Two semitrailers will form a set of doubles. Which loading order does the handbook recommend for the safest handling?
Correct answer: Put the more heavily loaded trailer immediately behind the tractor.
The handbook favors the heavier trailer first and the lighter trailer behind it. Correct order does not replace individual axle, coupling and gross-weight limits.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.2.1; Trailer loading order; physical PDF page 202 (printed 202); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
A driver makes a sudden lane change with doubles or triples. Which trailer is especially vulnerable to the crack-the-whip effect?
Correct answer: The last trailer.
Steering movements can be amplified toward the rear of the combination. Steer smoothly and anticipate hazards; no fixed rollover probability is implied.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.1.2; Last trailer amplification; physical PDF page 200 (printed 200); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
Why do doubles and triples require particular care on slippery roads and mountain routes?
Correct answer: The drive axles must pull more non-driven axles and a longer combination.
Additional trailers add length and non-driven axles. Slippery conditions can increase the chance of losing traction, so allow extra space and drive smoothly.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.1.6; Adverse-condition traction; physical PDF page 201 (printed 201); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
Which parking plan is best for a doubles combination?
Correct answer: Choose a pull-through space with a manageable exit.
Choose a layout that permits a manageable exit. The length and articulation of doubles can make a confined parking area difficult to escape.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.1.7; Pull-through parking; physical PDF page 201 (printed 201); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
What is the role of a converter dolly in a common doubles combination?
Correct answer: It provides axles and a fifth wheel to connect a rear semitrailer to the lead trailer.
The converter gear supports and couples the rear semitrailer, forming a multiple-trailer combination. Its own coupling, tires, brakes and connections need inspection.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.2.1; Figure 7.1; Converter dolly purpose; physical PDF page 202 (printed 202); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
You are preparing to couple a rear trailer and are uncertain whether its brakes will hold. What should you do?
Correct answer: Use suitable wheel chocks and verify the trailer is secured before coupling.
Use suitable wheel chocks whenever brake holding ability is uncertain, and verify the trailer is secured before coupling. Level pavement or an assumed spring-brake system is not sufficient.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.2.1; Rear trailer security; physical PDF page 202 (printed 202); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
Before backing a converter dolly under the rear semitrailer, how should it be positioned?
Correct answer: In line with the rear trailer’s kingpin.
Straight alignment helps the dolly enter the rear trailer correctly. Verify height, security and the equipment-specific coupling sequence as well.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.2.1, continued on page 203; Dolly alignment; physical PDF page 202 (printed 202); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
After a converter dolly couples to the rear trailer, which checks does the handbook call for?
Correct answer: A gentle coupling pull and a visual check of correct plate contact and kingpin locking.
Verify the mechanical connection, not just electrical operation. The visual check must confirm proper engagement after the rig is safely secured.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.2.1; Dolly coupling verification; physical PDF page 203 (printed 203); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
After hooking the converter dolly to the lead trailer, what should be done with the dolly’s support?
Correct answer: Secure it in the raised position for movement.
The support is used when positioning or parking the dolly. It must be properly raised and retained for travel so it does not strike the road or obstacles.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.2.1; Converter support for travel; physical PDF page 203 (printed 203); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
After draining and completing the checks, what position is required for the converter dolly air-tank drain valve before normal operation?
Correct answer: Closed.
The drain must be closed to retain usable air. Draining removes moisture; leaving the valve open during travel creates an air-loss problem.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.3.1; Dolly reservoir drain before travel; physical PDF page 206 (printed 206); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
In a jurisdiction permitting triples: In a set of triples, what should be the normal travel position of the rear air-line shut-off valves on the first and middle trailers?
Correct answer: Open so service and supply air can reach the following trailers.
Intermediate valves must pass air along the combination. The unused valves at the rear of the final trailer are the ones that stay closed. Maryland does not permit triple-trailer highway operation.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.3.1; Intermediate trailer valves; physical PDF page 206 (printed 206); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
In a jurisdiction permitting triples: On the last trailer in a set of triples, no further trailer is connected. How should its rear service and emergency shut-off valves be set for travel?
Correct answer: Both closed.
The air lines end at that trailer. Closing both unused outlets avoids air loss while the connected upstream lines supply the rig. Maryland does not permit triple-trailer highway operation.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.3.1; Final trailer isolation; physical PDF page 206 (printed 206); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
Why must the dolly be pulled clear of the rear trailer before its pintle connection to the lead trailer is released?
Correct answer: The loaded dolly tow bar could fly upward if released while still under the trailer.
Releasing the pintle too early can make the dolly tongue kick up violently. Follow the full uncoupling order and keep people clear of the coupling area.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.2.2; Dolly tongue kick-up; physical PDF page 204 (printed 204); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
In a jurisdiction permitting triples: Using the handbook’s common triple-trailer procedure, which trailer is uncoupled first?
Correct answer: The third trailer, followed by the remainder as a doubles rig.
The handbook describes removing the third trailer using the doubles method, then uncoupling the remaining rig. Other equipment requires its own approved procedure. Maryland does not permit triple-trailer highway operation.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.2.3; Triple uncoupling order; physical PDF page 204 (printed 204); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
Once the dolly is clear of the rear trailer, what must be done before releasing the pintle hook on the lead trailer?
Correct answer: Lower the dolly support and apply its spring brakes or chock its wheels.
Support and secure the dolly before disconnecting it. The handbook also calls for disconnecting its safety chains in the proper sequence.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.2.2; Dolly security before unhitching; physical PDF page 204 (printed 204); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
A spare tire is carried on the converter gear. What must be checked before departure?
Correct answer: It is securely retained on the dolly.
The spare is part of the load carried on the dolly and must be secured. It must not damage or obstruct air lines, electrical connections or safety equipment.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.3.1; Dolly spare tire; physical PDF page 206 (printed 206); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
During a correctly secured air-flow check, no air escapes from the opened supply-line valve at the back of the last trailer. What should you investigate?
Correct answer: A closed intermediate shut-off or another fault preventing air reaching the rear.
Air must reach every connected trailer. Check intermediate valves and diagnose the problem before driving; a charged tractor gauge alone does not prove rear-trailer supply.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.4.1; Missing supply air at the rear; physical PDF page 207 (printed 207); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
After confirming air at the last trailer’s service and supply outlets during the secured test, what should you do with those rear valves?
Correct answer: Close them again before normal operation.
The outlets are opened briefly for the prescribed check. They must be closed again because there is no following trailer to receive that air.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.4.1; Restoring valves after an air-flow test; physical PDF page 207 (printed 207); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
For a modern full-trailer hitch outside driveaway-towaway operations, where should the required safety device attach on the towing vehicle?
Correct answer: To the frame or its extension, independently of the pintle hook or tow-bar attachment device.
The safety connection must remain effective if the tow-bar attachment fails. This question uses modern equipment; the rule has a specific limited exception for certain pre-July 1973 forgings or castings.
Source: 49 CFR 393.70(d), (d)(1); Official eCFR XML dated September 11, 2026 checked against the linked rule; reviewed September 15, 2026. ↗
How much slack should a full-trailer safety device have under the cited federal coupling rule?
Correct answer: No more than is needed for the vehicles to turn properly.
Allow necessary turning movement without unnecessary slack. The safety device must also be arranged to keep a failed or disconnected tow bar off the ground.
Source: 49 CFR 393.70(d)(2), (4); Official eCFR XML dated September 11, 2026 checked against the linked rule; reviewed September 15, 2026. ↗
Under the federal rule for full trailers and converter dollies, the required safety device and its attachments must have what minimum ultimate strength?
Correct answer: At least the gross weight of the vehicle or vehicles being towed.
The specified ultimate strength is based on the gross weight being towed. This is not the working-load-limit calculation used for cargo tiedowns.
Source: 49 CFR 393.70(d)(3); Official eCFR XML dated September 11, 2026 checked against the linked rule; reviewed September 15, 2026. ↗
A converter dolly has the solid tongue and no-swivel arrangement specified in 49 CFR 393.70(d)(6). If it uses a single safety chain alone, how must that device be aligned?
Correct answer: Along the centerline of the trailer tongue.
The rule permits a single device for that specified dolly configuration and requires it to be in line with the tongue’s centerline. Do not generalize this exception to every trailer or hinged dolly.
Source: 49 CFR 393.70(d)(6)(i)–(iii); Official eCFR XML dated September 11, 2026 checked against the linked rule; reviewed September 15, 2026. ↗
Outside driveaway-towaway operations, an air-braked converter dolly was manufactured on March 1, 1998. What does the cited federal rule require?
Correct answer: An antilock braking system meeting the applicable FMVSS 121 requirements.
The relevant requirement applies to air-braked vehicles other than truck tractors manufactured on or after March 1, 1998. Later malfunction-signal circuit dates are a separate issue.
Source: 49 CFR 393.55(c)(2); see (e) for the exterior malfunction indicator; Official eCFR XML dated September 11, 2026 checked against the linked rule; reviewed September 15, 2026. ↗
What must a driver seeking Maryland’s T endorsement understand about testing and triple-trailer operation?
Correct answer: Pass the endorsement knowledge test; it does not authorize triples on Maryland highways.
Maryland’s manual requires the T-endorsement knowledge test but states that triples are not allowed in Maryland. Transportation §24-104.1(l) prohibits a truck tractor on a State highway in combination with more than two trailers or semitrailers, regardless of total length. An endorsement does not override that restriction.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7 introduction; physical PDF page 200 (printed 200); Maryland Transportation §24-104.1(l), official General Assembly text checked; 49 CFR 383.93(c)(1), official eCFR XML dated September 11, 2026; reviewed September 15, 2026. ↗
