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

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Brayton cycle

The ideal gas turbine cycle of compression, constant-pressure heat addition, expansion, and constant-pressure heat rejection; thermal efficiency depends mainly on pressure ratio and specific heat ratio.

Thermal efficiency formula

Ideal Brayton thermal efficiency is 1 minus 1 divided by pressure ratio raised to (gamma minus 1) over gamma; it rises with pressure ratio and is independent of turbine entry temperature.

Work ratio

Net work output divided by turbine work; a high pressure ratio raises thermal efficiency but lowers work ratio, which is why multi-spool arrangements are used.

Surge margin

The margin between the operating pressure ratio and the surge-line pressure ratio at the same corrected flow; it must remain positive throughout acceleration and transients.

Single-crystal turbine blade

A blade cast as one crystal with no grain boundaries, giving improved creep and thermal fatigue resistance at high turbine entry temperature compared with equiaxed castings.

Thermal barrier coating

A ceramic layer, typically yttria-stabilised zirconia, applied over a bond coat on turbine parts to reduce metal temperature and extend hot-section life.

Creep

Time-dependent plastic deformation under sustained load at high temperature; turbine blade life is limited by creep rupture and a small allowable creep strain.

Fir-tree root

A multi-tooth blade root shape that locates the blade in the disc and carries centrifugal load through mating serrations; wear and crack inspection is critical.

Axial compressor stage

A rotor blade row followed by a stator row; each stage gives a modest pressure rise, so many stages are needed for a high overall pressure ratio.

Rotating stall

A local region of stalled blades that rotates around the annulus at a fraction of rotor speed; it differs from surge, which is a whole-engine flow reversal.

Variable stator vanes

Compressor vanes whose angle is scheduled with engine speed to keep incidence correct, widen the surge margin, and improve part-speed efficiency.

Corrected mass flow

Mass flow multiplied by the square root of theta and divided by delta, where theta and delta are inlet temperature and pressure ratios to standard day; used to compare engine data.

Annular combustor

A single continuous annular flame tube around the engine axis; it gives even exit temperature, short length, and low pressure loss compared with can types.

Stoichiometric fuel-air ratio

The chemically correct mass ratio of fuel to air for complete combustion, about 15 to 1 for kerosene; the primary zone operates near this value for stability.

Jet A-1 fuel

A kerosene-grade turbine fuel with high flash point and low vapour pressure, widely used for civil operations; it is less volatile than wide-cut Jet B.

Combustor pressure loss

The drop in total pressure across the combustor, typically 3 to 8 percent of compressor delivery pressure; excessive loss reduces thrust and efficiency.

Turbine entry temperature

The gas temperature at the turbine inlet; it is the main limit on engine power and is controlled by blade cooling, coatings, and material capability.

Impulse turbine stage

A stage in which most of the pressure drop occurs in the nozzle guide vanes and little in the rotor; the reaction stage splits the pressure drop more evenly.

EGT spread

The difference between the highest and lowest exhaust gas temperature readings around the annulus; a large spread indicates fuel nozzle or combustor problems.

Convergent-divergent nozzle

A nozzle with a throat and divergent section that accelerates exhaust gas to supersonic speed; used when the nozzle pressure ratio is high enough.

Oil scavenge system

The part of the oil system that returns oil from bearing compartments to the tank; scavenge pump capacity normally exceeds pressure pump capacity to prevent flooding.

Oil debris monitoring

Inspection of magnetic chip detectors and filters for metal particles; quantity and size indicate bearing or gear wear and guide maintenance action.

Labyrinth seal

A non-contacting seal with a series of fins and lands that restricts oil leakage from bearing compartments; pressurised air keeps oil inside the compartment.

Capacitor discharge ignition

An ignition system in which a capacitor is charged and then discharged across the igniter plug, producing a high-energy spark for starting and relight.

Hung start

A start in which the engine lights but fails to accelerate to idle because of low fuel flow, weak starter, or ignition fault; the start must be aborted.

Hot start

A start in which turbine gas temperature exceeds limits because of excess fuel or insufficient airflow; fuel is cut and the engine is motored to cool.

FADEC

Full-authority digital engine control; it schedules fuel flow, variable geometry, and ignition, and provides protection against overspeed and overtemperature.

EGT margin

The difference between observed exhaust gas temperature and the limit at a given power setting; a decreasing margin indicates engine deterioration.

Trend monitoring

Comparison of engine parameters against a baseline over time; sudden changes indicate faults, while gradual changes indicate normal deterioration.

Vibration monitoring

Measurement of bearing housing vibration with accelerometers; a rising trend indicates imbalance, bearing wear, or blade damage and requires investigation.

Accessory gearbox

A gearbox driven from the HP spool that drives fuel and oil pumps, generators, hydraulic pumps, and the starter; its failure can affect several systems.

Borescope inspection

Inspection of internal compressor and turbine blades through ports without engine disassembly; findings such as cracks, burns, and tip rubs are graded against manual limits.