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Oklahoma CDL Combination study materials
Oklahoma CDL Combination study resources

Oklahoma CDL Combination practice questions

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Question 1Combination Vehicle Basics and Rollover Risks

Before driving a combination vehicle, you must check that the trailer air supply and trailer service lines are properly connected. Which statement describes the correct coupling?

  1. The service line is connected to the red glad hand and the emergency (supply) line to the blue glad hand.
  2. The service line is connected to the blue glad hand and the emergency (supply) line to the red glad hand. Correct answer.
  3. The lines should be left disconnected until the trailer brakes are released.
  4. Both lines connect to the same glad hand, and the color does not matter.

Explanation

In a standard tractor-trailer coupling, the blue glad hand carries the service line that applies the trailer brakes, and the red glad hand carries the emergency or supply line that charges the trailer air tanks and releases the spring brakes. Reversing them prevents proper brake operation and can leave the trailer brakes locked or unreleased.

Question 2Combination Vehicle Basics and Rollover Risks

You are preparing to back a tractor-trailer under a parked trailer. At what point should you stop and check the coupling height?

  1. After the trailer kingpin has locked into the fifth wheel.
  2. Only after the landing gear has been raised off the ground.
  3. After you pull forward to test the coupling.
  4. When the fifth wheel is just under the trailer apron, before the kingpin enters the jaws. Correct answer.

Explanation

You should stop when the fifth wheel is just beneath the trailer apron so you can confirm the trailer height matches the fifth wheel before the kingpin enters the jaws. If the trailer is too high, the kingpin can pass over the fifth wheel; if too low, it can strike the rear of the tractor.

Question 3Combination Vehicle Basics and Rollover Risks

After coupling to a trailer, how do you perform a proper pull test?

  1. Set the trailer brakes, then pull forward gently against them to confirm the kingpin is locked. Correct answer.
  2. Lower the landing gear and pull forward until the trailer moves.
  3. With the trailer brakes released, pull forward gently and feel for resistance from the locked trailer brakes.
  4. Release the parking brake and accelerate hard for several feet.

Explanation

The pull test is done with the trailer parking brakes set and the tractor pulling gently against them. If the kingpin is not locked, the trailer will not hold and the tractor will move forward, revealing a bad coupling.

Question 4Combination Vehicle Basics and Rollover Risks

Why must the trailer landing gear be fully raised before you drive away after coupling?

  1. To prevent the landing gear from striking the road or railroad tracks and damaging the trailer. Correct answer.
  2. To allow the trailer brakes to release.
  3. To keep the trailer electrical cord from dragging.
  4. To reduce aerodynamic drag on the trailer.

Explanation

Landing gear left down can catch on pavement, railroad crossings, or dock edges and be torn off or cause loss of control. Raising it completely is a required step in the coupling procedure.

Question 5Combination Vehicle Basics and Rollover Risks

When uncoupling a tractor-trailer on level ground, which step comes first?

  1. Pull the tractor forward slightly to clear the kingpin.
  2. Release the trailer spring brakes.
  3. Disconnect the glad hands and electrical cord.
  4. Lower the landing gear until it touches the ground. Correct answer.

Explanation

The landing gear must be lowered and firmly on the ground before the trailer is released from the tractor so the trailer does not drop. Only after the gear supports the trailer should you disconnect lines and pull out.

Question 6Combination Vehicle Basics and Rollover Risks

What is the main reason a combination vehicle is more likely to roll over than a single vehicle?

  1. Combination vehicles have a shorter stopping distance.
  2. The tractor's steering axle carries most of the cargo weight.
  3. Combination vehicles are exempt from rollover physics.
  4. The trailer has a higher center of gravity and the combination has a longer wheelbase. Correct answer.

Explanation

A loaded trailer raises the center of gravity, and the long combination responds slowly to steering input. The combination can roll over at lower speeds than a passenger car, especially in curves and during evasive maneuvers.

Question 7Combination Vehicle Basics and Rollover Risks

You are driving a loaded tractor-trailer through a curve posted at 35 mph. Your speed is 50 mph. What is the safest action?

  1. Slow down before entering the curve to a safe speed. Correct answer.
  2. Accelerate through the curve to stabilize the trailer.
  3. Maintain speed because the trailer will hold the road.
  4. Brake hard in the middle of the curve to scrub speed.

Explanation

Speed must be reduced before the curve, while the vehicle is still traveling straight. Braking hard inside a curve can cause a jackknife or loss of control, and carrying too much speed into a curve is a leading cause of rollover.

Question 8Combination Vehicle Basics and Rollover Risks

What is the correct way to check that the trailer brakes are working after coupling?

  1. Rely on the tractor brakes alone to stop the combination.
  2. Release all air from the system and watch the gauges.
  3. Apply the trailer hand valve or test knob and confirm the trailer brakes hold. Correct answer.
  4. Pull forward with the trailer brakes released and listen for air leaks.

Explanation

The trailer brakes must be tested separately to confirm they apply and hold. Applying the trailer hand valve or test knob while the combination is stationary or gently pulling confirms the trailer brakes are functioning.

Question 9Combination Vehicle Basics and Rollover Risks

During a pre-trip inspection of a combination vehicle, you notice the trailer air pressure drops faster than normal when the engine is off. What is the most likely concern?

  1. There is an air leak in the trailer or coupling lines. Correct answer.
  2. The trailer is overloaded.
  3. The fifth wheel is too far forward.
  4. The landing gear is too low.

Explanation

A rapid pressure drop with the engine off indicates an air leak somewhere in the system, such as the glad hands, lines, or trailer tanks. Air leaks reduce braking ability and must be repaired before driving.

Question 10Combination Vehicle Basics and Rollover Risks

You are driving a combination vehicle and the trailer begins to swing side to side. What should you do?

  1. Brake hard to stop the swing.
  2. Accelerate to pull the trailer straight.
  3. Steer sharply in the opposite direction.
  4. Ease off the accelerator and steer steadily to bring the trailer back in line. Correct answer.

Explanation

Trailer sway is corrected by easing off the accelerator and making smooth, steady steering corrections. Hard braking or sharp steering can worsen the sway and lead to a jackknife or rollover.

Question 11Combination Vehicle Basics and Rollover Risks

What is the proper way to adjust the trailer brakes when the combination is empty versus loaded?

  1. Adjust the trailer brakes to apply slightly before the tractor brakes when empty.
  2. Adjust the trailer brakes to apply slightly before the tractor brakes when loaded. Correct answer.
  3. Disconnect the trailer brakes when empty.
  4. Use the same brake adjustment regardless of load.

Explanation

When loaded, the trailer brakes should apply slightly before the tractor brakes so the trailer does not push the tractor. When empty, the trailer brakes should apply slightly after the tractor brakes to avoid locking the light trailer wheels.

Question 12Combination Vehicle Basics and Rollover Risks

Why should you avoid sudden lane changes in a combination vehicle?

  1. The trailer brakes will automatically apply.
  2. The electrical cord will disconnect.
  3. The trailer can swing out and the combination can roll over. Correct answer.
  4. The fifth wheel will unlock.

Explanation

Sudden steering inputs cause the trailer to swing and shift weight, which can lead to a rollover or jackknife. Smooth, gradual lane changes give the trailer time to follow the tractor.

Question 13Combination Vehicle Basics and Rollover Risks

What does the term "off-tracking" mean when driving a combination vehicle?

  1. The tractor's rear wheels lift off the ground.
  2. The trailer brakes apply later than the tractor brakes.
  3. The trailer follows a tighter path than the tractor in a turn. Correct answer.
  4. The trailer follows a wider path than the tractor in a turn.

Explanation

Off-tracking, or rearward amplification, means the trailer wheels track inside the tractor's path in a turn. Drivers must swing wide enough and watch the trailer wheels in mirrors to avoid striking curbs or objects.

Question 14Combination Vehicle Basics and Rollover Risks

When making a right turn in a combination vehicle, what should you do to compensate for off-tracking?

  1. Accelerate through the turn to keep the trailer in line.
  2. Turn sharply and quickly to clear the corner.
  3. Swing wide to the left before turning and watch the trailer wheels in the mirrors. Correct answer.
  4. Turn from the right lane without swinging wide.

Explanation

Swinging wide before a right turn and monitoring the trailer wheels in the mirrors helps keep the trailer from cutting the corner and hitting the curb or pedestrians. Turning sharply or quickly increases the risk of a rollover.

Question 15Combination Vehicle Basics and Rollover Risks

According to Oklahoma's CDL guidance, what is the recommended following distance for a combination vehicle traveling at 55 mph on dry pavement?

  1. One car length for each 10 mph of speed.
  2. Two seconds total regardless of speed.
  3. One second for each 10 feet of vehicle length, plus one additional second. Correct answer.
  4. One second total regardless of speed.

Explanation

Oklahoma's CDL guideline calls for one second of following distance for each 10 feet of vehicle length, plus one additional second at speeds above 40 mph. At 55 mph, include that extra second. Conditions may require a larger gap.

Question 16Combination Vehicle Basics and Rollover Risks

You are descending a long downgrade in a loaded combination vehicle. Which technique helps prevent brake fade and loss of control?

  1. Ride the brakes continuously to hold a steady speed.
  2. Select a lower gear before the grade and use engine braking with intermittent brake applications. Correct answer.
  3. Shift to neutral and coast down the grade.
  4. Accelerate to get down the grade quickly.

Explanation

Selecting a lower gear before the downgrade uses engine braking to control speed and reduces reliance on the service brakes. Continuous braking overheats the brakes and causes fade, which can lead to a runaway.

Question 17Combination Vehicle Basics and Rollover Risks

What is a jackknife, and what commonly causes it?

  1. The landing gear drops while driving.
  2. The tractor and trailer fold at an angle, often from hard braking or sudden steering on slippery surfaces. Correct answer.
  3. The fifth wheel locks the trailer in place.
  4. The trailer tires blow out from overloading.

Explanation

A jackknife occurs when the tractor and trailer fold at an angle, often because of hard braking that locks the drive wheels or sudden steering on a low-traction surface. Smooth braking and steering help prevent it.

Question 18Combination Vehicle Basics and Rollover Risks

Why should the heaviest cargo be placed as low as possible in a trailer?

  1. To reduce the trailer's tire pressure.
  2. To increase the trailer's aerodynamic drag.
  3. To make the trailer easier to couple.
  4. To lower the center of gravity and reduce rollover risk. Correct answer.

Explanation

Placing heavy cargo low keeps the center of gravity low, which reduces the tendency to roll over in curves or evasive maneuvers. High loads raise the center of gravity and increase rollover risk.

Question 19Combination Vehicle Basics and Rollover Risks

You are driving a combination vehicle on a wet road and need to stop quickly. What is the best approach?

  1. Pump the brakes rapidly and turn the wheel.
  2. Accelerate to get around the hazard.
  3. Release the accelerator and brake smoothly and firmly while keeping the steering steady. Correct answer.
  4. Brake hard and steer sharply to avoid the obstacle.

Explanation

On wet roads, smooth and firm braking with steady steering helps maintain control and reduces the chance of a skid or jackknife. Hard braking with sharp steering can cause the combination to lose traction.

Question 20Combination Vehicle Basics and Rollover Risks

What is the purpose of the trailer spring brakes, and what happens when air pressure is lost?

  1. They are disconnected during normal driving.
  2. They only work when the trailer is empty.
  3. They release automatically when air pressure drops, allowing the trailer to roll.
  4. They apply automatically when air pressure drops, stopping the trailer. Correct answer.

Explanation

Spring brakes are held off by air pressure and apply automatically when pressure drops, providing a fail-safe brake. This is why a trailer with a major air loss will lock its brakes rather than roll freely.

Question 21Coupling and Uncoupling

Before backing a tractor under a semi-trailer to couple, what should you do with the trailer?

  1. Raise the landing gear fully so the trailer rests on the tractor frame
  2. Lower the landing gear until the trailer nose touches the ground
  3. Remove the kingpin lock so the fifth wheel can slide under the trailer
  4. Chock the trailer wheels and confirm the trailer is at the correct height Correct answer.

Explanation

The trailer must be secured so it cannot roll, and the trailer apron should be raised or lowered so the fifth wheel will slide under the apron and contact the kingpin plate squarely. Chocking the wheels prevents the trailer from creeping during the push. A trailer set too high or too low will cause the tractor frame or fifth wheel to strike the trailer instead of coupling cleanly.

Question 22Coupling and Uncoupling

What is the correct order when coupling a tractor to a semi-trailer?

  1. Raise the landing gear first, back under the trailer, then secure the coupling
  2. Inspect the fifth wheel, back under the trailer, secure the coupling, then connect air lines and raise the landing gear Correct answer.
  3. Connect the air lines first, back under the trailer, then check the coupling
  4. Back under the trailer, connect air lines, then check the fifth wheel is locked

Explanation

Coupling begins with an inspection of the fifth wheel and kingpin area, then you back under the trailer until the fifth wheel locks. Only after the coupling is secure do you connect the air lines, build air pressure, and raise the landing gear. Doing steps out of order risks a dropped trailer or a trailer that rolls when the lines are connected.

Question 23Coupling and Uncoupling

After the fifth wheel closes on the kingpin, how do you confirm the coupling is secure?

  1. Have someone stand between the tractor and trailer to watch the gap close
  2. Listen for the air lines to pressurize and then release the tractor brakes
  3. Pull the tractor forward gently while watching the trailer, or tug-test with the tractor in a low gear Correct answer.
  4. Look at the release handle and assume it returned to the locked position

Explanation

A visual check of the release handle and a pull test are both required to confirm the kingpin is captured. With the trailer brakes set, you gently pull against the trailer and watch for any gap opening between the tractor and trailer. If the trailer moves toward you, the coupling did not lock and must be corrected before driving.

Question 24Coupling and Uncoupling

What must be done with the trailer air supply and parking brake valves before you begin uncoupling?

  1. Open the trailer air supply valve to drain the trailer tanks
  2. Disconnect the electrical cord before setting any brakes
  3. Apply the trailer parking brakes and confirm the trailer is chocked or otherwise secured Correct answer.
  4. Release the trailer parking brakes so the trailer can roll free

Explanation

The trailer must be held stationary before the tractor is pulled out from under it. Set the trailer parking brakes, and chock the wheels if the ground is sloped or the brakes are suspect. Releasing the brakes or draining the trailer air supply would let the trailer roll when the tractor leaves.

Question 25Coupling and Uncoupling

When uncoupling, why should you lower the landing gear until it touches the ground before pulling the tractor out?

  1. So the kingpin can be removed from the fifth wheel
  2. So the trailer apron is supported and the trailer does not drop when the tractor leaves Correct answer.
  3. So the trailer brakes release automatically
  4. So the air lines can be disconnected without losing pressure

Explanation

The landing gear carries the trailer's weight once the tractor is gone. Lower the gear firmly to the ground, and if the trailer is heavily loaded, crank it slightly to lift the trailer off the fifth wheel. If the gear is not down, the trailer nose will drop and may damage the trailer, the landing gear, or the tractor.

Question 26Coupling and Uncoupling

Which glad hand color connects to the trailer emergency (supply) line on a standard tractor-trailer?

  1. Green
  2. Yellow
  3. Red Correct answer.
  4. Blue

Explanation

On standard color-coded combinations the red glad hand is the emergency or supply line, and the blue glad hand is the service line. The lines are crossed so they cannot be swapped accidentally. Matching color to coupler prevents loss of trailer air supply and brake failure.

Question 27Coupling and Uncoupling

You are coupling and notice the trailer apron is too low, so the tractor frame will hit it. What is the correct action?

  1. Raise the trailer with the landing gear until the apron clears the tractor frame Correct answer.
  2. Back under faster so the fifth wheel lifts the trailer
  3. Lower the fifth wheel plate to slide under the apron
  4. Let some air out of the tractor tires to reduce frame height

Explanation

If the trailer is too low, the tractor frame or fifth wheel will strike the trailer apron instead of sliding under it. Use the landing gear to raise the trailer nose until the apron clears the tractor frame. Adjusting tire pressure or forcing the coupling can damage equipment and does not solve the height mismatch.

Question 28Coupling and Uncoupling

After coupling, you connect the air lines and build pressure. What should you check before raising the landing gear?

  1. That the trailer spring brakes are caged
  2. That the trailer brakes release and hold with the tractor protection valve Correct answer.
  3. That the electrical cord is unplugged
  4. That the fifth wheel release handle is tied open

Explanation

With the lines connected and pressure built, operate the trailer brakes to confirm air is reaching the trailer and the brakes apply and release properly. This verifies both the emergency and service lines are correctly connected. The landing gear stays down until the coupling and air system are confirmed good.

Question 29Coupling and Uncoupling

During a pull test after coupling, the trailer moves forward with the tractor. What does this indicate?

  1. The landing gear needs to be raised
  2. The trailer brakes are working correctly
  3. The air lines are crossed
  4. The fifth wheel jaws have not locked around the kingpin Correct answer.

Explanation

In a proper pull test the trailer brakes are set, so a locked coupling holds the trailer in place while the tractor strains forward. If the trailer follows the tractor, the kingpin is not captured and the coupling is unsafe. Stop, pull forward clear, and recouple before driving.

Question 30Coupling and Uncoupling

What is the proper sequence for disconnecting a tractor from a trailer?

  1. Disconnect air lines, lower landing gear, release trailer brakes, pull out
  2. Lower landing gear, release fifth wheel, disconnect lines, set trailer brakes
  3. Release fifth wheel, lower landing gear, disconnect lines, pull out
  4. Secure trailer, lower landing gear, disconnect air and electrical lines, release fifth wheel, pull out Correct answer.

Explanation

Uncoupling starts by securing the trailer with its parking brakes and chocks, then lowering the landing gear to the ground. Next disconnect the air lines and electrical cord and secure them on the tractor, then release the fifth wheel and pull the tractor slowly out. Releasing the fifth wheel before the trailer is supported can drop the trailer.

Question 31Coupling and Uncoupling

Why should you place the air line couplers and electrical cord in their holders on the tractor after uncoupling?

  1. To keep them clean and prevent damage from dragging or road debris Correct answer.
  2. To keep the trailer brakes applied
  3. To keep the fifth wheel jaws open
  4. To prevent the trailer from rolling

Explanation

Loose lines and cords can drag on the ground, fill with dirt, or be crushed by the tractor. Storing them in the holders keeps the couplers clean and the cord protected for the next coupling. This step does not affect trailer brakes, fifth wheel jaws, or trailer movement.

Question 32Coupling and Uncoupling

You are uncoupling on a slight slope. What additional precaution is required beyond setting the trailer parking brakes?

  1. Raise the landing gear completely off the ground
  2. Leave the tractor transmission in neutral
  3. Disconnect the electrical cord first
  4. Chock the trailer wheels Correct answer.

Explanation

On any slope, trailer parking brakes alone may not be enough if the brakes leak or are out of adjustment. Chocking the wheels gives a positive mechanical stop before the tractor is removed. Leaving the transmission in neutral or raising the gear off the ground would make the situation less safe.

Question 33Coupling and Uncoupling

What is the purpose of the fifth wheel release handle?

  1. To adjust the height of the fifth wheel plate
  2. To lock the landing gear in the raised position
  3. To connect the trailer air supply line
  4. To unlock the jaws so the kingpin can enter or leave the fifth wheel Correct answer.

Explanation

The release handle operates the locking mechanism that holds the kingpin in the fifth wheel. Pulling it opens the jaws so the kingpin can be released or inserted, and it must return to the locked position after coupling. It does not adjust height, connect air, or affect the landing gear.

Question 34Coupling and Uncoupling

After backing under a trailer, the fifth wheel locks but you cannot see a gap between the tractor and trailer. What should you do next in the coupling procedure?

  1. Immediately raise the landing gear and drive away
  2. Connect the air lines, build pressure, and check the trailer brakes Correct answer.
  3. Disconnect the electrical cord to test the coupling
  4. Release the fifth wheel and try again

Explanation

Once the coupling is locked and verified, the next steps are connecting the air lines, building air pressure, and testing that the trailer brakes apply and release. Raising the landing gear comes after the air system check. Driving away or releasing the coupling prematurely skips required safety checks.

Question 35Coupling and Uncoupling

Why is it important to check that the fifth wheel jaws completely surround the kingpin after coupling?

  1. The jaws operate the trailer parking brakes
  2. A partially engaged kingpin can release and drop the trailer while driving Correct answer.
  3. The jaws adjust the trailer height
  4. The jaws control the trailer air pressure

Explanation

The jaws must close fully around the kingpin shank to hold the trailer. If the kingpin is only partly engaged, vibration and load can cause the coupling to release, dropping the trailer. The jaws have no role in air pressure, height adjustment, or trailer brakes.

Question 36Coupling and Uncoupling

You are coupling and the trailer has a damaged or missing kingpin plate. What is the correct action?

  1. Connect only the service air line to compensate
  2. Do not couple the trailer until the plate is repaired or replaced Correct answer.
  3. Couple anyway and drive slowly
  4. Use the landing gear to lift the trailer onto the fifth wheel

Explanation

The kingpin plate distributes load and helps guide the fifth wheel onto the kingpin. A damaged or missing plate can cause the coupling to fail or the trailer to be damaged. The trailer should be taken out of service for repair rather than coupled with a workaround.

Question 37Coupling and Uncoupling

After uncoupling, you notice the trailer landing gear is sinking into soft ground. What should you do?

  1. Let the trailer rest on the fifth wheel until the ground freezes
  2. Lower the landing gear further into the ground for stability
  3. Pull the tractor away quickly before it sinks further
  4. Place a firm board or pad under the landing gear foot and re-crank it to a stable height Correct answer.

Explanation

Soft ground can let the landing gear foot sink, dropping the trailer nose and possibly damaging the gear. A wide, firm pad spreads the load and keeps the gear stable. Pulling away or letting the trailer rest on the fifth wheel leaves the trailer unsupported.

Question 38Coupling and Uncoupling

Why should you visually check the fifth wheel opening before backing under a trailer?

  1. To confirm the landing gear is raised
  2. To confirm the air lines are connected
  3. To confirm the trailer brakes are released
  4. To confirm the jaws are open and the area is free of debris or obstructions Correct answer.

Explanation

The fifth wheel must be open and clean so the kingpin can enter and the jaws can close. Debris, a closed jaw, or a foreign object can prevent full engagement and cause a false lock. Trailer brakes, landing gear, and air lines are checked at other points in the procedure.

Question 39Coupling and Uncoupling

During a pre-trip after coupling, you find the trailer service line disconnected from the glad hand. What is the effect on the combination?

  1. The fifth wheel will not lock
  2. Only the trailer marker lights will fail
  3. The trailer brakes will not release normally and service braking is lost Correct answer.
  4. The landing gear will not raise

Explanation

The service line carries the brake application signal from the tractor to the trailer. If it is disconnected, the trailer brakes will not respond normally and braking performance is compromised. The electrical cord controls lights, and the fifth wheel and landing gear are mechanical systems.

Question 40Coupling and Uncoupling

You couple to a trailer and the trailer brakes will not release even though the tractor pressure gauge shows normal. Which check is most appropriate first?

  1. Disconnect the electrical cord
  2. Verify the emergency glad hand is fully engaged and the trailer supply valve is open Correct answer.
  3. Raise the landing gear and drive
  4. Replace the fifth wheel jaws

Explanation

If the trailer brakes will not release, the emergency line may not be delivering supply air. A partially engaged glad hand or a closed trailer supply valve blocks air to the trailer and keeps the spring brakes applied. Checking the connection and valve is the first step before assuming a mechanical failure.

Question 41Coupling and Uncoupling

What is the correct way to release a fifth wheel when uncoupling a heavily loaded trailer?

  1. Raise the landing gear off the ground, then release the handle
  2. Release the handle while the trailer brakes are off
  3. Lower the landing gear firmly, crank slightly to take weight off the fifth wheel, then pull the release handle Correct answer.
  4. Release the handle and pull the tractor out in one motion

Explanation

With a heavy load, the kingpin can bind in the jaws. Lowering the landing gear and cranking until the trailer just lifts off the fifth wheel removes that binding so the release handle moves freely. Pulling out with the handle released but weight still on the coupling can damage the jaws and kingpin.

Question 42Coupling and Uncoupling

After coupling, you raise the landing gear but the crank turns hard and the gear does not retract. What is the most likely cause?

  1. The trailer parking brakes are set
  2. The glad hands are crossed
  3. The electrical cord is unplugged
  4. The trailer is still resting weight on the landing gear because the fifth wheel did not take the load Correct answer.

Explanation

If the fifth wheel has not properly taken the trailer's weight, the landing gear still carries the load and will be hard or impossible to crank up. This is a warning sign that the coupling may not be fully engaged. Stop, lower the gear, and recheck the coupling before proceeding.

Question 43Coupling and Uncoupling

Why must you avoid backing under a trailer at an angle when coupling?

  1. It causes the electrical cord to short
  2. It makes the trailer brakes release automatically
  3. It prevents the landing gear from being raised
  4. It increases the chance the kingpin will miss the jaws and damage the fifth wheel Correct answer.

Explanation

Backing at an angle can cause the trailer apron or kingpin to strike the fifth wheel jaws or the tractor frame instead of entering the throat. The result can be a missed coupling or bent components. Line up as straight as possible with the trailer before backing under.

Question 44Coupling and Uncoupling

You have coupled and connected the lines, but the tractor low-air warning sounds when you release the parking brake. What should you do?

  1. Release the trailer spring brakes manually
  2. Disconnect the trailer and leave it
  3. Continue driving and let the compressor catch up
  4. Stop, build air pressure, and check for leaks before moving Correct answer.

Explanation

A low-air warning means the system cannot safely hold the combination. Stop, let the compressor build pressure to the normal range, and check the lines and couplings for leaks. Driving with low air pressure risks losing the brakes, and manually releasing spring brakes bypasses a safety system.

Question 45Inspection of Combination Vehicles

During a pre-trip inspection of a combination vehicle, where should you check the air lines between the tractor and trailer?

  1. Only at the coupler plate where the trailer pivots
  2. Where they rest on the catwalk and connect to the glad hands Correct answer.
  3. Inside the cab at the trailer brake valve
  4. At the glad hands only, since the rest of the line is sealed

Explanation

Air lines must be checked along their full run, paying particular attention to where they cross the catwalk and connect at the glad hands, because that is where chafing, cuts, and loose couplings develop. A leak or severed line there will prevent the trailer brakes from releasing or applying properly.

Question 46Inspection of Combination Vehicles

You are coupling a tractor to a loaded trailer. After backing under the trailer, what should you do before pulling the fifth-wheel release handle?

  1. Lower the trailer apron onto the fifth wheel
  2. Secure the tractor by setting the parking brake and chocking wheels Correct answer.
  3. Raise the landing gear fully
  4. Back up slightly to test the kingpin

Explanation

The tractor must be secured with the parking brake and, when necessary, wheel chocks before you leave the cab or reach under the trailer. This prevents the tractor from rolling and injuring you while you work at the fifth wheel.

Question 47Inspection of Combination Vehicles

What is the purpose of the trailer spring brakes on a combination vehicle?

  1. To apply automatically if air pressure is lost Correct answer.
  2. To assist the tractor brakes during hard stops
  3. To keep the trailer level during loading
  4. To hold the trailer while the tractor engine idles

Explanation

Spring brakes are held off by air pressure; if pressure drops below a safe level, the springs apply the brakes automatically. This fail-safe design stops the trailer if a line breaks or pressure is otherwise lost.

Question 48Inspection of Combination Vehicles

When you perform a tug test after coupling, what are you confirming?

  1. That the kingpin is locked in the fifth wheel Correct answer.
  2. That the air lines are free of leaks
  3. That the trailer lights are wired correctly
  4. That the landing gear is raised

Explanation

A tug test involves pulling gently forward with the trailer brakes set; if the kingpin is properly locked, the tractor will not pull out from under the trailer. If the tractor moves forward freely, the coupling is not secure.

Question 49Inspection of Combination Vehicles

How should you check the electrical cord between tractor and trailer during a pre-trip inspection?

  1. Inspect for cracked insulation and verify the plug is seated and locked Correct answer.
  2. Check that the cord is longer than the air lines
  3. Verify the cord is tied to the air lines
  4. Confirm the cord is coiled neatly on the catwalk

Explanation

The electrical cord should be inspected for cracked or worn insulation and the plug must be fully seated and locked so the trailer lamps stay powered. A loose plug can cause lights to fail in transit.

Question 50Inspection of Combination Vehicles

What is the correct order for uncoupling a tractor from a trailer?

  1. Set the trailer brakes, lower the landing gear, disconnect the lines, then release the fifth wheel Correct answer.
  2. Release the fifth wheel, then lower the landing gear and disconnect the lines
  3. Lower the landing gear, disconnect the lines, then release the fifth wheel
  4. Disconnect the lines, release the fifth wheel, then lower the landing gear

Explanation

The trailer must be supported and secured before the tractor is released. You set the trailer parking brakes, lower the landing gear to the ground, disconnect the air and electrical lines, and only then pull the fifth-wheel release handle.