DTT HGV Passenger Safety and Air Brakes 4 — Questions and Answers
Question 1: What is 'jackknifing' and how can a truck driver prevent it?
- The trailer pivots around the fifth wheel creating a V-shape — prevented by gradual braking, avoiding over-braking the tractor while trailer brakes lock (Correct answer)
- The trailer detaches from the tractor on a sharp corner
- The truck's front axle locks under heavy braking
- The trailer swings sideways during a lane change at speed
Correct answer: The trailer pivots around the fifth wheel creating a V-shape — prevented by gradual braking, avoiding over-braking the tractor while trailer brakes lock
Jackknifing occurs when the trailer's braking locks the trailer wheels causing it to swing outward while the tractor continues forward. Prevent by using progressive braking, avoiding over-braking on slippery surfaces.
Question 2: What is 'trailer swing' and how does it differ from jackknifing?
- Trailer swing (snaking) is lateral oscillation of the trailer — usually caused by excessive speed for the condition, while jackknifing is a braking-induced fold at the coupling (Correct answer)
- Trailer swing and jackknifing are the same event described differently
- Trailer swing is a front axle phenomenon; jackknifing is rear axle
- Trailer swing only occurs on long trailers over 13.5 metres
Correct answer: Trailer swing (snaking) is lateral oscillation of the trailer — usually caused by excessive speed for the condition, while jackknifing is a braking-induced fold at the coupling
Trailer swing (snaking) is a side-to-side oscillation of the trailer that worsens with speed. It is typically caused by excessive speed for load/wind conditions. Jackknifing is a distinct braking-related fold at the coupling point.
Question 3: What is the correct response if trailer swing begins on a motorway?
- Ease off the accelerator gently and do not brake — gradual deceleration without braking often allows the trailer to stabilise (Correct answer)
- Brake firmly to stop the swing
- Accelerate to pull the trailer straight
- Apply the trailer brake separately to stop the swing
Correct answer: Ease off the accelerator gently and do not brake — gradual deceleration without braking often allows the trailer to stabilise
If trailer swing begins, ease off the accelerator smoothly — do not brake, as braking worsens the oscillation. If the swing is severe, progressive gentle braking after the speed has reduced may be needed.
Question 4: What should a bus driver do in an emergency evacuation of the vehicle?
- Stop safely, apply spring brakes and engine stop, open all emergency exits, direct passengers calmly to exit, assist mobility-impaired passengers, and call emergency services (Correct answer)
- Evacuate quickly — direct passengers to the nearest exit regardless of hazards outside
- Only evacuate if instructed by emergency services on scene
- Passengers self-evacuate — the driver focuses on keeping the vehicle stable
Correct answer: Stop safely, apply spring brakes and engine stop, open all emergency exits, direct passengers calmly to exit, assist mobility-impaired passengers, and call emergency services
In an emergency evacuation: secure the vehicle (handbrake, engine off), activate hazards, open all emergency exits, calmly direct passengers to safe exits, assist those unable to self-evacuate, and call 999.
Question 5: What is the function of load sensing valves in an air brake system?
- They adjust the braking force to the axle based on the load carried — preventing wheel lock on lightly loaded axles (Correct answer)
- They control maximum air pressure in the reservoir
- They prevent over-braking on the steering axle
- They monitor tyre pressure and adjust brake force accordingly
Correct answer: They adjust the braking force to the axle based on the load carried — preventing wheel lock on lightly loaded axles
Load sensing valves modulate the brake air pressure delivered to rear axles based on the suspension compression (which indicates load). This prevents wheel lock on empty or lightly loaded axles.
Question 6: What is the purpose of EBS (Electronic Braking System) on modern commercial vehicles?
- EBS integrates ABS, traction control, stability control, and optimised brake force distribution electronically — improving safety and brake efficiency (Correct answer)
- EBS replaces air brakes entirely with electronic actuators
- EBS is a driver-assist feature that automatically applies brakes in emergencies only
- EBS monitors brake pad wear and notifies the driver when replacement is due
Correct answer: EBS integrates ABS, traction control, stability control, and optimised brake force distribution electronically — improving safety and brake efficiency
EBS (Electronic Braking System) uses electronic control to optimise brake force distribution across axles, integrating ABS, traction control, and stability systems. It improves braking performance and reduces stopping distances.
DTT HGV Passenger Safety and Air Brakes 4
What is 'jackknifing' and how can a truck driver prevent it?