Ontario MTO Truck Knowledge Practice Test

If the tractor is jackknifing (if the tractor rear wheels slide sideways), what action should be taken?

1. Apply the service brakes only

2. Apply the trailer brakes only

3. Apply both the service and trailer brakes

4. Release all brakes

The correct answer is: Apply the trailer brakes only

When a tractor is jackknifing, applying the trailer brakes only is the most effective strategy to regain control of the vehicle. The jackknife situation occurs when the trailer pushes into the tractor unit, which can lead to a loss of control if not managed correctly. When you apply only the trailer brakes, it can help to stabilize the trailer and slow it down without locking up the tractor's wheels, which allows the driver to retain steering control of the tractor. This method can effectively mitigate the skid and help align the trailer and tractor for better handling. In contrast, using the service brakes alone could increase the potential for further sliding, as applying the front brakes can cause the rear to slide even more. Similarly, engaging both the service and trailer brakes could inhibit maneuverability and may exacerbate the skid. Releasing all brakes typically isn't advisable, as it may allow the trailer to exert excessive force on the tractor, worsening the jackknife situation. Thus, applying the trailer brakes is a critical tactical move to regain control during this dangerous scenario.