CAA NZ AME Licence Exam - Turbine Engines (Subject 6) cheat sheet

Brayton cycle: air compressed (1-2), heat added at constant pressure (2-3), expansion through turbine (3-4), heat rejected at constant pressure (4-1).

Ideal thermal efficiency = 1 - 1/(PR)^((γ-1)/γ); efficiency rises with pressure ratio and with γ, independent of turbine entry temperature.

Specific thrust depends on turbine entry temperature; raising TET increases work output but is limited by turbine blade material and cooling.

Isentropic compressor work per unit mass = cp·(T2 - T1) = cp·T1·[(PR)^((γ-1)/γ) - 1]; turbine work similarly with expansion ratio.

Work ratio = net work / turbine work; a high pressure ratio raises thermal efficiency but lowers work ratio, so a twin-spool or LP/HP split is used.

Compressor and turbine must produce equal work on the same shaft (plus accessory load); turbine work = compressor work + net output.

Surge margin = (PR at surge / PR at operating point) - 1; operating line must stay clear of the surge line at all speeds.

Thrust = mass flow × (jet velocity - flight velocity) + (exit pressure - ambient pressure) × exit area; fan engines trade jet velocity for mass flow.

Combustion pressure loss is typically 3-8% of compressor delivery pressure; a rise indicates blockage or damaged liner.

Hot-section alloys: nickel-based superalloys for turbine blades and vanes, often directionally solidified or single-crystal to resist creep.

Compressor and fan blades: titanium alloys (e.g. Ti-6Al-4V) for low weight and good strength-to-weight; steel for some shafts and discs.

Turbine blade cooling: internal convection, film cooling through holes, and thermal barrier coatings (YSZ) reduce metal temperature by 100-300 °C.

Creep is time-dependent deformation at high temperature and stress; design life is set by rupture and 0.2% creep limits.

Fatigue life is governed by stress cycles; low-cycle fatigue at start/shutdown dominates disc life, high-cycle fatigue from vibration.

Casing materials: aluminium or magnesium for cool sections, nickel or steel for hot sections; casings carry bearing loads and contain blades.

Fir-tree roots locate turbine blades in discs; blade tip clearance controls efficiency and is set by active clearance control or shroud design.

Fire zones: designated areas where hot air or fuel can collect; materials and drain paths must prevent ignition of leaked fluid.

Bolted joints in hot sections use stretch-controlled torque or torque-plus-angle to give consistent preload; re-use limits apply to fasteners.

Axial compressor stage = rotor row + stator row; pressure rise per stage is low (about 1.2-1.4), many stages needed for high overall PR.

Velocity triangles: work input = U·(Cw2 - Cw1); degree of reaction near 50% gives symmetric velocity triangles and low losses.

Surge is a low-frequency, whole-engine flow reversal; rotating stall is a local, higher-frequency cell rotating around the annulus.

Variable inlet guide vanes and variable stators schedule angle with speed to keep incidence correct and widen the surge margin.

Bleed valves open at low speed to unload the rear stages and prevent surge; they close as speed and pressure ratio rise.

Compressor map axes: corrected mass flow (m√θ/δ) on x, pressure ratio on y; surge line is the upper left boundary.

Centrifugal compressor: pressure rise per stage is high, used in small engines and as the last HP stage; diffuser converts velocity to pressure.

Compressor fouling raises TGT and reduces thrust; washing restores performance. Tip clearance and blade erosion degrade efficiency.

Corrected speed N/√θ and corrected flow m√θ/δ allow comparison of engine data at different inlet conditions.

Combustor types: can (multiple cans), can-annular (cannular), and annular; annular gives even exit temperature and shorter length.

Primary zone burns fuel-rich near stoichiometric for stability; dilution zone mixes cool air to produce an acceptable turbine entry temperature profile.

Combustion pressure loss is typically 3-8% of compressor delivery pressure; high loss reduces thrust and efficiency.

Fuel properties: Jet A-1 (kerosene) has high flash point and low vapour pressure; Jet B (wide-cut) is more volatile and has lower flash point.

Stoichiometric air/fuel ratio for kerosene is about 15:1 by mass; primary zone operates near this, overall engine FAR is about 1:60 to 1:100.

Ignition energy must exceed the minimum ignition energy of the fuel-air mixture; altitude relight is harder due to low pressure and temperature.

Fuel nozzles: pressure-swirl (duplex), air-blast, and vaporiser types; coking or blockage causes hot streaks and uneven temperature profile.

Fuel control: hydro-mechanical or FADEC; fuel flow is scheduled against N2, TGT, and compressor delivery pressure to avoid rich blowout or lean extinction.

Fuel system components: LP pump, HP pump, fuel/oil heat exchanger, fuel metering unit, flow divider, and shut-off valve.

Turbine stage = nozzle guide vanes (accelerate and swirl) + rotor blades (extract work); impulse stage has pressure drop mainly in NGV, reaction stage split roughly equally.

Turbine entry temperature is the limiting factor; blade cooling and coatings allow TET well above the melting point of the alloy.

Creep and thermal fatigue are the main life-limiting mechanisms in turbine blades; TGT is the monitored parameter.

Exhaust: convergent nozzle for subsonic, convergent-divergent for supersonic; thrust depends on pressure ratio across the nozzle.

Exhaust gas temperature profile (EGT spread) indicates combustor or fuel nozzle problems; a large spread suggests hot streaks.

Turbine clearance control uses cooling air on the casing to shrink clearance at cruise, improving efficiency.

Exhaust cone and struts support the rear bearing and straighten the gas flow; cracks or distortion affect performance and vibration.

Turbine blade tip shroud or tip clearance loss can be several percent of stage efficiency; tip rubs raise TGT and vibration.

Reverser (thrust reverser) uses blocker doors or buckets to redirect fan or jet flow forward; must not be deployed in flight.

Oil functions: lubricate bearings and gears, cool, clean, and protect against corrosion; also used as hydraulic fluid in some fuel control systems.

Oil system: tank, pumps (pressure and scavenge), filter, cooler (fuel/oil or air/oil), and bearing jets; scavenge capacity exceeds pressure pump capacity.

Oil pressure is regulated by a pressure relief valve; low pressure warnings may indicate pump wear, filter blockage, or bearing failure.

Oil temperature limits: high temperature degrades oil and indicates cooler or bearing problems; low temperature raises viscosity and reduces flow.

Oil debris monitoring (chip detectors, magnetic plugs) detects bearing and gear wear; metal particles are graded by size and quantity.

Cooling: turbine blades and vanes cooled by compressor bleed air; bearings cooled by oil; some engines use fuel as a heat sink.

Bearing compartments are sealed by labyrinth seals with pressurised air; seal failure causes oil loss and oil smell in bleed air.

Oil consumption limits are set by the manufacturer; rising consumption indicates seal or bearing wear.

Fire detection in the oil system uses thermal or pressure switches; oil mist in the bleed system can cause smoke in the cabin.

Ignition systems: high-energy capacitor discharge (CDI) or low-tension; exciter charges capacitor, spark discharges across igniter plug.

Ignition is used for start and for relight; continuous ignition is selected in heavy rain, icing, or turbulence to prevent flameout.

Starting sequence: starter motor cranks N2, fuel is introduced at a set N2, igniters fire, light-off confirmed by TGT rise, starter disengages at self-sustaining speed.

Starter types: air turbine, electric, and cartridge; air starters use bleed air and are common on large engines.

FADEC controls fuel flow, variable geometry, and ignition; it schedules acceleration to avoid surge and overtemperature.

Overspeed protection: independent overspeed governor or electronic trip cuts fuel at a set N2/N1 limit.

Hot start: TGT exceeds limit due to excess fuel or low airflow; corrective action is to cut fuel and motor the engine to cool.

Hung start: engine fails to accelerate to idle due to insufficient fuel, weak starter, or ignition problem; abort and investigate.

Engine control modes: N1 or N2 governing, TGT limiting, and acceleration scheduling; the lowest demand wins (min-select).

Primary parameters: N1 (fan/LP spool), N2 (HP spool), EGT/TGT, fuel flow, oil pressure and temperature, and vibration.

EGT/TGT thermocouples are distributed around the exhaust or turbine; the highest reading or an average with spread is displayed.

Vibration monitoring uses accelerometers on bearing housings; a rising trend indicates imbalance, bearing wear, or blade damage.

Trend monitoring compares parameters against a baseline to detect deterioration; sudden changes indicate faults, gradual changes indicate wear.

N1 and N2 are measured by magnetic or optical speed probes; a failed probe may cause FADEC to revert to a backup mode.

Fuel flow is measured by a turbine flowmeter or differential pressure transmitter; flow vs N2 indicates fuel control health.

EGT margin is the difference between the observed EGT and the limit at a given power setting; reduced margin indicates deterioration.

Cockpit indications use colour bands: green normal, amber caution, red limit; exceedance must be recorded and investigated.

Data recording (QAR/FDR) supports maintenance and trend analysis; exceedance events must be reported per the maintenance manual.

Accessory gearbox is driven from the HP spool and drives fuel pumps, oil pumps, generators, hydraulic pumps, and starters.

Power off-takes reduce available thrust; bleed air for cabin pressurisation and anti-icing is taken from the compressor.

Bleed air sources: HP bleed (high pressure) and LP/IP bleed (low pressure); a pressure regulator and shut-off valve control flow.

APU provides bleed air for starting and ground operation, and electrical power; APU is not used for thrust in flight.

Hydraulic pumps are engine-driven; a pump failure requires the other engine or an electric backup pump.

Anti-ice systems use hot bleed air on engine inlet and airframe surfaces; leakage can cause overheating and must be checked.

CSD/IDG (constant speed drive / integrated drive generator) maintains constant generator frequency from variable engine speed.

Engine-driven generator failure causes electrical load shedding; the APU or battery provides backup.

Accessory gearbox oil and bearing condition are monitored; gearbox failures can cause loss of multiple systems.

Maintenance is planned per the maintenance manual and approved data; all work must be recorded and signed by authorised personnel.

Borescope inspection of compressor and turbine blades is done without disassembly; findings are graded against limits in the manual.

Torque values and sequences are specified in the manual; over-torque damages threads and fasteners and requires rework.

Lockwire, cotter pins, and self-locking nuts are used to secure fasteners against vibration; correct installation is verified by inspection.

FOD prevention: covers, caps, and tool control are mandatory when the engine is open; foreign objects cause blade damage.

Fluid servicing: oil and fuel must meet the specified specification; mixing of grades is not permitted unless approved.

Preservation and storage: engines are preserved with oil and desiccant and rotated per the manual to prevent corrosion.

Engine wash and cleaning restore compressor efficiency; cleaning agents must be approved and rinsed per the manual.

Configuration control: parts must be approved and traceable; unapproved parts are not permitted on the engine.

Troubleshooting follows the manual fault isolation charts; start with the simplest and most likely cause and confirm with tests.

High EGT with normal N2 suggests hot-section deterioration, fuel nozzle blockage, or compressor fouling.

Low oil pressure with normal temperature suggests pump wear, filter blockage, or a leaking line; stop the engine if below limits.

Vibration with a rising trend suggests imbalance, bearing wear, or blade damage; borescope and balance check are indicated.

Surge or stall on acceleration suggests a faulty bleed valve, variable vane schedule, or compressor damage.

Fuel nozzle check: uneven EGT spread, soot, or hot start indicates a blocked or leaking nozzle; replace per manual.

Oil consumption rising indicates seal wear or bearing failure; check chip detectors and oil filter for metal.

Borescope findings: cracks, burns, tip rubs, and coating loss are graded; limits are in the maintenance manual.

After any exceedance, record the event, inspect per the manual, and return the engine to service only when within limits.