50 Combination Vehicles Practice Questions & Answers

1. 1. Before you back the tractor under a semitrailer, what condition must the fifth wheel be in?

   • A.Tilted down toward the rear, jaws locked shut, and the sliding mechanism loose so the plate centers
   • B.Tilted down toward the rear of the tractor, jaws open, and the safety handle in the automatic lock position✓ Answer
   • C.Level with the tractor frame, jaws locked shut, and the safety handle set in the automatic lock position
   • D.Tilted down toward the front of the tractor, jaws open, and the safety handle pulled to released

   The coupling procedure starts with the fifth wheel tilted down toward the rear so the trailer apron rides up onto the plate, the jaws open so they can snap around the kingpin, and the safety unlocking handle in the automatic lock position so the jaws latch on their own. A sliding fifth wheel must also be locked before you back under.

   Source: AAMVA Model CDL Manual, Combination Vehicles — Coupling Tractor-Semitrailers, Step 1Report a problem with this question

2. 2. Why must the tractor be lined up directly in front of the trailer instead of backing under at an angle?

   • A.Backing in at an angle overheats the fifth wheel plate, so the grease burns off and the jaws seize on the pin
   • B.Backing in at an angle shifts the axle loads, so the tractor rides on the steer axle and steers heavily
   • C.Backing in at an angle can shove the trailer sideways and break the landing gear before the kingpin seats✓ Answer
   • D.Backing in at an angle twists the glad hand seals, so the service line leaks once the trailer is charged

   A trailer resting on its landing gear has no lateral support. A tractor backing in crooked pushes the trailer sideways instead of sliding the apron onto the plate, and the sideways load can buckle or snap the landing gear legs, dropping the nose of the trailer.

   Source: AAMVA Model CDL Manual, Combination Vehicles — Coupling Tractor-Semitrailers, Step 3Report a problem with this question

3. 3. How should the trailer be set for height before the tractor backs under it?

   • A.High enough that the fifth wheel passes cleanly under the apron, then lowered onto the plate after
   • B.Level with the fifth wheel so both plates meet flat, with the kingpin resting on the closed jaws
   • C.Low enough that the tractor raises it slightly as it backs under, with kingpin and fifth wheel aligned✓ Answer
   • D.Low enough that the apron drags on the plate, which scrubs off grease and seats the pin more firmly

   The trailer must sit slightly below the fifth wheel so that backing under lifts it a little. That small lift proves the plate is carrying the apron and lets the kingpin drop into the throat of the jaws. Too high and the pin can ride over the top of the plate; too low and the tractor hits the trailer nose.

   Source: AAMVA Model CDL Manual, Combination Vehicles — Coupling Tractor-Semitrailers, Step 6Report a problem with this question

4. 4. What is likely to happen if the trailer sits too low when you try to couple?

   • A.The tractor strikes and damages the nose of the trailer instead of sliding in under the apron✓ Answer
   • B.The fifth wheel slides under the apron but the jaws close on the head of the kingpin, not the shank
   • C.The kingpin passes over the fifth wheel and the trailer looks coupled while resting on the jaws
   • D.The trailer rolls forward off its chocks because the apron rides up over the lip of the fifth wheel

   A trailer set below the leading edge of the fifth wheel gives the tractor nothing to slide under. The back of the tractor frame and the plate itself run into the nose of the trailer, which is why the manual has you check and adjust trailer height before backing under rather than after.

   Source: AAMVA Model CDL Manual, Combination Vehicles — Coupling Tractor-Semitrailers, Step 6Report a problem with this question

5. 5. After you supply air to the trailer and the system charges, how do you check for crossed air lines?

   • A.Shut the engine off, then apply and release the trailer brakes and listen for them setting and releasing✓ Answer
   • B.Pull the trailer air supply knob out and see whether the tractor protection valve stays in normal position
   • C.Leave the engine at fast idle and watch the dash gauge for a drop of more than three psi in one minute
   • D.Open the emergency line shut-off valve at the rear of the last trailer and listen for escaping air

   Crossed lines send supply air into the service line, so the trailer tanks never charge and the brakes do not respond to the control. Shutting the engine off removes engine noise so you can hear the trailer brakes actually apply and then hear air escape as they release; silence means the lines are crossed or the trailer is not getting air.

   Source: AAMVA Model CDL Manual, Combination Vehicles — Coupling Tractor-Semitrailers, Step 8Report a problem with this question

6. 6. You push the trailer air supply control in, but the trailer spring brakes do not release. What should you suspect first?

   • A.The tractor protection valve is stuck open, so air bleeds past it and the low air warning stays lit
   • B.The service line is blocked, so the foot valve cannot send a signal down to the trailer relay valves
   • C.The trailer tank drain is open, so the compressor cannot build the tanks above the governor cut-in
   • D.The air lines are crossed, so supply air went to the service line and never charged the trailer tanks✓ Answer

   Spring brakes are held off by air that reaches the trailer through the emergency (supply) line. If the glad hands are reversed, that air is routed into the service line instead, the trailer tanks stay empty, and the springs stay applied. The manual tells you to check the air line connections when the spring brakes will not release.

   Source: AAMVA Model CDL Manual, Combination Vehicles — Hose Couplers (Glad Hands)Report a problem with this question

7. 7. In the coupling sequence, what do you do with the trailer air supply control just before backing under the trailer?

   • A.Push it in and leave it in, so the trailer brakes stay released and the trailer settles onto the plate
   • B.Leave it in the emergency position and use the trailer hand valve to hold the trailer on its chocks
   • C.Pull it out and then push it back in, so the relay valves reseat before the kingpin enters the jaws
   • D.Pull it out, so the trailer brakes stay locked and the trailer cannot roll as the tractor backs in✓ Answer

   Air is supplied first only long enough to test the trailer brakes. Before backing under, the knob is pulled back out so the trailer brakes are locked; the trailer must stay put while the tractor drives the fifth wheel under the apron, otherwise the trailer is pushed away instead of being coupled.

   Source: AAMVA Model CDL Manual, Combination Vehicles — Coupling Tractor-Semitrailers, Step 9Report a problem with this question

8. 8. What is the correct way to make the tug test after the kingpin locks into the fifth wheel?

   • A.Leave the landing gear down, set the tractor parking brake, and rock the trailer with the hand valve
   • B.Raise the landing gear fully, release the trailer brakes, and pull forward hard enough to feel the jaws hold
   • C.Lower the landing gear onto blocks, then back up firmly against the kingpin to seat the jaws on the shank
   • D.Raise the landing gear slightly, then pull gently forward in low gear with the trailer brakes still locked✓ Answer

   The tug test works only if the trailer is held by its own brakes and its weight is on the fifth wheel. Raising the landing gear slightly puts the load on the plate; a gentle pull in low gear against locked trailer brakes then proves the jaws are closed on the kingpin rather than merely appearing shut.

   Source: AAMVA Model CDL Manual, Combination Vehicles — Coupling Tractor-Semitrailers, Step 11Report a problem with this question

9. 9. Before you go under the trailer with a flashlight to inspect the coupling, what must you do?

   • A.Leave the engine running to hold air pressure, set the parking brakes, and chock behind a drive tire
   • B.Put the transmission in neutral, set the parking brakes, shut the engine off and take the key with you✓ Answer
   • C.Set the trailer hand valve, leave the tractor in gear against the kingpin, and shut off the trailer air
   • D.Put the transmission in low reverse, idle the engine, and leave the key in so the rig can be moved fast

   The hazard under the trailer is the vehicle moving while you are beneath it. Neutral and parking brakes keep it from creeping, and taking the key with you is what stops another person from starting and moving the truck while you are out of sight underneath it.

   Source: AAMVA Model CDL Manual, Combination Vehicles — Coupling Tractor-Semitrailers, Step 12Report a problem with this question

10. 10. You see a space between the upper and lower fifth wheel after coupling. What does that tell you?

    • A.The fifth wheel plate is short of grease, so the plates stand apart until the trailer settles
    • B.The trailer is loaded toward the rear, so the nose is light and the gap closes once you move
    • C.The kingpin may be sitting on top of the closed jaws, so the trailer could come loose very easily✓ Answer
    • D.The air suspension has not dumped yet, so the gap disappears as soon as the trailer bags inflate

    A properly coupled kingpin drops through the throat and the jaws close behind it, leaving the apron flat on the plate with no gap. Any visible space means the pin is likely resting on top of the jaws rather than captured by them, and a trailer held that way can separate from the tractor.

    Source: AAMVA Model CDL Manual, Combination Vehicles — Coupling Tractor-Semitrailers, Step 13Report a problem with this question

11. 11. When you look into the back of the fifth wheel, where must the locking jaws have closed?

    • A.Around the shank of the kingpin, below its head, with no play between the jaws and the pin✓ Answer
    • B.Around the throat of the fifth wheel, behind the kingpin, so the pin is wedged at the front
    • C.Around the head of the kingpin, above the shank, so the head cannot lift out of the throat
    • D.Around the apron plate itself, on both sides of the kingpin, which spreads the pulling load

    The kingpin has a narrow shank under a wider head, and the jaws are shaped to capture that shank so the head cannot pull up through them. Jaws closed on the head instead look locked from above but give the pin room to climb out, which is why the manual makes you look into the rear of the fifth wheel.

    Source: AAMVA Model CDL Manual, Combination Vehicles — Inspecting a Combination Vehicle, coupling system areasReport a problem with this question

12. 12. How far must the landing gear be raised before you drive a coupled tractor-semitrailer?

    • A.Just clear of the pavement, so the pads act as a backstop if the fifth wheel should release
    • B.To the height of the rear tractor tires, so the crank handle clears the frame during sharp turns
    • C.Halfway up, leaving the pads low enough to catch the nose if the kingpin lets go in traffic
    • D.All the way up, with the crank handle secured, since part-way gear can catch on railroad tracks✓ Answer

    Landing gear left part way down hangs below the trailer and can strike raised railroad crossings, curbs or debris, tearing the legs off and dropping the nose. The manual requires the gear all the way up and the crank handle secured so it cannot swing out into passing traffic.

    Source: AAMVA Model CDL Manual, Combination Vehicles — Coupling Tractor-Semitrailers, Step 15Report a problem with this question

13. 13. When uncoupling, how do you take the pressure off the fifth wheel locking jaws?

    • A.Dump the tractor air suspension so the fifth wheel drops away from the apron before unlatching
    • B.Raise the trailer with the landing gear until its weight lifts clear of the fifth wheel plate
    • C.Back up gently against the locked trailer brakes, then set the parking brakes while still pushing✓ Answer
    • D.Pull forward gently against the locked trailer brakes, then set the parking brakes while still pulling

    Pulling forward loads the jaws against the kingpin and binds the release handle. Backing gently while the trailer brakes hold pushes the tractor into the pin from the other side, and setting the parking brakes during that push keeps the jaws unloaded so the release handle moves freely.

    Source: AAMVA Model CDL Manual, Combination Vehicles — Uncoupling Tractor-Semitrailers, Step 2Report a problem with this question

14. 14. Why must you chock the wheels of a trailer that has no spring brakes before you uncouple from it?

    • A.The landing gear alone can hold the trailer on a grade only when the pads rest on firm pavement
    • B.Air can build in the trailer tank and force the emergency brakes on, which would drag the tires flat
    • C.The trailer service brakes stay applied after uncoupling and will bleed off within a few minutes
    • D.Air can leak out of the trailer tank and release its emergency brakes, leaving the trailer free to roll✓ Answer

    Trailers without spring brakes rely on air stored in the trailer tank to hold their emergency brakes on. That stored air bleeds down over time, so a trailer that is braked when you drop it can be rolling free hours later. Chocks are the only mechanical hold that does not depend on air.

    Source: AAMVA Model CDL Manual, Combination Vehicles — Uncoupling Tractor-Semitrailers, Step 3Report a problem with this question

15. 15. You are uncoupling from a loaded trailer. What do you do after the landing gear makes firm contact with the ground?

    • A.Turn the crank a few extra turns in low gear to take some weight off, without lifting the trailer✓ Answer
    • B.Stop cranking at once and let the fifth wheel carry the load until the tractor is driven out
    • C.Turn the crank several more turns in high gear to raise the trailer clear of the fifth wheel plate
    • D.Back the tractor hard to force the apron down onto the pads before you pull the release handle

    A few extra turns in low gear transfer part of the load from the fifth wheel to the landing gear, which unbinds the jaws and makes the release handle pull easily and the next coupling simpler. Cranking far enough to lift the trailer off the plate is wrong, because it overloads the gear and can topple the trailer.

    Source: AAMVA Model CDL Manual, Combination Vehicles — Uncoupling Tractor-Semitrailers, Step 4Report a problem with this question

16. 16. After you pull the release handle, why do you pull the tractor only partially clear of the trailer and stop?

    • A.So the trailer brakes have time to bleed down and set before the trailer is left standing alone
    • B.So the tractor frame stays under the trailer and catches it if the landing gear collapses or sinks✓ Answer
    • C.So the glad hands can be reached from the ground and hung on the dummy couplers before leaving
    • D.So the kingpin can drop free of the jaws without dragging the apron across the fifth wheel plate

    The landing gear is the only thing holding the nose of the trailer once the fifth wheel is out from under it. Stopping with the tractor frame still under the trailer gives the nose something to land on if a leg buckles or a pad sinks into soft ground, instead of letting the trailer drop to the pavement.

    Source: AAMVA Model CDL Manual, Combination Vehicles — Uncoupling Tractor-Semitrailers, Step 7Report a problem with this question

17. 17. After disconnecting the lines from a trailer you are dropping, how should the glad hands and electrical cord be handled?

    • A.Leave the glad hands open to dry out and tape the cord flat against the rear cab wall
    • B.Let the glad hands rest on the deck plate and hang the cord with the plug pointing up
    • C.Tie the glad hands to the trailer nose and coil the cord inside the trailer's front rack
    • D.Put the glad hands on the dummy couplers and hang the cord with the plug pointing down✓ Answer

    Dummy couplers seal the glad hands so water and dirt cannot get into the air lines, where moisture later freezes and causes brake failure. Hanging the electrical plug downward keeps rain from running into the pins and corroding the connector.

    Source: AAMVA Model CDL Manual, Combination Vehicles — Uncoupling Tractor-Semitrailers, Step 5Report a problem with this question

18. 18. What is the first step in uncoupling a trailer that is coupled with a pintle hook?

    • A.Open the latch and remove the lock pin, then set the trailer brakes with the hand valve
    • B.Park on a firm, level surface and block the trailer tires before anything is disconnected✓ Answer
    • C.Disconnect the safety chains first so the drawbar eye can lift freely off the pintle horn
    • D.Extend the jack until the tongue weight is off the hook, then look for a level place to park

    Everything after this step removes something that was holding the trailer, so the trailer must already be immobilized. Parking on firm level ground keeps the jack from sinking and blocking the tires keeps the trailer from rolling once the chains, brake lanyard and coupler are released.

    Source: AAMVA Model CDL Manual, Combination Vehicles — Uncoupling a Pintle Hook, Step 1Report a problem with this question

19. 19. What problem does a fifth wheel plate that is not kept greased cause?

    • A.Steering problems, because friction between tractor and trailer resists every turn✓ Answer
    • B.Braking problems, because the plate binds and holds the trailer brakes partly applied
    • C.Suspension problems, because the dry plate feeds road shock straight into the apron
    • D.Coupling problems, because the jaws cannot close fully around the shank of the kingpin

    The trailer pivots on the fifth wheel plate every time the rig turns. Without grease, that steel-on-steel contact resists rotation, and the resistance is felt through the steering, which is why adequate grease on the plate is an inspection item.

    Source: AAMVA Model CDL Manual, Combination Vehicles — Coupling Tractor-Semitrailers, Step 1Report a problem with this question

20. 20. What is the correct way to connect a pair of glad hands?

    • A.Clean the seals, hook the couplers over the dummy mounts, and let air pressure seat them
    • B.Clean the seals, push the couplers straight together in line, and snap the locking clip shut
    • C.Wet the seals, hold the couplers at a forty five degree angle, and press until they click
    • D.Clean the seals, press them together with the couplers at a ninety degree angle, and turn✓ Answer

    Glad hands lock by a quarter turn: the two rubber seals are pressed face to face with the couplers held at ninety degrees to each other, then one is rotated so the lugs slide behind each other and clamp the seals. Cleaning first matters because grit on a seal makes the joint leak.

    Source: AAMVA Model CDL Manual, Combination Vehicles — Hose Couplers (Glad Hands)Report a problem with this question

21. 21. What extra check does a sliding fifth wheel need that a fixed one does not?

    • A.That the slide is set to its rearmost stop, which puts the most weight on the drive axles
    • B.That the slide rails are free of grease, so the plate can move under load without binding
    • C.That the locking pins are all present and fully engaged, and that air slides show no leaks✓ Answer
    • D.That the release handle is unlatched, so the plate can shift and center itself in turns

    A sliding fifth wheel carries the whole coupling load through its locking pins. If a pin is missing or only partly engaged, the plate can slide under braking and shift the trailer load without warning, so the inspection checks that every pin is present and locked and that air-operated slides are not leaking.

    Source: AAMVA Model CDL Manual, Combination Vehicles — Inspecting a Combination Vehicle, sliding fifth wheelReport a problem with this question

22. 22. How must the air lines and electrical cord between tractor and trailer be secured?

    • A.Supported clear of moving parts and undamaged, with enough slack to let the rig turn fully✓ Answer
    • B.Pulled tight between the units, so they cannot droop low enough to catch on the drive tires
    • C.Taped together into a single bundle, so one strap can hold the pair against the cab wall
    • D.Coiled around the catwalk rail, which keeps them off the deck plate and out of the grease

    The tractor swings a long way relative to the trailer in a tight turn, so the lines need slack to follow that movement; pulled tight they tear out of the glad hands or the socket. They also must be routed and supported so they never contact the driveline, frame or tires.

    Source: AAMVA Model CDL Manual, Combination Vehicles — Inspecting a Combination Vehicle, air and electric linesReport a problem with this question

23. 23. During a walk-around you find the landing gear crank handle bent and hanging loose. What is the correct action?

    • A.Tie the handle up with a bungee cord and write it on the post-trip report at the end of the run
    • B.Treat it as a defect to be repaired, since a loose handle can swing out and strike passing traffic✓ Answer
    • C.Ignore it while the trailer is loaded, since the gear is fully raised and will not be cranked
    • D.Remove the handle and stow it in the cab, so nothing at all protrudes from the side of the trailer

    The inspection standard for landing gear is that it is fully raised, undamaged, with no missing parts and the crank handle in place and secured. A bent handle that will not stay secured fails that standard, and a handle that swings out into the next lane is a hazard to other traffic, so the defect must be written up and repaired.

    Source: AAMVA Model CDL Manual, Vehicle Inspection — Landing Gear and Clearance; 49 CFR 396.11Report a problem with this question

24. 24. Under the federal periodic inspection standards, how much horizontal movement between the upper and lower fifth wheel halves is a failure?

    • A.More than two full inches of movement
    • B.Any horizontal movement that can be felt
    • C.More than one half of an inch of movement✓ Answer
    • D.More than one eighth of an inch of movement

    The federal minimum periodic inspection standards list coupling device defects, and for fifth wheels horizontal movement between the upper and lower halves of more than one half inch is a rejection item. The looseness lets the trailer surge fore and aft against the tractor and accelerates wear on the jaws and kingpin.

    Source: 49 CFR Part 396, Appendix A — Minimum Periodic Inspection Standards, Coupling Devices (fifth wheels)Report a problem with this question

25. 25. On a rig set up to tow another trailer, which shut-off valves must be closed?

    • A.The valves on both sides of each converter dolly in the line
    • B.The valves at the rear of the last trailer in the combination✓ Answer
    • C.The valves at the front of every trailer in the combination
    • D.The valves at the rear of the first trailer in the combination

    Shut-off valves sit in the service and supply lines at the rear of trailers built to tow another unit. They must be open everywhere air still has to travel farther back, and closed only at the very rear of the last trailer so the system holds pressure. A valve left closed farther forward leaves the trailers behind it with no brakes.

    Source: AAMVA Model CDL Manual, Combination Vehicles — Shut-off ValvesReport a problem with this question

26. 26. The Combination Vehicles section names two things that do the most to keep a tractor-trailer from rolling over. What are they?

    • A.Keep the cargo high over the axles and brake hard in the curve, so weight settles on the springs.
    • B.Keep the cargo loose so it can slide and hold the posted advisory speed, which is set for trucks.
    • C.Keep the cargo as low as possible and take turns slowly, because speed in a curve rolls a rig.✓ Answer
    • D.Keep the cargo pressed against the front wall and take turns quickly, so the load cannot shift.

    Piling cargo higher raises the center of gravity and speed in a turn supplies the sideways force, so the manual's two rollover defenses are a low load and slow cornering. Braking hard inside a curve or hurrying through it adds to the sideways force instead of reducing it.

    Source: AAMVA Model CDL Manual, Combination Vehicles section (Rollover Risks)Report a problem with this question

27. 27. How does the rollover risk of a fully loaded rig compare with that of an empty one, according to the Combination Vehicles section?

    • A.About ten times as likely to roll, because the load raises the rig's center of gravity.✓ Answer
    • B.About the same, because rollover depends only on curve speed and never on how much is loaded.
    • C.About twice as likely to roll, because the extra weight presses the tires harder into the road.
    • D.About half as likely to roll, because the added weight holds the trailer down in a curve.

    The manual states that fully loaded rigs are ten times more likely to roll over in a crash than empty rigs, because the cargo sits above the frame and lifts the center of gravity. Weight does press the tires down, but that grip is exactly what lets a high load pivot the vehicle over instead of sliding it sideways.

    Source: AAMVA Model CDL Manual, Combination Vehicles section (Rollover Risks)Report a problem with this question

28. 28. A shipper loads your trailer so that most of the weight sits along one side. What does that do to the way the combination handles?

    • A.It shifts weight off the steering axle, so the front tires lose grip and the rig will not turn.
    • B.It puts far more weight on one side's tires and springs, so the rig can roll over toward that side.✓ Answer
    • C.It has no effect on handling as long as each axle stays within its legal weight limit.
    • D.It makes the trailer track wider, so the rear wheels swing out farther on every right turn.

    The manual tells the driver to keep the load centered and spread out, because a load to one side makes the trailer lean and a rollover more likely. Staying under the legal axle weights does not help here: the total weight can be legal while its side-to-side placement is still dangerous.

    Source: AAMVA Model CDL Manual, Combination Vehicles section (Rollover Risks)Report a problem with this question

29. 29. You are leaving a freeway on a curving exit ramp posted with an advisory speed, pulling a loaded trailer. What does the manual tell you to do?

    • A.Enter the ramp faster and let the ramp slow the rig, so no braking is needed inside the curve.
    • B.Slow to well under the posted advisory speed, because that figure is worked out for cars.✓ Answer
    • C.Hold the posted speed and pull the trailer hand valve, so the trailer stays stretched out.
    • D.Take the ramp at the posted advisory speed, which engineers set with loaded trucks in mind.

    Rollovers happen when you turn too fast, and the manual specifically tells combination drivers to go slowly around corners, on ramps and on off ramps. An advisory speed is set for ordinary passenger vehicles with a low center of gravity, so a loaded rig can reach its tipping point at or below that number.

    Source: AAMVA Model CDL Manual, Combination Vehicles section (Rollover Risks)Report a problem with this question

30. 30. Why does stacking cargo higher in the trailer make a combination easier to turn over?

    • A.It lengthens the rig's off-track path, so the trailer wheels climb the curb in a tight turn.
    • B.It raises the center of gravity, but the real danger is added wind load on the trailer roof.
    • C.It adds weight to the steering axle, so the front tires bite and pull the rig into the turn.
    • D.It raises the center of gravity, so less sideways force in a turn is needed to tip the rig.✓ Answer

    The manual explains that when more cargo is piled up, the center of gravity moves higher up from the road and the truck becomes easier to turn over. A higher center of gravity means the cornering force has a longer lever arm, so the rig reaches its tipping point at a lower speed.

    Source: AAMVA Model CDL Manual, Combination Vehicles section (Rollover Risks)Report a problem with this question

31. 31. A five-axle double is shown with a rearward amplification of about 2.0. What does that number mean for the driver?

    • A.The rear trailer feels twice the sideways force the tractor does and rolls over much sooner.✓ Answer
    • B.The rear trailer carries twice the load of the front trailer whenever the set is loaded right.
    • C.The rear trailer needs twice the steering input the tractor does to follow the same curve.
    • D.The rear trailer's brakes apply with twice the force of the tractor's during a hard stop.

    Rearward amplification is the crack-the-whip measure: the manual says a value of 2.0 means the rear trailer is twice as likely to turn over as the tractor. The number describes how a steering movement grows as it travels back through the set, not how the load or the braking effort is divided.

    Source: AAMVA Model CDL Manual, Combination Vehicles section (Steer Gently)Report a problem with this question

32. 32. In a set of triples, which trailer does the manual say is most likely to turn over?

    • A.In a set of triples the middle trailer goes first, whipped from the front and the rear.
    • B.In a set of triples the last trailer is the one most likely to turn over in a quick move.✓ Answer
    • C.In a set of triples all three go over together, since the dollies lock them into line.
    • D.In a set of triples the first trailer rolls first because it carries the heaviest load.

    Rearward amplification grows toward the back of the set, and the manual puts triples at about 3.5, meaning the last trailer can be rolled roughly 3.5 times as easily as a five-axle tractor-semitrailer. Because the whip motion is largest at the tail, the rear trailer is the one that goes over first.

    Source: AAMVA Model CDL Manual, Combination Vehicles section (Steer Gently)Report a problem with this question

33. 33. A driver makes a quick lane change to miss debris. The tractor stays upright but the rear trailer goes over. What explains this?

    • A.The crack-the-whip effect multiplies each steering movement toward the rear of the rig.✓ Answer
    • B.The trailer's air bags inflate unevenly in a swerve and lift one side of it off the road.
    • C.The tractor has more axles than the trailer, so it always recovers from a sudden swerve.
    • D.The trailer brakes grab before the tractor's, and the drag pulls the trailer onto its side.

    The manual warns that a quick lane change can turn the trailer over through the crack-the-whip effect, and notes there are many crashes in which only the trailer overturned. The sideways motion the driver starts at the steering wheel is amplified as it passes rearward, so the trailer exceeds its tipping point while the tractor does not.

    Source: AAMVA Model CDL Manual, Combination Vehicles section (Steer Gently)Report a problem with this question

34. 34. You are driving a 60-foot combination at 55 mph on a dry highway. What following distance does the manual's rule give you?

    • A.Ten seconds, one for each 10 feet plus four more because the vehicle is a combination.
    • B.Six seconds, one for each 10 feet of length, since the extra second applies only below 40 mph.
    • C.Seven seconds, one for each 10 feet of length plus one more for the speed above 40 mph.✓ Answer
    • D.Four seconds, the fixed gap the manual gives to any vehicle running on a dry highway.

    The manual's rule is at least one second for each 10 feet of vehicle length, plus another second above 40 mph. Sixty feet gives six seconds, and 55 mph adds the seventh, so the correct gap is seven seconds measured against a fixed roadside object.

    Source: AAMVA Model CDL Manual, Combination Vehicles section (Steer Gently); Driving Safely section (Space Ahead)Report a problem with this question

35. 35. Pulling a 50-foot combination at 35 mph, what following distance does the same second-per-10-feet rule give?

    • A.Three seconds, the fixed gap the manual sets for city driving in a combination.
    • B.Six seconds, one for each 10 feet plus one more that applies at every road speed.
    • C.Four seconds, one for each 10 feet after the tractor is subtracted from the length.
    • D.Five seconds, one for each 10 feet of length, with no extra second needed below 40 mph.✓ Answer

    Fifty feet of vehicle gives five seconds under the one-second-per-10-feet rule, and the additional second is only added above 40 mph, so at 35 mph the gap stays at five. The length used is the whole combination, tractor included, not the trailer alone.

    Source: AAMVA Model CDL Manual, Combination Vehicles section (Steer Gently); Driving Safely section (Space Ahead)Report a problem with this question

36. 36. Why does a large combination vehicle generally need more distance to stop when it is empty than when it is fully loaded?

    • A.Empty rigs roll on stiff springs that press the tires down harder and lengthen every skid.
    • B.Empty rigs have less tire traction and brakes sized for a full load, so the wheels lock early.✓ Answer
    • C.Empty rigs ride higher, so the brake drums run cooler and cannot build up enough friction.
    • D.Empty rigs carry less air in their tanks, so the brakes apply with much lower chamber pressure.

    The manual says that when lightly loaded, the very stiff suspension springs and strong brakes give poor traction and make it very easy to lock up the wheels. A locked wheel is sliding rather than braking, and a sliding tire stops the vehicle over a longer distance and can let the trailer swing out.

    Source: AAMVA Model CDL Manual, Combination Vehicles section (Brake Early)Report a problem with this question

37. 37. How does a bobtail tractor compare with a loaded tractor-semitrailer when it comes to stopping?

    • A.A bobtail can take longer to stop than the same rig fully loaded, despite weighing far less.✓ Answer
    • B.A bobtail stops in the same distance, since its brakes are matched to the tractor's weight.
    • C.A bobtail stops shorter only on wet roads, where the light rear lets the tires clear water.
    • D.A bobtail always stops in about half the distance, since it carries none of the load.

    The manual states that tests have shown bobtails can be very hard to stop smoothly and take longer to stop than a tractor-semitrailer loaded to maximum gross weight. With no trailer weight over the drive axles, those tires lock easily and the braking effort cannot be used.

    Source: AAMVA Model CDL Manual, Combination Vehicles section (Brake Early)Report a problem with this question

38. 38. With air brakes, what does the manual say total stopping distance is made up of?

    • A.Reaction distance plus braking distance alone, because air brakes add nothing to the total.
    • B.Braking distance doubled, a figure that already covers reaction time and the valve lag.
    • C.Perception distance plus reaction distance plus brake lag distance plus braking distance.✓ Answer
    • D.Perception distance plus braking distance, both counted from the moment the wheels lock.

    Air does not reach the brake chambers instantly, so the manual adds a fourth term, brake lag distance, to the perception, reaction and braking distances used for other vehicles. Leaving lag out understates how far a combination travels before it is stopped.

    Source: AAMVA Model CDL Manual, Air Brakes section (Braking with Air Brakes)Report a problem with this question

39. 39. About how much distance does air-brake lag add at 55 mph, according to the manual?

    • A.About 200 feet, the distance covered before the trailer brakes begin to hold.
    • B.About 32 feet, the distance covered during roughly half a second of brake lag.✓ Answer
    • C.About 90 feet, the distance covered while the compressor rebuilds lost pressure.
    • D.About 12 feet, the distance covered in the tenth of a second the valves need.

    The manual puts air-brake lag at about one half second, which at 55 mph works out to roughly 32 feet of extra travel before the brakes even begin to slow the vehicle. That lag is why a combination's total stopping distance runs past 450 feet at highway speed.

    Source: AAMVA Model CDL Manual, Air Brakes section (Braking with Air Brakes)Report a problem with this question

40. 40. What is the earliest and best way to know that your trailer has started to skid?

    • A.Watch the mirrors during any hard braking, because the trailer swinging out shows there first.✓ Answer
    • B.Listen for the trailer brakes to chatter, because that noise comes well before any visible swing.
    • C.Watch the tractor's hood line, because the nose drops the moment the trailer loses its grip.
    • D.Feel the steering wheel, because it tugs to the left the instant the trailer wheels slide.

    The manual says the earliest and best way to recognize that the trailer has started to skid is by seeing it in your mirrors, and that you should check them any time you apply the brakes hard. Once the trailer has swung out of your lane it is very difficult to prevent a jackknife, so the mirror check has to come first.

    Source: AAMVA Model CDL Manual, Combination Vehicles section (Prevent Trailer Skids)Report a problem with this question

41. 41. During a hard stop the rear of your trailer swings out to one side. What does the manual tell you to do?

    • A.Steer sharply into the swing and hold the brake so the tractor lines up with it.
    • B.Use the trailer hand valve so the trailer is pulled back into line behind you.
    • C.Press the brakes harder so the trailer wheels lock and drag the rig back straight.
    • D.Release the brakes so the trailer wheels can roll again and get their traction back.✓ Answer

    The manual's skid procedure is to stop using the brake: release them to get traction back, and once the trailer wheels grip the road again the trailer will start to follow the tractor and straighten out. Locked wheels have no steering or tracking force at all, so more braking only keeps the trailer sliding.

    Source: AAMVA Model CDL Manual, Combination Vehicles section (Prevent Trailer Skids)Report a problem with this question

42. 42. Why does the manual warn against using the trailer hand valve to straighten out a jackknifing rig?

    • A.Using it bleeds down the emergency line, so the trailer brakes release altogether.
    • B.Braking the trailer by itself keeps its wheels locked, and that makes the skid worse.✓ Answer
    • C.The hand valve only works above 20 mph, so it does nothing at jackknife speeds.
    • D.The hand valve applies the tractor brakes as well, which locks the drive wheels.

    The manual says this is the wrong thing to do because the brakes on the trailer wheels caused the skid in the first place. The hand valve works the trailer brakes alone, so pulling it reapplies exactly the force that is already sliding the trailer.

    Source: AAMVA Model CDL Manual, Combination Vehicles section (Prevent Trailer Skids)Report a problem with this question

43. 43. On a slick road the tractor's drive wheels lock up under braking. What is happening, and what does the manual tell you to do?

    • A.The trailer lifts off the fifth wheel, so the kingpin has to be re-seated before going on.
    • B.The tractor plows straight ahead with no steering, and only the trailer brakes can slow it.
    • C.The tractor can swing sideways into a jackknife, so the brakes have to be released at once.✓ Answer
    • D.Very little happens, because the trailer's rolling wheels keep the tractor lined up behind.

    The manual warns that with poor traction the tractor can jackknife very quickly, because locked drive wheels have no side grip and the trailer's momentum pushes the rear of the tractor around. The recovery is the same as for any wheel lockup: release the brakes so the wheels roll and regain traction.

    Source: AAMVA Model CDL Manual, Combination Vehicles section (Brake Early; Prevent Trailer Skids)Report a problem with this question

44. 44. What does the term 'off-tracking' describe in a combination vehicle?

    • A.The rear wheels follow a path outside the front wheels', sweeping wider on every turn.
    • B.The trailer slides sideways toward the low side whenever the road surface is crowned.
    • C.The drive wheels turn faster than the trailer wheels when the pavement is slippery.
    • D.The rear wheels follow a different and tighter path inside the path of the front wheels.✓ Answer

    The manual defines off-tracking, also called cheating, as the rear wheels following a different path than the front wheels when the vehicle goes around a corner, cutting inside the front path. Longer vehicles off-track more, and in a multi-trailer set the rear wheels of the last trailer off-track the most.

    Source: AAMVA Model CDL Manual, Combination Vehicles section (Turn Wide)Report a problem with this question

45. 45. You reach a right turn too tight to complete inside one lane. How does the manual tell you to make it?

    • A.Begin close to the curb and swing into the left lane only as you complete the turn.✓ Answer
    • B.Swing wide to the left before you start the turn, so the trailer has room to come around.
    • C.Straddle both lanes from the start, which keeps other drivers from passing you.
    • D.Stop short of the corner and back up a few feet, then take the turn in one lane.

    The manual says that if you cannot complete the turn without entering another traffic lane, turn wide as you complete the turn, which is better than swinging wide to the left before starting it. Swinging left first opens a gap on your right that another driver will try to use.

    Source: AAMVA Model CDL Manual, Combination Vehicles section (Turn Wide)Report a problem with this question

46. 46. Why does the manual say to keep the rear of your vehicle close to the curb while making a right turn?

    • A.It keeps the drive wheels from climbing the curb, which could tip the loaded rig over sideways.
    • B.It keeps another driver from slipping in on your right and being caught in the turn.✓ Answer
    • C.It keeps the trailer tires from scrubbing and wearing away against the curb face.
    • D.It keeps the steering axle loaded, which is what gives the front tires their grip.

    The manual pairs turning the front end wide with keeping the rear close to the curb, and gives the reason plainly: this will stop other drivers from passing you on the right. A car in that gap disappears from your mirrors just as the trailer sweeps across it.

    Source: AAMVA Model CDL Manual, Combination Vehicles section (Turn Wide)Report a problem with this question

47. 47. What is the trailer hand valve, also called the trolley valve, properly used for?

    • A.Use it as the normal service brake, since it saves wear on the tractor's linings.
    • B.Use it whenever the road is slick, so only the trailer wheels do the braking work.
    • C.Use it only to hold the rig on a grade, since it is the trailer's parking brake control.
    • D.Use it only to test the trailer brakes, never as a brake while the rig is rolling.✓ Answer

    The manual says the trailer hand valve should be used only to test the trailer brakes and must not be used in driving because of the danger of making the trailer skid. It is also never a parking device: on a trailer without spring brakes the air can leak off and release the brakes entirely.

    Source: AAMVA Model CDL Manual, Combination Vehicles section (Trailer Hand Valve)Report a problem with this question

48. 48. What job does the tractor protection valve, worked by the red trailer air supply knob, do?

    • A.It saves the tractor's air if the trailer breaks away, closing in the 20 to 45 psi range.✓ Answer
    • B.It meters air so the trailer brakes always apply a moment before the tractor's brakes.
    • C.It drains water from the trailer air tanks each time the service brakes are applied.
    • D.It holds the trailer brakes released while parked, so the rig can be towed with air off.

    The manual says the tractor protection valve keeps air in the tractor brake system should the trailer break away or develop a bad leak, and that it closes automatically when pressure falls into the 20 to 45 psi range. When it closes it also dumps the trailer emergency line, which sets the trailer emergency brakes.

    Source: AAMVA Model CDL Manual, Combination Vehicles section (Tractor Protection Valve; Trailer Air Supply Control)Report a problem with this question

49. 49. What does the emergency air line, the red line between tractor and trailer, do?

    • A.It carries air to the trailer's suspension bags to level the trailer under a load.
    • B.It carries only the pressure from the foot pedal that applies the trailer service brakes.
    • C.It charges the trailer air tanks and sets the trailer emergency brakes if air is lost.✓ Answer
    • D.It feeds the trailer lights and the antilock system through the same coiled line.

    The manual gives the emergency line two purposes: it supplies air to the trailer air tanks and it controls the emergency brakes on combination vehicles. A loss of pressure in it, from a breakaway or a burst hose, applies the trailer emergency brakes and pops the trailer air supply knob out.

    Source: AAMVA Model CDL Manual, Combination Vehicles section (Trailer Air-lines)Report a problem with this question

50. 50. How does steering while backing a tractor-semitrailer differ from backing a straight truck?

    • A.Turn the wheel the way you want the trailer's rear to go, exactly as in a straight truck body.
    • B.Keep the wheel straight and use the tractor's brakes to nudge the trailer into place.
    • C.Turn the wheel the opposite way at first, then follow the trailer once it starts around.✓ Answer
    • D.Turn the wheel the opposite way and hold it there until the trailer reaches the dock.

    The manual says that when backing a trailer you turn the steering wheel in the opposite direction, and once the trailer starts to turn you must turn the wheel the other way to follow it. Holding the first input in place would keep the trailer swinging past the intended line and into a jackknife.

    Source: AAMVA Model CDL Manual, Combination Vehicles section (Backing with a Trailer)Report a problem with this question

Practice questions based on the AAMVA/FMCSA model CDL manual. Requirements vary by state — study your state's official CDL manual before your exam. Official CDL info →