J E T      S Y S T E M S

 G l o s s a r y

Here are some terms that are commonly used to describe jet systems. We will define them such that they may take some of the mystery out of turbine powered flying machines.  This secton has been given to many people in order that they might prepare to upgrade to a jet aircraft.

AMPERE - The term Ampere, or "AMP" describes the flow rate of electrons in the circuit, similar to "Gallons per Minute" in a hydraulic system.  One "AMP" of current is one "Coulomb" of electrons per second flowing through the system. A coulomb is a number with many zero's to the left of the decimal point.  Imagine that a generator is a pump.  The ampere tells us how many electrons the generator pushes through the wire in a given unit of time, not how hard it pushes them.  The "Volt" tells us that.  The alternator in your car, if operating normally, puts out between 13 and 14 volts.  When you are driving during the day, and not using any of the car's accesories, the alternator may be producing between 2 and 5 "amps".  On a hot summer night with the air conditioner, stereo, electric cooling fans and God knows what else, as much as 50 or 60 amps may be the case.  How does this translate to power?  One volt at one amp equals 1 watt!  Multiply volts times amps and get watts.  There are 746 watts in one horsepower.  That alternator producing 50 amps at 13 volts is producing 650 watts, or almost a horsepower.

AMPERE HOUR - An Amp Hour, sometimes abbreviated "AH", is a unit used to describe the capacity of a battery. A battery that can put out 1 AMP for 1 hour prior to becoming fully discharged is a one Amp Hour battery. The same battery should be able to put out 2 Amps for half an our, or one half an amp for 2 hours. Amps x Time equals Capacity. When the drain on a battery becomes extreme, the capacity will usually be less than it would be at a lower rate of discharge.  To determine how much energy a battery contains, you must also know the voltage.  A 12 volt 10 ampper hour battery has one half the energy as a 24 volt battery that has a 10 ampere hour rating.

ANTI-ICE - Systems that are designed to prevent the formation of ice on the aircraft are referred to as "Anti-Ice". This may be done by heating the area or device that you wish to protect from ice formation. Heat may be applied through the use of electric heating elements, or by pumping hot engine bleed air through whatever it is you wish to keep free from ice. Chemical ice protection systems also exist. Alcohol applied to a surface will prevent or retard the formation of ice. This type of ice protection is used as an alternate system for many aircraft windshields, and as a primary wing anti-ice system on aircraft such as the HS-125 and the Citation S2. Anti-ice is intended for ice PREVENTION not ice removal. It should be activated PRIOR to entering icing conditions.

BLEED AIR- Bleed air is nothing more than air that has been compressed by one or more of the compressor stages of a turbine engine. Bleed air is hot, as a result of being compressed. The more compressor stages that have compressed the bleed air, the higher the temperature and pressure will be. There are bleed valves on some turbine engines that serve only to improve the engine's idle and acceleration characteristics. These will operate automatically, and are of little concern to the pilot unless they malfunction, causing an unstable idle RPM, or a compressor stall.

In most turbine powered aircraft, bleed air is used for pressurization, air conditioning, engine anti-ice, windshield anti-ice, de-ice boot inflation, to power a venturi system, (creates vacuum to control outflow valves and power gyroscopic instruments), and on some aircraft, to anti-ice the wing and tail. Remember, when bleed air is extracted from an engine, some power loss will result, as well as an increase in exhaust gas temperature. When operating bleed air powered wing heat, engine, or windshield anti-ice systems, aircraft performance and engine power settings will change. A loss of performance will result. There will most likely be a substantial reduction in your maximum takeoff or landing weight if anti-ice is used. Check your performance charts!

BUS- A bus is nothing more than a metal bar that receives electrical power from one or more sources, and supplies that power to various electrical devices and / or other busses. The term "Main Bus" usually describes a bus that is supplied by the generator, alternator, or inverter as the case may be. It will usually then supply power to some high load items (landing lights, boost pumps ect.), and to additional busses in the system. An extension bus is usually powered by the main bus, and supplies power to items located in parts of the aircraft away from the main busses. In the case of the Citation Jet, the main busses are in the tailcone compartment, and the extension busses are in the cockpit. The term "Crossover Buss" describes a bus on one side of the aircraft that is powered from the opposite side. This allows circuit breakers to be grouped by system instead of bus.

CRANKING POWER - The cranking power of a battery is the maximum number of amps that it can deliver. This is a function of how fast the chemical reaction can occur inside the battery itself.  In the olden days, Ni-cad batteries were the only batteries that produced the "cranking power" to start a jet engine, while sitll being small enough and light enough for use in an aircraft.  This is no longer the case.  Lead acid batteries have come a long way, and can now compete.

CROSS GENERATOR START - A cross generator start is nothing more than starting the second or subsequent engine(s) with the operating generator on line to assist the battery, thus making the start quicker. There are limitations associated with this procedure on many aircraft. If improperly done, the cross generator start may blow fuses (current limiters) and possibly damage the starter/generators themselves. Strict adherence to the flight manual procedures is a must. Serious damage may result from an improperly performed cross generator start.

CURRENT LIMITER - Current limiters are nothing more than big "Slow Blow" type fuses. When installed in electrical circuits they will self destruct, or "BLOW" if the current flow it exceeds its design value for more than a very brief period. When blown they must be replaced. They are placed in electrical circuits to prevent severe overloads from causing damage other than the loss of the equipment powered by the particular circuit. Citation's do not just blow current limiters if nothing else is wrong with the system. You must have a malfunction in the electrical system in order to blow a current limiter in this aircraft.

DE-ICE - Those ice protection devices that are intended to REMOVE existing ice are "De-ice" systems. They are activated only AFTER a prescribed amount of ice has been allowed to form. Premature activation of de-ice boots may cause the ice to form in such a manner as to make the de-ice boots ineffective. Wait until there is 1/4 to 1/2 inch of ice accumulation prior to cycling the boots. Be good to your de-ice boots. Keep them clean and treated with the proper chemicals. Do not cycle them without cause at very high altitudes and low temperatures. This can cause them to crack. Ice will generally not build up on an aircraft at temperatures below -20º C. At high altitudes, ice will usually evaporate, (sublimation), as a result of the extremely low atmospheric pressure.

ENGINE DRIVEN FUEL PUMP - The engine driven fuel pump takes fuel from the tank (assisted by a fuel boost pump or ejector pump) and boosts the pressure from less than 50 PSI to a very high pressure ranging from 500 to 1500 PSI depending on the type of engine. The fuel control requires some very high pressures in order to operate. If the engine driven fuel pump fails, the engine will quit! The other pumps in the fuel system will not supply enough pressure to keep the engine running, even at reduced thrust.

EPR - Engine pressure ratio, sometimes referred to as exhaust pressure ratio. It is the ratio of pressure at the engine exhaust as compared to the engine inlet. A higher EPR means more thrust. EPR is the primary engine instrument used to set thrust on straight turbojet, and some turbofan jet engines.

FLOW CONTROL VALVE - A flow control valve is a bleed air valve that is designed to maintain a relatively constant flow of air regardless of changes in engine power. The changes in engine power will result in a change in the pressure and temperature of the bleed air. This can be annoying to the aircraft occupants. The flow control valve adjusts it's position toward open or closed to minimize changes in cabin airflow. The pressurization outflow valves also compensate for changes in cabin air supply although they do not act as quickly as the flow control valves.

FUEL BOOST PUMP - The term fuel boost pump usually refers to an electric boost pump located at one or more places in an aircraft fuel system. The primary purpose of these pumps is to deliver fuel to the inlet of the engine driven fuel pump. Additionally they provide a means to transfer, crossfeed or dump fuel. Fuel dumping is performed by opening the required "Fuel Dump Valves" in the system and pumping the fuel overboard with the electric fuel pumps.

FUEL CONTROL - Proper fuel flow is very important in a gas turbine engine. Unlike an engine with a carburetor, the amount of air flowing into a jet engine is not regulated. Jet engines suck in whatever air is available. Engine power is controlled by the amount of fuel that enters the engine's combustion chamber. At sea level, only about 25% of the air entering the engine is burned. The other 75% just flows through the engine, absorbs heat and provides mass to be routed through the turbines. The proper fuel flow for a given engine RPM varies with altitude. The same fuel flow will result in much different engine RPM and temperature at different altitudes. The end result of all this is thrust in the case of a jet engine, or horsepower in the case of a turbo shaft engine. In general, the proper fuel flow for a given situation depends on the following:

1. Thrust lever position
2. Compressor discharge pressure (P3)
3. Engine inlet temperature (T2)
4. Engine RPM and engine acceleration
This would be just a bit much for a pilot to consider while attempting to keep the airplane right side up. Abrupt thrust lever (throttle) changes or improper fuel flow might cause excessive engine temperatures, or even an engine flameout due to excessive or insufficient fuel flow. Engine starting would be much more a function of technique, thus more prone to pilot induced errors.

The fuel control is a hydro mechanical device that regulates the amount of fuel the engine receives. The above mentioned parameters are the data the fuel control uses to decide how much fuel is delivered to the combustion chamber. The amount of fuel the engine gets for a given set of circumstances is called the "Fuel Schedule". Engine starting phase has it's own fuel schedule.

Some of the more complex jet engines have an electronic fuel computer that enables the fuel control to operate more efficiently. These fuel computers make the fuel control do more, but most of these type units do not meet certification requirements for other than emergency of ferry operations if the fuel computer fails. The engine will still run, but it will not meet the minimum engine acceleration criteria for part 25 certification. The designs of various fuel controls vary, but the primary functions of the units remain the same.

The result of all this is that the pilot merely tells the fuel control unit how much thrust is desired by moving the thrust lever. The fuel control delivers the proper amount of fuel to the combustion chamber for optimum operation of the engine. Rapid movement of the thrust levers DOES NOT damage the engine. Most jet engines in use today have some type of Overspeed governor or device to limit engine power. Full throttle operation will result in reduced service life and possible additional engine maintenance, but will not cause an immediate engine failure. Remember this if it looks like you will collide with an obstacle. The impact will cause more damage than briefly overtaxing the engine. Don't break the airplane to be easy on the engine.

FUEL DENSITY CONTROL - The fuel density control is an adjustment on the fuel control unit or fuel computer as the case may be. Chemically, the proper fuel to air ratio for combustion is a function of weight, not volume. The fuel pump and fuel control think in terms of volume. In order to determine how much fuel the engine requires for a given situation, the fuel control must know the DENSITY or specific gravity of the fuel. Specific gravity is merely the weight of the fuel, in percent, as compared to an equal volume of water. Water has a specific gravity of 1.0, as it is the standard by which other materials are rated. Jet fuel varies from about .78 to .812 and Av Gas is usually between .69 and .73 specific gravity. From this one might conclude that if the engine thought it was getting jet fuel, (more dense) and was actually getting Av Gas, (less dense) the acceleration and performance of the engine would be substandard. By a minor adjustment of the fuel density control, compensation is made for the difference in fuel density. This should only be done by a certificated aircraft mechanic who is qualified to work on the particular type of turbine engine. If this is to be done, check the aircraft flight manual, as in most cases, restrictions as to the aircraft's maximum altitude, ambient temperature, time in service and flight procedures apply.

GENERATOR CONTROL UNIT- The generator control unit, or GCU, is a device which controls the output of the generator. It regulates the voltage, controls whether the generator is on or off, and protects the generator from damage by tripping it off in case of a short or other problem that may cause the generator or other equipment to be damaged.

GENERATOR FIELD - Generators make electricity because current will be caused to flow through a coil of wire as that coil of wire is passed through a magnetic field. Without this magnetic field, the generator would not produce electricity. The armature of a generator has many coils of wire within it. The magnetic field through which these coils will pass may be created by a permanent magnet or electromagnet. An electromagnet is a coil of wire through which electricity flows and creates a magnetic field. This magnetic field may be strengthened or weakened in order to regulate the voltage output of the generator or alternator. Field weakening is also used to prevent starter overspeed in the case of a broken drive shaft connected to a starter/generator. The lack of load on such a powerful motor may otherwise cause the device to reach an RPM that would cause it to explode. This would not be good. Generators with an electromagnetic field will not operate unless the "Field" is energized, providing the magnetic field through which the coils in the armature will pass.

HEAT EXCHANGER - A heat exchanger is a simple device. Heat flows from hot to cold. The radiator in an automobile is a heat exchanger. Hot water contained in the radiator will transfer heat to the cooler air that flows through the radiator. The radiator may be referred to as a "water to air heat exchanger". The air and water do not mix, but the heat transfers from the water to the air. Air to air heat exchangers are used on aircraft environmental systems. Bleed air transfers heat to ambient air. Oil coolers are oil to fuel or oil to air heat exchangers. The efficiency of a heat exchanger can be regulated to control the temperature of the air or liquid flowing through it.

INVERTER - An inverter is a device that takes DC electricity and converts it to AC. In the old days, inverters were merely DC motors that turned Alternators (AC generators) at a constant rpm in order to produce "Constant Frequency" 400 cycle AC. Now most inverters are solid state and have few if any moving parts. Solid state inverters are light weight and extremely reliable.  The old rotary types require much more power to run them.  I have heard them refered to as "Flintstone Inverters".

JET PUMP - A jet pump is a pump that uses fuel pressure that is directed through a venturi.  The fuel flowing through the venturi causes a low pressure area inside the venturi section of  the jet pump.  Fuel from the surrounding area is sucked into the jet pump and expelled out the jet pump nozzle.  The surounding fuel slows down the flow of the high pressure fuel, and in turn is sucked out the nozzle with it, but at a much lower pressure.  The general idea is high pressure low volume in, and high volume at low pressure out, kind of like an electrical step down transformer, only with liquid, not electrons.  Jet pumps are controled by valves, or are designed to be on all the time.  They require no electrical power to operate.   For example, the Citation has two types of jet pumps, primary and secondary.  The primary jet pumps have an input of 500 psi and an output of 20 to 30 psi.  The secondary jet pumps have in input pressure of 20 to 30 psi, and an output of 3 to 4 psi.

LEAD ACID BATTERY - A lead acid battery is a battery very similar to the one in your family automobile. It is rechargeable, and requires little maintenance except for keeping the fluid at the proper level. Lead acid batteries have their good and bad points when it comes to their use in turbine powered aircraft.

Good points: Safe, cheap to purchase and maintain, requires no special training or procedures to operate.

Bad Points: Voltage drops as battery discharges, poor shelf life, less than optimum performance in cold climates, can be ruined if left in a discharged state for too long, less powerful than a Ni-Cad battery for the same size and weight.  They a also do not last as long as the Ni-Cad.

NI-CAD BATTERY - The Nickel-Cadmium (Ni-Cad) battery uses a different chemical process to generate electricity. Ni-Cad's are generally found in applications where high amounts of current are required, such as starting jet engines. The Ni- Cad can charge and discharge much more rapidly than a lead acid type battery. It will also maintain very near full output until almost dead. If improperly operated or maintained, the Ni-Cad may experience what is called a "Thermal Runaway". This can happen if the charging rate is excessive for too long, as may be the case if the battery were severely discharged, then charged rapidly. The heat generated by the excessive charge rate may cause the internal resistance in the battery to decrease, allowing more current to flow, generating more heat. Then you have a vicious circle. The solution is to isolate the battery from it's charging source, or better yet, observe the batteries operating limitations and prevent the thermal runaway in the first place. Good Points: High power output, lighter weight, smaller size, better shelf life, faster re-charge, maintains rated voltage until almost dead, better performance in cold climates.

Bad Points: More expensive to purchase and maintain, thermal problems if operated improperly.

MACH NUMBER - The speed of sound for a given atmospheric condition is expressed as the number one (1.0), or 100% (of the speed of sound). If an airplane is flying at 75% of the speed of sound, then it is said to be flying at .75 Mach (spoken as, "point seven-five Mach"). In order to understand what this "number" is all about, you must first comprehend what the speed of sound is in the first place.

a= Square root of (49.022 X T)

where a= the speed of sound (in fps). T= the temperature of the air mass (Fahrenheit). An in-depth mathematical look into thermodynamics will reveal that air pressure and density vary directly; therefore, they have no effect on sound waves. However, variances in temperature can appreciably effect density without effecting pressure. This means that sound waves will speed up in warmer temperatures and slow down in colder temperatures. Just take this at face value and don't worry about it too much; an explanation of the thermodynamic principles involved can be found at your nearest library. In short, temperature is the measure of the kinetic energy of the air molecules. The higher the temperature, the faster these molecules can transfer energy, and the faster sound waves will travel. Thank you Jack Bayt. Since air temperature decreases with an increase in altitude, the higher you go, the slower these pressure waves (sound) will travel. This holds true up to an altitude of about 37,000 feet. Then the temperature remains constant at a frosty -57º C.

MACHMETER - Because the speed of sound varies with the temperature--and altitude (inasmuch as the air gets colder as you go higher), it is obvious that we need something more than an airspeed indicator. This simple instrument doesn't care how cold it is, and doesn't give us an accurate indication of our "relative" speed. It is therefore essential to use an instrument which gives the airplane's speed as a percentage of the speed of sound under the atmospheric conditions in which the airplane is flying. This instrument is the Machmeter. So why all the fuss? Well, the pilot needs to know when he is approaching his airplane's Mmo (Maximum Mach Operating) speed. This is because certain aerodynamic phenomenon occur in this speed range, and most commercial aircraft have to avoid it. Also, a great percentage of a jet's operating parameters involve the use of Mach speeds. Believe me, you'll use it as a matter of routine.

RAM RISE - The rise in air temperature that occurs at high speed. This is due to the compressibility of the air.  The faster you go, the more the temperature rises. This can be seen on your OAT gauge. At about 180 knots indicated airspeed,  you will notice about a 3º Centigrade rise in the temperature above the actual temperature of the air at that altitude. At about Mach .77 (440 kts TAS, 220 kts IAS at 41,000 Ft) the temperature rise will be about 25º Centigrade. This is why the OAT in a Lear reads -29º or so at cruise instead of -54º, the actual temperature.  There are tables where you can calculate the SAT, or static air temperature if you know the "indicated" or Ram Air Temperature and the mach number.

RELAY - A relay is nothing more than an electrical valve. When the relay is closed, allowing electricity to flow, the valve would be considered "open". If the relay is open, the valve would be "closed". If the amount of current required to start a jet engine were to pass through the battery switch, the switch would probably get red hot and maybe even melt. To avoid this, the "Start Relay" is used to get the electricity from the battery to the starter. Just as when you start your car, the starter is not powered through the ignition switch. When you turn the key, the ignition switch, supplies power to a small coil or solenoid that closes the "start relay". This provides a path for large amounts of electricity to flow from the battery to the starter, bypassing the ignition switch entirely. If this were  not done this way, your car keys would get very hot indeed!  When a generator is placed on line, the generator may charge the battery through the battery relay.  The battery relay does not care which way the current flows through it. There is a type of relay that will allow current to flow only one way. This is called a "Reverse Current Relay". Reverse current relays (RCR's) are used when the designer wishes to allow current to flow only one way.  Most aircraft have reverse current relays between the generators and their respective busses to prevent one generator from motorizing another generator that might have a lower voltage. A large capacity diode may also be used for this purpose. A diode is the electrical equivalent of a "one way check valve".  A reverse current relay assures that the generator will either be a generator like it is supposed to be, or be electrically disconnected from the system in the event it's voltage is less than the battery voltage for whatever reason.

ROSEMONT PROBE - a temperature sensing probe that measures the true air temperature without being affected by ram rise. The Rosemont Probe can measure the actual air temperature, even when mounted on a fast moving aircraft.  These probes are most commonly found on aircraft equipped with some form of long range nav system. True air temperature, or TAT, is needed to solve for TAS. If the INS, Laser Ref, GPS or VLF knows the TAT and the IAS, it can solve for TAS. The wind may be computed using TAS, Ground speed, heading and track.  These days, the VLF is a boat anchor, as many of the stations have been shut down.

STARTER - GENERATOR - Aircraft engines must be started in order to be of much use. Some small aircraft engines (piston) may be started by turning the prop by hand. This, however, is not very practical when it comes to most larger piston type engines, and impossible with jets or turboprops. Piston engines, and all small jet engines use an electric motor of some kind to turn the engines over fast enough to start.  Look at an electric motor like a propeller. If you blow enough air through the propeller, it will turn the engine due to the force the air exerts on the prop. If the engine is running, the prop will pump air and produce thrust.  DC electric motors act in a similar manner. If you apply electricity to the motor, the motor turns over and produces power. If you mechanically turn the motor, it will produce electricity. The starter-generator is one device that will perform the functions of a starter or a generator as needed. The starter-generator is connected to one of the engine's compressor sections via the accessory gearbox. The starter in this case, does not engage, start the engine, then disengage. It is always connected. It functions as a starter or generator as appropriate.  Jet engines that require 40 or so horsepower to start them, usually have an air driven starter.  The smaller jet engines, TFE 731, CJ-610, JT-12 and CF 700 can use the generator to start the engine, as well as be the generator once the engine is running, therefore the term "Starter - Generator".

STATOR VANES - Stator vanes or "Stators" are small airfoils between various stages of a turbine engine. Their purpose is to reduce turbulence, stabilize and direct the airflow within a jet engine. Stators in the compressor section of an engine may require ice protection, as they do not move like the compressor blades do. Stator anti-ice is provided by engine bleed air. Stators in the turbine or hot section of the engine obviously do not need ice protection.

TRANSFORMER - A transformer is an electrical device that is used to "transform" one electrical voltage to another.  This is possible because of the fact that electricity flowing through a coil of wire produces a magnetic field.  Also, when a coil of wire is passed through a magnetic field, it produces electricity in the coil of wire.  If I have a transformer with a 100 turn winding on the end that is powered, and a 50 turn winding on the other end, I will get half the voltage on the other side, but twice the number of amps!  This means that the total amount of energy is the same, but the volts and amps may be changed or "transformed".  A transformer can be up to about 98% eficient.  The 2% is lost to heat!

WINDMILL AIRSTART - Most jet engines may be started without using the starter if the aircraft has enough airspeed to windmill the engine at a high enough rpm. With the engine in the proper RPM range, add fuel and ignition, and the engine will start without the use of the starter. Charts or other required data related to this procedure may be found in the aircraft flight manual.

UNITS - You have seen the term "Kilowat", "Miliamp", "Megahertz", and many similar terms.  "Kilo" means 1,000 of whatever is being described.  A "Kilogram" equals 1,000 grams!  "Mili" means devided by 1,000, "Mega" means a million times, micro means one millionth!  KW means "Kilowatts", as does KVA, kilo volt amps.

VOLT - A volt is a unit of "Electro Motive Force". How many volts tells you how hard the electrons are being pushed through the wire, not how many of them are making the trip!  Volts to an electrical system are like PSI or pressure to a hydraulic system. The more pressure or "Volts", the more electricity tries to flow.  I believe that the "Volt" is named after a Frenchman named Voltiare!

WATT - A watt is a unit of power, like horsepower. The power, or watt's may be computed by multiplying the VOLTS times the AMP's. Just for your information, one horsepower is equal to 746 Watts. Considering the fact that nothing is 100% efficient, it takes about one kilowatt of electric power to drive a one horsepower electric motor.   One hundred percent efficiency is not possible yet, but the loss must show up somewhare. Where did the other 254 watts go? Heat, that's where.