• Total pressure is equal to the static pressure plus the dynamic pressure

• Bernoulli’s theorem states that as any incompressible fluid passes through a convergent opening its velocity increases and pressure decreases

• A subsonic nozzle is convergent, it increases velocity and decreases pressure
• A subsonic diffuser is divergent, it decreases velocity and increases pressure

• A supersonic nozzle is divergent, it increases velocity and decreases pressure
• A supersonic diffuse is convergent, it decreases velocity and increases pressure

• A gas generator has a compressor, combustion chamber, and a turbine

• The Brayton Cycle consists of four events occurring simultaneously: intake, compression, combustion, and exhaust

• The gas generator has the Brayton Cycle which occurs simultaneously throughout the gas turbine engine
• Compression takes place in the compressor, combustion in the combustion chamber, and exhaust through the turbine producing thrust

• Pressure increases from the inlet, through the compressor into the diffuser, and it decreases from the burner through the turbine into the exhaust

• Temperature increases from the inlet, through the compressor and the diffuser into the burner, and decreases through the turbine into the exhaust

• Velocity remains virtually constant through the inlet and compressor, into the diffuser and increases through the burner and turbine and peaks in the exhaust

• As temperature increases thrust decreases

• As pressure increases thrust increases

• As altitude increases thrust decreases

• As airspeed increases thrust decreases

• Ram effect has a neutral effect on thrust at low airspeed but a slight increase in thrust at subsonic airspeeds

• The turbine pressure discharge indicator (TPDI) or the engine pressure ratio (EPR) gauge are used to measure thrust for turbojets and turbofans
• Propeller or rotor driven aircraft use a torquemeter gauge to indicate power available

• The inlet duct is designed to provide the proper amount of high pressure, turbulence-free air to the compressor
• Single-entrance inlet ducts are the simplest and most effective inlet design
• Has very smooth airflow characteristics
• Engine is mounted midship
• Divided-entrance inlet ducts are split ducts that can cause some distortion of air flow
• Allows pilot to sit lower in the fuselage
• Reduces friction loss due to length
• Skin friction may distort air flow

• Subsonic inlets are divergent
• Because of the relative incompressibility of subsonic airflow, the shape of the subsonic inlet will increase airflow pressure while reducing its velocity

• Supersonic inlets are convergent – divergent inlet ducts
• Supersonic airflow is highly compressible
• At supersonic speeds, sonic shock waves are developed and if these are not controlled high duct losses will result and could create inlet buzz

• The variable geometry inlet duct utilizes mechanical devices such as ramps, wedges, or cones to change the shape of the inlet duct as the aircraft speed varies between subsonic and supersonic

• The primary function of the compressor section is to supply enough air to satisfy the requirements of the combustion section
• Specifically, the compressor increases the pressure of the airflow from the air inlet duct and directs it to the burners in the quantity and at the pressures required

CENTRIFUGAL FLOW COMPRESSOR

• Centrifugal flow compressors consist of three main components; an impeller, a diffuser, and a manifold
• Advantages are good power output over a wide range of RPM’s, low cost, ruggedness, and it produces the best pressure increase per stage
• Disadvantages are a large frontal area is required and it is impractical for multiple stages

• Axial flow compressors have to main elements; rotor blades and stator vanes
• Advantages are efficient use of multiple stages producing very high overall compression ratios
• Disadvantages include being susceptible to foreign object damage, and it is very expensive and complex

• Dual spool axial flow compressors are divided into two completely independent rotor spools, each driven by its own turbine and drive shaft; one is the high pressure compressor and one is the low pressure compressor
• Advantages are greater flexibility, power, small frontal area reducing drag, high peak efficiencies, straight through flow, allowing for high ram efficiency, and increased combustion efficiency
• Disadvantages are possible stalls at slow speeds, ram effect may increase inlet temperatures, good efficiencies only possible over a narrow rotational speed, difficult to manufacture and costly, and they have high starting power requirements

• Axial-centrifugal flow compressors utilizes the combination of the centrifugal and axial flow compressors
• Advantage is the large pressure increase yet small size which is useful on helicopters and small aircraft
• Disadvantage is that it is only capable on small aircraft

• The burner section contains the combustion chamber, and provides the means for proper mixing of the fuel and air to assure good combustion

CAN COMBUSTION CHAMBER

• The can combustion chamber is used mainly on older centrifugal compressor engines
• Airflow is ducted to individual combustion cans that are arranged around the circumference of the burner section
• Each can contains its own fuel nozzle, burner liner, and casing
• Advantages include its strength and durability, combined with the ease of maintenance
• Disadvantages include poor use of space in the chamber, greater pressure loss, and uneven heat distribution to the turbine section

• Annular combustion chambers consist of a circular, continuous, inner and outer shroud around the outside of the compressor drive shaft
• Fuel is introduced through a series of nozzles where it is mixed and ignited with the incoming air
• Advantages include providing a uniform heat distribution across the face of the turbine section which aids in the prevention of heat warping or turbine blade failure, and for better mixing of air and fuel along with better use of space
• Disadvantages include not being able to be removed without complete disassembly of the engine from the aircraft, and possible structural problems may arise due to the large-diameter, thin-wall cylinder required with this type of chamber
• Often found in helicopters

• Can-annular type chambers combine the ease of maintenance of the can type with the outstanding thermodynamics of the annular type
• Contains cans at the front where the fuel and air are mixed and burned
• Advantages are even temperature distribution, and eliminates the possibility of cold spots by nozzles clogging, it also has greater structural stability and lower pressure loss than that of the can type
• Disadvantage is that it is expensive

• The turbine section drives the compressor and the accessories and is also designed to increase airflow velocity

• Thermal stress on turbine components causes "creep", or blade elongation, which is caused by excessive temperatures over long periods causing permanent deformation

• Fir tree is an attachment method preventing the thin metal blades from cracking at the attachments points by allowing them to expand when heated

• Creep is elongation of blades under extreme heating

• The increasing turbine inlet temperature results in an increase in specific thrust with a corresponding decrease in fuel consumption

• The exhaust section must direct the flow of hot gases rearward to cause a high exit velocity to the gases while preventing turbulence

• Subsonic exhaust nozzles are convergent because as the gas velocity increases, the ability of pressure to push the molecules from behind decreases

• Supersonic exhaust nozzles are convergent / divergent because the divergent section allows for pressure buildup to propel the gases forward to supersonic speeds

• The afterburner is used in turbojets and turbofans to increase maximum thrust available from an engine by 50% or more

• The afterburner has four main components; fuel spray bars which introduce fuel to the afterburner, the flame holders which is located downstream of the fuel spray bars and is where turbulent eddies are formed allowing for the proper mixture of fuel and air combustion, the screech liner controls screech which is characterized by loud noise and vibration, and the variable exhaust nozzle which can be convergent or divergent and prevents a back pressure which could cause a stall

• Two vector components that make up the relative wind in the compressor are the compressor rotation (RPM) and the inlet airflow

• A stall occurs when the airflow over the airfoil breaks away causing the airflow to lose lift due to excessive angle of attack

• Angle of attack is defined as the angle formed between the chordline of the airfoil and the airfoil’s relative wind
• The angle of attack of the compressor blades is changed by changing the rotation speed of the rotors during engine operation and/or changing the velocity of the inlet airflow
• A low angle of attack will result in a low compression ratio and the compressor will be inefficient
• A high angle of attack will result in a possible stall

• Indications of a compressor stall can range from mild pulsation with minimum indications to aircraft vibration and loud bangs or noises
• With constant PCL / throttle position, indications of a compressor stall include RPM decay, and/or interstage turbine temperature (ITT) rise, along with possible loud noises

• Compressor stalls are mainly the result of airflow distortions or mechanical malfunctions

• Airflow distortions can cause compressor stalls by the breakdown of the airflow through a few stages of the compressor
• Airflow distortion to the compressor can be a result of the aircraft attitude and airspeed
• The compressor blade angle of attack is affected by the velocity and direction of the airflow entering the compressor and the rotational velocity of the compressor rotor blades
• The instances when airflow distortions may induce compressor stall include:
• Abrupt changes in aircraft attitude
• Encountering air turbulence
• Deficiency of air volume, caused by atmospheric conditions
• Rapid throttle movement

• Failure to change the angle of attack will cause too much or too little airflow at low engine speeds

• The FCU determines the correct amount of fuel to be introduced into the combustion chamber
• If the FCU fails too much or too little fuel could added

• FOD is caused when an object damages the delicate blades of the compressor

• If the variable exhaust nozzle fails to open, an excessive back pressure will be produced which could lead to a compressor stall

• Erratic or abrupt power control lever (PCL) movements should be avoided, especially at low airspeeds or high angles of attack
• The PCL should be advanced or retarded in a smooth fashion
• The pilot should maintain at least the prescribed minimum airspeed and avoid abrupt changes in aircraft attitude to allow the proper amounts of smooth air to enter the inlets

• Variable inlet guides and variable stator vanes are installed so the angle of attack is changed at low engine speed
• They are automatically positioned by the stator vane actuator (SVA) using fuel pressure via the fuel control unit
• This action maintains the velocity of the air within acceptable limits for low airflow conditions and permits high airflow with a minimum of restrictions

• Dual spool axial flow compressors may be incorporated which allows the front rotor to turn at a slower RPM than the rear rotor
• This allows the front rotor to turn without being choked by the low airflow

• Bleed valves are installed near the middle or rear of the compressor to "bleed" air into the atmosphere and increase airflow in the front of the compressor at low speeds

• Variable exhaust nozzle is used to unload the pressure during afterburner operation

• The first step if a stall occurs is to reduce the attitude of the aircraft which will reduce the inlet’s angle of attack
• The PCL should be retarded to just below stall threshold to allow the engine to "catch up" with the inlet airflow

• The components of a turbojet engine are the inlet, compressor, burner, turbine, and exhaust

• TSFC is the amount of fuel required to produce one pound of thrust

• The total energy of the airflow within the gas generator is altered as it passes through each section
• Altering the airflow causes an imbalance of forces within the engine which provides the propulsive means to the turbojet engine

• As speed increases the turbojet engine becomes increasingly more efficient
• Efficiency of the turboprop increases with speed to a maximum efficiency and decreases efficiency with increasing speeds beyond the maximum efficiency speed
• A turbofan engine increases in efficiency with the increase in speed, but is more efficient that the turbojet engine at every stage

• The TSFC of a turbojet is relatively high at low velocity and low altitude
• The TSFC of a turbofan is lower than that of a turbojet

• Low propulsive efficiency at low forward airspeeds
• Relatively high TSFC at low altitude and low airspeeds
• Long takeoff roll required
• Lightest specific weight (weight per pound of thrust produced)

• The components of a turbofan engine are a duct-enclosed fan, gas generator, inlet, compressor, burner, turbine, and exhaust

• The duct-enclosed fan is driven by the gas generator providing additional thrust by accelerating a fairly large mass of air around the gas generator
• The increased airflow helps to cool the engine without altering the percentages for the secondary airflow in the combustion chamber

• Higher thrust at low airspeeds
• Lower TSFC due to less fuel to produce the same thrust
• Considerable noise reduction
• Heavier takeoff weights and shorter takeoff distances
• Increased performance at operational altitudes when compared to the turbojet

• The fan produces 30 to 60 percent of the total thrust of a turbofan engine

• The gas generator produces between 40 and 70 percent of the total thrust

• The fan is driven by a free or power turbine, which is a turbine aft of the gas generator turbines and is not connected to the gas generator
• Another way to drive the fan is with the gas generator turbine which is done by attaching the fan directly to the compressor

• Bypass ratio is a ratio of the amount of air that bypasses the gas generator in comparison with the amount of air that passes through the gas generator

• A higher bypass ratio results in a higher TSFC as the efficiency of the engine is increased

• The turboprop engine couples a gas generator with a propeller, which is driven by the turbine section

• A turboprop uses a propeller to provide the majority of the thrust and imparts a small amount of acceleration to a large mass of air

• The blades are installed into the hub

• The hub is the barrel assembly and is then attached to the propeller shaft

• The pitch change/dome assembly is the mechanism that changes the blade angle of the propeller

• The reduction gear box is located between the propeller assembly and the gas generator and is basically a one speed transmission
• This assembly prevents the propeller blades from reaching supersonic speeds
• It converts the high RPM and low torque of the gas generator to low RPM, high torque necessary for efficient propeller operation

• The torquemeter assembly is a set of shafts (the torque shaft and the reference shaft) located between the gas generator and reduction gear box
• It is used on some turboprop engines to transmit and measure the power output from the gas generator to the reduction gear box
• The torquemeter operates on the principle of accurately measuring the torsional deflection that occurs in any power transmitting shaft, commonly called the torque shaft

• The propeller assembly may be connected to the gas generator by being attached to the front of the compressor drive shaft or attached to the free/power turbine

• The turboprop accelerates a large amount of air to a moderate speed
• The propeller assembly maintains the propeller at a constant 100 percent RPM
• The propeller adjusts the angle of its blades to accommodate the fluctuations of fuel flow resulting in increased and decreased power in order to maintain constant propeller RPM

• In the alpha range, also known as the flight range, the PCL can be positioned from flight idle to full power
• The PCL send signals to the FCU for fuel flow
• The FCU works with the prop governor to maintain constant propeller RPM by adjusting the blade angle

• The beta range is only used during ground operations
• This range is from flight idle to max reverse
• Allows the pilot direct control of blade angle
• Reversing blade angle results in decreased landing distances and greater ground mobility

• The turboshaft engine consists of a gas generator and a free/power turbine section

• The free/power turbine section is mechanically independent from the gas generator
• Exhaust gases from the gas generator turbine drive the power turbine

• In the turboshaft, the propulsive energy from the exhaust is negligible, that is, all of the remaining energy is extracted by the free or power turbine to drive the rotor assembly
• In the turboshaft engine, virtually all of the pressure energy is converted into shaft horsepower

• Pascal’s Law states that pressure applies to a confined liquid is transmitted equally in all directions without the loss of pressure and acts with equal force on equal surfaces
• The shape of the container has no effect on the pressure or force relationships

• Force is simply a push or pull

PRESSURE

• Pressure is the amount of force per unit area

AREA

• Area, for hydraulic systems, is given in square inches

• Pressure is equal to the amount of force per unit area
• Equation is given as F = P / A

• Linear displacement or distance traveled is exchanged for the change in force
• Linear displacement is proportional to the multiplied force

• Hydraulic systems are required on aircraft because they are used to operate flight controls as well as up to a dozen other systems on the aircraft

• The reservoir functions as a storage tank for the hydraulic fluid required in the system
• It also serves as an overflow basin for excess hydraulic fluid forced out of the system by thermal expansion, heat dissipation, allow air bubbles to be purged, and separate some foreign matter from the system
• Pressurized reservoirs exist on high altitude aircraft

• Hand pumps are used in hydraulic systems to supply fluid under pressure to subsystems such as landing gear, flaps, canopy, cargo doors, bomb bay doors, and to charge brake accumulators
• Hand pump systems are referred to as emergency systems
• Power pumps are normally driven by the engine but may be electric-motor driven
• They displace a constant or a variable amount of fluid

• The pressure regulator or unloader valve always works in conjunction with the constant displacement pump
• Pressure regulator maintains a set pressure in the system
• The unloading valve diverts all pump flow back to the reservoir when the preset system pressure is met

• The check valve allows for one way flow in a hydraulic system
• Allows free flow from the pumps but prevents a back flow

• The accumulator serves as a cushion or shock absorber by absorbing pressure surges in the system
• Supplements the pump’s output when the pump is under peak load by storing energy in the form of fluid under pressure

• Filters ensure delivery of contaminant free hydraulic fluid by preventing dust, grit, and undesirable impurities from entering the system

• Relief valves are simply a pressure limiting device
• The valves prevent seals from bursting or becoming damaged due to pressure build-up

• The pressure gauge indicates the amount of pressure in the hydraulic system

• Pressure switches are used to indicate a hydraulic drop that falls below allowable limits
• Located in the lines leading from the pump

• Hydraulic fuses are safety devices that are installed at strategic locations throughout a hydraulic system
• They are designed to detect or gauge ruptures, failed fittings, or other leak-producing failures or damage

• Selector control valves are used in a hydraulic system to direct the flow of fluid
• Directs fluid under pressure to the desired working port of an actuating unit
• At the same time it directs the return fluid from the opposite working port of the actuator to the reservoir

• Actuators convert fluid under pressure into linear or reciprocating mechanical motion
• Usually installed with the piston shaft end attached to the mechanism and the other end to the aircraft structure

• Fluid is pumped out of the reservoir through the check valve past the pressure regulator
• Pressure is then released to the actuating cylinder allowing function of the piston
• Fluid is returned through the selector valve and the filter to the reservoir

• AC is a form of electricity that reverses its direction
• AC is used in our homes

• DC is a form of electricity that only flows in one direction
• DC is used in our boats, cars, and even small electrical devices such as watches

• DC units are heavy compared to their power output capabilities
• DC units are often not reliable and they increase maintenance
• DC units also use an inverter for conversion to alternating current for AC powered equipment
• AC power requires less current because of higher voltage and a ground neutral system
• AC components are lightweight, simple, and reliable

• Generators are often used as the main source for AC and DC power
• DC generators are called generators while AC generators are called alternators
• Generators require a constant rotational input speed regardless of engine RPM
• A constant speed drive is a hydro-mechanical linkage between the engine and the generator

• An inverter is an electro-mechanical device that transforms direct current into alternating current
• On DC electrical systems, inverters are used to power AC equipment

• A transformer rectifier is an electrical device which transforms AC power into DC power
• The rectifier’s DC current capability is high and is largely dependent on the cooling ability of its fan

• The battery provides direct current power
• This DC voltage is primarily used as a source of emergency power should the generators fail and also for starting the aircraft’s engines
• Nickel cadmium is the preferred battery over the lead-acid battery
• Nickel cadmium batteries recharge in a short amount of time
• They hold charge for long periods
• Worn cells can be replaced
• Delivers a large amount of power

• The electrical distribution network in an aircraft provide the various electrical components with their power requirements through several buses
• The essential bus routes power to equipment required for flight safety
• The primary bus routs power to equipment devoted to the aircraft’s intended mission
• The monitor or secondary bus routes power to convenience circuits
• The starter bus routes power to start the aircraft’s engine(s)
• Manual or automatic control over the flow of electrical power comes from fuses, switches, and circuit breakers

• The main generators are powered from the aircraft engine’s accessory drive section via the constant speed drive
• Warning lights are designed to alert the crew to any system malfunction
• A series of buses is incorporated to provide the electrical distribution based on equipment type and current required
• Emergency power is available to energize the essential busses in the event of generator failure
• Electrical power for the starter bus is obtained from either an external source, the battery, or the auxiliary power unit (APU)

• Volatility is the measurement of a liquid’s ability to convert to a vaporous state

• Flash point is the lowest temperature of a combustible substance that would ignite with a momentarily application of a flame

• A fuel’s flashpoint and volatility rating are inversely related
• As the flash point increases, the temperature at which fuel would ignite decreases

• JP-4 is a wide cut blend of kerosene with some naphtha fractions and gasoline
• Highly volatile with a flashpoint of –35 degrees farenheight
• Easier starting, slower acceleration, lower operating temperatures, higher tendency to vapor lock, and shorter range compared to JP-5

• The Navy, Marine Corps, and Coast Guards primary jet fuel
• JP-5 is a heavy kerosene to be blended with gasoline
• Thermally stable with a high heat content per gallon
• Low volatility and a flashpoint of 140 degrees farenheight permits storage aboard ships

• Similar to JP-5 except for the flashpoint, which is 100 degrees farenheight and permits storage aboard ships
• Air force’s primary jet fuel
• Advantages include fuel handling and operational safety

• The fuel tank is the starting point for fuel
• The tank is a reservoir, or holding cell, for the jet propellant

• The boost pump is an integral unit composed of a centrifugal pump and electric motor
• Submerged in fuel tanks, they insure adequate supply of fuel to the engine-driven fuel pump
• A critical function of the boost pump is to prevent aeration of the fuel supply which may result from a rapid pressure change incurred during a climb

• Used to regulate stability of the fuel load by transferring fuel to even out the load

• When the pilot pulls the emergency handle, the emergency shutoff valve shuts off fuel to the engine electrically and mechanically

• Located in the cockpit, the fuel pressure gauge receives signals from a pressure sensor at the boost pump outlet

• The low pressure fuel filter is usually a paper cartridge type filter, located downstream of the boost pump to strain impurities from the fuel
• The minute openings make this type of filter susceptible to clogging, therefore, a bypass valve is a necessary safety factor to ensure a positive supply of fuel to the engine

• The engine-driven pump is a high pressure pump designed to deliver fuel to the control unit in excess amounts

• The fuel control unit is the "brain" of the engine fuel system
• The FCU is a hydromechanical or electrical device that consists of fuel computing and fuel metering systems
• To ensure proper fuel flow the system incorporates various inputs to include:
• PCL position – inputs from the aviator
• Compressor inlet temperature (CIT) – measures ambient air density
• RPM’s - compressor speed
• Turbine temperature – prevent turbine damage

NORMAL RATED THRUST (NRT)

• NRT is the thrust produced at the maximum continuous turbine temperature with no time limitation
• This rating is for cruising

• MRT is the thrust produced at the maximum turbine temperature for a limited time, normally 30 minutes
• The maximum temperature for MRT is higher than for NRT, however, the time constraints ensure blades are not damaged
• This rating is for takeoff or when additional thrust is needed

• CRT is thrust produced with the afterburner in operation, and is not based on turbine temperature limitations

• The fuel transfer valve, which is mounted on the body of the engine driven fuel pump, supplies fuel to the afterburner fuel control unit

• The afterburner fuel control unit meters fuel to the afterburner spray bars and excess fuel is returned to the fuel pump inlet

• Synthetic lubricants are less volatile, has a stronger chemical stability, and a lower tendency to leave cooking deposits
• Multiple synthetics may not be derived from the same base and therefore cannot be mixed
• Synthetic oils are never to be mixed with petroleum based lubricants

• Viscosity is the property of a fluid that resists the force tending to cause the fluid to flow
• Viscosity is inversely related to temperature

• Oils should have the following characteristics to lubricate properly:
• It must be low enough viscosity to flow, yet high enough to protect
• Must not break down under high heat or pressure
• Must flow readily when starting under extremely low temperatures
• Must have a high flashpoint so it does not burn or vaporize
• Should not form or deposit excessive amounts of gum, carbon, or varnish

• Lubricants must cool moving parts by carrying heat away from gears and bearings
• Temperatures in excess of 1700 degrees farenheight so this is important

• Lubricants must carry dirt, metal, and carbon away from moving parts to the filter
• This is a vital function

• The lubrication system pumps oil around moving parts of the engine
• As the oil is pumped through the engine it removes heat, dirt, carbon, and metal from moving parts while lubricating at the same time

• The wet sump system is used on aircraft that require a limited supply of oil and limited cooling
• Oil is stored in a sump which limits the amount of oil carried
• Oil is difficult to cool in this system and it is not adaptable for unusual flight attitudes for extended periods of time

• Oil is carried in a dry sump mounted to the engine or in the airframe
• More oil can be stored and temperature is readily controlled
• Three subsystems of the dry sump system are:
• Pressure subsystem supplies lubricating oil from the tank to the main engine bearings and the accessory drives
• Scavenge subsystem removes the oil from the main bearings and accessory drives through the oil coolers and returns it to the tank, completing the flow cycle
• The breather pressurizing subsystem connects the individual bearing components and the oil tank with the breather pressurizing valve to help minimize oil leakage

• Pressure subsystems normally employ an engine driven, gear type, pressure pump
• Oil is sprayed at a constant pressure despite engine speed

• The oil pump supplies oil under pressure to the parts of the engine that must have lubrication

• The oil pressure gauge displays oil pump discharge pressure
• The oil temperature gauge displays the temperature of the oil prior to entering the engine bearing compartments

• Filters remove any foreign particles that may be present in the oil
• The filter bypass valve allows oil to flow around the filter in the event the filter gets clogged
• The magnetic-chip detector is a metal plug with magnetized contacts, and is placed in the scavenged oil path
• The oil pressure relief valve is the pressure oil line to limit the maximum pressure within the system
• The relief valve is preset to relieve pressure by bypassing oil back to the pump inlet whenever the pressure exceeds safe limits

• The scavenge subsystem removes oil from the main bearing compartments and accessory gear drives

• The breather pressurizing subsystem provides the following functions
• Minimizes internal oil leakage by encasing the oil sumps
• Ensures proper spray patterns of oil across the bearing by mixing pressurized air with the oil to form a fine oil mist for the bearings

• Accessories for gas turbine engines can be divided into two categories: those driven by bleed air and those driven mechanically

• Compressor discharge air at high pressure is bled from the engine through ports or valves at intervals along the compressor case and at the end of the diffuser
• It is used as a source of power for operating air conditioning units, cockpit pressurization, and engine anti-icing to name a few

• Driven by a geared drive taken directly from the main shaft connecting the turbine to the compressor
• This is used for tachometers, hydraulic pumps, generators, alternators, and other accessories

• Interstage bleed air is required to maintain and ensure compressor stability

• As soon as the starter has accelerated the compressor sufficiently to establish airflow through the engine, the ignition is activated and then the fuel is added

• A hot start is defined as exceeding the maximum allowable temperature for the turbine section during start

• A hung start describes a situation where the temperature within the turbine section continues to rise, and the compressor RPM stabilizes below normal

• A false start occurs when compressor RPM stabilizes below normal, and the turbine temperature remains within limits

• A wet start is a situation in which the fuel-air mixture does not light off initially, but has the capability to ignite
• The wet start is an ignition problem and is the most dangerous abdominal start

• The electric starter is mechanically connected to the compressor and is mounted on either the engine accessory gear box or the front frame of the engine
• A battery, auxiliary power unit, or external electrical source may be used to supply electric current to the start motor

• An air turbine starter is a small, geared, air turbine motor attached to the engine
• Air is directed to the air turbine which accelerates the compressor

• A basic aircraft ignition system is a high energy, capacitor-type ignition system that provides both high voltage and an exceptionally hot spark which gives an excellent chance of lighting the fuel-air mixture at reasonably high altitudes

• The annular-gap plug protrudes slightly into the combustion chamber line to provide an effective spark

• The constrained-gap does not closely follow the face of the plug, it jumps in an arc which carries it beyond the face of the chamber liner