A trailer jackknife occurs when the truck reduces speed, but for whatever reason the trailer doesn’t and the trailer tires break traction. It can occur on bare dry pavement, but then it is almost always caused by improper loading of the trailer (all of the weight up front in the trailer) and badly out-of-adjustment brakes. Recovering from this particular scenario is pretty easy. Just release the brake pedal, and everything should line up properly again. In adverse weather conditions though, it gets a lot more complicated. Again, you have 2 or 3 seconds to react properly, or you’re just along for the ride.
First and foremost, always drive on the road and in weather conditions. Just because the posted speed limit is 90 km/h doesn’t mean you need to drive that fast. If the road is covered in snow or ice, SLOW DOWN! If you notice your trailer is trying to slide out from behind you, you will need to fight the urge and instinct to apply the brakes. Doing so will only make the situation worse, and will make recovery impossible. Some people will tell you to apply light pressure to the trailer brakes using the hand valve. In this case, it won’t help at all, as the trailer tires have already lost all traction with the road surface. Your only hope is to gently apply power and hope it will pull the trailer back in line behind you. Again, if the jack knife exceeds 15°, it’s probably too late. One situation to watch for during a trailer jackknife is that it could quickly also become a truck jack knife, as the trailer is usually heavier than the truck and it could easily cause the drive axles to break traction. If this happens and you have both the trailer axles and the truck axles breaking traction, recovery is impossible. You’re simply at the mercy of the laws of physics.
The best situation is to avoid getting into a jackknife in the first place. Reduce your speed during inclement weather, and on less-than-perfect road conditions. Check your brake adjustments daily and drain the moisture from your air tanks. Moisture in the air lines could cause a valve to freeze up rendering your brakes inoperative. Avoid hard brake applications, as this could cause the tires to break traction. Avoid using the engine brake and cruise control on anything but bare dry pavement. The engine brake slows the engine, and through the drive line it slows the drive tires but does nothing to trailer tires, and this can put you into an unrecoverable jackknife. The cruise control likewise can cause the drive axles to break traction, and make the jack knife totally unrecoverable.