2. Engines
Why does a jet aircraft climb as high as possible?
"Jet aircraft climb as high as possible (i.e., to their service ceiling) because the gas turbine (bypass) engines are most efficient when their compressors are operating at a high rpms—approximately 90 to 95 percent. This high-rpm speed results in the engine's optimum gas flow condition that achieves its best specific fuel consumption (SFC). However, this high-rpm speed can be achieved only at high altitudes because only at high altitudes, where the air density is low, will the thrust produced be low enough to equal the required cruising thrust. The primary reason for designing an engine's optimum operating condition at approximately 90 to 95 percent rpms is to make it coincident with the best operating conditions of the airframe, namely, minimum cruise drag. Therefore at high altitudes, there are two main consequences: 1. Minimum cruise airframe drag. This is experienced at high altitudes because drag varies only with equivalent airspeed (EAS), i.e., as EAS decreases, drag decreases. At very high altitudes, i.e., above 26,000 ft, the Mach number (MN) speed becomes limiting and therefore EAS and true airspeed (TAS) are reduced for a constant MN with an increase in altitude. (See Q: Describe Mach number? page 122[...
What is a fuel injection system, and what are its advantages and disadvantages?
A fuel injection system delivers metered fuel directly into the induction manifold and then into the combustion chamber (or cylinder of a piston engine) without using a carburetor. Normally, a fuel control unit (FCU) is used to deliver metered fuel to the fuel manifold unit (fuel distributor). From here, a separate fuel line carries fuel to the discharge nozzle in each combustion chamber (or cylinder head in a piston engine, or into the inlet port prior to the inlet valve). With fuel injection, a separate fuel line can provide a correct mixture.
Describe an engine hot start and its causes, indications, and actions.
A hot start is one in which the engine ignites and reaches self-sustaining rpms, but the combustion is unstable and the exhaust gas temperature (EGT) rises rapidly past its maximum limit. Causes of a hot start include 1. Overfueling (throttle open) 2. Air intake/exhaust blocked 3. Tailwind, causing the compressor to run backward 4. Seized engine, e.g., ice blockage Indication of a hot start is an EGT rising rapidly toward its maximum limit. Actions required for a hot start are 1. Close fuel lever/stop fuel delivery before the EGT limit has been reached. 2. When the engine rpms have slowed to the reengagement speed, motor over the engine to blow out the fuel (approximately 60 seconds).
Describe an engine hung start and its causes, indications, and actions.
A hung start occurs when the engine ignites but does not reach its self-sustaining rpms. (Self-sustaining speed is an rpm engine speed at and above which the engine can accelerate on its own without the aid of the starter motor.) The cause of a hung start is insufficient airflow to support combustion due to the compressor not supplying enough air because of one or a combination of the following, but not restricted too: 1. High altitude, low-density air 2. Hot conditions, low-density air 3. Inefficient compression 4. Low starter rpms Indications of a hung start include 1. High exhaust gas temperature (EGT), above normal 2. Engine rpm below normal self-sustaining speed Actions required for a hung start are 1. Close fuel lever/stop fuel delivery. 2. Motor over the engine to blow out the fuel (for approximately 60 seconds) Note: To gain a successful start in hot and high conditions, you have to introduce more air into the engine. Adjusting the fuel supply does not help. Increasing fuel = rpm decreases & EGT increases Decreasing fuel = rpm increases & EGT decreases
What is a jet engine surge, what causes it, and what are the indications?
A surge is the reversal of airflow through an engine, where the high-pressure air in the combustion chamber is expelled forward through the compressors, with a loud bang and a resulting loss of engine thrust. A surge is caused when 1. All the compressor stages have stalled, e.g., bunt negative-g maneuver. 2. An excessive fuel flow creates a high pressure in the rear of the engine. The engine will then demand a pressure rise from the compressors to maintain its equilibrium, but when the pressure rise demanded is greater than the compressor blades can sustain, a surge occurs, creating an instantaneous breakdown of the flow through the machine. A surge is indicated by 1. Total loss of thrust. 2. A large increase in TGT. The required actions in response to an engine surge are 1. Close the throttles smoothly and slowly. 2. Adjust the aircraft's attitude to unstall the engines, which lead to the surge. 3. Slowly and smoothly reopen the throttles.
Describe a triple-spool turbofan engine, e.g., the RB211, and its advantages.
A triple-spool turbofan engine such as the Rolls Royce RB211 is a further development of the fan engine that has two distinct differences from the twin-spool fan engine (see Q: Describe the fan engine and its advantages, page 66): 1. The triple-spool turbofan engine has three independent compressor spools: N1, the low-pressure compressor spool or fan N2, the intermediate-pressure compressor N3, the high-pressure compressor spool and they are each driven by their own turbine and connecting shafts. 2. The front turbofan, or N1 low-pressure compressor spool, is not connected to any other compression stages. The turbofan on a triple-spool engine is further improved because it is not restricted to the size of other compressor spools (as it is on a twin-spool engine) and it is driven at its optimal speed by its own turbine. This allows it to have a larger frontal area that consists mainly of a giant ring of large blades, which act more like a shrouded prop than a fan. It is responsible for producing an even larger bypass ratio (i.e., 5:1), which generates approximately 75 percent of the engine's thrust in the form of bypass[...]
Describe a typical aircraft fire detection and protection system.
A typical engine fire detection and protection system would consist of the following: 1. Overheat and fire detection loop(s) with multisensors. Note that normally a minimum of two separate systems (loops) exist per unit. These are coupled with visual (lights) and aural indications for both overheat and fire detected conditions for a. Each engine b. Auxiliary power units (APUs) c. Wheel wells (not on all aircraft types) 2. Fault monitoring system of overheat and fire detection systems. This is coupled with visual indications for a. Engines b. APUs 3. Fire extinguishers and firing circuits for a. All engines. Note that usually there are a minimum of two extinguisher bottles that can fire into each engine so as to provide a second extinguishing supply to all engines. This is coupled with visual indications of bottle discharge. b. APUs. Note that usually there is a separate single fire extinguisher bottle. This is coupled with visual indications of bottle discharge. 4. Testing facility of a. Firing circuits for (1) Engine fire extinguisher(s) (2) APU fire extinguisher b. Fault monitoring system c. Overheat/fire detection loops for (1) Each engine (2) APUs 5. Lavatory/cargo holds smoke detection system."
Explain a venturi.
A venturi is a practical application of Bernoulli's theorem, sometimes called a convergent/divergent duct. A venturi tube has an inlet that narrows to a throat, forming a converging duct and resulting in (1) velocity increasing, pressure (static) decreasing, and (3) temperature decreasing. The outlet section is relatively longer with an increasing diameter, forming a diverging duct and resulting in (1) velocity decreasing, (2) pressure (static) increasing, and (3) temperature increasing. For a flow of air to remain streamlined, the mass flow through a venturi must remain constant. To do this and still pass through the reduced cross section of the venturi throat, the speed of flow through the throat must be increased. In accordance with Bernoulli's theorem, this brings about an accompanying drop in pressure and temperature. As the venturi becomes a divergent duct, the speed reduces, and thus the pressure and temperature increase.
What is an engine windmill start, and when is it used?
A windmill start occurs when the engine is started without the aid of the starter because the compressors are being turned by a natural airflow when airborne. This delivers the air charge to the combustion chambers, where fuel and an ignition spark are introduced as normal for a stable engine relight. This effect is known as windmilling, and as such, windmill starts are used to relight an engine when airborne.
Describe an engine wet start and its causes, indications, and actions.
An engine wet start is otherwise known as a failure to start after the fuel has been delivered to the engine. The cause of a wet start normally is an ignition problem. Indications of a wet start are 1. Exhaust gas temperature (EGT) does not rise. 2. Revolutions per minute (rpms) stabilize at starter maximum. Actions required for a wet start include 1. Close the fuel lever/supply as soon as a wet start is diagnosed (usually at the end of the starter cycle). 2. Motor over the engine to blow out the fuel (approximately 60 seconds).
When and where is a jet/gas turbine (bypass) engine at its most efficient, and why?
At high altitudes and high rpm speeds.
Why do gas turbine engines have auto igniters, and how do they work?
Auto igniters are used in gas turbine engines to protect against disturbed/turbulent airflow upsetting the engine. This condition is particularly common with rear-mounted engines during some abnormal and even some rather normal flight maneuvers because rear-mounted engines are placed ideally to catch any disturbed airflow generated by the wing when the airflow pattern brakes down as a result of either a high incidence of attack (e.g., prestall buffet), high-g maneuvers (e.g., steep turns), or high Mach number effect. Auto igniters work by sensing a particular value of incidence of the aircraft, via the incidence-sensing (probe) system (which is also used to activate the stick shaker and pusher), and automatically signals on the ignition system before the disturbed airflow generated by the wing affects the engines, thus ensuring that the engines at least continue to run, although in some cases they might surge a little.
Explain Bernoulli's theorem
Bernoulli's theorem is that the total energy in a moving fluid or gas is made up of three forms of energy: 1. Potential energy (the energy due to the position) 2. Pressure/temperature energy (the energy due to the pressure) 3. Kinetic energy (the energy due to the movement) When considering the flow of air, the potential energy can be ignored; therefore, for practical purposes, it can be said that the kinetic energy plus the pressure/temperature energy of a smooth flow of air is always constant. Thus, if the kinetic energy is increased, the pressure/temperature energy drops proportionally, and vice versa, so as to keep the total energy constant. This is Bernoulli's theorem.
Why are bleed valves fitted to gas turbine engines?
Bleed valves are fitted to gas turbine engines for two main reasons: 1. To provide bleed (tap) air for auxiliary systems. For example: a. Air-conditioning and cabin heating/pressurization/EFIS cooling/cargo heating b. Engine cooling, especially (1) The combustion chamber (2) The turbine section c. Accessory cooling (generator, gearbox, and other engine-driven systems) d. Engine and wing anti-icing systems 2. To regulate the correct airflow pressures between different engine sections."
What is bypass ratio?
Bypass ratio in an early single- or twin-spool bypass engine is the ratio of the cool air mass flow passed through the bypass duct to the air mass flow passed through the high-pressure system. Typically, this early evolution of the bypass engine has a low bypass ratio, i.e., 1:1. Figure 2.7 Bypass airflow for a single- or twin-spool gas turbine engine. (Reproduced with kind permission of Rolls-Royce plc.) Alternatively, bypass ratio for a fan-ducted bypass engines is the ratio of the total airmass flow through the fan stage to the airmass flow that passes through the turbine section/high-pressure (engine core) system. A high bypass ratio, i.e., 5:1, is usually common with ducted fan engines.
What are the limitations of a variable/reduced thrust (flex) takeoff?
Clearly, a variable/reduced (flex) thrust takeoff can only be used when full takeoff thrust is not required to meet the various performance requirements on the takeoff and initial climb-out. Therefore, the limitations of using a reduced thrust takeoff are 1. Not maximum takeoff weight limited by a. Takeoff field length b. Takeoff weight-altitude-temperature (WAT) curve (engine-out climb gradient at takeoff thrust) c. Net takeoff flight path (engine-out obstacle clearance) 2. Maximum outside air temperature (OAT) limitation Where the proposed takeoff weight is such that none of the preceding considerations are limiting, then the takeoff thrust may be reduced until one of the considerations listed below becomes limiting. Note: Based upon one engine inoperative. 3. Furthermore, reduced (flex) thrust takeoffs are limited by a. The reduced (flex) thrust must not be reduced by more than a set amount (engine specific), e.g., 25 percent below the maximum takeoff weight full-rated takeoff thrust. Note: This is normally restricted by a maximum flex or assumed temperature constraint, i.e., Tmax flex. That is, Flex temperature < Tmax b. The reduced (flex) temperature cannot be lower than TREF (flat rating cutoff temperature that guarantees a constant rate of thrust[...]
What causes a jet/gas turbine upset, and how do you correct it?
Disturbed or turbulent airflow will cause a jet/gas turbine engine to be upset and to stall. This occurs because a jet/gas turbine engine is designed to operate using a clean uniform airflow pattern that it obtains within the aircraft's normal operating attitude. However, beyond the aircraft's normal angles of incidence and slip and/or in extremely severe weather turbulence, the engines can experience a variation in the ingested air's pressure/density, volume, angle of attack, and velocity properties. This changes the incidence of the air onto the compressor blades, causing the airflow over the blades to break down and/or inducing aerodynamic vibration. This upsets the operation of the engine causing it to stall. The stall can be identified by (1) increases in total gas temperature (TGT), (2) engine vibration, and (3) rpm fluctuations.
What is exhaust gas temperature (EGT), and why is it an important engine parameter?
EGT is exhaust gas temperature and is an important engine parameter because it is a measure/indication of the temperatures being experienced by the turbine. The only real operating threat to the engine's life is excessive turbine temperatures. The maximum temperature at the turbine is critical because if the EGT limit is exceeded grossly on startup (hot start), the excessive temperatures will damage the engine, especially the turbine blades. Also, if the cruise EGT limit is exceeded slightly for a prolonged period, this will shorten the engine's life.
What is engine pressure ratio (EPR)?
Engine pressure ratio (EPR) is the ratio of air pressure measurements taken from two or three different engine probes and displayed on the EPR gauge for the pilot to use as a parameter for setting engine thrust. The EPR reading is the primary engine thrust instrument, with the temperature of the turbine stage governing the engine's maximum attainable thrust. Normally, EPR on a gas turbine-powered aircraft is a ratio measurement of the jet pipe pressure to compressor inlet ambient pressure or sometimes the maximum compressor cycle pressure to compressor inlet ambient pressure. However, on a fan engine, the EPR is normally a more complex ratio measurement of an integrated turbine discharge and fan outlet pressure to compressor inlet pressure.
What is FADEC?
FADEC is full authority digital engine control and is a system that automatically controls engine functions, i.e., start procedures, engine monitoring, fuel flow, ignition system, and power levels required. FADEC computers can be found on the A320 aircraft's engine and on the EJ200 engines used on military aircraft.
What happens to engine pressure ratio (EPR) on the takeoff roll?
For the purpose of this question, let us assume that EPR is a measure of the jet pipe exhaust pressure (P7) against compressor inlet ambient pressure (P2). Prior to opening the throttle levers, the EPR reading will be very low, if not a 1:1 ratio. Now, when the throttle levers are advanced, the EPR reading will decrease initially because the P2 pressure increases against a constant or slowly increasing P7 value; therefore, the ratio decreases before steadily increasing to its takeoff setting. There are two reasons for this. First, the engine compressor, combustion, and turbine stages are in series, and this leads the engine to suffer from a slow response (or lag) to a throttle input. This is so because the greater amount of air induced into the engine takes time to move through the compressor, combustion, and turbine stages before it is expelled from the engine with a resultant/reaction forward force. Second, a consequence of the engine's slow response rate or lag is its effect on the EPR reading because the reading of the (P7) jet pipe exhaust pressure is taken from the rear of the engine and the (P2) compressor inlet pressure reading is taken[...]
What is the theory of a jet/gas turbine engine?
Frank Whittle described the theory behind the jet engine as the balloon theory: "When you let air out of a balloon, a reaction propels the balloon in the opposite direction." This, of course, is a practical application of Newton's third law of motion. A jet/gas turbine produces thrust in a similar way to the piston engine/propeller combination by propelling the aircraft forward as a result of thrusting a large weight of air rearward. Thrust = air mass × velocity Early jet engines adopted the principle of taking a small mass of air and expelling it at an extremely high velocity. Later gas turbine engines have evolved into taking and producing a large mass of air and expelling it at a relatively slow velocity (e.g., high-bypass engine).
Why was the jet/gas turbine engine invented?
Frank Whittle invented the jet aircraft engine as a means of increasing an aircraft's attainable altitude, airspeed, reliability, and, to a lesser extent, maneuverability for the military. Frank Whittle designed the jet/gas turbine engine for two main reasons: 1. To achieve higher altitudes and thus airspeed because propeller aircraft had limited altitude and speed capabilities. 2. As a more simplistic and therefore reliable engine because the piston engine was a very complicated engine with many moving parts and thus was unreliable.
How is jet/gas turbine engine noise controlled or reduced?
Jet/gas turbine engine noise can be reduced by the following: 1. Bypass engines. This reduces the sheer effect between the displaced slower bypass engine air and the ambient air that reduces noise. 2. Reduced-thrust takeoff. Jet/gas turbine engine noise can be controlled by the following: 3. Maximum-angle climb after takeoff. This allows the aircraft to get above any noise-control zones.
Describe maximum continuous thrust.
Maximum continuous thrust is simply the maximum permissible engine thrust setting for continuous use, expressed either as an N1 or engine pressure ratio (EPR) figure.
Describe maximum takeoff thrust and its limitations
Maximum takeoff thrust is simply the maximum permissible engine thrust setting for takeoff, expressed either as an N1 or engine pressure ratio (EPR) figure. Maximum takeoff thrust is the highest thrust setting of the aircraft's engine when the highest operating loads are placed on the engine. However, as a protection to the engine, maximum takeoff thrust settings have a time limit on their use, namely, 5 minutes for all engines working and 10 minutes with an engine failure. Note: Some authorities allow a 10-minute time limit with all engines operating.
What are the indications of thermal expansion and use of the fire bottles on the side of the aircraft fuselage?
Separate disks, i.e., one for extinguishent release due to thermal expansion and one to indicate use, normally are found on the side of the aircraft's fuselage. If they are intact, they indicate that the extinguishent is still in the fire bottle. Note: The color and location of such disks are aircraft type-specific.
What is specific fuel consumption (SFC)?
Specific fuel consumption is the quantity/weight (lb) of fuel consumed per hour divided by the thrust of an engine in pounds:
Why does engine pressure ratio (EPR) need to be set by 40 to 80 knots on the takeoff role?
The EPR for takeoff has to be set by 40 to 80 knots (the exact speed is type-specific) for the following main reasons: 1. So that the pilot is not chasing rpm needles on the takeoff roll. 2. To ensure an adequate aircraft acceleration so that the performance-calculated V1 and VR speeds are achieved by the takeoff run required (TORR) rotate point for the given aircraft weight and ambient conditions.
What advantages does a jet-engined aircraft gain from flying at a high altitude?
The advantages a jet engine gains from flying at high altitudes are 1. Best specific fuel consumption (SFC)/increased (maximum) endurance Note: Endurance is the need to stay airborne for as long a time as possible for a given quantity of fuel. Therefore, the lowest SFC in terms of pounds of fuel per hour is required. 2. Higher true airspeed (TAS) for a constant indicated airspeed (IAS), providing an increased (maximum) attainable range Note: Maximum attainable range is the greatest distance over the ground flown for a given quantity of fuel, or the maximum air miles per gallon of fuel. 1. Best SFC and thus increased (maximum) endurance are achieved at high altitudes because of two effects: a. Minimum cruise drag is experienced at high altitudes because the Mach number (MN) speed becomes limiting above approximately 26,000 ft, and for a constant MN (as is the normal operating practice), the TAS and equivalent airspeed (EAS) decrease with altitude, and drag varies only with EAS. As such, the EAS is reduced progressively to a level closer to the aircraft's best endurance speed, the higher the altitude, which is obviously where drag is least. Note: Minimum drag speed (VIMD), broadly speaking[...]
What are the advantages of a wide-chord fan engine?
The advantages of a wide-chord fan engine are better fuel economy, more thrust, and less weight and noise. A wide-chord fan engine is a term used to describe a modern turbofan jet engine having a ducted fan with specific blade geometry, namely wider blades. This technology was pioneered by Rolls Royce in the 1970s. Designers refined the blade design by making the blade chord wider, altering the blade geometry, manufacturing them with hollow cross-sections, and by using lighter materials, such as titanium, to extract more thrust for any given fan area.
What is the combustion cycle of a jet/gas turbine engine?
The combustion cycle of a jet/gas turbine engine is induction, compression, combustion, expansion, and exhaust. In a jet/gas turbine engine, combustion occurs at a constant pressure, whereas in a piston engine, it occurs at a constant volume.
What is the compression ratio of a gas turbine engine?
The compression ratio of a gas turbine engine is a ratio measure of the change in air pressure between the inlet and outlet parts of either an individual compressor stage or the complete compressor section of the engine. Individual compressors, either centrifugal or axial-flow types, are placed in series so that the power compression ratio accumulates. For example:
Describe the fan engine and its advantages.
The fan engine can be regarded as an extension of the bypass engine principle (see Q: What is bypass ratio? page 66) with the difference that it discharges its cold bypass airflow and hot engine core airflow separately. The turbine-driven fan is in fact a low-pressure axial-flow compressor that provides additional thrust. Normally, the fan is mounted on the front of the engine and is surrounded by ducting that controls the high supersonic airflow speeds experienced at the blade tips, preventing them from suffering from compressibility effect losses. The fan is either coupled to the front of a number of core compression stages (twin spool engine), which restricts the width size of the fan, and its bypass air is ducted overboard at the rear of the engine through long ducts, or it is mounted on a separate shaft driven by its own turbine (triple spool engine) where the bypass airstream is ducted overboard directly behind the fan through short ducts, hence the term ducted fan. For example, the CFM56-3 is a twin spool fan engine, and the Rolls Royce RB211 is a triple spool fan engine. The fan design reflects the specific requirements of the[...]
Why is a fan engine flat rated?
The fan engine is flat rated to give it the widest possible range of operation, keeping within its defined structural limits, especially in dense air. Note: Flat rating guarantees a constant rate of thrust up to a fixed temperature, namely, the warmest temperature at which the engine can produce its maximum-rated thrust. This temperature usually corresponds to the performance TREF temperature.
What fuels are used commonly for civil jet aircraft?
The fuels used for gas turbine civil aircraft engines are 1. Jet A1 (Avtar). This is a kerosene-type of fuel with a normal specific gravity (SG) of 0.8 at 15°C. It has a medium flash point and calorific value, a boiling range of between 150 and 300°C, and a waxing point of -50°C. 2. Jet A. This is similar to A1, but its freezing point is only -40°C. Note: Jet A normally is available only in the United States.
Describe how a jet/gas turbine engine works.
The jet engine or aerothermodynamic duct, to give it its real name, has no major rotating parts and consists of a duct with a divergent entry and convergent or convergent/divergent exit. When forward motion is imparted to it from an external source, air is forced into the engine intake, where it loses velocity or kinetic energy and therefore increases its pressure energy as it passes through the divergent duct. The total energy is then increased by the combustion of fuel, and the expanding gases accelerate to atmosphere through the outlet converging duct, thereby producing a propulsive jet.
What are the main engine instruments?
The main primary engine instruments usually are 1. Engine pressure ratio (EPR) gauge (thrust measurement) 2. N1 gauge (low compressor rpms) 3. Exhaust gas temperature (EGT) or total gas temperature (TGT) (engine temperature) Other possible primary engine instruments are 4. N2 gauge (intermediate compressor rpms) 5. Fuel flow (fuel flow indicator) Secondary engine instruments usually are 6. Oil temperature gauge, pressure gauge, and quantity gauge 7. Engine vibration meter
How do jet/gas turbine engines generate noise?
The noise generated by a jet/gas turbine engine is from the sheer effect of different displaced air velocities. The sheer is the difference between the jet's faster displaced air and the slower ambient air around it.
What is the principle of the bypass engine?
The principle of the bypass engine is an extension of the gas turbine engine that permits the use of higher turbine temperatures to increase thrust without a corresponding increase in jet velocity by increasing the air mass/volume intake and discharge to atmosphere via the bypass ducts. Remember, Thrust = air mass × velocity The bypass engine involves a division or separation of the airflow. Conventionally, all the air entering into the engine is given an initial low compression, and a percentage is then ducted to bypass the engine core. The remainder of the air is delivered to the combustion system in the usual manner. The bypass air is then either mixed with the hot airflow from the engine core in the jet pipe exhaust or immediately after it has been discharged to atmosphere to generate a resulting forward thrust force. The term bypass normally is restricted to engines that mix the hot and cold airflow as a combined exhaust gas. This improves (1) propulsive efficiency and (2) specific fuel consumption and (3) reduces engine noise (this is due to the bypass air lessening the shear effect of the air exhausted through the engine core)."
What is the purpose of engine relight boundaries?
The purpose of engine relight boundaries is to ensure that the correct proportion of air is delivered to the engine's combustion chamber to restart the engine in flight. For this reason, the aircraft's flight manual outlines the approved relight envelope of airspeed against height. This ensures that within the limits of the envelope, the airflow ingested into the engine will rotate the compressor at a speed that generates and delivers a sufficient volume of air into the combustion chamber to relight the engine successfully. Note: The approved flight envelope usually will be subdivided into starter assist and windmill boundaries.
Explain the jet/gas turbine engine's thrust-to-thrust lever position.
The thrust lever produces more engine thrust from its movement near the top of its range than the bottom. An engine's operating cycle and gas flow are designed to be at their most efficient at a high-rpm speed, where it is designed to spend most of its life. (See Q: When and where is a jet/gas turbine engine at its most efficient and why? page 69.) Therefore, as rpms rise, mass flow, temperature, and compressor efficiency all increase, and as a result, more thrust is produced, say, per 100 rpm, near the top of the thrust lever range than near the bottom. In practical terms, this translates to differing thrust output per inch of thrust lever movement; i.e., at low-rpm speed (near the bottom of its range), an inch movement of the thrust lever could produce only 600 lb of thrust, but at a high-rpm speed (near the top of its range), an inch movement of the thrust lever typically could produce 6000 lb of thrust. For this reason, if more power is required at a low thrust lever setting, then a relatively large movement/opening of the thrust levers is required, i.e., when initiating[...]
Why do you use variable/reduced thrust (flex) takeoffs in a jet aircraft?
There are two main reasons for using a variable/reduced thrust takeoff: 1. To protect engine life and to improve engine reliability. Variable/reduced thrust takeoffs reduce the stress and attrition of the engine during the takeoff period when the highest loads are placed on the engine. 2. To reduce the noise generated by the aircraft. (Noise suppression of this type normally is used for takeoff and occasionally on approaches over noise abatement areas, as well as for nighttime flying noise restriction.)
Is there a critical engine on a jet/gas turbine aircraft (nonpropeller driven)?
There is no critical gas turbine engine if the engines are positioned symmetrically with opposing revolution direction. The term critical engine is historically associated with propeller-driven aircraft whose mechanically driven propellers were not symmetrical (in particular blade effect) and gave rise to different thrust line and moment arm values. Therefore if the jet/gas turbine engines are positioned symmetrically with opposing revolution direction, then they have no design in balance and therefore no "mechanical" critical engine can exist. However, aerodynamically, there can be a critical "failure" where a crosswind exists, or if you like a critical failure engine. Then yes, there is a critical engine, but technically this is an incorrect term, as it is a function of environmental properties, i.e., crosswind effect, rather than the mechanical properties of the engine. Hence the term critical failure is more appropriate. (See Q: How does a crosswind affect the critical engine/failure? page 60.) Note: There can be a governor engine, i.e., an engine that is the master that sets the rpm speed for the others. But this is not a critical engine and should not be confused as such.
What is thermodynamics?
Thermodynamics is the study of heat/pressure energy or the behavior of gases (including air) and vapors under variations of temperature and pressure.
What are thrust reverses, and how do they work?
Thrust reverses on jet/gas turbine engine reverse the airflow forward, thereby creating a breaking action. There are two types of thrust reverses: (1) blockers or bucket design and (2) reverse flow through the cascade vane.
Why is the risk per flight decreased with a reduced-thrust takeoff?
When a reduced thrust is used for takeoff, the risk per flight is decreased because of the following main reasons: 1. The assumed/flexible temperature method of reducing thrust to match the takeoff weight does so at a constant thrust-weight ratio, making the actual takeoff distance and takeoff run distance from the reduced-thrust setting less than that at full thrust and full weight by approximately 1 percent for every 3°C that the actual temperature is below the assumed temperature. 2. The acceleration-stop distance is further improved by the increased effectiveness of full-reverse thrust at the lower temperature. 3. The continued takeoff after engine failure is protected by the ability to restore full power on the operative engine.
Can a maximum takeoff weight aircraft use a reduced takeoff technique?
Yes, a reduced-thrust takeoff can be used even when an aircraft is at its maximum takeoff structural weight, providing the TOR/D is not limiting. This is so because you can trade momentum gained from a longer TOR/D to achieve the V1 and VR speeds at the performance-limiting conditions for a lower thrust setting.
What is a variable/reduced thrust takeoff (flex)?
variable/reduced thrust takeoff uses the takeoff thrust (EPR/N1) required for the aircraft's actual takeoff weight, which is a reduced thrust value from the maximum takeoff weight thrust value that meets the aircraft's takeoff and climb performance requirements with one engine inoperative. The full takeoff thrust is calculated against an aircraft's performance-limited (either field length, WAT, tire or net flight path, obstacle clearance climb profile) maximum permissible takeoff weight and not its actual takeoff weight. The reduced takeoff thrust is the correct thrust setting for the aircraft's actual takeoff weight that achieves the aircraft's takeoff and climb, one engine inoperative, performance requirements. A reduced/variable takeoff thrust is calculated by using the assumed/flexible temperature performance technique (see Q: What is an assumed/flexible temperature? page 201). Using this variable takeoff engine pressure ratio (EPR) means that the aircraft is now operating at or near a performance-limiting condition, whereby following an engine failure the whole takeoff would still be good enough in terms of performance. In fact, a lower weight aircraft using a variable thrust will mirror the takeoff run/profile of a maximum takeoff weight aircraft using a full thrust setting. Note: A function of[...]
