CDL Combination Vehicles — Uncoupling Procedure and Air Lines

Federal regulations require ABS on air-braked tractors built on or after March 1, 1997, and on air-braked trailers built on or after March 1, 1998. Hydraulic-braked vehicles have similar requirements. Vehicles built before these dates are not required to have ABS, but many have been retrofitted. An ABS-equipped vehicle has a yellow ABS warning light that comes on briefly at startup and goes out once the system has self-tested; a steady or recurring yellow ABS light indicates a malfunction. ABS prevents wheel lockup during heavy braking, preserving steering control and often reducing stopping distance on slick surfaces. Drivers should brake firmly with ABS, not pump the brakes.

If the ABS warning light illuminates while driving, the ABS is malfunctioning but the conventional brakes continue to work. You can drive carefully to your destination or to a repair shop. Adjust your driving to compensate: brake earlier and more gradually to avoid the lockup conditions ABS would normally prevent, increase following distance, and avoid panic braking that could cause uncontrolled skids. The yellow ABS light is distinct from the brake warning light (which is red and indicates a more serious problem). Have the ABS repaired at the next reasonable opportunity, and note the malfunction in your inspection log. ABS does not work in reverse on most heavy vehicles regardless of fault status.

The vast majority of cargo tank rollovers are caused by driver error, most often excessive speed in curves. A tanker's high center of gravity and the surge of liquid cargo make it tip easily when entering a curve too fast. The posted curve speed on signs is for cars; tankers need to enter substantially slower, often 5 to 15 mph below the posted advisory speed. The risk is greatest in two situations: highway ramps (off-ramps and cloverleaf interchanges, where drivers underestimate the curve and over-rely on momentum) and city turns (where the curb-side wheels can come off the ground without the driver realizing). Even seemingly mild curves can roll a half-empty tanker due to liquid surge. Slow down before the curve, not in it.

A high center of gravity is the strongest predictor of rollover. Cargo loaded high in a trailer raises the center of gravity, which means smaller side forces (from a curve or a swerve) are enough to tip the vehicle. Tankers have inherently high centers of gravity. Stacking cargo high in box trailers — for example, light loads piled to the ceiling rather than spread on the floor — makes the same trailer much more prone to rollover than the same weight loaded low. Heavy cargo should always be loaded low. Even loads of similar weight can produce very different rollover thresholds depending on how they are arranged. Tall narrow loads roll more easily than short wide loads of equal weight.

A jackknife occurs when the trailer pivots out of alignment with the tractor, folding to one side like a folding knife. The most common causes are trailer wheel lockup during hard braking (the trailer continues forward by momentum while the tractor decelerates, pushing the tractor sideways) and abrupt steering or braking on slippery surfaces. Once a jackknife begins, recovery is extremely difficult; prevention is the only reliable defense. Prevention measures include: ABS on tractor and trailer, smooth braking (especially on slick roads), avoiding sudden steering inputs, maintaining safe following distance to eliminate the need for panic braking, and proper load distribution. If the trailer begins to swing during braking, ease off the brake and let the trailer wheels regain traction.

Glad hands are the coupling devices that connect the service line and emergency (supply) line air hoses between the tractor and trailer. They have rubber seals (grommets) that compress to form an airtight connection. The two glad hands are typically color-coded: red for the emergency/supply line and blue for the service line, often with shapes or labels to prevent cross-connection. Always check the grommets for damage before connecting; a torn or missing grommet causes air leaks. After connecting, listen for air leaks and watch the trailer air pressure gauge to confirm the trailer is being supplied. Always disconnect by pulling the glad hands straight off, not by twisting, to preserve seal integrity.

The correct technique is to slow down before entering a curve and maintain steady speed (or gentle acceleration) through it. Braking inside a curve transfers weight forward, reduces grip on the rear, and increases jackknife risk. Accelerating inside the curve adds force that can push the trailer outward. Slowing before the curve gives you time to brake straight (when braking is most effective and safest), then carrying steady speed through lets the trailer follow the tractor smoothly. Choose your entry speed conservatively — posted curve advisories are for cars, not loaded tractor-trailers. If you have to brake hard in a curve, the curve was too tight for your entry speed.

When uncoupling a trailer on soft surfaces — wet ground, asphalt softened by heat, gravel, or unpaved lots — the landing gear can sink into the surface under the trailer's weight, causing the trailer to drop after the tractor pulls out. This can damage the trailer, cargo, and surrounding property, and can injure people. Prevention: park trailers on hard, level surfaces whenever possible. On softer surfaces, place landing gear feet on wide boards or steel plates to spread the load. Avoid parking on grades; if you must, chock the wheels in both directions. After lowering landing gear, look to confirm it is firmly contacting the ground before disengaging the fifth wheel.

The rollover threshold is the level of lateral acceleration (cornering force) at which a trailer will tip over. It is expressed in terms of gravity (g). A typical empty box trailer has a rollover threshold around 0.5g; a typical loaded box trailer around 0.4g; a typical loaded tanker around 0.35g; and a tanker with partial liquid load may be as low as 0.25g. A passenger car has a rollover threshold around 1.0g — well above the limits of road adhesion, so cars rarely roll. Trucks have thresholds within or below the limits of grip, which is why curves and lane changes that pose no rollover risk to cars can roll heavy vehicles. Drivers reduce rollover risk by reducing speed and avoiding sudden steering.

On ABS-equipped vehicles, do not pump the brakes. Apply firm, steady pressure and let the ABS system modulate brake pressure automatically (dozens of times per second) to prevent wheel lockup. Pumping defeats the ABS and reduces braking effectiveness. The vehicle may feel and sound different during ABS-active braking — pulsation in the pedal, mechanical noise — but this is normal operation, not malfunction. Old habits from non-ABS vehicles (pumping, stab braking) actively work against ABS. The exception is a true brake failure where pumping is a last resort to recover hydraulic pressure, but this does not apply to air brake systems and is not normal operating practice on ABS systems.