.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.1.5 Prevent Trailer Skids — Recognize the Skid, printed 6-3 (physical PDF page 109) ↗
A trailer without functioning ABS has locked wheels and starts swinging during braking. What action addresses the skid?
Correct answer: Release the brakes to restore traction
Release the service brakes so the locked trailer wheels can regain grip. Do not apply the trailer hand valve to try to straighten the rig; added trailer braking can worsen this skid. Follow the vehicle's ABS instructions when equipped.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.1.5 Prevent Trailer Skids — Stop Using the Brake, printed 6-3 (physical PDF page 109) ↗
What does off-tracking mean in a turn?
Correct answer: Rear wheels cut inside the front-wheel path
Long vehicles’ rear wheels cut inside the path of their front wheels, with the last trailer cutting in most.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.1.6 Turn Wide, printed 6-3 (physical PDF page 109) ↗
If straight-line trailer backing is not possible and a curved path is necessary, which side generally gives the driver a better view?
Correct answer: The driver’s side
Driver-side backing provides the clearest view; straight-line backing is preferable whenever possible.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.1.7 Backing with a Trailer, printed 6-4 (physical PDF page 110) ↗
What is the proper routine use of a trailer hand valve?
Correct answer: Testing the trailer brakes
The hand valve is for testing trailer brakes; normal braking should use the foot pedal.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.2.1 Trailer Hand Valve, printed 6-5 (physical PDF page 111) ↗
What is the tractor protection valve designed to do?
Correct answer: Preserve tractor air after a major trailer leak
The valve closes to protect the tractor air supply when trailer-side pressure is lost.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.2.2 Tractor Protection Valve, printed 6-5 (physical PDF page 111) ↗
On a newer tractor, what does pulling out the red eight-sided trailer-air-supply knob do?
Correct answer: Shuts off trailer air and applies trailer emergency brakes
Pulling the red knob closes the supply; pushing it in supplies air to the trailer.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.2.3 Trailer Air Supply Control, printed 6-5 (physical PDF page 111) ↗
What are the two air lines connecting the tractor and trailer in an air-braked combination?
Correct answer: Service and emergency
The service line carries brake-control pressure, while the emergency line supplies tanks and controls emergency braking.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.2.4 Trailer Air Lines, printed 6-5 (physical PDF page 111) ↗
What happens when pressure is lost from the trailer emergency line?
Correct answer: Trailer emergency braking and tractor protection activate
Loss of supply-line pressure triggers trailer emergency braking and protects the tractor air supply.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.2.4 Trailer Air Lines — Emergency Air Line, printed 6-6 (physical PDF page 112) ↗
Which color normally identifies the trailer emergency air line?
Correct answer: Red
Emergency or supply lines are normally red; service lines are normally blue.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.2.4 Trailer Air Lines; 6.2.5 Hose Couplers (Glad Hands), printed 6-6 (physical PDF page 112) ↗
What danger can result from crossing the trailer service and emergency air lines?
Correct answer: Tanks may not charge; brakes may fail
Crossed connections may prevent spring-brake release or, on older trailers, allow movement without trailer brakes.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.2.5 Hose Couplers (Glad Hands), printed 6-6 (physical PDF page 112) ↗
During normal operation—not an airflow test—how should trailer air-line shut-off valves be positioned in a multi-trailer combination?
Correct answer: Open through the rig; closed at the final rear
Air must pass through each unit, so intermediate shut-off valves remain open. Close the unused service and supply outlets at the rear of the last trailer. Opening those final valves temporarily to test airflow is a separate inspection step.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.2.7 Shut-off Valves; airflow-test distinction at 6.5.2, printed 6-16 / physical 122, printed 6-7 (physical PDF page 113) ↗
Why must wheel chocks be used when parking an older trailer without spring brakes?
Correct answer: Stored air can leak away and release its emergency brakes
Air-operated emergency brakes on such trailers hold only while stored pressure remains.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.2.8 Trailer Service, Parking and Emergency Brakes, printed 6-7 (physical PDF page 113) ↗
Where is a trailer’s yellow ABS malfunction lamp normally located?
Correct answer: Left-side front or rear corner
A trailer ABS malfunction lamp is yellow and located on the left side near a front or rear corner. Do not confuse it with the red trailer-air-supply control in the tractor cab.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.3.1 Trailers Required to Have ABS, printed 6-8 (physical PDF page 114) ↗
Why must a fifth-wheel plate that requires grease be properly lubricated before coupling?
Correct answer: Excess friction can create steering problems
Where lubrication is required, excess friction between the tractor and trailer can cause steering problems. Learn the coupling details and maintenance requirements for the equipment you operate.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.4.1 Coupling Tractor-Semitrailers — Step 1 Inspect Fifth Wheel, printed 6-9 (physical PDF page 115) ↗
How should the trailer height be set before backing fully under it?
Correct answer: Low enough to be raised slightly by the tractor
Set the trailer so the tractor raises it slightly while backing under. Too high can prevent proper coupling; too low can damage the trailer nose. Check kingpin and fifth-wheel alignment.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.4.1 Coupling Tractor-Semitrailers — Step 6 Check Trailer Height, printed 6-9 (physical PDF page 115) ↗
How is the tractor-trailer connection initially tested after the kingpin locks?
Correct answer: With supports slightly raised, gently tug against locked trailer brakes
A gentle pull against the locked trailer brakes is an initial coupling check, not complete proof. Secure the vehicle and also visually check that the fifth-wheel plates meet and the jaws are locked around the kingpin shank before driving.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.4.1 Coupling Tractor-Semitrailers — Step 11 Check Connection for Security; Steps 12–13, printed 6-10 (physical PDF page 116) ↗
Where should the fifth-wheel locking jaws be after proper coupling?
Correct answer: Around the shank of the kingpin
Secure the vehicle before looking underneath. Check that the jaws have closed around the kingpin shank, not its head, and that there is no gap between the upper and lower fifth-wheel plates. Also inspect the locking lever and safety latch.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.5.1 Coupling System Areas and Figure 6.11, printed 6-15 (physical PDF page 121); secure-vehicle procedure at 6.4.1 steps 12–13, printed 6-10 / physical 116 ↗
Before driving a coupled tractor-semitrailer, how far should the landing gear be raised?
Correct answer: Fully raised with the crank secured
Partly raised gear can strike tracks or road objects and must be fully stowed.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.4.1 Coupling Tractor-Semitrailers — Step 15 Raise Front Trailer Supports, printed 6-10 (physical PDF page 116) ↗
While uncoupling a tractor-semitrailer, why stop with the fifth wheel clear but the tractor frame still under the trailer?
Correct answer: Support the trailer if its landing gear fails
Keeping the tractor frame under the trailer provides temporary support while the landing gear is checked.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.4.2 Uncoupling Tractor-Semitrailers — Step 7 Pull Tractor Partially Clear of Trailer, printed 6-11 (physical PDF page 117) ↗
On a combination equipped with a trailer hand valve, how are the trailer service brakes checked before normal operation?
Correct answer: Move slowly and test with the trailer hand valve
Check normal air pressure, release the parking brakes and move forward slowly in a safe test area. Apply the trailer hand control, if equipped, and feel the trailer brakes engage. During normal driving use the foot pedal to control service brakes at all wheels.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.5.2 Combination Vehicle Brake Check — Test Trailer Service Brakes, printed 6-16 (physical PDF page 122) ↗
You find dirt on a glad-hand coupling and its rubber seal before connecting a trailer. What should you do?
Correct answer: Clean the coupler and seal before making the connection
Clean couplers and seals before connecting them. The rubber seal prevents air from escaping, and keeping contamination out protects the air system.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.2.5 Hose Couplers (Glad Hands), printed 6-6 (physical PDF page 112) ↗
After disconnecting a trailer, where should the tractor’s air-line couplers be attached if dummy couplers are provided?
Correct answer: To the dummy couplers to keep dirt and water out
Dummy or dead-end couplers protect unused air lines from water and dirt. Use them when the hoses are not connected to a trailer.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.2.5 Hose Couplers (Glad Hands), printed 6-6 (physical PDF page 112) ↗
When you press the foot brake, what determines how much pressure the relay valves send from the trailer reservoirs to its brakes?
Correct answer: The pressure in the service line
Service-line pressure communicates the braking demand to the relay valves. The trailer’s own reservoirs supply the air sent to its brakes.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.2.6 Trailer Air Tanks, printed 6-7 (physical PDF page 113) ↗
A major trailer service-line leak may become obvious only when you apply the brakes. What can happen then?
Correct answer: Air pressure can fall rapidly and eventually apply the trailer emergency brakes
A service-line leak may not be apparent until braking pressurizes that line. The resulting air loss can rapidly lower tank pressure; at sufficiently low pressure, trailer emergency brakes can apply.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.2.8 Trailer Service, Parking and Emergency Brakes, printed 6-7 (physical PDF page 113) ↗
A tractor-trailer has ABS on the trailer but not on the tractor. Which statement is correct?
Correct answer: Trailer ABS still helps, but the tractor can still lose control under braking
Trailer ABS can reduce trailer swing, but it cannot prevent tractor wheel lock. Use only the force needed to stop safely. If steering control is lost or a tractor jackknife starts, ease off the brakes if safe until control returns. ABS does not justify a shorter following gap.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.3.2 Braking with ABS, printed 6-8 (physical PDF page 114) ↗
Does an automatic drain eliminate the handbook’s advice to check trailer air tanks for accumulated moisture?
Correct answer: No; open the drains to check for remaining moisture
The handbook says to drain each trailer air tank daily and still open drains to check tanks with automatic drains. Water and oil accumulation can interfere with correct brake operation.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.2.6 Trailer Air Tanks, printed 6-7 (physical PDF page 113) ↗
Before backing a tractor under a trailer, what must prevent the trailer from rolling?
Correct answer: Applied spring brakes or appropriate wheel chocks
Coupling forces can move an unsecured trailer. Check the area and its securement first.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.4.1 Coupling Tractor-Semitrailers — Step 2 Inspect Area and Chock Wheels, printed 6-9 (physical PDF page 115) ↗
Why should the tractor be straight in front of the trailer before coupling?
Correct answer: An angled approach can damage the landing gear
Align the units rather than use the tractor to force the trailer sideways.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.4.1 Coupling Tractor-Semitrailers — Step 3 Position Tractor, printed 6-9 (physical PDF page 115) ↗
How should air lines be arranged before you finish backing under the trailer?
Correct answer: Supported clear of crushing and snagging points
Line support must protect the connections during coupling and later movement.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.4.1 Coupling Tractor-Semitrailers — Step 7 Connect Air Lines to Trailer, printed 6-9 (physical PDF page 115) ↗
What reverse gear and speed are appropriate for the final coupling movement?
Correct answer: Lowest reverse gear at a slow, controlled speed
Avoid striking the kingpin hard while completing the connection.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.4.1 Coupling Tractor-Semitrailers — Step 10 Back Under Trailer, printed 6-10 (physical PDF page 116) ↗
Before going underneath to inspect the coupling, how should you secure the tractor?
Correct answer: Parking brakes on, transmission neutral, engine off, and key with you
Prevent someone moving the rig while you inspect underneath.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.4.1 Coupling Tractor-Semitrailers — Step 12 Secure Vehicle, printed 6-10 (physical PDF page 116) ↗
You see a gap between the trailer’s upper plate and the fifth-wheel plate after coupling. What should you do?
Correct answer: Do not drive until the coupling problem is corrected
A gap can mean the kingpin is resting above the jaws rather than being correctly locked.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.4.1 Coupling Tractor-Semitrailers — Step 13 Inspect Coupling, printed 6-10 (physical PDF page 116) ↗
Besides checking the jaws around the kingpin, which locking parts need inspection?
Correct answer: The locking lever and its safety latch
Verify the complete locking mechanism rather than relying on a tug test alone.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.4.1 Coupling Tractor-Semitrailers — Step 13 Inspect Coupling, printed 6-10 (physical PDF page 116) ↗
Once the trailer’s weight rests on the tractor, which clearances must you check?
Correct answer: Tractor frame to landing gear and tractor tires to trailer nose
Turning or suspension movement must not cause contact between the units.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.4.1 Coupling Tractor-Semitrailers — Step 15 Raise Front Trailer Supports, printed 6-10 (physical PDF page 116) ↗
Before uncoupling a loaded trailer, what must you know about the parking surface?
Correct answer: It can support the trailer and landing gear
Landing gear can sink or collapse on an unsuitable surface.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.4.2 Uncoupling Tractor-Semitrailers — Step 1 Position Rig, printed 6-11 (physical PDF page 117) ↗
You are unsure whether the parked trailer has spring brakes. What should you do before uncoupling?
Correct answer: Chock its wheels
Stored air can leak away; use chocks when spring-brake protection is uncertain.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.4.2 Uncoupling Tractor-Semitrailers — Step 3 Chock Trailer Wheels, printed 6-11 (physical PDF page 117) ↗
When uncoupling a loaded trailer, how far should you lower the landing gear after firm ground contact?
Correct answer: A few extra low-gear turns, without lifting off the fifth-wheel
Support some weight to ease release without creating an unsafe gap or over-lifting.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.4.2 Uncoupling Tractor-Semitrailers — Step 4 Lower the Landing Gear, printed 6-11 (physical PDF page 117) ↗
How should the disconnected trailer electrical cable be stored?
Correct answer: Supported, plug downward to keep moisture out
Protect the plug from moisture and the cable from moving parts or road damage.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.4.2 Uncoupling Tractor-Semitrailers — Step 5 Disconnect Air Lines and Electrical Cable, printed 6-11 (physical PDF page 117) ↗
While operating the fifth-wheel release, where should your legs and feet be?
Correct answer: Clear of the tractor’s rear wheels
Unexpected vehicle movement can cause severe injury near the wheels.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.4.2 Uncoupling Tractor-Semitrailers — Step 6 Unlock Fifth Wheel, printed 6-11 (physical PDF page 117) ↗
You have pulled the fifth-wheel clear but kept the tractor frame under the trailer. What should you verify before pulling away fully?
Correct answer: The landing gear is sound and the ground supports the trailer
After pulling partly clear, set the tractor parking brake and select neutral. Check that the ground supports the trailer and the landing gear is undamaged before releasing the brake and pulling fully clear.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.4.2 Uncoupling Tractor-Semitrailers — Steps 8–9 Secure Tractor / Inspect Trailer Supports; Step 10, printed 6-11 (physical PDF page 117) ↗
