Module 15 Gas Turbine Engine cheat sheet

Brayton cycle: compression (diffuser/compressor), constant-pressure heat addition (combustor), expansion (turbine/nozzle); ideal thermal efficiency η = 1 − 1/PR^((γ−1)/γ), rising with pressure ratio and γ.

Station numbering (SAE ARP755 style): 1 inlet entry, 2 compressor inlet (face), 3 compressor discharge, 4 combustor exit/turbine inlet, 5 turbine exit, 7 nozzle entry, 8 nozzle throat, 9 nozzle exit; 13/23/25 etc. denote bleed ports off a station.

Net thrust Fn = ṁ(Vj − V0) + (pj − p0)Aj; for a choked convergent nozzle pj = p0 and thrust reduces to ṁ(Vj − V0).

Gross thrust = ṁVj + (pj − p0)Aj; ram drag = ṁV0. Thrust-specific fuel consumption TSFC = fuel flow / net thrust.

Spool layouts: single spool (one compressor–turbine rotor), twin spool (LP and HP independent, N1/N2), three spool (LP, IP, HP; e.g. RB211/Trent family). Free-turbine turboshaft/turboprop decouples power turbine from gas generator.

Specific thrust = Fn/ṁ (high for pure turbojet, low for high-bypass turbofan); bypass ratio BPR = ṁbypass/ṁcore, typically 0.3–1.0 low-bypass military, 5–12 high-bypass civil.

Pressure ratio and TET trade: raising turbine entry temperature increases specific work but is limited by blade material and cooling; overall pressure ratio of modern civil engines reaches 40–60:1.

Ideal vs real: compressor and turbine isentropic efficiencies (0.85–0.92 typical), combustor pressure loss 3–6 percent, intake and nozzle losses reduce cycle work; polytropic efficiency describes stage-by-stage behavior.

Thrust lapse: net thrust falls with altitude (density) and rises with Mach number; TSFC generally improves at high altitude until temperature limits derate the engine.

Rated thrust is defined at sea-level static, ISA conditions (15 °C, 1013.25 hPa); flat-rated engines hold constant thrust up to a break temperature, then thrust decays roughly 1 percent per 1 °C above it.

Takeoff thrust is time-limited (commonly 5 minutes, extendable to 10 minutes for some one-engine-inoperative cases); MCT (max continuous thrust) is the only rating with no time limit.

Derating reduces takeoff thrust to extend hot-section life: assumed temperature method (use a fictitious higher OAT) or fixed derate (percentage of rated thrust), both increasing takeoff distance.

Thrust lapse with altitude approximated by Fn ∝ σ (density ratio) for a fixed corrected parameter; corrected thrust and fuel flow are plotted against corrected N1 or EPR.

Specific fuel consumption: TSFC = Wf/Fn, units kg/(N·h) or lb/(lbf·h); lower is better. Cruise SFC for high-bypass turbofans is about 0.5–0.6 lb/(lbf·h).

Engine pressure ratio EPR = p5/p2 (turbine exit total pressure over compressor inlet total pressure) is used as a thrust-setting parameter on many Rolls-Royce and Pratt & Whitney engines; N1 is typical on GE/CFM.

Thrust is proportional to mass flow and jet velocity excess; fan pressure ratio and bypass ratio govern propulsive efficiency ηp = 2/(1 + Vj/V0).

Ratings ladder: takeoff/go-around (T/O), max climb, max cruise, max continuous, idle (ground and flight). Flex/assumed temperature takeoff is not permitted when performance-limited conditions or contamination exist.

Acceleration and bleed extraction: customer bleed (ECS, anti-ice) and power extraction reduce available thrust and increase EGT; performance manuals correct for both.

Axial compressor stages: rotor blades accelerate air, stator vanes diffuse (convert velocity to pressure); stage pressure ratio is modest (1.1–1.4), so 10–15 stages achieve overall ratios of 20–40:1.

Velocity triangles: relative velocity enters rotor at angle β1, leaves at β2; diffusion in the rotor and stator must be limited to avoid boundary-layer separation that leads to stall.

Surge is a global, axisymmetric flow breakdown with violent flow reversal and loud bangs; rotating stall is a localized, rotating cell of stalled blades causing vibration and asymmetric pressure.

Surge margin = (PRstall − PRoperating)/PRoperating; reduced by throttle transients, inlet distortion, bleed extraction, and fouling; variable stator vanes and bleed valves protect low-speed operation.

Variable stator vanes (VSVs) and variable inlet guide vanes schedule with corrected N1 to keep the front stages from stalling at low speed; bleed valves open at low power to unload front stages.

Compressor map axes: corrected mass flow (ṁ√θ/δ) vs pressure ratio, with corrected speed lines (N/√θ) and surge line; operating line must stay below it with margin.

Inlet types: pitot (subsonic), convergent-divergent (supersonic, with normal/oblique shocks), and S-ducts; total pressure recovery and distortion ( circumferential and radial ) matter for stall margin.

Compressor blade damage modes: FOD, erosion, tip rub, and blade creep; wash and salt-corrosion effects reduce efficiency and increase EGT for a given thrust.

Twin-spool matching: LP and HP compressors run at different corrected speeds; the HP spool is more sensitive to back-pressure and surge during rapid throttle chops.

Primary zone: fuel is atomized, vaporized, mixed and burned near stoichiometric for stability; intermediate (secondary) zone completes CO and soot burnout; dilution zone brings exit temperature to a turbine-safe profile.

Typical combustor pressure loss is 3–6 percent of inlet total pressure; the diffuser slows air to about 30–40 m/s before the dome to stabilize the flame.

Chamber types: can (individual cans), can-annular (cans in an annular casing, common on large engines), annular (single continuous liner, shorter and lighter), and reverse-flow annular (small engines, e.g. turboshafts).

Fuel nozzles: pressure-swirl (simplex/duplex), air-blast/air-assist (atomization by high-velocity air, good for high pressure ratios), and vaporizers; duplex nozzles use primary and secondary orifices for low/high flow.

Ignition: high-energy igniters (capacitor discharge, 4–20 J) light only a few cans or a local region; flame propagates through crossfire tubes or annular crossover.

Combustor exit temperature profile (pattern factor) must be controlled so turbine nozzle and blade temperatures stay within limits; hot streaks reduce blade life.

Emissions: NOx rises with flame temperature (lean, staged combustion and rich-quench-lean designs reduce it); CO and unburned hydrocarbons rise at low power and lean blowout.

Liner cooling: film cooling, effusion (transpiration) cooling, and thermal barrier coatings protect the liner; cracking and hot spots are common inspection findings.

Lean blowout and rich blowout limits define the stable operating envelope; combustor rumble (low-frequency instability) and screech (high-frequency) are combustion dynamics phenomena.

Turbine nozzle guide vanes (NGVs) accelerate and turn hot gas to the correct angle; they are usually cooled and may be a single cast vane ring or individual segments.

Turbine blade cooling: internal convection through serpentine passages, film cooling holes, impingement inserts, and thermal barrier coatings (TBC, yttria-stabilized zirconia); cooling air is taken from HP compressor stages.

Single-crystal and directionally solidified nickel superalloys with TBC allow gas temperatures above the alloy melting point; turbine entry temperatures can exceed 1,700 K in modern engines.

Turbine blade tip clearance is critical: excessive clearance increases leakage and SFC; active clearance control (ACC) uses cooling air on the casing to shrink clearance at cruise.

Turbine blade creep and thermal fatigue: blades stretch (creep) and develop cracks at leading/trailing edges; borescope inspection and EGT margin trend detect degradation.

Turbine blades are retained by fir-tree roots in the disk; blade platforms and shrouds reduce leakage; shroud interlocks and tip shrouds damp vibration.

Turbine cooling air is metered by internal orifices and modulated by active clearance control and cooling-air valves; too much cooling air costs cycle efficiency.

Turbine nozzle and blade erosion from particulates and hot corrosion from sulfur/salt attack are life limiters; EGT margin (EGT at takeoff vs redline) is the key on-wing health indicator.

LP turbine drives the fan through a shaft (or free power turbine); HP turbine drives the HP compressor; the number of turbine stages is roughly matched to the compressor work split.

Exhaust system: exhaust cone (tail cone) and struts support the rear bearing and smooth the flow; the exhaust nozzle converts remaining pressure into velocity.

Convergent nozzle: choked when nozzle pressure ratio exceeds about 1.85–1.9 for γ ≈ 1.33; beyond that, thrust no longer increases with pressure ratio in a fixed convergent nozzle.

Convergent-divergent (C-D) nozzle: used in supersonic engines and afterburning engines; variable throat and exit area schedule with Mach number and afterburner operation.

Thrust reversers: cascade (bucket) type, target (clamshell) type, and cold-stream (fan) reversers; hot-stream reversers on low-bypass engines, fan reversers on high-bypass.

Reverser operation: translating sleeve moves aft, blocker doors deploy into the fan duct, cascades turn the flow forward; thrust reverse is normally limited to idle-to-part power and disabled in flight.

Reverser interlocks: thrust lever cannot be moved to reverse unless the reverser is fully deployed; deploy inhibit above a set airspeed and engine power above idle; EEC/FADEC controls the sequence.

Exhaust gas temperature (EGT) measured at station 4.5 or 5 (turbine exit) is the primary indicator of exhaust system and hot-section health; a rising EGT at fixed thrust signals degradation.

Exhaust noise: chevrons and serrated nozzles mix fan and core streams to reduce jet noise; noise certification is a design driver for high-bypass engines.

Exhaust cone cracks and strut distress are inspection items; reverser cascade blockage or damaged blocker doors cause asymmetric thrust and must be checked before dispatch.

Rolling-element bearings: ball bearings carry radial and some axial loads; roller bearings carry high radial loads; thrust (ball) bearings take axial loads from compressor/turbine pressure differences.

Bearing numbering and location: No. 1 (fan), No. 2 (LP compressor), No. 3 (HP compressor front), No. 4 (HP turbine), No. 5 (LP turbine); exact arrangement varies by engine.

Squeeze-film dampers and intershaft bearings reduce vibration; bearing cages and materials (M50 steel, ceramic hybrid balls) resist high DN and temperature.

Lubrication: dry-sump system with oil tank, pressure pump, scavenge pumps, oil cooler (fuel/oil or air/oil), filter, and chip detectors; oil also cools bearings and seals.

Oil types: MIL-PRF-23699 (standard turbine oil, 5 cSt class) and MIL-PRF-7808 (low-temperature); oil pressure and temperature limits are in the AFM.

Seals: labyrinth (knife-edge) seals for air/oil separation, carbon face seals for main shaft, brush seals and honeycomb seals for tight clearances; seal leakage affects oil consumption and EGT.

Pressurized bearing compartments: buffer air from the compressor keeps oil in and contaminants out; loss of buffer pressure causes oil loss and possible fire.

Oil analysis and chip detection: spectrometric oil analysis (SOAP) trends wear metals; magnetic chip detectors and master chip detectors indicate bearing or gearbox distress.

Oil consumption limits and oil pressure low/high annunciations; oil system failures can lead to bearing seizure and engine shutdown.

Fuel system chain: tank boost pumps, LP fuel valve, fuel/oil heat exchanger, LP and HP pumps (gear or centrifugal plus gear), fuel metering unit (FMU), fuel flow transmitter, fuel nozzles, and fuel shutoff valve.

Fuel metering: hydro-mechanical FMU or FADEC-controlled metering valve; fuel flow is scheduled as a function of throttle resolver angle, N1/N2, EGT, and pressure altitude.

FADEC: dual-channel digital control with hydromechanical backup; inputs include throttle position, N1, N2, EGT, TAT/TIT, altitude, Mach, and bleed status; outputs drive fuel metering, VSVs, bleed valves, and igniters.

FADEC protections: N1/N2 overspeed, EGT redline, fuel flow limits, and surge detection; thrust lever angle maps to a thrust parameter via the rated thrust schedule.

Fuel types: Jet A/A-1 (kerosene), JP-8 (military), JP-5 (carrier), and Jet B/JP-4 (wide-cut, high vapor pressure); water in fuel causes icing and flameout, so fuel heaters and anti-ice additives are used.

Fuel nozzles: duplex or air-blast, with flow number and spray pattern affecting combustor pattern factor; clogged nozzles cause hot streaks and EGT spread.

Fuel flow indication and fuel used are computed by the FADEC/EEC and displayed; fuel temperature is monitored to prevent waxing (freeze point about −40 °C for Jet A).

Fuel control modes: starting (fuel schedule with ignition), acceleration (fuel/air limits to avoid surge or blowout), steady-state governing, and deceleration (minimum fuel to prevent flameout).

Fuel system leak checks and the fuel shutoff valve test are preflight items; the HP fuel valve closes on shutdown and fire-handle pull.

Bleed sources: HP compressor stages (e.g., 5th/9th) through bleed valves; LP bleed for low-power operation, HP bleed for high-power; bleed air is used for ECS, anti-ice, and engine starting.

Bleed effects: extracting compressor air reduces thrust and increases EGT; performance manuals correct for bleed and power extraction.

Cooling air: HP compressor air cools turbine blades and vanes, active clearance control, and accessory gearbox; cooling air valves modulate flow with power setting.

Anti-ice: engine inlet anti-ice uses hot bleed air on the nose cowl and inlet guide vanes; nacelle anti-ice valves are controlled by the anti-ice switch and FADEC.

Starting: air turbine starter (ATS) driven by bleed air or APU, or electric starter; starter cutout at a set N2 (e.g., 45–55 percent) with ignition and fuel on.

Start sequence: motoring (dry crank) to purge, ignition on, fuel on at minimum N2, light-off with EGT rise, acceleration to idle, starter disengagement, and stabilization.

Start malfunctions: hot start (EGT exceeds limit), hung start (fails to accelerate), no light-off, and starter failure; abort actions are in the AFM.

Cabin air and ECS bleed: engine bleed or APU bleed; bleed valve malfunctions cause loss of cabin pressure and must be handled per checklist.

Crossbleed starting uses bleed from a running engine to start the other; isolation valves and crossbleed valves route air through the pneumatic manifold.

Ignition types: capacitor-discharge exciter (high energy, 4–20 J) and low-tension ignition; igniter plugs are either air-gap or surface-discharge types.

Ignition selection: both igniters for start, takeoff, and anti-ice/precipitation conditions; one igniter for normal continuous ignition in some engines.

Ignition exciter receives 115 V AC and produces high-voltage pulses to the igniter plug; a faulty exciter causes no light-off or intermittent ignition.

Starter types: air turbine starter (bleed-driven), electric starter, and cartridge starters (older); starter duty cycle limits prevent overheating.

Starting fuel schedule: fuel is introduced at a minimum N2 (e.g., 20–25 percent) and modulated to avoid over-temperature and to ensure light-off; FADEC controls the schedule.

Auto-start and manual start: FADEC auto-start monitors N2, EGT, and light-off, aborting on hot start; manual start requires crew monitoring of EGT and N2.

Igniter maintenance: check gap, cleanliness, and electrode erosion; igniter leads and exciter connectors are frequent fault points.

Wet start (fuel without ignition) purges fuel before retry; dry motoring is used to clear the combustor and reduce fire risk.

Starting envelope: crossbleed, APU, or ground cart air; high-altitude airstarts require windmill or starter assist and are limited by the flight envelope.

Primary indications: N1, N2, EGT, fuel flow, and thrust parameter (EPR or N1); secondary: oil pressure, oil temperature, oil quantity, vibration, and fuel temperature.

EGT margin: difference between the EGT at takeoff thrust and the EGT redline; a declining margin indicates hot-section degradation and drives shop visit planning.

Vibration monitoring: accelerometers measure fan and core vibration; units are typically mils (displacement) or inches per second (velocity); high vibration can result from fan imbalance, bearing wear, or blade damage.

Trend monitoring: records of EGT, fuel flow, N1/N2, and vibration versus corrected parameters over time; used for condition-based maintenance and detection of degradation.

Engine health monitoring (EHM) systems record exceedances and trend data; data is downloaded for analysis and can trigger maintenance actions.

EGT redline exceedance requires a maintenance action and possibly a borescope; hot-section distress is often first seen as a rising EGT at fixed thrust.

Vibration limits: fan imbalance is corrected by trim balancing; core vibration can indicate bearing or seal distress; high vibration may require an engine change.

FADEC fault codes and built-in test (BITE) help troubleshoot; maintenance messages are displayed on the engine indication and crew alerting system (EICAS) or equivalent.

Oil analysis and chip detector trends complement vibration and EGT data for early detection of bearing and gearbox failures.