Combination Vehicles
Coupling, uncoupling, trailer air controls and rollover risk.
5.1 Coupling and uncoupling
- Inspect the fifth wheel and area; chock trailer wheels and set brakes.
- Make sure the trailer is slightly lower than the center of the fifth wheel.
- Back slowly until the fifth wheel locks onto the kingpin.
- Do a tug test in low gear with trailer brakes set.
- Get out and look: there must be no space between the upper and lower fifth wheel, and the locking jaws must be closed around the shank of the kingpin (not the head).
- Connect air and electrical lines, raise landing gear fully, remove chocks.
Manba: AAMVA / FMCSA — Commercial Driver License Manual (AAMVA model manual used by state CDL manuals) (Model manual — verify your state edition). Yangilangan: 2026-10-09
5.2 Trailer air controls
- The trailer air supply control (usually a red, eight-sided knob) supplies air to the trailer and is used to set the trailer emergency brakes.
- The tractor protection valve keeps air in the tractor if the trailer breaks away or develops a bad leak. It closes automatically when pressure drops into the 20–45 psi range.
- The trailer hand valve (trolley valve) applies trailer brakes only. Never use it for parking.
Diqqat: Rearward amplification ("crack-the-whip") makes the last trailer most likely to roll over during quick lane changes.
Manba: AAMVA / FMCSA — Commercial Driver License Manual (AAMVA model manual used by state CDL manuals) (Model manual — verify your state edition). Yangilangan: 2026-10-09