ATPL Powerplant Questions

Within an axial-flow compressor, a single 'stage' is best described as:

• A ring of rotor blades followed by a ring of stator blades
• A ring of stator blades followed by a ring of rotor blades
• A single row of stator blades only, with no rotating element
• A combustion can paired with a single turbine disc

Explanation: Each axial-compressor stage consists of a rotating rotor blade row followed by a fixed stator blade row; the rotor accelerates the air and imparts kinetic energy, and the following stator row diffuses that flow, converting velocity into a rise in static pressure. Each stage typically produces only a modest pressure ratio, which is why many stages are needed in series to reach a useful overall compression ratio.

Powerplant - Axial Compressors

In a typical turbine engine fuel system, the high-pressure (HP) fuel shut-off valve is normally commanded closed:

• By the engine master/fuel control switch when the crew selects the engine off for a normal shutdown
• Automatically whenever flight idle power is selected
• Only when the engine fuel booster pump fails
• Automatically whenever the aircraft touches down on landing

Explanation: The HP fuel shut-off valve, at the engine fuel inlet, is normally closed by the flight-deck fuel control/master switch when the crew selects the engine off for a normal shutdown, stopping fuel flow into the engine's fuel manifold. A separate low-pressure shut-off valve further upstream is typically closed instead by the fire handle for an emergency shutdown.

Powerplant - Fuel System Shutoff Valves

On a piston engine with a manual mixture control, as altitude increases the mixture is normally leaned in order to:

• Reduce fuel flow, compensating for the reduced air density entering the engine
• Increase fuel flow, compensating for the reduced air density entering the engine
• Increase fuel flow, compensating for increased air density at altitude
• Have no real effect, since mixture setting only matters at sea level

Explanation: As altitude increases, air density falls, so a fixed fuel flow would produce an increasingly fuel-rich mixture. Leaning the mixture reduces fuel flow to restore an appropriate fuel-to-air ratio for the thinner air, keeping combustion efficient and avoiding an overly rich, rough-running, fuel-fouling mixture.

Powerplant - Piston Engine Mixture Control

The 'octane rating' of aviation gasoline (AVGAS) is a measure of:

• The fuel's resistance to detonation (knocking) under compression
• The fuel's ability to absorb and disperse water contamination
• The fuel's freezing or waxing point at altitude
• The fuel's volatility, or tendency to vaporise

Explanation: Octane rating measures a fuel's resistance to premature, uncontrolled combustion (detonation, or 'knock') under the heat and pressure of compression. A higher-octane fuel tolerates a higher compression ratio or manifold pressure before knocking becomes a risk, which is why higher-performance piston engines require higher-octane AVGAS grades.

Powerplant - AVGAS Octane Rating

On a typical turbofan/turbojet engine, Exhaust Gas Temperature (EGT) is measured at the:

• High-pressure turbine discharge, just ahead of any lower-pressure turbine stages
• Combustion chamber inlet
• High-pressure compressor outlet
• Very tip of the final exhaust nozzle

Explanation: EGT probes are positioned at the high-pressure turbine's discharge - upstream of any low-pressure turbine stages and the final exhaust nozzle - a location hot enough to give a meaningful, closely-monitored limit reference without being exposed to the full combustion chamber temperature further upstream.

Powerplant - EGT Measurement Location

In a centrifugal compressor, the airflow path through the impeller is generally:

• Air enters axially at the impeller eye (centre) and leaves radially/tangentially at the periphery (tip)
• Air enters tangentially at the periphery and leaves axially at the eye
• Air enters radially at the eye and leaves axially at the periphery
• Air flows straight through axially with no change of direction

Explanation: A centrifugal compressor's impeller draws air in axially at its centre (the eye); centrifugal force then accelerates and turns the flow outward through the impeller vanes, so it leaves the impeller tip travelling radially/tangentially at high velocity. A diffuser downstream converts that velocity into a pressure rise.

Powerplant - Centrifugal Compressor Airflow

On a single-spool gas turbine engine, the compressor rotates:

• At exactly the same rotational speed as the turbine, since both are mounted on one common shaft
• Faster than the turbine, via a reduction gearbox
• Slower than the turbine, via a step-up gearbox
• Independently of the turbine, coupled only aerodynamically

Explanation: A single-spool design has one shaft connecting a single compressor section directly to a single turbine section, so both necessarily rotate at exactly the same speed. A multi-spool engine instead has separate, mechanically independent shafts (such as N1 and N2), each running its own compressor-turbine pair at its own speed.

Powerplant - Single-Spool Engine Architecture

A pilot notices slightly higher-than-normal engine oil pressure immediately after starting in very cold weather. This is generally considered:

• Acceptable, provided the indication returns to a normal value as the oil warms during the start/warm-up sequence
• A sign the oil filter is blocked and must be changed before flight
• Abnormal, but not serious enough to require shutting the engine down
• A definite sign an oil change is required before the next flight

Explanation: Cold oil is significantly more viscous than warm oil, commonly producing a higher-than-usual pressure indication right after a cold start. This is generally acceptable as long as the pressure decreases into the normal range as the oil warms during the start and warm-up period, rather than remaining abnormally high or continuing to rise.

Powerplant - Cold Weather Starting