Jackknifed tractor-trailer: why it happens, and what recovery involves

What a jackknife actually is

A tractor-trailer bends at one point: the fifth wheel, where the trailer's kingpin locks onto the tractor. In normal driving that joint lets the rig turn corners. In a jackknife it keeps bending until the tractor and trailer fold toward each other, like the blade of a folding knife closing, and the rig slides sideways or comes to rest at a sharp angle across the lanes.

There are two different ways it happens, and the difference matters for understanding the cause. In one, the tractor's drive wheels lose their grip and the trailer, still moving forward, pushes the back of the tractor around. In the other, often called trailer swing, the trailer's own wheels lock or lose grip and the trailer slides out to the side while the tractor carries on. Both end with a rig that is no longer lined up behind itself.

What causes it

• Braking hard on a slippery surface. Rain on a dry, dusty road, ice, snow, gravel or wet leaves reduce traction, and hard braking locks wheels far more easily.
• Braking in a curve. A rig that is already turning has less traction available for braking, so braking hard mid-curve is a common trigger.
• Too much speed for the conditions. Speed turns a small skid into a large one, and leaves less time and room to recover it.
• A light or empty trailer. An empty trailer has little weight pressing its tires onto the road, so its wheels lock more readily, and it is more easily pushed around by the tractor or by wind.
• The engine brake on a slick road. An engine brake slows only the drive wheels, and on ice or snow that can be enough to break their grip and start a drive-wheel skid.
• Brake problems. Brakes that are out of adjustment or unevenly balanced between axles, or an anti-lock system that is not working, make lockup more likely. Our post on the ABS light on a tractor or trailer explains what a lit ABS warning means for this.
• Sudden steering or a shifted load. A sharp swerve, or cargo that moves inside the trailer, can unsettle a rig enough to start the fold.

Preventing it is mostly about traction

Almost every jackknife starts with wheels that stopped rolling or lost their grip, so almost all prevention is about keeping traction available. Slow down before a curve rather than in it. Increase following distance on wet and icy roads so there is room to brake gently. Be cautious with the engine brake when the road is slick, and follow the manufacturer's and your fleet's guidance about switching it off in those conditions. Keep brakes properly adjusted and the ABS working, and take extra care when running empty.

Commercial driver training covers how to respond if a skid starts, and the principle is the same throughout: a locked wheel has no steering and little grip, and the way to regain traction is to let the wheels roll again. That is not something to learn from a blog post on the day, but it is worth knowing why brakes are released rather than stamped on once a skid begins.

Right after a jackknife

Once the rig has stopped, the priorities are the same as any crash scene. If anyone is hurt, or the rig is blocking live lanes, call 911 first. Turn on the hazard flashers and put out warning triangles as soon as it is safe; our guide to where to place warning triangles covers how. Do not try to drive the rig straight again before it has been checked. From a safe position, and only without stepping into traffic, look over the points below and note what you find, because it decides what kind of help is sent.

• The lines between tractor and trailer. A sharp fold can stretch or tear the air lines and the electrical pigtail. A torn air line will apply the trailer brakes and can leave the rig unable to move.
• The fuel tanks. In a tight jackknife the corner of the trailer can strike the tractor's fuel tanks. Look for leaking diesel and keep any flame or spark away.
• The fifth wheel and kingpin area. Contact between the trailer and the back of the cab or the frame can damage the coupling.
• The load. A violent sideways slide can shift cargo inside the trailer, which changes how the rig can safely be moved.
• Where it is sitting. On pavement, on a soft shoulder, in a median or in a ditch, which decides the kind of recovery it needs.

How a jackknifed rig is recovered

Recovery depends on how badly the rig is folded and where it ended up. A tractor-trailer that jackknifed on firm pavement with its lines, tanks and coupling intact can sometimes be straightened out carefully once it has been inspected, and moved on. More often the lines are damaged or the angle is too tight to back out safely, and the rig is dealt with in pieces: the trailer is supported on its landing gear and uncoupled, the tractor is moved clear, and the two are reconnected or taken away separately. If police are investigating the crash, the recovery works to their direction, and moving the rig may have to wait until they release the scene.

When the rig has gone off the pavement into a median, a ditch or soft ground, the job becomes a winch-out, with heavy wreckers anchored on firm ground pulling the tractor and trailer back onto a surface that will carry them. If the load has shifted, it may have to be dealt with before the trailer can be moved without causing more damage. Our post on what happens after a rollover covers the recovery planning that applies when a jackknife ends with the trailer on its side.

What to tell dispatch

The details below decide whether the call needs a service truck to repair torn lines, a wrecker to separate and move the rig, or a full recovery. Jackknifed tractor-trailers are part of the winch-out and recovery work we run across Kansas City, northwest Missouri and northeast Kansas, with WreckMaster-certified operators.

• Where the rig is. Mile marker and direction of travel, and which lanes it is blocking.
• How it is sitting. On the pavement, on the shoulder, in the median or in a ditch, and how sharp the fold is.
• Damage you can see. Torn air or electrical lines, leaking fuel, cab or trailer damage.
• The load. Loaded or empty, what it is, and whether it seems to have shifted.
• Whether anyone is hurt and whether police are already on scene.

Related reading

After a Semi Rollover: What Happens, Step by Step — After a rollover, people and the scene come first: emergency services, traffic control, and any leaking fuel or hazardous cargo.

Semi Stuck in Mud, Snow or Soft Ground? Stop Digging — Stop spinning the wheels.

Winch-Out vs Recovery: What Is the Difference? — A winch-out pulls an otherwise undamaged vehicle back onto the pavement — off a soft shoulder, out of mud or a shallow ditch — and is usually a single truck and a straight pull.