48 Combination Vehicles Practice Questions & Answers

1. 1. In Step 1 of the coupling procedure, what position must the fifth wheel jaws be in before you back the tractor under the trailer?

   • A.Closed and locked, so the kingpin cannot slip out
   • B.Either position works, because the kingpin forces the jaws open
   • C.Half closed, so the kingpin can be centered by hand
   • D.Open, with the safety unlocking handle in the automatic-lock position✓ Answer

   The jaws must be open so the kingpin can slide in and trip them shut as you back under; the safety unlocking handle must be in the automatic-lock position for that to happen. Backing under with the jaws closed makes the trailer ride up on top of the fifth wheel (a high hitch) instead of coupling.

   Source: AAMVA Model CDL Manual §6.4.1, Coupling Step 1Report a problem with this question

2. 2. During the fifth wheel inspection you find the plate is dry, with no grease. What problem does a dry fifth wheel plate cause?

   • A.Faster wear of the trailer service brake linings
   • B.Air leaks at the glad hands
   • C.Steering problems, because of friction between the tractor and trailer✓ Answer
   • D.Loss of pressure in the emergency line

   The trailer apron must pivot freely on the fifth wheel plate as the rig turns; without grease the two steel surfaces bind, and that friction is transmitted back through the kingpin and shows up as hard or erratic steering. The manual therefore lists 'enough grease' as a required fifth wheel inspection item.

   Source: AAMVA Model CDL Manual §6.4.1 Step 1 and §6.5.1 (fifth wheel/skid plate inspection)Report a problem with this question

3. 3. Why must you position the tractor directly in front of the trailer instead of backing under at an angle?

   • A.Backing at an angle prevents the electrical cord from reaching the socket
   • B.Backing at an angle pushes the trailer sideways and can break the landing gear✓ Answer
   • C.Backing at an angle overheats the fifth wheel locking jaws
   • D.Backing at an angle crosses the service and emergency air lines

   The landing gear legs are designed to carry vertical load, not side load; a tractor backing in at an angle contacts one side of the trailer first and shoves it sideways, bending or collapsing the legs. Step 3 of the coupling procedure therefore requires the tractor to be lined up squarely with the kingpin.

   Source: AAMVA Model CDL Manual §6.4.1, Coupling Step 3Report a problem with this question

4. 4. How should the trailer be set before the tractor backs under it?

   • A.Low enough that the fifth wheel plate rests against the trailer nose
   • B.Low enough that the trailer is raised slightly when the tractor backs under it✓ Answer
   • C.Exactly level with the top of the fifth wheel plate
   • D.High enough that the fifth wheel passes under the apron with several inches of clearance

   The trailer must sit slightly below the fifth wheel so that backing under lifts it a little, which guarantees the kingpin drops into the jaws rather than riding over them. Too low and the tractor strikes and damages the trailer nose; too high and the unit will not couple correctly.

   Source: AAMVA Model CDL Manual §6.4.1, Coupling Step 6Report a problem with this question

5. 5. You have connected the air lines, supplied air to the trailer, and checked the system. What must you do with the trailer air supply knob immediately before backing under the trailer?

   • A.Push it in and out three times to seat the glad hand seals
   • B.Pull it out only after the kingpin locks in the jaws
   • C.Pull it back out, so the trailer brakes are locked while you back under✓ Answer
   • D.Leave it pushed in, so the trailer can roll back if you hit it too hard

   Step 9 of the procedure is to lock the trailer brakes by pulling the air supply knob back out; a braked trailer cannot roll away as the kingpin drives into the jaws, so the jaws latch instead of just shoving the trailer backward. Only after that do you back under in the lowest reverse gear.

   Source: AAMVA Model CDL Manual §6.4.1, Coupling Steps 8–10Report a problem with this question

6. 6. After supplying air to the trailer during coupling, how do you check that the air lines are not crossed?

   • A.Feel both glad hands for equal temperature
   • B.Watch the trailer ABS lamp on the left side of the trailer
   • C.Open the drain valve on the trailer air tank and listen for air
   • D.Shut the engine off and listen for the trailer brakes applying and releasing✓ Answer

   With the engine off the cab is quiet enough to hear the trailer brakes react when you push in and pull out the air supply control; if the lines are crossed, supply air goes to the service line, the tanks never charge, and you will not hear the brakes apply and release normally. The manual pairs this with a gauge check for major air loss.

   Source: AAMVA Model CDL Manual §6.4.1, Coupling Step 8Report a problem with this question

7. 7. How is the tug test (checking the connection for security) performed after the kingpin locks?

   • A.Raise the landing gear all the way up and accelerate hard in low gear
   • B.Release the trailer brakes and pull forward while a helper watches the kingpin
   • C.Leave the landing gear on the ground and rock the tractor forward and back
   • D.Raise the landing gear slightly off the ground and pull gently forward against the locked trailer brakes✓ Answer

   Raising the landing gear only slightly means the trailer is supported by the fifth wheel but can still be caught by the legs if the coupling fails, and pulling gently against locked trailer brakes loads the jaws just enough to reveal a bad connection. Jerking or accelerating hard can damage the coupling or the trailer if the kingpin is not truly captured.

   Source: AAMVA Model CDL Manual §6.4.1, Coupling Step 11Report a problem with this question

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

   • A.Chock the tractor steer axle and leave the transmission in gear
   • B.Set only the trailer brakes so the tractor can be moved out if needed
   • C.Leave the engine idling so you can hear an air leak while you are underneath
   • D.Put the transmission in neutral, set the parking brakes, shut off the engine and take the key with you✓ Answer

   Step 12 requires securing the vehicle and taking the key so that no one can start or move the rig while your body is between the tractor and trailer. An idling engine or an unsecured key leaves the possibility of the vehicle moving and crushing you.

   Source: AAMVA Model CDL Manual §6.4.1, Coupling Step 12Report a problem with this question

9. 9. You shine a flashlight into the coupling and see a space between the upper and lower fifth wheel. What does this mean?

   • A.It is normal clearance that lets the trailer pivot in turns
   • B.The fifth wheel simply needs more grease
   • C.The kingpin may be resting on top of the closed jaws, so the rig must not be driven✓ Answer
   • D.The trailer air suspension is over-inflated and will settle on its own

   There must be NO space between the upper and lower fifth wheel; a gap means the kingpin never entered the jaws and is sitting on top of them (a high hitch), a coupling that can drop the trailer at the first bump. The manual is explicit that if the coupling is not right you do not drive the vehicle.

   Source: AAMVA Model CDL Manual §6.4.1 Step 13; §6.5.1 (no visible space between apron plate and lower fifth wheel skid plate)Report a problem with this question

10. 10. When you inspect the coupling from underneath, the locking jaws must be closed around which part of the kingpin?

    • A.The head of the kingpin, which is the widest part
    • B.The apron plate just outside the kingpin
    • C.The shank of the kingpin, with the safety latch over the locking lever✓ Answer
    • D.Either the head or the shank, as long as the lever is in the lock position

    The jaws are shaped to grip the narrow shank below the kingpin head; if they close on the head instead, the trailer is held only by a wedge that can pop free under load. The safety latch must also be over the locking lever, and on some fifth wheels that latch has to be closed by hand.

    Source: AAMVA Model CDL Manual §6.4.1 Step 13 and §6.5.1Report a problem with this question

11. 11. Which statement about raising the landing gear at the end of the coupling procedure is TRUE?

    • A.Raise it before doing the tug test so the trailer weight is on the fifth wheel
    • B.Raise the landing gear all the way up and secure the crank handle before driving✓ Answer
    • C.Leave it partway down as a safety backup in case the coupling fails
    • D.Raise it only until it just lifts off the pavement, to save time

    Step 15 requires the landing gear to be fully raised, using low gear range until the weight is off and then high range, with the crank handle stowed and secured. Gear left partway down can catch on a raised railroad crossing or other obstruction and tear off, and the slight raise used for the tug test is only temporary.

    Source: AAMVA Model CDL Manual §6.4.1, Coupling Step 15Report a problem with this question

12. 12. When uncoupling, what must be done to ease pressure on the fifth wheel locking jaws before you pull the release handle?

    • A.Release both the tractor and trailer brakes and let the rig roll back on a grade
    • B.Shut off the trailer air supply to lock the trailer brakes, back up gently, then set the tractor parking brakes while it is still pushing against the kingpin✓ Answer
    • C.Pull forward gently until the kingpin is against the front of the jaws, then set the brakes
    • D.Raise the landing gear fully so all the weight sits on the fifth wheel

    Backing gently against a braked trailer pushes the kingpin toward the back of the jaws so the jaws are no longer being pulled tight by the trailer's weight and drag; with the tractor parking brakes set while it is still pushing, the release lever will move instead of binding. This is Step 2 of the uncoupling procedure and it must come before unlatching the fifth wheel.

    Source: AAMVA Model CDL Manual §6.4.2, Uncoupling Step 2Report a problem with this question

13. 13. Why must you chock the wheels of a trailer that has no spring brakes before uncoupling it?

    • A.Chocks are needed to keep the landing gear from sinking into the pavement
    • B.The trailer ABS will not hold the trailer once the electrical cord is unplugged
    • C.Its emergency brakes are held only by air stored in the trailer tank, which can leak off and release the brakes✓ Answer
    • D.Chocks keep the kingpin centered in the jaws while the tractor pulls out

    A trailer without spring brakes has no true parking brake: its emergency brakes are applied by air already in the trailer tank, and as that air bleeds down over hours the brakes release and the trailer can roll. Chocks are a mechanical hold that does not depend on air, so the manual requires them whenever the trailer lacks spring brakes or you are unsure.

    Source: AAMVA Model CDL Manual §6.4.2 Step 3; §6.2 (trailer air tanks and emergency brakes)Report a problem with this question

14. 14. You are uncoupling a LOADED trailer. How far should the landing gear be lowered?

    • A.Until it makes firm contact with the ground, then a few extra turns in low gear — but never far enough to lift the trailer off the fifth wheel✓ Answer
    • B.Only until the pads touch the ground, since the fifth wheel still carries the load
    • C.Until the trailer is lifted clear of the fifth wheel so the tractor slides out easily
    • D.All the way down in high gear range, as fast as the crank will turn

    The extra turns in low gear transfer some of the load off the tractor so the fifth wheel jaws are not pinched and the tractor pulls out cleanly, but lifting the trailer completely off the fifth wheel puts the whole load on the landing gear and can collapse it. Step 4 of the uncoupling procedure states this limit explicitly.

    Source: AAMVA Model CDL Manual §6.4.2, Uncoupling Step 4Report a problem with this question

15. 15. After the fifth wheel is unlocked, why do you pull the tractor only PARTIALLY clear of the trailer and stop?

    • A.So the kingpin can be re-engaged quickly if the trailer starts to roll
    • B.So the trailer ABS can complete its self-test before the cord is unplugged
    • C.So the air lines stay connected until the trailer brakes bleed off
    • D.So the tractor frame is still under the trailer and can catch it if the landing gear collapses or sinks✓ Answer

    Step 7 has you stop with the tractor frame still under the trailer so that a leg that buckles or sinks into soft ground drops the trailer onto the frame instead of onto the pavement. By that point the air and electrical lines have already been disconnected and stored, so they play no part in this step.

    Source: AAMVA Model CDL Manual §6.4.2, Uncoupling Step 7Report a problem with this question

16. 16. How should the air line glad hands and the electrical cord be handled once they are disconnected from the trailer?

    • A.Store the glad hands on dummy couplers or lock them together, and hang the electrical plug facing down✓ Answer
    • B.Lay both on the catwalk so they are easy to reach at the next coupling
    • C.Tape the glad hands shut and coil the electrical cord around the air lines
    • D.Leave the glad hands hanging free so any trapped water can drain out

    Dummy (dead-end) couplers or locking the two glad hands together seals the open air lines so water and dirt cannot enter the air system, where moisture freezes valves and contamination ruins brake components. Hanging the electrical plug facing down uses gravity to keep rain out of the pin contacts.

    Source: AAMVA Model CDL Manual §6.4.2 Step 5; §6.1 (glad hands / dummy couplers)Report a problem with this question

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

    • A.Hook them at a 45-degree angle and pull down until the seal seats
    • B.Clean the seals, hold the two couplers at a 90-degree angle to each other, press the seals together and turn to lock✓ Answer
    • C.Wet the rubber seals and press them together with the couplers parallel
    • D.Push the couplers straight together face to face until they snap

    Glad hands are cam-type couplers: presenting them at 90 degrees lets the lugs pass each other, and rotating them into line forces the rubber seals together to make an airtight joint. Cleaning the couplers and seals first is required because grit under the seal is a common source of air leaks.

    Source: AAMVA Model CDL Manual §6.1, Glad Hands (air line connectors)Report a problem with this question

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

    • A.The trailer service brakes are out of adjustment
    • B.The electrical cord is not plugged in
    • C.The air lines are crossed, so supply air is going to the service line and the trailer tanks are not charging✓ Answer
    • D.The trailer ABS lamp circuit has failed

    Spring brakes release only when the trailer tanks are charged through the emergency (supply) line; if the glad hands are crossed, that air is routed into the service line instead, so the tanks stay empty and the springs stay applied. On older trailers with no spring brakes the same crossed connection lets you drive away with no trailer brakes at all, which is why the trailer brakes are always tested before moving.

    Source: AAMVA Model CDL Manual §6.1, Crossed Air LinesReport a problem with this question

19. 19. Which of these is a required check on a SLIDING fifth wheel that does not apply to a fixed one?

    • A.That the plate is greased and securely mounted to the frame
    • B.That the locking jaws close around the kingpin shank
    • C.That the kingpin is not bent or broken
    • D.All locking pins present and locked, and the fifth wheel not so far forward that the tractor frame hits the landing gear or the cab hits the trailer in a sharp turn✓ Answer

    A sliding fifth wheel adds a slide mechanism whose locking pins must all be present and engaged, otherwise the whole coupling can shift fore and aft under braking. Its adjustable position also creates a clearance hazard the fixed unit does not have: too far forward and the tractor frame strikes the landing gear or the cab strikes the trailer during a sharp turn. The other three items are checked on every fifth wheel.

    Source: AAMVA Model CDL Manual §6.5.1, Sliding Fifth WheelReport a problem with this question

20. 20. When you inspect the air and electrical lines running to the trailer, how should they be secured?

    • A.Pulled tight and strapped down so they cannot sag or move at all
    • B.Coiled and tied to the trailer landing gear leg
    • C.Supported and secured with enough slack for turns, and free from damage or crushing✓ Answer
    • D.Left long and loose so they can drag lightly on the frame

    The distance between tractor and trailer connection points grows in a sharp turn, so lines pulled tight will stretch, pull out of the glad hands, or burst, while lines with no support drag and chafe through. The inspection standard is lines supported so they cannot be crushed or caught, with enough slack to allow full turns.

    Source: AAMVA Model CDL Manual §6.5.1, Air and Electric Lines to TrailerReport a problem with this question

21. 21. During a combination walk-around you find the landing gear crank handle bent and not secured in its holder. What is the correct action?

    • A.Fold it against the leg and drive, since the crank is only used when parked
    • B.Tie it up with wire and note it on tomorrow's inspection report
    • C.Treat it as a defect to be reported and repaired, because a loose or damaged handle can vibrate down, snag an obstruction, and may leave you unable to lower the gear to uncouple✓ Answer
    • D.Ignore it as long as the landing gear is fully raised for the trip

    The inspection standard for landing gear is fully raised, no missing/bent/damaged parts, and the crank handle in place and secured, so a bent unsecured handle already fails. It is also a practical safety issue: the handle can drop into the path of obstructions, and a damaged crank may not turn when you need to set the trailer down. Coupling devices are among the items a driver must report under the driver vehicle inspection report rules.

    Source: AAMVA Model CDL Manual §6.5.1 (Landing Gear); 49 CFR 396.11 (coupling devices on the DVIR)Report a problem with this question

22. 22. Under the federal periodic inspection standards for coupling devices, a fifth wheel must be rejected when horizontal movement between the upper and lower fifth wheel halves exceeds how much?

    • A.When it exceeds 1/2 inch✓ Answer
    • B.When it exceeds 1 inch
    • C.When it exceeds 1/8 inch
    • D.When it exceeds 3/8 inch

    Appendix A to 49 CFR Part 396 rejects a fifth wheel for horizontal movement between the upper and lower halves exceeding 1/2 inch, because that slack means the jaws or kingpin are worn enough to let the coupling shift and eventually separate. The 3/8 inch figure in the same appendix applies to movement at pivot bracket pins or between the slider bracket and its base, not to the two fifth wheel halves.

    Source: Appendix A to 49 CFR Part 396, Coupling Devices — Fifth WheelsReport a problem with this question

23. 23. When uncoupling a trailer that is towed by a pintle hook, what must be done before you unlock the coupler?

    • A.Disconnect only the electrical connector; chains and the breakaway lanyard come off after the tow vehicle pulls away
    • B.Extend the tongue jack first so the drawbar eye lifts off the horn
    • C.Unlock the coupler first, then remove the safety chains while the eye is still on the horn
    • D.Park on a firm level surface, block the tires, then disconnect the electrical connector, the breakaway brake switch lanyard and the safety chains✓ Answer

    The trailer must be immobilized before anything is released, and the electrical connector, breakaway lanyard and safety chains must come off while the eye is still captured on the horn — otherwise a trailer that starts to move can yank the lanyard, set its brakes unexpectedly, or drag connectors and chains. The jack is extended only after you confirm the ground will support the tongue load.

    Source: AAMVA Model CDL Manual §6.4, Pintle Hook UncouplingReport a problem with this question

24. 24. On a combination set up to tow another trailer, which air line shut-off (cut-out) valves must be CLOSED?

    • A.Only the ones at the rear of the last trailer; all the others must be open✓ Answer
    • B.The ones at the rear of the first trailer, so the second trailer gets no service air
    • C.All of them, so air stays in the tractor system
    • D.Only the ones on the converter dolly

    Air must reach the brakes on every unit, so each shut-off valve in the line must be open except the pair at the very rear of the last trailer, which is closed to seal the system and stop the air from escaping. That is also why the combination brake check has you walk to the rear of the last trailer, open the emergency line valve and listen for escaping air, then check the service line the same way — if you hear nothing, a valve somewhere ahead is closed and those brakes will not work.

    Source: AAMVA Model CDL Manual §6.2 (shut-off valves) and §6.5.2 (check air flows to all trailers)Report a problem with this question

25. 25. According to the CDL manual, what two things help most to prevent a rollover in a combination vehicle?

    • A.Shift into a higher gear before a curve and keep the trailer air supply knob pulled out
    • B.Load the heaviest freight over the trailer's rear axles and use the engine brake in every curve
    • C.Inflate the tires above their rated pressure and slow the rig with the trailer hand valve
    • D.Keep the cargo as close to the ground as possible and drive slowly around turns✓ Answer

    A rollover happens when the lateral force of a turn overcomes the rig's stability. Lowering the cargo lowers the center of gravity, and reducing speed reduces the cornering force, so both directly shrink the overturning moment. Keeping cargo low matters even more in combination vehicles than in straight trucks.

    Source: AAMVA Model CDL Manual, Section 6.1.1 (Rollover Risks)Report a problem with this question

26. 26. Why does piling cargo higher inside a trailer make the rig easier to turn over?

    • A.It shortens the effective wheelbase of the tractor-semitrailer
    • B.It moves the center of gravity higher above the road, so less side force is needed to tip the vehicle✓ Answer
    • C.It causes the trailer's rear wheels to off-track farther on every turn
    • D.It reduces the air pressure that reaches the trailer brake chambers

    The manual states 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 sideways force generated in a turn acts through a longer lever arm, so the rig reaches its tipping point at a lower speed.

    Source: AAMVA Model CDL Manual, Section 6.1.1 (Rollover Risks)Report a problem with this question

27. 27. Compared with empty rigs, how much more likely are fully loaded rigs to roll over in a crash?

    • A.About five times as likely
    • B.About ten times as likely✓ Answer
    • C.About three times as likely
    • D.About twice as likely

    The manual states that fully loaded rigs are ten times more likely to roll over in a crash than empty rigs, because the added cargo raises the center of gravity and adds mass that keeps moving sideways once the rig starts to tip. That is also why more than half of truck driver deaths in crashes result from rollovers.

    Source: AAMVA Model CDL Manual, Section 6.1.1 (Rollover Risks)Report a problem with this question

28. 28. A load has been stacked toward one side of the trailer so that the trailer visibly leans. What is the effect on handling?

    • A.The trailer will brake better on the loaded side, shortening stopping distance
    • B.A rollover becomes more likely, because the off-center weight already tilts the rig toward its tipping point✓ Answer
    • C.It only matters when backing, because the trailer will drift toward the heavy side
    • D.No effect, as long as the axle weights are within legal limits

    The manual instructs drivers to keep the load centered and spread out, and warns that if the load is to one side so it makes the trailer lean, a rollover is more likely. The lean consumes part of the roll margin before the driver even enters a turn, so a smaller cornering force is enough to tip the trailer.

    Source: AAMVA Model CDL Manual, Section 6.1.1 (Rollover Risks)Report a problem with this question

29. 29. You are driving a loaded tractor-semitrailer and approach a curved highway off-ramp posted with a 35 mph advisory speed. What should you do?

    • A.Hold 35 mph through the ramp, since posted advisory speeds already account for loaded trucks
    • B.Enter at 35 mph and brake hard partway through if the trailer starts to lean
    • C.Use the trailer hand valve to hold the rig back while you are in the curve
    • D.Slow to a safe speed below the posted advisory before entering the ramp, because rollovers happen when you turn too fast✓ Answer

    Posted advisory speeds on ramps are set for passenger cars, which have a far lower center of gravity than a loaded trailer. The manual directs drivers to slow down to a safe speed before going into a turn and to drive slowly around corners, on-ramps and off-ramps, because braking hard once the rig is already leaning in the curve can trip a rollover or start a skid.

    Source: AAMVA Model CDL Manual, Section 6.1.1 (Rollover Risks)Report a problem with this question

30. 30. In the manual's rearward amplification chart, a value of 2.0 for a combination means what?

    • A.The rear trailer off-tracks twice as far as the tractor's drive wheels
    • B.In a quick lane change, the rear trailer is twice as likely to turn over as the tractor✓ Answer
    • C.The trailer takes twice as long to come to a complete stop as the tractor alone
    • D.The rig needs twice the normal following distance at highway speed

    Rearward amplification measures how much the steering input at the tractor is magnified by the time it reaches the rear of the combination, which is what produces the crack-the-whip effect. A value of 2.0 means the rear trailer is twice as likely to turn over as the tractor in a quick lane change.

    Source: AAMVA Model CDL Manual, Section 6.1.2 (Steer Gently), Figure 6.1Report a problem with this question

31. 31. What does the manual say about rollover risk in a set of triple trailers?

    • A.All three trailers respond to steering identically, so risk is spread evenly
    • B.Triples have a rearward amplification of 3.5, so the last trailer can be rolled 3.5 times as easily as a five-axle tractor✓ Answer
    • C.The lead trailer is most likely to roll because it carries the greatest share of the load
    • D.Triples have a rearward amplification of 1.5, less than a set of doubles

    The manual's chart gives triples a rearward amplification of 3.5, meaning you can roll the last trailer of triples 3.5 times as easily as a five-axle tractor. Because each coupling adds lateral overshoot, the last trailer in any multi-trailer combination is the unit most likely to turn over.

    Source: AAMVA Model CDL Manual, Section 6.1.2 (Steer Gently), Figure 6.1Report a problem with this question

32. 32. A driver swerves quickly at highway speed to miss debris. The tractor stays upright, but the trailer overturns. What best explains this?

    • A.The trailer service brakes locked up, which always tips the trailer onto its side
    • B.The crack-the-whip effect caused by rearward amplification magnified the steering input at the rear of the rig, tipping the trailer while the tractor stayed upright✓ Answer
    • C.The emergency air line lost pressure and applied the trailer emergency brakes
    • D.Off-tracking pulled the trailer's rear wheels off the edge of the pavement

    The manual warns that a quick lane change can produce the crack-the-whip effect, and that there are many crashes in which only the trailer has overturned. Rearward amplification means the trailer experiences a larger lateral acceleration than the tractor, so the trailer can exceed its rollover threshold while the tractor never does. The countermeasure is to steer gently and smoothly and look far enough ahead to avoid sudden lane changes.

    Source: AAMVA Model CDL Manual, Section 6.1.2 (Steer Gently)Report a problem with this question

33. 33. You are driving a 60-foot combination vehicle at 55 mph in good conditions. What is the minimum following distance the manual's rule calls for?

    • A.7 seconds✓ Answer
    • B.6 seconds
    • C.9 seconds
    • D.5 seconds

    The rule is at least one second for each 10 feet of vehicle length, plus one additional second when travelling over 40 mph. A 60-foot rig needs 6 seconds for length, and 55 mph is above 40 mph, so one more second is added for a total of 7 seconds. The extra cushion exists because a long combination cannot be steered or stopped abruptly without risking a jackknife or rollover.

    Source: AAMVA Model CDL Manual, Section 6.1.2 (Steer Gently)Report a problem with this question

34. 34. Why do large combination vehicles generally take LONGER to stop when they are empty than when they are fully loaded?

    • A.An empty trailer has a higher center of gravity, which lifts weight off the steering axle
    • B.Rolling resistance is greater on an empty trailer, which pushes the rig forward under braking
    • C.The brakes, tires, springs and shock absorbers are designed to work best fully loaded; when lightly loaded the very stiff springs and strong brakes give poor traction and the wheels lock up easily✓ Answer
    • D.Air pressure builds more slowly in the system when the trailer is empty

    Heavy-vehicle brakes, tires, springs and shocks are engineered for the loaded condition. Empty, there is little weight pressing the tires onto the road, so the same braking force locks the wheels; a locked wheel slides instead of gripping, which lengthens the stop and can swing the trailer out or jackknife the tractor very quickly.

    Source: AAMVA Model CDL Manual, Section 6.1.3 (Brake Early)Report a problem with this question

35. 35. How does a bobtail tractor (a tractor with no semitrailer attached) compare with a tractor-semitrailer loaded to maximum gross weight when it comes to stopping?

    • A.The bobtail stops noticeably shorter, which is why bobtails are considered safer in traffic
    • B.Stopping distance is essentially the same for both
    • C.Tests show bobtails are very hard to stop smoothly and take LONGER to stop than the fully loaded tractor-semitrailer✓ Answer
    • D.The bobtail stops in roughly half the distance because it weighs far less

    The manual states that tests have shown bobtails can be very hard to stop smoothly and that it takes them longer to stop than a tractor-semitrailer loaded to maximum gross weight. With no trailer weight over the drive axles, the tractor's powerful brakes lock the rear wheels easily, so a driver running bobtail must allow extra following distance and brake early.

    Source: AAMVA Model CDL Manual, Section 6.1.3 (Brake Early)Report a problem with this question

36. 36. Which formula gives total stopping distance for a vehicle with AIR brakes?

    • A.Perception Distance + Brake Lag Distance + Effective Braking Distance
    • B.Reaction Distance + Brake Lag Distance + Effective Braking Distance
    • C.Perception Distance + Reaction Distance + Brake Lag Distance + Effective Braking Distance✓ Answer
    • D.Perception Distance + Reaction Distance + Effective Braking Distance

    Hydraulic brakes act essentially instantly, so the three-term formula suffices for cars. Air brakes need a half second or more for air to travel through the lines to the brake chambers, so the air-brake formula adds a fourth term, brake lag distance, between reaction and effective braking.

    Source: AAMVA Model CDL Manual, Section 5.4.4 (Stopping Distance)Report a problem with this question

37. 37. At 55 mph on dry pavement, about how much distance does air brake lag add, and roughly what is the resulting total stopping distance for an average driver?

    • A.About 60 feet, for a total of about 600 feet
    • B.About 20 feet, for a total of about 350 feet
    • C.About 32 feet, bringing the total to over 450 feet✓ Answer
    • D.About 12 feet, for a total of about 300 feet

    The manual gives an air brake lag distance of about 32 feet at 55 mph on dry pavement, added to roughly 142 feet of perception distance, about 61 feet of reaction distance and about 216 feet of braking distance. That is why total stopping distance for an air-braked vehicle at 55 mph is over 450 feet, well beyond the length of a football field, and why combination drivers must look far ahead and brake early.

    Source: AAMVA Model CDL Manual, Section 5.4.4 (Stopping Distance)Report a problem with this question

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

    • A.Hearing the trailer tires squeal on the pavement
    • B.Watching for the low air pressure warning buzzer to sound
    • C.Seeing the trailer swinging out of position in your mirrors✓ Answer
    • D.Feeling the steering wheel go light in your hands

    The manual's first step for stopping a trailer skid is to recognize it, and it says the earliest and best way is by seeing it in your mirrors. That is why you should check the mirrors any time you brake hard: once the trailer has swung out of your lane, a jackknife is very difficult to prevent, so early mirror detection is what makes recovery possible.

    Source: AAMVA Model CDL Manual, Section 6.1.5 (Prevent Trailer Skids)Report a problem with this question

39. 39. You see in your mirror that your trailer has begun to swing out during hard braking. What is the correct recovery action?

    • A.Apply the trailer hand valve firmly to pull the trailer back in line
    • B.Release the brakes so the trailer wheels regain traction and the trailer starts to follow the tractor again✓ Answer
    • C.Press harder on the foot brake so the tractor slows faster than the trailer
    • D.Steer sharply in the opposite direction to counter the swing

    The manual's second step is to stop using the brake: release the brakes to get traction back. A locked wheel produces no directional grip, so the trailer keeps sliding sideways; once the trailer wheels roll again they generate side force, and the trailer will start to follow the tractor and straighten out.

    Source: AAMVA Model CDL Manual, Section 6.1.5 (Prevent Trailer Skids)Report a problem with this question

40. 40. Why must you NOT use the trailer hand valve to try to 'straighten out the rig' during a trailer jackknife?

    • A.Because the hand valve also applies the tractor's steering-axle brakes and would cause a tractor jackknife
    • B.Because the brakes on the trailer wheels caused the skid in the first place, so more trailer braking keeps those wheels locked and makes the swing worse✓ Answer
    • C.Because operating the hand valve opens the tractor protection valve and dumps the trailer air supply
    • D.Because the hand valve drains the trailer air tanks, leaving no air for the emergency brakes

    The trailer hand valve operates the trailer brakes only. In a trailer jackknife those same brakes have already locked the trailer wheels and caused the skid, so applying them harder simply keeps the wheels sliding and lets the trailer continue around. The manual explicitly calls this the wrong thing to do; the correct action is to release the brakes.

    Source: AAMVA Model CDL Manual, Section 6.1.5 (Prevent Trailer Skids)Report a problem with this question

41. 41. On a slick surface the tractor's DRIVE wheels lock up and the driver loses steering control while the trailer stays roughly in line. What is happening?

    • A.A tractor jackknife, caused by locked drive wheels — as distinct from a trailer jackknife, in which the trailer wheels lock and the trailer swings around✓ Answer
    • B.Rearward amplification, because the steering input has been magnified toward the rear
    • C.Off-tracking, because the rear wheels are following a tighter path than the front wheels
    • D.A trailer jackknife, because any loss of control in a combination originates at the trailer

    The two failures are distinct. Locked drive wheels rob the tractor of directional control and let the tractor pivot, which is a tractor jackknife; locked trailer wheels let the trailer swing around behind you, which is a trailer jackknife recognized in the mirrors. In either case, letting up on the brakes to restore rolling traction is the recovery, and the manual notes that with trailer-only ABS you should still back off the brakes if you start a tractor jackknife.

    Source: AAMVA Model CDL Manual, Sections 6.1.3, 6.1.5 and 6.3.2Report a problem with this question

42. 42. What is off-tracking (also called 'cheating')?

    • A.The tractor's drive wheels spinning on a slippery surface as power is applied
    • B.The trailer sliding sideways down the crown of a banked or crowned roadway
    • C.The rear wheels follow a tighter path than the front wheels through a turn, so the rig sweeps a path wider than the vehicle itself; longer vehicles off-track more, and the last trailer's rear wheels off-track the most✓ Answer
    • D.The rear overhang of the trailer swinging outward, away from the direction of the turn

    Because the trailer's wheels are set well behind the pivot point, they cut inside the arc the steering axle describes, so the total swept path is wider than the rig. The rear wheels of the powered unit off-track some, the trailer's rear wheels off-track more, and with multiple trailers the last trailer's rear wheels off-track the most, which is why the driver must steer the front end wide enough that the rear does not climb the curb or strike pedestrians. Option four describes tail swing, a different effect.

    Source: AAMVA Model CDL Manual, Section 6.1.6 (Turn Wide)Report a problem with this question

43. 43. You are making a right turn at an intersection that is too tight to complete without using part of another lane. What does the manual say to do?

    • A.Swing wide to the left before starting the turn so the trailer has room to come around
    • B.Keep the tractor tight against the curb and allow the trailer wheels to ride over the curb
    • C.Keep the rear of your vehicle close to the curb and turn wide as you COMPLETE the turn, so other drivers cannot slip past you on the right✓ Answer
    • D.Stop short of the intersection and back up to reposition before turning

    Swinging wide to the left first opens a gap on your right that another driver will move into, putting that vehicle exactly where the trailer will off-track. Keeping the rear close to the curb blocks that gap, and turning wide only as you complete the turn gives the trailer's rear wheels room without inviting anyone alongside.

    Source: AAMVA Model CDL Manual, Section 6.1.6 (Turn Wide)Report a problem with this question

44. 44. What is the correct use of the trailer hand valve (also called the trolley valve or Johnson bar)?

    • A.It should be applied together with the foot brake on every long downgrade to spare the tractor brakes
    • B.It is the primary service brake control for the combination and should be used for routine stops
    • C.It serves as the parking brake for trailers that have no spring brakes
    • D.It should be used only to TEST the trailer brakes; do not use it while driving, and never use it for parking✓ Answer

    The hand valve works the trailer brakes only, so using it in motion applies braking to the trailer alone and can make it skid and jackknife; the foot brake sends air to all the brakes on the combination, which is far less likely to cause a skid. It must never be used for parking either, because on a trailer without spring brakes the air can leak away and unlock the brakes — use the parking brakes, and wheel chocks if the trailer has no spring brakes.

    Source: AAMVA Model CDL Manual, Section 6.2.1 (Trailer Hand Valve)Report a problem with this question

45. 45. What is the purpose of the tractor protection valve?

    • A.It keeps air in the tractor's brake system if the trailer breaks away or develops a bad leak, and it closes automatically when air pressure drops into the 20 to 45 psi range✓ Answer
    • B.It regulates the governor cut-out pressure so the compressor stops loading the system
    • C.It automatically drains moisture and oil from the trailer air tanks
    • D.It prevents the trailer spring brakes from applying if the service line is damaged

    Without this valve, a trailer breakaway or ruptured hose would empty the entire system and leave the tractor with no brakes at all. When the valve closes it stops air from leaving the tractor and also lets air out of the trailer emergency line, which applies the trailer emergency brakes — with possible loss of control, so the driver should expect the rig to grab. Federal rule 49 CFR 393.43 fixes the automatic operating point no lower than 20 psi and no higher than 45 psi.

    Source: AAMVA Model CDL Manual, Section 6.2.2 (Tractor Protection Valve); 49 CFR 393.43Report a problem with this question

46. 46. On a newer tractor, the trailer air supply control is a red, eight-sided knob. How does it work?

    • A.It is the yellow, diamond-shaped knob; pulling it out releases the trailer brakes so the rig can be towed
    • B.Pull it OUT to supply the trailer with air; push it IN to apply the trailer emergency brakes
    • C.It must be held out while driving so the trailer service line stays charged
    • D.Push it IN to supply the trailer with air; pull it OUT to shut the air off and apply the trailer emergency brakes. It also pops out on its own when system pressure falls into the 20 to 45 psi range✓ Answer

    The red octagonal knob is the driver's control over the tractor protection valve, so pushing it in opens the supply path and charges the trailer, while pulling it out closes the valve and dumps the emergency line, setting the trailer emergency brakes. Because it is tied to the same automatic function, it pops out by itself in the 20 to 45 psi range. Do not confuse it with the yellow, diamond-shaped parking brake knob.

    Source: AAMVA Model CDL Manual, Section 6.2.3 (Trailer Air Supply Control)Report a problem with this question

47. 47. Which statement correctly describes the emergency (supply) air line in a combination vehicle?

    • A.It carries only the signal that tells the relay valves how hard to apply the trailer brakes
    • B.Its single job is to charge the trailer air tanks; the emergency brakes are controlled separately
    • C.It is color coded blue to distinguish it from the red service line
    • D.It has two jobs — supplying air to the trailer air tanks and controlling the trailer emergency brakes — so a loss of pressure in it applies those emergency brakes and closes the tractor protection valve, popping the air supply knob out✓ Answer

    The emergency line, coded red, both fills the trailer air tanks and acts as the control that holds the trailer emergency brakes off. That dual role is what makes a breakaway self-protecting: if the trailer tears loose or a hose bursts, the pressure loss automatically sets the trailer emergency brakes and closes the tractor protection valve. The blue service (control or signal) line is the one whose pressure varies with how hard you press the foot brake or hand valve.

    Source: AAMVA Model CDL Manual, Section 6.2.4 (Trailer Air-lines)Report a problem with this question

48. 48. How does steering while backing a tractor-semitrailer differ from backing a car, straight truck, or bus?

    • A.There is no difference; you turn the top of the wheel in the direction you want the rear of the vehicle to go
    • B.You turn the steering wheel in the OPPOSITE direction from where you want the trailer to go, and once the trailer begins to turn you turn the wheel back the other way to follow it✓ Answer
    • C.You hold the wheel straight and control the trailer's direction with the trailer hand valve
    • D.You should always back on a curved path toward the passenger side, since the trailer responds faster in that direction

    Because the trailer pivots at the fifth wheel, moving the tractor's rear to the left pushes the trailer's nose left and swings the trailer's tail to the right, so the steering input is reversed. Once the trailer starts to turn you must steer back the other way to follow it, back slowly, use the mirrors on both sides, and correct any drift immediately by turning the top of the wheel in the direction of the drift. Position the rig to back in a straight line whenever possible; if a curved path is unavoidable, back toward the driver's side so you can see.

    Source: AAMVA Model CDL Manual, Section 6.1.7 (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 →