EASA ATPL — Aircraft General Knowledge (021/022) Exam Prep

Correct answer: B. To transfer hydraulic power between systems without transferring fluid

A PTU transfers hydraulic power (pressure energy) between two independent systems without mixing the hydraulic fluids. It consists of a hydraulic motor on one system driving a hydraulic pump on another. This maintains system isolation while providing redundancy. This is commonly seen on the A320 between the Green and Yellow systems.

Correct answer: B. To regulate cabin pressure by controlling the rate of air discharged overboard

The outflow valve is the primary means of controlling cabin pressure. By modulating how much air is discharged overboard, it controls the cabin altitude and rate of change. The pressurisation controller adjusts the outflow valve position to maintain the selected cabin altitude and a comfortable rate of pressure change during climb and descent.

Correct answer: C. 6,000 ft

Cabin pressure = ambient pressure + max differential = 3.14 + 8.6 = 11.74 psi. Standard sea-level pressure is 14.7 psi. A cabin pressure of 11.74 psi corresponds to approximately 6,000 ft cabin altitude (using the ISA pressure-altitude relationship). Maximum cabin altitude is typically limited to 8,000 ft for passenger comfort and safety.

Correct answer: C. Halon 1301 (or approved HFC replacement)

Halon 1301 (bromotrifluoromethane) has been the standard fire extinguishing agent for engine nacelles and cargo compartments in transport aircraft due to its effectiveness, low toxicity, and clean discharge. As Halon is being phased out under the Montreal Protocol, approved HFC replacements (like HFC-125 / Halotron) are being adopted.

Correct answer: B. The flight envelope protections (alpha floor, bank angle limits) are lost, but some stability augmentation remains

In Alternate Law, the fly-by-wire system loses its high angle-of-attack protection (alpha floor/alpha max), load factor limitation may be degraded, and bank angle protection is replaced by a spiral stability function. However, the system still provides some stability augmentation. The aircraft does not revert to Direct Law unless further degradation occurs. The low-speed stability (stall warning replaces alpha protection) becomes the pilot's responsibility.

Correct answer: B. To cool and condition hot bleed air using a turbine-compressor cycle

A bootstrap air cycle machine (ACM) uses a compressor and turbine on a common shaft. Hot bleed air is first passed through a primary heat exchanger, then compressed further by the ACM compressor (raising temperature), cooled again in a secondary heat exchanger, and finally expanded through the ACM turbine where work extraction dramatically drops the temperature. This produces cold air for cabin conditioning.