DTT HGV Passenger Safety and Air Brakes 5 — Questions and Answers
Question 1: What is the air pressure threshold at which the low-pressure warning alarm activates in an air brake system?
- Typically around 4.5-5.5 bar — the alarm warns the driver before the pressure drops to spring brake activation level (around 2.5 bar) (Correct answer)
- When the pressure drops to zero
- Only when the brakes are physically failing to respond
- At 7 bar — the maximum pressure of the system
Correct answer: Typically around 4.5-5.5 bar — the alarm warns the driver before the pressure drops to spring brake activation level (around 2.5 bar)
The low-pressure warning alarm activates before pressure drops to the critical level where spring brakes apply automatically. This gives the driver time to pull over safely before losing control of the braking.
Question 2: How does the air brake system maintain pressure during a long journey?
- An engine-driven compressor continuously tops up the air reservoirs as air is used during braking (Correct answer)
- The reservoirs are filled only at the depot — they must last the full journey
- Atmospheric air is drawn in through filters when the brakes are released
- The system recycles exhaust pressure to top up the reservoirs
Correct answer: An engine-driven compressor continuously tops up the air reservoirs as air is used during braking
An air compressor driven by the engine continuously replenishes the air reservoirs. A governor valve stops the compressor when maximum pressure is reached and restarts it when pressure drops — maintaining consistent operating pressure.
Question 3: What is the purpose of drainage taps on air brake reservoirs?
- To drain water condensation that accumulates in the air system — water can freeze in cold weather and cause brake failure (Correct answer)
- To release excess pressure if the safety valve fails
- To allow emergency manual brake application
- To drain old brake fluid when replacing it with new
Correct answer: To drain water condensation that accumulates in the air system — water can freeze in cold weather and cause brake failure
Air compressors generate moisture from compressed atmospheric air. This moisture accumulates in reservoirs and can freeze in cold weather, blocking the air system and causing brake failure. Reservoirs must be drained regularly.
Question 4: When a bus picks up wheelchair users, what specific safety procedures apply?
- Wheelchair must be positioned and secured with approved anchoring systems — facing rearward or forward depending on vehicle specification — before moving off (Correct answer)
- Wheelchairs are secured at the driver's discretion
- Only the wheelchair user is responsible for securing their own chair
- Wheelchair spaces are designated zones — no additional securing is required
Correct answer: Wheelchair must be positioned and secured with approved anchoring systems — facing rearward or forward depending on vehicle specification — before moving off
Wheelchair users must be positioned in the designated wheelchair space and the wheelchair must be secured using all provided restraints (tie-down straps and anti-tip). The driver must confirm security before moving.
Question 5: How does tyre pressure affect air-braked heavy vehicle braking performance?
- Under-inflated tyres increase braking distance, reduce lateral stability, and increase the risk of overheating and blowout under heavy braking (Correct answer)
- Tyre pressure has minimal effect on air-braked vehicle stopping distance
- Higher tyre pressure reduces stopping distance — inflate tyres above recommended levels for better braking
- Tyre pressure only affects fuel consumption — not braking
Correct answer: Under-inflated tyres increase braking distance, reduce lateral stability, and increase the risk of overheating and blowout under heavy braking
Under-inflated tyres on a heavy vehicle create a larger contact patch but with reduced structural integrity. Under braking, the tyre can flex excessively, generating heat and increasing stopping distances. Overloaded under-inflated tyres are at high blowout risk.
Question 6: What is 'brake proportioning' and why is it important in truck-trailer combinations?
- Distributing braking force between tractor and trailer axles to prevent jackknifing — too much tractor braking relative to trailer braking causes jackknife (Correct answer)
- A system for adjusting brake sensitivity to road conditions
- The balance of front and rear axle braking to prevent front axle lock
- A calibration process carried out at the CVRT test centre
Correct answer: Distributing braking force between tractor and trailer axles to prevent jackknifing — too much tractor braking relative to trailer braking causes jackknife
Brake proportioning ensures braking effort is shared correctly between tractor and trailer. If the tractor brakes harder than the trailer, the trailer pushes the tractor sideways — causing jackknife. EBS manages this electronically.
DTT HGV Passenger Safety and Air Brakes 5
What is the air pressure threshold at which the low-pressure warning alarm activates in an air brake system?