AH-64E Auxiliary Power Unit (APU) Comprehensive Study Notes

Overview and purpose

• AH-64E Auxiliary Power Unit (APU) GTCP36-155 provides self-sustaining, field power for pneumatic, hydraulic, and electrical systems, eliminating the need for external power sources when operating in the field. It delivers pressurized bleed air to IPAS and drives the main transmission accessory section to supply electrical and hydraulic power.
• APU is a self-contained, fully automatic gas turbine engine that requires only DC power, fuel, and input signals from the helicopter. Electromechanical controls manage starts, acceleration, operation, and shutdown.
• ECU oversees sequencing, PTO clutch operation, normal operation, and APU protection.
• Location: right side of the aft equipment bay under a five-piece Kevlar/epoxy enclosure, aft of the main rotor support. The enclosure includes air cooling and a network that directs extinguishing agent in a fire.
• APU is attached to the deck by three mounts: Tripod truss (right forward), Bipod truss (left forward), and Aft support strut (aft center).

APU characteristics (major specs and limits)

• Model: GTCP36-155; Engine type: gas turbine.
• Manufacturer: Garrett Auxiliary Power Division, Allied Signal Aerospace.
• Dry weight: 129.8 ext{ lb}.
• Shaft power: 81 ext{ SHP}.
• Speeds: ext{Turbine rotor (full load)} = 59{,}566 ext{ RPM}; ext{PTO clutch} = 8{,}216 ext{ RPM}. Back-driven by pre-Block III AGB CCW: 8{,}334 ext{ RPM}; 0 RPM typical; 8{,}046 RPM maximum (depends on seal drag).
• Electrical system: 14–30 ext{ V}_{dc}.
• Lubrication: MIL-L-23699 or MIL-L-7808; sump capacity ≈ 2 ext{ qt}.
• Fuel system: JP-4, JP-5, JP-8, and F-24.
• Power take-off (PTO) connection: drives the transmission accessory gear train; clutch engagement is governed by oil pressure and control valves.
• Operating envelope: sea level to 10{,}000 ext{ ft}; temperature range: -65^ ext{F} ext{ to } 135^ ext{F} (−54° C to 57° C). Winterization improves starting in -25^ ext{F} to -64^ ext{F}.
• The APU is designed with automatic shutdown features including overspeed, overcurrent, low oil pressure, and overtemperature protections.

APU system overview (major sections)

• Major sections of the APU: Power section, Air inlet plenum, Compressor section, Turbine plenum, Load control valve (LCV), Combustor, Turbine section, Exhaust section. These are followed by the APU Airflow path through the turbine to drive the compressor and the accessory gearbox.
• Accessory gearbox section houses the oil sump, oil pump, oil filter, PTO clutch, fuel control unit, and various sensors and mounts. It also provides air/oil separation via a vent port.
• The APU power train is a single-shaft engine that drives the compressor and the accessory section via the turbine wheel.

APU Power Section (the drive and air supply core)

• The power section provides drive for the transmission accessory section and supplies IPAS bleed air. It sits behind the accessory section.
• Subsystems/components within the Power Section include:
• Air Inlet Plenum Assembly: directs inlet air to the compressor; isolates APU intake from enclosure cooling airflow; installed around the inlet housing; one-piece fiberglass.
• Compressor Section: includes inlet housing, inlet face shroud, impeller, diffuser, and deswirl assembly. Inlet housing features debris protection (holes ≥ 0.25 in) to prevent FOD.
• Turbine Plenum Assembly: houses the combustor and turbine cooling; self-aligning bearing; connects to deswirl/turbine plenum path.
• Load Control Valve (LCV): normally closed; opens when Ng reaches or exceeds 95%; bleeds air to IPAS; modulates airflow for cooling; located on lower left of turbine plenum.
• Combustor Section: mixes compressor discharge air with fuel and ignites; reverse-flow annular design for uniform combustion and cooling air supply.
• Turbine Section: converts heat energy to mechanical energy via turbine nozzle and turbine wheel; nozzle accelerates gases to optimal angle; wheel drives compressor and accessories.
• Exhaust Section: directs exhaust gases to exhaust duct and helps create cooling airflow over the enclosure.

APU air inlet, compressor, diffuser, deswirl, turbine, exhaust flow path (APU airflow chain)

• Air is drawn through a screen on the enclosure; air goes through Air Inlet Plenum to the Compressor Inlet Housing.
• Inlet housing features debris protection; air flows into the impeller, which accelerates air outward to the diffuser.
• Diffuser lowers velocity and increases static pressure; directs air into Deswirl Assembly.
• Deswirl removes swirl, straightens airflow into the Turbine Plenum.
• Combustion liner receives airflow; combustion occurs, producing high-temperature gases that accelerate the Turbine Nozzle.
• Turbine Nozzle directs gases onto the Turbine Wheel; turbine drives the compressor and accessory gearbox.
• Exhaust Nozzle and Exhaust Duct vent exhaust and promote cooling airflow around enclosure and components.

APU Load Control Valve (LCV) and fuel/air control interactions

• LCV (bleed control valve) provides bleed air to the IPAS whenever Ng > 95%. It remains closed during start (Ng < 95%), when FIRE/READY pressed, or during shutdown.
• LCV is a normally closed, electrically opened, 28 Vdc solenoid-operated, pneumatic diaphragm valve. It opens when Ng reaches 95% and remains open during normal operation, modulating at an EGT = 1283 ext{ }^ ext{F} ext{ extpm } 25^ ext{F} (≈ 1283 ext{°F}; ?C value ≈ 704 ext{°C}) to provide more airflow for cooling. If engine is off or Ng is low, it remains closed.
• The LCV is integrated with ECU so that, during normal operation, it remains open; the valve is also tied to 100% Ng operation and shutdown conditions.

APU fuel system (pressurized, metered fuel for start and operation)

• Purpose: deliver metered fuel to the APU for light-off, acceleration, steady operation, and shutdown sequences. Fully automatic; no operator control needed.
• Primary components:
• Fuel Shutoff Valve (FSOV): 24/28 Vdc motor-driven ball-type valve; located on upper right aft fuselage; isolates fuel from the fuel cell during start/shutdown or fire.
• Boost Pump: 24/28 Vdc vane-type pump; located upper center aft fuselage; delivers positive fuel flow to the fuel control unit at roughly 10 ext{ psi} ext{ (±3 psi)}. Shuts off when pushbutton is pressed for shutdown or FIRE/READY.
• Fuel Control Unit (FCU): metering system controlled by ECU; contains high-pressure pump, metering valve, differential relief valve, ultimate relief valve, 70-μm screen, and other internal passages.
• Fuel Solenoid Shutoff Valve (FSOV) – separate from FSOV on the fuel cell; normally closed until ECU energizes during start.
• Fuel Nozzle/Manifold Assembly: three primary and three secondary nozzles; flow divider directs high-pressure fuel to the correct nozzles during start vs normal operation.
• Plenum Chamber Drain Check Valve: drains residual fuel after shutdown.
• FCU internal components described: pump inlet pressure test fitting; high-pressure pump; 70-μm screen; pump discharge test port; metering valve; torque motor; differential pressure relief valve; ultimate relief valve.
• Operation flow: boost pump pressurizes fuel to ~10 ext{ psi}; fuel flows through inlet filter (3-μm) to high-pressure pump; high-pressure pump raises to 250–300 psi across metering valve (valve area controlled by torque motor); differential relief maintains a constant 25 psi drop across metering valve; ultimate relief bypasses excess pressure back to pump inlet; fuel goes to SOV and to FCU, then to fuel nozzle manifold; starting at Ng ≈ 5 extrm{–}60 ext{%} sequence; light-off initiated with ignition and fuel solenoid status signals; during start, no fuel to nozzle until Ng reaches ~5%.
• Operational details: differential pressure across metering valve maintained at 25 ext{ psi}; ultimate relief ensures relief path if pressure cannot be relieved. If FCU or SOV faults occur, advisory/fault reporting occurs via ECUs/MP and DMS.

Fuel nozzle/atomizer and plenum details

• Fuel Nozzle/Manifold Assembly distributes and atomizes fuel to combustor; located around aft portion of power turbine plenum; includes three primary and three secondary nozzles; flow divider is a pressure-operated poppet valve.
• Fuel Nozzle/Atomizer: primary and secondary nozzles with thermostatically controlled air blast to help shape the spray; air assist prevents carbonization; high-pressure fuel enters through an internal screen and passes to the primary nozzle; as speed increases, the flow divider opens, allowing secondary nozzle operation.
• Combustor is a reverse-flow annular design; igniter plug and fuel nozzle nozzles protrude into combustor.

APU exhaust, cooling, and ducting

• Exhaust nozzle directs gases into exhaust duct; cone-shaped nozzle attached to turbine plenum by a V-band clamp; high-pressure gas creates a low-pressure area around intake, aiding cooling airflow.
• Exhaust duct connects to aft panel of enclosure and airframe; insulated with heavy-duty aluminum, wire mesh screen; bell-shaped at enclosure attachment point; transitions to rectangular duct.
• Plenum drain valve drains residual fuel after shutdown.

APU lubrication system (self-contained oil system)

• Purpose: lubrication and cooling of vital engine components; sludge removal; supplies oil to PTO clutch; oil sump forms lower portion of the accessory gearbox.
• Oil sump (wet-sump, 2 qt capacity) includes oil level sight gauge, magnetic chip/drain plug, mounting bosses, and filler cap with chain.
• Magnetic chip/drain plug: contains permanent magnet to attract ferrous particles; drain plug allows oil drain; not wired to warning system; inspect visually; a check valve prevents oil loss when removing the plug.
• Forward mounting bosses provide alignment and attachment to sump.
• Oil filler cap with dipstick for level checks.
• Oil pump (internal to the accessory gearbox; not line replaceable): two-spur gear positive-displacement; flow ≈ 3 ext{ gpm}; driven by same shaft as FCU; has oil passages to main bearings, gear meshes, and gear shafts; mist lubrication for low-load components; pressure regulator maintains 95–105 psi (2–4 gpm flow); not field adjustable.
• Oil filter: disposable 10 μm nominal/25 μm absolute, mounted on top of gearbox; no bypass; a clogged filter causes oil pressure loss and APU shutdown.
• Low Oil Pressure (LOP) switch: monitors oil pressure; located on top of the accessory gearbox; ECU waits ~10 ±0.05 s after Ng reaches 95% to report fault; oil pressure during acceleration 76–90 psi opens switch; the ECU grounds MP logic on open; if 95% Ng for 10 seconds with switch closed, APU shuts down.
• PTO Clutch: connects APU to main transmission via input quill and output shaft; clutch is hydraulically actuated by the APU lubrication system through the TLCV; engages around 60% Ng; torque-limiting feature: 1400 in-lb; primary bearing set grease-lubricated; back-up bearings included; Shaft Displacement Detector (SDD) protects against loss of centerline, can be overridden; engaging/disengaging controlled by dual oil chambers and a bellows spring; friction disk life ~3000 engagements at 60% Ng and 0% accessory gearbox speed; lubrication-controlled by TLCV.
• PTO clutch output housing contains a friction disk inspection hole and a weep hole; PTO clutch is a Line Replaceable Unit (LRU).
• PTO clutch engagement: ECU-controlled; standard engagement at ~60% Ng; cold weather engagements delay until Ng = 95% if main transmission oil temp < 0°F; engagement achieved by hydraulic oil pressure acting on arm/piston against the bellows spring; friction disk engages via splined adapter; solar and mechanical details describe clutch assembly (input quill, output flange, stationary plate, friction disk).

APU bearings

• Bearings support main rotating components; present in the accessory gearbox and forward end of the power section; two ball bearings support PTO clutch; ball bearings and roller bearings support main rotating shaft; planetary gears supported by two ball bearings; main shaft supported by ball and roller bearings; radial and thrust loads managed by different bearings in different locations.

APU start system (hydraulic start system)

• Start system components: Utility Hydraulic Accumulator, APU Start Manifold and Start Solenoid Valve, Hydraulic Start Motor, Sprag Clutch, APU Pushbutton.
• Utility Hydraulic Accumulator stores hydraulic pressure for starting and emergency control; mounted on aft equipment bay deck; cylinder wrapped with Kevlar; two chambers separated by a piston; one with hydraulic fluid at 3000 psi; the other with nitrogen precharge at 1650 psi at 70°F.
• Start manifold and solenoid valve direct hydraulic pressure to APU hydraulic start motor during start. The start solenoid valve is normally closed and energized by ECU during start; after Ng reaches 60%, ECU de-energizes the start solenoid and engages PTO clutch.
• Hydraulic Start Motor: attached to upper right side of APU gearbox; nine-piston motor; hydraulic pressure from accumulator drives pistons, rotating the APU drive train.
• Sprag Clutch: couples starter with gear train to drive APU until self-sustaining; decouples as APU speeds up beyond self-sustain; located in the APU gearbox; outer race overruns inner race as speed increases, allowing sprags to disengage.
• APU pushbutton: pilot initiates start; MP signals to energize ECU; 24/28 Vdc to ECU powers start sequence; pushbutton also used to stop APU.

APU electrical system and ASM (Aircraft Subsystem Management)

• Electrical system provides automatic control of APU operations; includes circuit breakers, APU pushbutton, Nr (Main Rotor) sensor, APU speed sensor, ignition system, EGT thermocouple, elapsed-time indicator, APU ECU.
• APU electrical power path: MP commands APU start/stop via ELC No. 2; APU loads are powered through ELC No.2; MP monitors via AIU No. 6.
• Nr sensor (Main Rotor RPM): Monopole magnetic pickup mounted on aft center of the main transmission accessory section; counts gear teeth to determine rotor speed; NR signal used for PTO clutch engagement control (engage when Nr < 95%).
• APU Speed Sensor: Monopole, non-contact, variable-reluctance sensor on the APU; detects APU RPM by sensing gear teeth; used to illuminate APU ON pushbutton at 95% Ng and to detect overspeed condition or loss of signal.
• AIU No. 6: MUX/RT for status/control of Nr sensor and APU ECU signals; used to update MP display and warnings; NR and APU signals used to engage PTO clutch.
• Ignition System: Provides electrical current for combustion during start; includes Ignition Unit (sealed capacitor-discharge), Igniter Lead, Igniter Plug; energized by ECU during start from ~5% Ng until ~4 seconds after Ng reaches 95%.
• EGT Thermocouple: mounted at turbine plenum to measure exhaust gas temperature; used by ECU to monitor operation and prevent overheating.
• APU ECU: solid-state unit with internal power supply; monitors Nr, APU speed sensor, LOP switch, EGT; outputs control signals to Ignition Unit, Fuel Solenoid Shutoff Valve, PTO clutch solenoid, hydraulic start solenoid, FC torque motor, LCV solenoid, and MP/AMI indications; enables automatic shutdown for overspeed, overcurrent, low oil pressure, overtemperature, loss of temperature signal, and loss of speed signal.
• Electrical interconnects: battery power through PDP, relays KB1, KB201, and related buses; 24 Vdc to APU ECU via ELC No. 2; battery start sequence via Master Ignition switch and bus connections.

Start system (detailed sequence)

• Start sequence begins with the APU pushbutton; MP commands 24/28 Vdc to ECU via ELC No. 2; ECU energizes hydraulic start solenoid valve to pressurize hydraulic accumulator to about 3000 psi; hydraulic start motor drives sprag clutch via start gear train; APU drive shaft begins to accelerate; APU speed sensor (monopole) provides RPM signal back to ECU; ECU monitors EGT and RPM; ignition system is energized from ~5% Ng and remains active until ~4 seconds after Ng hits 95%; fuel solenoid and fuel boost pump energize to prime the fuel system; at approximately 60% Ng, PTO clutch is engaged to drive main transmission accessory gear train; as Ng increases, ECU modulates FC torque motor to regulate fuel flow; the APU reaches 60–95% Ng; at 95% Ng, ignition/solenoids adjust to normal operation; the PTO clutch engages, transmitting drive to the main transmission accessory gear train; APUs are capable of self-sustaining operation once the rotor engages; the APU continues to be monitored by ECU for faults.
• Important thresholds and control signals:
• Ng thresholds: 5% light-off; 60% engage PTO; 95% ready for normal operation; 100% Ng is full speed; 10-second LOP hold after startup, etc.
• Oil pressure: 90 psi threshold to confirm adequate lubrication; LOP switch signals to MP that oil pressure is sufficient to continue.
• Fire/Ready position inhibits fuel boost and shutoff valve to stop fuel flow in case of fire.
• If start fails, ECU reports faults; MP can abort/start again; APU ON indicator and start advisory reflect status.

APU ASM (Aircraft Subsystem Management) functions

• ASM provides basic control and monitoring to reduce pilot workload and automate tasks. MP processes APU power, start, and stop commands; MP monitors via AIUs and issues APU shutdown if failures occur.
• APU Start, Power, and Stop commands originate from MP and are carried to ELC No. 2 to power APU ECU, FCU, boost pump, and FSOV; EUFD/MPD display APU start, ON, STOP statuses; faults generate APU FAIL advisories and DMS fault entries.
• Power-on sequence: battery power via PDP through Master Ignition switch to circuitry that energizes APU equipment; after pushbutton is pressed, the MP commands start and acknowledges via EUFD; APU ON is indicated on EUFD when Ng reaches 95% and APU started.
• Safety and fault handling: if AC power is not present, EUFD shows advisories; if AC power is present, faults are shown in DMS Fault page; the APU can be started with engines running or AGPU, or battery power; the battery is the source during battery start sequence.

APU warnings, cautions, and advisories (EUFD/MPD)

• Warnings: APU FIRE (only warning). Master Warning light and ENG page auto-page; audible warning.
• Cautions: APU ON (APU is ON in flight or in-air states, off-squat switch).
• Advisories (EUFD/MPD): APU START, APU PWR ON, APU ON, APU STOP, APU FAIL, plus text-to-MPD equivalents.
• When AC power is off, certain advisories display on EUFD only; if AC is on, fault messages appear on DMS Fault page.
• Advisory mapping examples (EUFD to MPD):
• APU START → APU START (EUFD); APU PWR ON → APU PWR ON (EUFD); APU ON → APU ON (EUFD/MPD); APU STOP → APU STOP; APU FAIL → APU FAIL with faults listed.

Faults and troubleshooting (faults, advisories, and diagnostic procedures)

• Faults (non-exhaustive): APU FUEL SHUTOFF VALVE - CLOSE/OPEN FAIL; GAS GENERATOR RPM OVRSPD; OIL PRESSURE LOW; OVERTEMPERATURE; OVC; STARTER CNTLR FAIL; IGNITION CNTLR FAIL; PTO CNTL FAIL; STOP CNTL FAIL; START/STOP CMD FAIL; RPM decrease; NO IGN SPARK; EXHAUST GAS TEMP SENSOR FAIL; OIL SWITCH FAIL; APU PTO CNTL FAIL, etc.
• Faults also map to DMS faults and provide descriptions that match corresponding advisories.
• Troubleshooting IETM (APU SYSTEM) steps: use System/Subsystem index to access AUXILIARY POWER UNIT MAINTENANCE OPERATIONAL CHECK; run MOC/FIP; if ECU IGN CNTLR FAIL: ECU not replaced; check wires; determine malfunctioning component; rerun FIP with different options to explore other causes.
• APU Run/PTO Clutch does not Engage MOC/FIP: check whether PTO clutch solenoid valve and APU ECU have been replaced; check connections between ELC No. 2 and ECU; verify resistance and opens; check for opens between various J1/J2 terminals; if faults persist, perform different option tests to identify root cause.
• APU MOC (Maintenance Operational Check) includes leak checks (fuel), start sequence checks, advisory checks, and end-of-check shutdown;
• Post-check: review summary reinforces the need to navigate IETM and use publications to identify faults and corrective actions.

APU related system details (lubrication, oil, and bearing specifics)

• Lubrication system purpose: self-contained; stores/ distributes oil for lubrication and cooling; cleans contaminants; provides oil pressure to PTO clutch; oil sump forms part of accessory gearbox.
• Oil sump: wet-sump, 2 quarts; includes dipstick, magnet/draint plug, and mounting bosses.
• Magnetic chip/drain plug: permanent magnet attracts ferrous particles; drain plug permits oil drain; not wired to warning system; visually inspect; check valve prevents oil loss when removing plug.
• Oil sump fillers and controls: oil filler cap with chain; dipstick integrated; level checks done via dipstick; forward mounting bosses help attach to sump.
• Oil pump (internal to accessory gearbox): 3 gpm capacity; driven by FCU shaft; positive-displacement two-spur gear; oil passages to main bearings and gear meshes; mist lubrication for some parts; pressure regulator 95–105 psi; not field-adjustable.
• Oil filter: 10 μm nominal/25 μm absolute; disposable; no bypass; clogged filter reduces oil pressure and can cause shutdown.
• LOP switch: located on top of the accessory gearbox; signals ECU when oil pressure is low; 10 s delay after Ng reaches 95% before faulting; during acceleration 76–90 psi triggers open; if 95% Ng for 10 s with switch closed, ECU shuts down APU.
• PTO clutch: details covered under lubrication section (see above). The clutch is line replaceable as an LRU.
• Oil system test and monitoring: the oil passages and jets lubricate main bearings, gear meshes, and gear shafts; the system maintains 95–105 psi with 2–4 gpm flow; oil is cooled and returned to sump via relief valves.

APU fuel system operation details

• Fuel system is automatic and designed to ensure startup and operation without operator adjustments. It includes: Fuel Shutoff Valve, Boost Pump, Fuel Control Unit, Fuel Solenoid Shutoff Valve, Fuel Nozzle/Manifold Assembly, and Plenum Chamber Drain Check Valve.
• Start sequence: when pushbutton is pressed, FSOV and boost pump energize; boost pump pressurizes fuel to ~10 ext{ psi}; fuel passes through a 3-μm inlet filter; high-pressure pump boosts fuel, delivering fuel to metering valve (250–300 psi); metering valve is ECU-controlled with torque motor; differential pressure relief valve maintains a 25 psi drop; excessive pressure flows back via ultimate relief valve.
• SOV isolates fuel until ECU enables; during start, fuel is directed to primary atomizers; as RPM increases and flow divider opens, fuel reaches secondary nozzles for normal operation.
• The FCU includes a 70 μm screen to filter wear debris; pump discharge test fittings; pump inlet pressure test fittings; differential relief valve and ultimate relief valve for pressure regulation.
• The FCU operation is governed by ECU to avoid overspeed (108% Ng). If overspeed occurs, the ECU commands the FC torque motor to a closed position, providing dual shutdown capability.
• Fuel Flame/Ignition: ignition system energizes during start; high-pressure fuel from FCU is directed to the fuel nozzle; the engine starts with ignition exciter signals; after ignition, the ECU monitors EGT to ensure safe operation.

APU start and control flow (simplified sequence)

• Start pushbutton activated; MP provides APU START CMD to ELC No. 2; APU ECU powers start sequence; APU Boost pump primed; APU Start Solenoid energized; hydraulic accumulator pressurizes to ~3000 ext{ psi}; Hydraulic Start Motor turns APU driveshaft; RPM sensors provide feedback to ECU; Ignition system energizes at approx. 5% Ng; Fuel solenoid opens to allow fuel after Ng reaches light-off threshold; PTO clutch engages around 60% Ng to drive main transmission accessory gear train; APU climbs to 100% Ng with ECU controlling fuel flow; APU ON advisory appears on EUFD/MPD when Ng reaches 95% and the system is on.
• If ignition fails or faults occur, the APU may fail to start; MP clears advisories, logs faults, and may shut down the sequence.

APU start sequence in layman terms (summary)

• Battery power path to ECU via PDP and master ignition switch enables ECU and start logic.
• APU pushbutton triggers: ECU PWR/START, energizes APU ECU, energizes boost pump and FSOV, and supplies hydraulic pressure from the accumulator to the hydraulic start motor.
• Sprag clutch couples starter to gear train; as speed increases, clutch disengages when self-sustaining speed is achieved.
• APU RPM increases; APU speed sensor detects Ng → 95%; ignition and fuel flow are validated; PTO clutch engages to drive accessories.
• APU ON, 100% Ng, and APU start advisory statuses appear on the EUFD/MPD; LCV modulates for cooling as needed; APU remains under ECU control with continuous monitoring.

APU shutdown (normal and fault-driven)

• Normal shutdown: APU pushbutton pressed; MP issues STOP CMD; ECU power to boost pump and FSOV is removed; if ECU does not respond, MP may log APU STOP CMD fault; MP confirms OFF status via APU ON input.
• Fault shutdown: if faults occur (overspeed, overtemperature, low oil pressure, loss of signals, etc.), the MP sends STOP CMD, removes ECU power, closes FSOV, and turns off boost pump; APU indicators on EUFD/FAULT pages reflect fault.
• Start or run faults are logged with fault messages and faults mapped to DMS entries; APU crew must diagnose and remediate per IETM and LSA.

Important numerical references and formulas (highlights)

• APU shaft power: 81 ext{ SHP}.
• Turbine rotor speed (full load): 59{,}566 ext{ RPM}; PTO clutch speed: 8{,}216 ext{ RPM}; backup speed values: 8{,}334 ext{ RPM}; max speed ~8{,}046 ext{ RPM} depending on seal drag.
• Electrical system: 14–30 ext{ V}_{dc}.
• APU dry weight: 129.8 ext{ lb}; APU sump capacity: 2 ext{ qt}.
• Fuel: fuel grades ext{JP-4}, ext{JP-5}, ext{JP-8}, ext{F-24}; normal fuel consumption: 160 ext{ lb/hr}; full load: 225 ext{ lb/hr}; no load: 100 ext{ lb/hr}; start consumption around 175 lb/hr (approximate).
• APU temperatures: overspeed limit 108 ext{% Ng}; critical EGTs: 1785 ext{°F (}974 ext{°C)} at 60% Ng; 1340 ext{°F (}727 ext{°C)} at 100% Ng.
• LCV operation: bleed air control above 95 ext{% Ng}; LCV closed during start; LCV modulates air flow with EGT around 1283 ext{°F} ext{ (} ext{approx} ext{ } ext{ } )}.
• PTO clutch engagement torque limit: 1400 ext{ in-lb}.
• PTO clutch actuation: oil pressure and a TLCV control; SDD (Shaft Displacement Detector) used to protect against loss of centerline control.
• Oil system pressures: LOP threshold around 75 ext{ psig} minimum; check values highlight that 75 psi minimum holds for 5 seconds above 95% Ng.
• Differential relief: metering valve ΔP ≈ 25 ext{ psi}.
• Fuel system pressures: metering valve delivers fuel at 250 ext{–}300 ext{ psi}; differential relief valve maintains 25 psi drop; ultimate relief allows bypass to pump inlet if pressure cannot be relieved.
• Hydraulic accumulator: 3000 psi hydraulic pressure stored; nitrogen pre-charge 1650 psi at 70 °F; 9-piston hydraulic start motor as part of start system.
• Oil sump capacity: 2 quarts; 2 oil circuits: front and main; components include oil level sight gauge; magnetic chip/drain plug; forward mounting bosses.
• 25–30 Vdc power rails from PDP to APU and start circuitry; battery start path is used when AC power is unavailable.

Connections to aircraft systems and world relevance

• ASM links APU to MP via AIU No. 6; MP uses APU data to generate EUFD advisories and DMS faults; the APU can start with engines running or on battery, enabling continued operation during alternate power scenarios.
• The APU’s electrical, fuel, and lubrication subsystems are integrated into a robust control loop: ECU monitors Nr, Ng, EGT, and LOP to regulate ignition, fuel metering, and PTO clutch engagement; the MP automates start/stop sequences and fault handling, reducing pilot workload.
• The IETM troubleshooting guidance emphasizes stepwise checks of ECU, wiring, and connectors; the HMI (EUFD/MPD) messaging ensures crew awareness of APU status and faults; the MS (Master Warning, Master Caution) signals are used to convey critical conditions.

Practical implications and takeaways

• The APU provides critical self-contained power for mission operations, enabling independent electrical power, IPAS air, and hydraulic support when external sources are not available.
• The APU’s complex interlocks (Ng, RPM, EGT, LCV, SOV, SDD) and start/stop sequencing require proper sequencing to ensure safe operation and to avoid damage to the engine or helicopter systems.
• Regular checks of the LOP switch, FCU components, and PTO clutch status are essential to prevent unplanned shutdowns in flight or during ground ops.
• Correct fault diagnosis relies on the IETM data and the DMS fault/advisory mapping to identify the root cause and recommended corrective actions.

Quick reference questions (from the check-on-learning sections)

• What is the shaft horsepower (SHP) of the APU? 81 ext{ SHP}.
• What is the approximate fuel consumption for the APU? Roughly 160 ext{–}175 ext{ lb/hr} depending on load; normal about 160 ext{ lb/hr}.
• What is the purpose of the LCV? Provide bleed air to IPAS when Ng > 95%; closed during start and shutdown.
• What drives the PTO clutch engagement? Oil pressure modulated by the TLCV via ECU control; engagement typically around 60% Ng.
• When is the APU ignition system energized? From about 5 ext{% Ng} during start to roughly a few seconds after Ng reaches 95%; maintained until stable operation.
• When will the APU ECU de-energize the start solenoid valve? After the PTO clutch engages and the start sequence moves toward normal operation (e.g., around 60% Ng and prior to full operation).

Note on equations and constants used in the course material (LaTeX-formatted)

• Turbine rotor speed (full load): 59{,}566 ext{ RPM}.
• PTO clutch speed: 8{,}216 ext{ RPM}.
• Engine oil pressure thresholds: 75 ext{ psig} minimum (LOP activation threshold), with 5 seconds above 95% Ng.
• Overspeed limit: 108 ext{% Ng}.
• Fuel metering valve differential pressure: riangle P = 25 ext{ psi} across the metering valve.
• Light-off and ignition thresholds: Ng reaches approximately 5 ext{%} for light-off; ignition is controlled from ECU until ~95% Ng.
• Combustion and exhaust temperatures: 1785 ext{°F} (≈ 974 ext{°C}) at 60% Ng; 1340 ext{°F} (≈ 727 ext{°C}) at 100% Ng.
• Accumulator pressure: 3000 ext{ psi}; nitrogen pre-charge: 1650 ext{ psi} at 70°F.
• Fuel pressure to metering valve: 250 ext{–}300 ext{ psi}.
• Sump capacity: 2 ext{ qt}.
• Oil pressure relief range and regulator: 95–105 psi operating range for the oil system flow of 2 ext{–}4 ext{ gpm}.

How to study from these notes

• Use the APU system map (Power Section → Air Inlet Plenum → Compressor → Deswirl → Turbine → Exhaust) to trace the flow of air and energy during operation.
• Memorize critical thresholds (Ng percentages, EGT levels, oil pressure) and the corresponding control actions (LCV opening, PTO engagement, ignition enabling).
• Understand the interaction between the ECU and MP for automatic sequencing and fault management; know where data is displayed (EUFD/MPD) and how faults map to DMS pages.
• Review the IETM troubleshooting flows for common faults (ECU ignition control fail, PTO clutch engage fail, etc.) and the recommended corrective actions.

Summary takeaway

• The AH-64E APU is a compact, self-contained gas turbine that provides essential helicopter power in the field. Its operation relies on tightly integrated subsystems (fuel, lubrication, ignition, start, electrical control, and timing) managed by the ECU and MP through a robust ASM framework. Proper understanding of its components, operating limits, sequencing, and fault reporting is critical for safe, efficient helicopter operation and mission success.