09 Aircraft Systems & Propulsion

What are the basic energy-conversion stages of a gas-turbine engine?

Intake → compression → combustion → turbine expansion → exhaust/nozzle; the turbine extracts enough energy to drive the compressor/fan, while the remaining energy produces thrust.

Why does a turbofan produce most of its thrust efficiently through bypass flow?

It accelerates a large mass of air by a relatively small velocity increase, which is more propulsively efficient at subsonic transport speeds than accelerating a smaller mass much more.

What is bypass ratio?

The ratio of mass airflow bypassing the engine core to mass airflow passing through the core.

What is the purpose of multiple compressor/turbine spools?

They allow different compressor sections to rotate at their most efficient speeds and improve starting, stability and overall engine performance.

What do N1 and N2 broadly represent on a two-spool turbofan?

N1 represents low-pressure spool/fan speed; N2 represents high-pressure spool speed.

What does EPR represent?

The ratio of engine exhaust pressure to inlet pressure, used on some engines as an indication of thrust.

Why does a compressor increase both pressure and temperature?

Mechanical work is added to the airflow, increasing its total pressure and total temperature.

What powers the compressor in steady engine operation?

The turbine extracts energy from the hot combustion gases and transmits it mechanically through the engine shaft.

Why is only part of the oxygen entering a turbine engine used for combustion?

A large amount of air is needed for cooling, dilution and turbine-temperature control in addition to combustion.

What is FADEC responsible for?

Automatic engine control including fuel scheduling, thrust management, start sequencing and protection/limit functions according to the engine design.

What is the main operational benefit of FADEC?

It reduces pilot workload and prevents many engine parameters from being exceeded through inappropriate manual fuel control.

What is the difference between a compressor stall and compressor surge?

A stall is local airflow separation on compressor blades; surge is a major breakdown of compressor flow that may involve violent pressure oscillation or temporary flow reversal through the engine.

What can promote compressor stall or surge?

Disturbed inlet airflow, rapid power changes, foreign-object damage, icing or compressor/control-system malfunction.

What indications may accompany a severe compressor stall/surge?

Bang or rumble, engine vibration, fluctuating parameters, loss of thrust and possibly increased EGT.

What is a hot start?

An engine start in which turbine temperature exceeds the permitted start limit.

What is a hung start?

The engine stabilises below normal self-sustaining idle speed instead of accelerating normally.

What is a wet start?

Fuel enters the engine during start but does not ignite, leaving unburned fuel in the combustion section/exhaust.

Why must turbine temperature be closely monitored during engine start?

Airflow is initially low, so excessive fuel or insufficient compressor acceleration can quickly overheat turbine components.

What is the main purpose of an APU on a transport aircraft?

To provide electrical power and/or pneumatic air independently of the main engines, particularly on the ground and as applicable in flight.

Why may APU capability become more restricted with altitude?

Lower air density reduces compressor mass flow and therefore pneumatic and power capability.

What is bleed air commonly used for on conventional transport aircraft?

Air conditioning, pressurisation, engine starting and thermal anti-icing, depending on aircraft design.

Where is engine bleed air normally taken from?

From compressor stages, with the selected source varying with engine operating conditions.

Why is high-stage bleed not used continuously when low-stage bleed is sufficient?

Higher-pressure bleed extracts more useful energy from the engine and carries a greater performance penalty.

What is the basic function of an air-conditioning PACK?

To cool, condition and regulate compressed air before it enters the aircraft cabin.

What primarily controls cabin pressure in a conventional pressurisation system?

The amount of air leaving the pressure vessel through the outflow valve.

Why is the outflow valve normally more important for pressure control than changing PACK inflow?

Cabin inflow is relatively continuous; controlled restriction of outflow allows precise regulation of cabin pressure.

What is differential pressure?

The difference between cabin pressure and ambient outside pressure.

Why is maximum differential pressure structurally limited?

The fuselage pressure vessel must not be subjected to loads beyond its certified structural limit.

What prevents excessive positive or negative cabin differential pressure if normal control fails?

Pressure-relief valves.

Why does cabin altitude rise during a depressurisation at high altitude?

Cabin pressure falls toward the much lower ambient pressure, so the equivalent pressure altitude inside the cabin increases.

What is the basic architecture principle of transport-aircraft electrical systems?

Multiple independent sources and buses are arranged so essential equipment can remain powered after individual failures.

What are the usual primary electrical sources in flight?

Engine-driven generators, with the APU generator, batteries and emergency sources available depending on aircraft design.

What is the function of a transformer rectifier unit (TRU)?

It converts AC electrical power to DC.

What is the purpose of an aircraft battery beyond engine/APU starting?

It provides stored DC energy for essential equipment when normal generation is unavailable and supports electrical transients.

What is a static inverter used for?

To convert DC electrical power into AC for selected essential loads when normal AC generation is unavailable.

What is electrical load shedding?

Automatic or procedural removal of non-essential electrical loads to preserve power for essential systems after generation loss.

Why are electrical buses separated during normal operation but capable of being interconnected?

Separation limits propagation of faults; interconnection allows a remaining source to power additional buses after a failure.

What is the purpose of a RAT on aircraft equipped with one?

To provide limited emergency hydraulic and/or electrical power after major loss of normal energy sources.

Why are hydraulic systems widely used on transport aircraft?

They can transmit very large forces with compact actuators and precise control.

What is the difference between hydraulic pressure and hydraulic flow?

Pressure provides the potential force; flow is required to move an actuator and determines how quickly hydraulic work can be performed.

Why can a hydraulic system show normal pressure but still fail to operate an actuator properly?

A restriction, low fluid quantity or failed pump may prevent sufficient flow despite pressure being indicated.

What is the purpose of a hydraulic accumulator?

To store hydraulic energy, damp pressure fluctuations and provide a limited reserve for specific functions.

What is the purpose of a PTU?

To transfer hydraulic power between systems without transferring hydraulic fluid between them.

Why are transport-aircraft hydraulic systems normally separated and redundant?

A leak or component failure in one system should not disable all flight-control or landing functions.

Why may hydraulic reservoirs be pressurised?

To ensure positive fluid supply to pumps and reduce cavitation, particularly at altitude.

What is the basic purpose of fuel crossfeed?

To allow an engine or fuel system to be supplied from another tank/feed path when required.

Why is fuel crossfeed different from fuel transfer?

Crossfeed changes the source feeding an engine; transfer physically moves fuel from one tank to another.

Why is fuel imbalance undesirable?

It creates lateral mass imbalance and may increase structural loads, drag or control requirements.

Why are boost pumps used in aircraft fuel systems?

They provide positive fuel pressure to the engine feed system and reduce the risk of vapour formation or interruption of supply.

Why must fuel temperature sometimes be monitored in flight?

Very low fuel temperature can approach the fuel freezing limit and restrict safe operation.

What is the fundamental difference between mechanical flight controls and fly-by-wire?

Mechanical systems transmit pilot input through physical linkages; fly-by-wire converts pilot commands into electrical signals processed by computers before commanding actuators.

Does fly-by-wire mean the control surfaces are electrically powered?

Not necessarily. Commands are electrical, while the actuators may be hydraulic, electro-hydraulic or electric.

Why does fly-by-wire require extensive redundancy?

A single electrical, computing or sensor failure must not cause loss of essential flight-control capability.

What is the basic purpose of anti-skid braking?

To regulate brake pressure near optimum tyre slip and prevent wheel lock, improving directional control and braking effectiveness.

Why can a locked wheel provide less useful braking on a wet runway?

It loses optimal tyre-road friction and becomes much more susceptible to skidding or hydroplaning.

What is the basic principle of an autobrake system?

It automatically modulates braking to achieve a selected deceleration target rather than simply applying a fixed brake pressure.

Why can brake temperature become operationally significant after a high-energy stop?

Excessive brake energy can damage brakes/tyres and may create a fire risk or require cooling before further operation.

How do thermal and pneumatic ice-protection systems differ?

Thermal systems prevent/remove ice using heat; pneumatic boots remove accumulated ice by cyclically deforming the protected surface.

Why are pitot probes and similar air-data sensors heated?

To prevent ice from blocking or corrupting pressure measurements used for airspeed and other flight data.

What is the purpose of fire-detection loops in engines/APUs?

To detect excessive heat or fire while providing redundancy and reducing false warnings.

What is the general effect of activating an engine fire-isolation control?

It isolates major fuel, pneumatic, hydraulic and/or electrical connections as designed and arms or enables extinguishing capability; exact actions are aircraft-specific.

Why are engine fire bottles not simply discharged immediately for every fire warning?

Initial isolation and shutdown remove fuel/energy sources; extinguishing agent is then used according to the aircraft procedure if required.