DTT HGV Passenger Safety and Air Brakes 5

Question 1

What is the air pressure threshold at which the low-pressure warning alarm activates in an air brake system?

Accepted 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)

Answer

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?

Accepted Answer

An engine-driven compressor continuously tops up the air reservoirs as air is used during braking

Answer

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?

Accepted Answer

To drain water condensation that accumulates in the air system — water can freeze in cold weather and cause brake failure

Answer

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?

Accepted Answer

Wheelchair must be positioned and secured with approved anchoring systems — facing rearward or forward depending on vehicle specification — before moving off

Answer

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?

Accepted Answer

Under-inflated tyres increase braking distance, reduce lateral stability, and increase the risk of overheating and blowout under heavy braking

Answer

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.

Irish DTT Truck/Bus Practice Test

Irish DTT Truck/Bus Practice Test

DTT HGV Passenger Safety and Air Brakes 5