CAAS SAR-66 AML Module 14 — Propulsion

In a gas turbine engine operating on the ideal Brayton cycle, which thermodynamic process occurs inside the combustion chamber?

• AIsentropic compression
• BConstant pressure heat addition
• CIsothermal expansion
• DConstant volume combustion

An engine exhaust nozzle has a total pressure (Pt7) of 45 psia and the engine inlet total pressure (Pt2) is 15 psia. What is the indicated Engine Pressure Ratio (EPR)?

• A0.33
• B3.00
• C30.0
• D60.0

A turbofan engine ingests air at a mass flow rate of 120 kg/s. The aircraft is flying at a true airspeed of 150 m/s, and the exhaust gas exit velocity is 400 m/s. Assuming a fully expanded convergent nozzle (Pj = Pa), calculate the net thrust (Fn).

• A18,000 N
• B30,000 N
• C48,000 N
• D66,000 N

What happens to the net thrust produced by a gas turbine engine as aircraft flight altitude increases at a constant true airspeed and engine RPM?

• ANet thrust increases due to lower ambient temperature.
• BNet thrust decreases due to the primary effect of reduced ambient air density.
• CNet thrust remains constant because ram pressure recovery compensates for altitude loss.
• DNet thrust increases continuously up to the tropopause.

The propulsive efficiency (eta_p) of a jet engine is expressed by which of the following mathematical relations, where Va is aircraft speed and Vj is jet velocity?

• Aeta_p = (Vj - Va) / (Vj + Va)
• Beta_p = 2 * Va / (Vj + Va)
• Ceta_p = Va^2 / Vj^2
• Deta_p = (Vj + Va) / (2 * Va)

In a dual-spool high bypass turbofan engine, the bypass ratio is defined as the ratio of:

• AMass flow rate through the fan bypass duct to mass flow rate passing through the engine c…
• BFuel mass flow rate to total ingested air mass flow rate.
• CTurbine exhaust gas velocity to compressor inlet air velocity.
• DLow pressure turbine work output to high pressure compressor power absorption.

Which change occurs to total air pressure and static air pressure as subsonic airflow passes through a divergent duct (diffuser) in an engine intake?

• AStatic pressure increases, dynamic pressure decreases, and total pressure remains constan…
• BStatic pressure decreases, dynamic pressure increases, and total pressure increases.
• CStatic pressure increases, dynamic pressure increases, and total pressure decreases.
• DStatic pressure remains constant, while dynamic pressure and total pressure decrease.

What is the primary operational advantage of a high-bypass turbofan over a turbojet engine producing equal total thrust?

• ALower overall weight per unit of engine length
• BLower Specific Fuel Consumption (SFC) and reduced noise emissions
• CHigher maximum exhaust gas velocity at supersonic flight speeds
• DElimination of the low-pressure compressor and turbine spools

If a gas turbine exhaust nozzle becomes choked under high pressure ratio conditions, what is the Mach number of the gas flow at the nozzle throat area?

• AMach 0.5
• BMach 0.85
• CMach 1.0
• DMach 2.0

How does an increase in ram air pressure ratio (due to increasing forward aircraft speed) affect the overall performance of a gas turbine engine?

• AIt decreases compressor air inlet density, reducing net thrust.
• BIt increases compressor inlet total pressure (Pt2), increasing mass air flow and gross th…
• CIt reduces turbine entry temperature, causing an immediate engine flameout.
• DIt eliminates the necessity for compressor variable stator vanes.

In a gas turbine core, thermal efficiency (eta_th) is primarily governed by which thermodynamic parameter ratio?

• AEngine overall pressure ratio (OPR) and turbine entry temperature (TET)
• BBypass ratio and fan diameter
• CExhaust pipe length and nozzle exit diameter
• DFuel density and oil pressure differential

If Pt7 is 52 psia and Pt2 is 20 psia, calculate the EPR. If an icing condition blocks the Pt2 probe and causes it to trap a false low pressure of 16 psia while Pt7 remains 52 psia, what will the false indicated EPR be?

• ATrue EPR = 2.60; False Indicated EPR = 3.25
• BTrue EPR = 2.60; False Indicated EPR = 2.08
• CTrue EPR = 3.25; False Indicated EPR = 2.60
• DTrue EPR = 0.38; False Indicated EPR = 0.30

Calculate the gross thrust (Fg) of a jet engine if mass air flow rate is 80 kg/s, fuel flow rate is 2 kg/s, jet exit velocity (Vj) is 500 m/s, nozzle exit area (Aj) is 0.2 m^2, exit static pressure (Pj) is 120 kPa, and ambient static pressure (Pa) is 100 kPa.

• A41,000 N
• B45,000 N
• C49,000 N
• D51,000 N

What is the primary function of the diffuser section located between the axial compressor and the combustion chamber?

• ATo increase air velocity while decreasing static pressure prior to fuel injection.
• BTo reduce airflow velocity and increase static pressure to prepare air for stable combust…
• CTo impart a high rotational swirl to the air to prevent turbine blade overheating.
• DTo bleed off excess high-pressure air directly into the exhaust nozzle.

Specific Fuel Consumption (SFC) for a turbofan engine is defined as:

• AMass of fuel burned per unit of time per unit of thrust generated.
• BTotal fuel volume consumed per nautical mile flown.
• CRatio of gross thrust to net engine weight.
• DPercentage of air mass flow converted into combustion gas.

Which operating state represents compressor stall in an axial flow compressor?

• AComplete reverse flow of gases through the entire engine core accompanied by loud detonat…
• BLocal aerodynamic breakdown of airflow over compressor rotor blades caused by excessive a…
• COverspeed of the high pressure turbine spool due to fuel control valve failure.
• DUncontrolled combustion in the secondary air bypass duct.

During engine acceleration from ground idle to takeoff thrust, how does the FADEC prevent compressor stall or surge?

• ABy scheduling fuel flow strictly in proportion to compressor discharge pressure (P3) and…
• BBy overriding the pilot thrust lever and shutting down the low-pressure spool.
• CBy locking the fuel metering valve in a fixed position until N2 reaches 100%.
• DBy opening the exhaust nozzle area to maximum reverse pitch.

In a triple-spool turbofan engine (such as the Rolls-Royce Trent series), what does N1, N2, and N3 designate respectively?

• AN1 = Fan/LP spool speed, N2 = IP spool speed, N3 = HP spool speed
• BN1 = HP spool speed, N2 = IP spool speed, N3 = Fan speed
• CN1 = Combustion pressure, N2 = Turbine temp, N3 = Fuel pressure
• DN1 = Starter speed, N2 = Main gear speed, N3 = Alternator speed

An engine has a compressor pressure ratio of 25:1. If ambient inlet pressure Pt2 is 14.0 psia, what is the theoretical total compressor discharge pressure Pt3?

• A175 psia
• B250 psia
• C350 psia
• D500 psia

Under constant cruise speed and altitude conditions, how does an increase in ambient outside air temperature (OAT) affect engine thrust and EGT?

• AThrust decreases because air density drops, and EGT increases for a given thrust lever se…
• BThrust increases because warmer air contains more internal kinetic energy.
• CThrust remains unchanged while EGT drops significantly.
• DThrust increases and EGT drops to maintain constant density.

In an axial-flow compressor, what is the primary structural and aerodynamic function of the compressor stator vanes?

• ATo accelerate the airflow and increase dynamic velocity.
• BTo convert velocity energy into static pressure and direct airflow into the next rotor st…
• CTo filter out particulate matter and moisture before combustion.
• DTo mechanically lock the rotor disk to the LP turbine shaft.

Which mechanical component allows Variable Stator Vanes (VSVs) to rotate simultaneously through precise angles on an axial compressor casing?

• AAn actuating ring driven by hydraulic fuel actuators linked to crank arms on each vane sp…
• BA flexible speedometer cable driven directly by the accessory gearbox
• CA pneumatic diaphragm connected to the exhaust tailcone
• DA magnetic clutch mounted on the front fan spinner

How is primary air distributed relative to secondary and tertiary air inside a typical annular combustion chamber?

• APrimary air (~20-25%) supplies combustion at stoichiometric ratio; secondary and tertiary…
• BPrimary air (~80%) dilutes exhaust gases; secondary air (~20%) enters the igniter plug.
• CPrimary air (~100%) enters through the fuel nozzle; no secondary air is permitted.
• DPrimary air is routed through bypass ducts directly to the turbine.

High pressure turbine (HPT) rotor blades operate under extreme thermal and centrifugal stress. Which cooling technique passes compressor bleed air through internal blade passages and out tiny leading-edge holes to form a protective boundary layer?

• AConvective internal cooling only
• BFilm cooling
• CRadiative shield cooling
• DThermoelectric peltier cooling

What type of bearing is typically employed to absorb main rotor axial thrust loads in a gas turbine engine?

• ADeep-groove ball bearing
• BCylindrical roller bearing
• CNeedle roller bearing
• DTapered sleeve journal bearing

Which non-contact seal configuration utilizes a series of fine knife-edge ridges and expansion chambers to limit oil and air leakage along rotating engine shafts?

• ACarbon face seal
• BLabyrinth seal
• CSynthetic O-ring seal
• DHydraulic lip seal

Active Clearance Control (ACC) systems modulate cooling air onto the HPT turbine casing primarily to achieve which operational goal?

• ATo expand the casing during engine start to prevent blade rub
• BTo shrink the casing during cruise to minimize blade tip clearance and maximize turbine e…
• CTo cool the accessory gearbox oil lines
• DTo prevent combustor liner thermal distortion during reverse thrust

What structural feature on modern fan blades helps prevent destructive aerodynamic flutter and resistance to Foreign Object Damage (FOD)?

• AMid-span shrouds or wide-chord hollow titanium/composite blade construction
• BSolid lead core filling
• CUn-tapered rectangular planform geometry
• DExternal stainless steel drag hinges

Which aerodynamic design characteristic distinguishes impulse turbine blades from reaction turbine blades?

• AImpulse blades experience a static pressure drop across the rotor blades, whereas reactio…
• BImpulse blades experience no static pressure drop across rotor blades (pressure drop occu…
• CImpulse blades operate exclusively in subsonic flow, while reaction blades require supers…
• DImpulse blades are hollow for cooling, whereas reaction blades are always solid.

What is the primary cause of turbine blade 'creep' during long-term engine operation?

• AHigh frequency acoustic vibration occurring during ground idle
• BPermanent plastic elongation of blade material caused by combined high temperature and co…
• CChemical corrosion from sulfur in jet fuel at low power settings
• DRapid thermal contraction during engine shutdown

Single-crystal (SX) superalloy turbine blades provide superior high-temperature strength compared to conventionally cast blades primarily because they eliminate:

• AGrain boundaries perpendicular to and along stress axes, preventing intergranular creep f…
• BThe need for protective ceramic thermal barrier coatings (TBC)
• CInternal cooling air passages
• DRotor blade attachment fir-tree roots

What type of combustor design is formed by interconnected individual burner cans arranged radially inside a single outer air casing?

• ACan-annular (tubo-annular) combustion chamber
• BMultiple-can type combustion chamber
• CReverse-flow annular combustion chamber
• DSingle centrifugal combustor

How are compressor rotor blades mounted into the compressor disk to accommodate thermal expansion and prevent fatigue concentration?

• AWelded permanently to the disk perimeter without clearance
• BFitted into dovetail or fir-tree slots with slight clearance and retained by tab locks or…
• CBolted rigidly through flange holes using high-tensile steel fasteners
• DGlued using epoxy resin structural adhesive

What is the function of interconnecting flame tubes between combustion cans in a multiple-can or can-annular combustor system?

• ATo equalize fuel pressure across all fuel nozzles
• BTo allow flame to propagate from lighted cans to unlighted cans during engine starting
• CTo route secondary cooling air into the turbine nozzle guide vanes
• DTo drain unburned fuel overboard after engine shutdown

In a cascade-vane thrust reverser installed on a high-bypass turbofan, what mechanical movement directs bypass airflow forward?

• ATranslating sleeves move aft, deploying blocker doors inside the bypass duct and exposing…
• BTarget clamshell doors swing closed behind the core turbine exhaust nozzle.
• CVariable stator vanes pivot 180 degrees inside the core compressor.
• DFan blades reverse their pitch angle across flat zero-pitch.

Why are turbine nozzle guide vanes (NGV) designed with convergent passages between adjacent vanes?

• ATo decelerate gas velocity and increase static pressure prior to entering the turbine rot…
• BTo accelerate hot exhaust gas velocity and direct it at the optimum angle onto turbine ro…
• CTo reverse gas flow direction back into the combustor
• DTo trap heavy carbon deposits before reaching turbine blades

What is the primary function of abradable shroud linings installed on compressor and turbine inner casings?

• ATo absorb engine oil leakage and prevent fire
• BTo permit tight blade tip clearances by allowing blade tips to cut minor grooves into the…
• CTo insulate outer engine cowlings from internal radiation
• DTo act as electrical ground paths for lightning strikes

Integrally Bladed Rotors (Blisks) offer significant advantages in modern compressors. What is a primary disadvantage of a blisk compared to a conventional disk and blade assembly?

• AHigher weight per stage
• BDifficulty in repairing or replacing an individual damaged blade in the field
• CLower resistance to centrifugal rotational stress
• DInability to operate at high spool RPM

What defines a Full Authority Digital Engine Control (FADEC) system in modern Category B2 avionics architecture?

• AA hydromechanical governor with an auxiliary electronic trim limiter that can be overridd…
• BA dual-channel electronic control system with complete authority over engine parameters a…
• CA single-channel flight deck indicator unit that advises pilots when to move thrust levers
• DA pneumatic pressure regulator that directly positions fuel metering valves

In a dual-channel FADEC (EEC Channel A and Channel B), how are the two channels configured during normal engine operation?

• AChannel A and B simultaneously command separate fuel metering valves.
• BOne channel is active controlling the engine while the other channel is in hot standby, c…
• CChannel A controls idle power, while Channel B controls high thrust power.
• DChannel A controls fuel flow, while Channel B controls aircraft cabin pressurization.

Under what condition will a dual-channel FADEC automatically initiate a channel handover from Channel A (active) to Channel B (standby)?

• AWhen Channel A detects a self-test failure, loss of a critical sensor signal, or loss of…
• BEvery time the aircraft transitions from climb to cruise altitude
• COnly when the pilot manually toggles an overhead engine switch
• DWhen fuel temperature drops below 0 degrees Celsius

What is the primary electrical power source for a FADEC/EEC when the gas turbine engine is running above approximately 10-15% N2?

• AThe aircraft 115V AC main bus
• BA dedicated engine-driven Permanent Magnet Alternator (PMA / PMG)
• CThe aircraft 28V DC main battery
• DAn APU generator dedicated feeder cable

If the dedicated engine-driven PMA fails completely during flight while the engine is running at high power, how does the FADEC maintain operation?

• AThe engine immediately shuts down due to fuel cutoff.
• BFADEC automatically switches power input to the aircraft 28V DC emergency/essential bus w…
• CFADEC reverts to mechanical throttle cable control.
• DThe engine locked-in fuel flow drops to ground idle.

How does the FADEC / EEC interface with the Hydro-Mechanical Unit (HMU) to position the main Fuel Metering Valve (FMV)?

• ABy sending variable current signals to electro-hydraulic torque motors or stepper motors…
• BBy physically turning a flexible push-pull cable connected to the cockpit thrust lever
• CBy varying high-voltage AC frequency to a main fuel pump induction motor
• DBy opening and closing pneumatic bleed air solenoid valves

In a FADEC system, what is the purpose of the dual-channel crosstalk data bus existing between EEC Channel A and EEC Channel B?

• ATo allow the two channels to cross-validate sensor health, exchange health status, and sy…
• BTo send radar navigation data to the flight deck
• CTo balance battery charging currents between engine spools
• DTo transmit cabin temperature demands to the air conditioning packs

What is the primary function of the Pressure Regulating / Pressure Drop valve (delta P valve) inside a Hydro-Mechanical Unit (HMU)?

• ATo maintain a constant pressure differential across the Fuel Metering Valve (FMV), ensuri…
• BTo shut off fuel flow instantly when cabin altitude exceeds 10,000 feet
• CTo mix fuel with hydraulic fluid to lubricate fuel nozzles
• DTo measure exhaust gas temperature changes

What role does the Engine Rating Plug (or Data Programming Plug) play when attached to a FADEC Electronic Engine Control (EEC) unit?

• AIt provides a physical electrical resistor/memory network that identifies specific engine…
• BIt acts as a primary 115V AC fuse for the ignition spark igniter
• CIt mechanically locks the engine fuel metering valve during ground towing
• DIt measures high-pressure turbine blade vibration

If both primary N1 speed sensors fail on a FADEC system operating in EPR target control mode, how does the system respond?

• AThe engine immediately shuts down.
• BFADEC automatically transitions to a degraded (N2 or unrated) control mode and generates…
• CThe thrust lever locks physically in the reverse thrust position.
• DThe PMA switches off and igniters fire continuously.