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1
Q

Applied Laws

What is thermodynamics?

A

Thermodynamics is the study of heat/pressure Energy or the behavior of gases (including air) and vapors under variations of temperature and pressure.

2
Q

Applied Laws

Explain Bernoulli’s theorem.

A

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.

3
Q

Applied Laws

Explain a venturi.

A

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,

     (2) 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.

4
Q

Piston Engines

What is the combustion cycle of an aeropiston engine?

A

Induction, compression, combustion (expansion), and exhaust.

The combustion of a piston engine occurs at a constant volume.

5
Q

Piston Engines

What is compression ratio in a piston Engine?

A

Compression ratio in a piston engine is the ratio of the total volume enclosed in a cylinder with the piston at bottom dead center (BDC) to the volume remaining at the end of the compression stroke with the piston at top dead center (TDC).
CR = BDC/TDC

6
Q

Piston Engines

What produces the ignition in a piston engine?

A

Magnetos are used on internal combustion engines to supply the high-tension voltage necessary to cause an electric spark at the spark plug.

7
Q

Piston Engines

What does blue, black, and white exhaust smoke indicate?

A

Blue exhaust smoke - indicates an oil burn in the cylinders, probably due to broken piston rings that allow oil seepage into the combustion chamber.

Black exhaust smoke - indicates carbon granules burning in the cylinders.

This occurs if the mixture is too rich, resulting in some of the fuel not being burnt and turning into carbon granules, which are then exhausted as black smoke.

White exhaust smoke indicates a high
water content in the combustion chamber, which is exhausted as white “steam” smoke.

8
Q

Piston Engines

What is engine torque?

A

Torque is a force causing rotation.

In particular, torque is the force created within an engine, which causes rotation of the rotating parts, e.g., the crankshaft.
Torque is a measure of the load experience, expressed in pounds per
inch or feet.

A quantity of torque, or twisting moment, is involved in the measurement of the engine brake horsepower (bhp).

Torque = force X distance (at right angles to the force)

9
Q

Piston Engines

What is a normally aspirated piston engine?

A

A normally aspirated engine works as a result of “the breathing of the cylinder is due to the pressure differential below the standard sea level 14.7 Ib/in2.”

In other words, it only uses the atmospheric air density that is available to produce a charge in its cylinders and is not boosted by a supercharger; therefore, the normally aspirated piston engine’s power output is restricted by its cylinder capacity.

10
Q

Piston Engines

What are the disadvantages of a piston engine?

A

A piston engine suffers from three main disadvantages:

  1. A lack of power output, especially with increased altitudes
 2. A low produced airspeed due to propeller rpm limitations
 3. Mechanical inefficiency
11
Q

Piston Engines

What is a supercharged (piston) engine?

A
A supercharger simply increases the air delivered to the engine cylinder above its normally aspirated capacity by compressing the intake air, which in turn requires more fuel to be delivered to the carburetor to maintain the correct mixture ratio, which in turn produces a greater
power output (horsepower).

Therefore, a supercharged engine is capable of producing a greater power output than a normally aspirated engine
of the same cylinder size.

12
Q

Piston Engines

How is the (piston) engine power output increased to compensate for low atmospheric pressure?

A

Superchargers are used to artificially raise the engine manifold pressure to compensate for low atmospheric pressure either
(1 ) to increase engine power output for takeoff and the initial climb (ground-boosted engine) or
(2) to maintain mean sea level (MSL) engine power at high altitudes (altitude-boosted engine).

13
Q

Piston Engines

What regulates the supercharger to deliver a constant boost/manifold absolute pressure (MAP)?

A

The auto boost control (ABC) keeps the boost pressure constant
during a climb or a descent.

14
Q

Piston Engines

How is engine power monitored?

A

There are two main engine power monitoring/indication systems:

(1 ) - manifold absolute pressure (MAP) and…
(2) - boost pressure.

15
Q

Piston Engines

What is carburetor icing?

A

Ice formation can occur in the engine induction system and in the carburetor
of piston engines, particularly in the venturi and around the throttle valve, where acceleration of the air can produce a temperature fall by as much as 25°C.

This, combined with the heat absorbed as the fuel evaporates, can cause serious icing, even when there is no visible moisture present.

Such a buildup of ice in a carburetor can disturb or even prevent the flow of air and fuel into the engine manifold, causing
it to lose power, run roughly, and even to stop the engine in extreme circumstances.

This effect is called carburetor icing.
Throttle icing (i.e., around the throttle valve) is more likely to occur at
low power settings (e.g., descents), when the partially closed butterfly
creates its own venturi cooling effect.

16
Q

Piston Engines

When would you expect carburetor icing in a piston engine?

A

Carburetor icing should be expected in a piston engine when the outside air temperature (OAT) is between -1O°C to +30°C with a high humidity and/or visible moisture present in the air.

However, carburetor / throttle icing is most likely to occur between + 10 and + 15°C with a relative humidity greater than 40 percent.

Note: Carburetor icing can be found on a warm day in moist air, especially with descent power settings.

17
Q

Piston Engines

What actions should you take to prevent or remove carburetor/throttle icing?

A

You should use the carburetor heat system (hot air) at regular intervals
to treat icing in the carburetor when in carburetor icing conditions.

The carb heat system delivers hot air from the engine compartment into the carburetor that melts the buildup of ice.

Note: It is also advisable to apply carb heat at the start of a descent to protect against throttle icing occurring and thereby ensuring that full power is available in the event of a go-around engine application.

However, carb heat always should be switched off before the throttle is advance to ensure maximum thrust delivery, and therefore, it should
be off in the final landing approach in anticipation of an emergency go around
near to the ground.

However, the use of carburetor heat should be avoided when the OAT is colder than -1OC because by applying carburetor heat you raise the air temperature in the carburetor into the icing temperature range, i.e., -1OC to +30C.

18
Q

Propellers

What advantages does an aircraft gain from a propeller?

A

The propeller provides the following advantages:
1. The propeller creates a high-energy slipstream, which has three
main effects on the aircraft:
a. The slipstream creates extra lift over the wing.
b. The slipstream suppresses the stall speed of the aircraft.
c. The slipstream makes the fin/rudder more effective.
2. The propeller/piston engine has a quick response rate to a throttle input, which gives an early application of the slipstream effects on the aircraft.

        Therefore, a propeller-driven aircraft has good slow-speed recovery.
19
Q

Propellers

What is the main disadvantage of a propeller?

A

A propeller-driven aircraft suffers from a lack of airspeed due to propeller rpm limitations as a result of propeller compressibility losses.

This is so because the propeller suffers the effects of compressibility when the speed of the propeller blade tips becomes sonic.

Therefore, propellers have a revolution speed limited to just below sonic speed,
which limits the thrust force produced by propellers and therefore the aircraft’s airspeed.

This results in…
(1 ) a lower true airspeed (TAS) and
(2) a shorter range.

20
Q

Propellers

What produces thrust on a propeller-driven aircraft?

A

The forward thrust (horizontal lift) of a propeller-driven aircraft is a result of Newton’s third law. “The application of a force on a body will cause an equal and opposite reaction.”

The propeller drives an accelerated
mass of air rearward, and the thrust force developed by the propeller is equal to the mass of the air and the rate of change in
momentum given to the air, which has the reaction of driving the aircraft forward.

The amount of the force created by the propeller is governed by two factors:

     1. The output of the engine, which drives the prop shaft.
    2. The blade angle, angle of attack, and pitch of the propeller.
21
Q

Propellers

What restrictions does the propeller design have?

A

The degree to which the propeller design can be increased to absorb engine power is restricted in the following areas:

  1. Blade length is restricted because of the following: 
        a. The need for adequate ground and fuselage clearance.
        b. The need to maintain subsonic blade-tip speeds (the longer the propeller blade, the greater are the tip speeds, and supersonic propeller speeds are inefficient).

  2. Blade chord size is restricted because of the following:
       a. An increase in the chord size will reduce the aspect ratio (blade diameter/chord), and as with the wing, this gives a lower efficiency output than the optimal ratio.
      b. Larger chord lengths also increase the centrifugal twisting moment, which tends to twist the blade to a finer pitch, causing...
            (1 ) Higher loads on the root fittings.
            (2) Higher torque values, which give a lower resultant force / lower thrust output (forward thrust force minus torque force equals resultant force).

 3. The number of blades can be increased to a certain value, but with more blades, the hub diameter and weight become excessive, and blade interference begins to reduce efficiency.
22
Q

Propellers

How does the propeller convert engine horsepower to produce thrust?

A

The propeller is connected to the engine via a prop shaft that rotates the propeller, which generates an accelerated mass of air rearward, thereby converting the shaft horsepower of the engine into a thrust force.

Thrust force = air mass x velocity

Air mass is determined by
( 1) blade angle,
(2) blade angle of attack and
(3) pitch.

Air velocity is determined by propeller rpm, which is set by
( 1) engine power output and
(2) blade angle of attack.

23
Q

Propellers

Why is the propeller blade twisted?

A

The propeller blade is twisted along its length to maintain a constant blade angle of attack.

24
Q

Propellers

How do you define propeller efficiency?

A

The propeller efficiency in producing thrust to propel an aircraft forward is determined as the ratio of the useful work done by the propeller (propeller thrust) in moving the aircraft to the work supplied by the engine (engine bhp).

Prop Efficency = Prop Thrust (Air-Mass x Velocity) / Engine BHP

25
Q

Propellers

What are the disadvantages of a fixed-pitch propeller?ngle of attack,

A

The failing of a fixed-pitch propeller is that it only produces its maximum
efficiency (i.e., best blade angle/pitch for rpm speed) at one predetermined
engine rpm, altitude, and forward airspeed condition.

This is so because the forward airspeed affects the blade angle of attack (an increase in forward airspeed causes a decrease in blade angle of attack,which reduces the rearward air displacement and thereby decreases the thrust produced), and therefore, propeller efficiency is reduced away from its single predetermined condition.

(See Q: How do you define propeller efficiency? page 56.)

Therefore, for a fixed-pitch propeller, a single setting must be made between the blade angle (and therefore the angle of attack and pitch) desirable for takeoff and that required for cruise conditions.

And if a blade angle for high-speed cruise is chosen (coarse), then the aircraft
has the disadvantage of poor propeller performance for takeoff, and vice versa.

(See Q: What is a variable-pitch propeller, and why is it used? page 57.)

26
Q

Propellers

What is a variable-pitch propeller, and why is it used?

A

Variable-pitch propellers are a development of the fixed-pitch propeller
that have a variable and controllable blade angle between the coarse and fine positions.

This is used to adjust the blade angle of
attack to its optimal setting in order to maintain propeller efficiency and aircraft thrust over a wide range of aircraft speeds, phases of flight, and differing operating conditions.

A variable-pitch propeller maximizes the propeller’s efficiency through a large speed range by maintaining a constant blade angle of attack that thereby produces a constant thrust value.

27
Q

Propellers

What controls the propeller blade angle/speed?

A

A constant-speed unit (CSU) (sometimes called a governor unit) controls the
propeller’s blade angle/pitch to maintain an rpm speed.

28
Q

Propellers

Why is a turboprop aircraft better suited for short regional operations?

A

Turboprop aircraft in general are better suited for short regional operations
because short sector routes normally have a restricted cruise altitude, which the turboprop aircraft is normally better suited to for the following reasons:
1. A jet engine is suited to high altitudes (i.e., 30,000 ft and above),
which an aircraft is not capable of reaching during a short sector route.

        2. The turboprop engine is designed to operate at its most efficient at a medium altitude, which is associated with short regional operations.

       3. In addition, many short regional operations operate out of restrictive airfields, and the turboprop aircraft's high-lift straight wing is capable of meeting the field lengths and climb and descent gradients of these restrictive airfields.

      4. In addition, short-range sectors usually are flown more frequently and with a smaller passenger demand per trip. Therefore, a turboprop aircraft, which typically has a smaller passenger capacity than a jet aircraft, is a better economical design for this type of short sector demand, i.e., 20- to 60-passenger loads.

However, it should be noted that this trend is changing, with 30 to 70 seat jet airliners now being built that are efficient at medium altitudes over short regional routes and as such are replacing the turboprop fleets operated by regional airlines.

29
Q

Propellers

Is there a critical engine on a propeller aircraft?

A

Yes, especially on aircraft with propellers rotating in the same direction.

A critical engine is the engine that determines the critical control speed
of the aircraft.

30
Q

Propellers

Why is the number 1 engine the critical engine on a multi-engine propeller
aircraft?

A

There are two main reasons why the number 1 engine is the critical
engine on an aircraft with propellers rotating in the same direction:
( 1) slipstream effect and
(2) asymmetric blade effect.

31
Q

Propellers

How does a crosswind affect the critical engine?

A

A crosswind, depending on its direction, can either help to restore or aggravate the yawing moment of an aircraft with a failed critical engine.

For instance, a failed critical number 1 engine will cause a yaw to the left.

A crosswind component from the left will apply a restoring force to the aircraft’s fuselage, whereas a crosswind from the right will aggravate the yawing moment further to the left due to the sideways
force experienced on the right side of the aircraft’s fuselage (which is from the right to the left).

Therefore, a crosswind landing is of even
greater importance with a critical engine failure.

(See Q: If you had an engine failure between Vi and VR and you had a maximum crosswind, which engine would be the best to lose, i.e., upwind or downwind engine?
page 195.)

32
Q

Propellers

What is a wind milling propeller?

A

Windmilling is experienced when the rpm is reduced but the airspeed is maintained, which eventually will cause the blade angle of attack to become negative.

When this occurs, the resulting force will act in a rearward direction. This is known as windmilling.

A windmilling propeller causes a drag force, which is opposite to the
direction of flight.

This drag can be quite high, and in addition to its decelerating effect, it also can cause a large yawing moment on a multi-engined aircraft with asymmetric thrust.

33
Q

Propellers

Can you obtain ground reverse / braking thrust from propellers?

A

It is possible to increase the braking effect on some aircraft types by reversing the pitch of the propeller (negative angle), causing it to produce thrust in the opposite direction.

34
Q

Propellers

What is propeller feathering, and why is it used?

A

Feathering the propeller is accomplished in the event of an inflight engine failure or fire.

Feathering a propeller is the term given to the arrangement of the propeller blades turned beyond the coarse pitch until they are edge on (i.e., the chord line is parallel) to the airflow, thereby causing the propeller not to windmill (or create extra drag) or hinder the extinguishing of an engine fire.

35
Q

Propellers

How do propeller aircraft generate noise?

A

The noise generated by a propeller aircraft is from the sheer effect of different
displaced air velocities.

That is, the sheer is the difference between the propeller’s faster displaced air and the slower ambient air around it.

36
Q

Propellers

How is propeller noise controlled or reduced?

A

The following can reduce propeller noise:

      1. Increase the number of propeller blades, i.e., four blades instead of three.  

This increases the mass of airflow; therefore, its velocity can
be reduced to maintain the same thrust.

This results in reducing
the sheer effect between the displaced propeller air and the ambient
air and reduces noise.

   2. Reduced-thrust takeoff. The following can control propeller noise: maximum-angle climb after takeoff. This allows the aircraft to get above any noise control zones.
37
Q

Jet/Gas Turbine Engines

What is the theory of a jet/gas turbine engine?

A

Its a practical application of Newton’s third law of motion.

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.”

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).

38
Q

Jet/Gas Turbine Engines

What is specific fuel consumption (SFC)?

A

Specific fuel consumption is the quantity/weight (lb) of fuel consumed
per hour divided by the thrust of an engine in pounds:

SFC = Fuel per hour/ thrust

39
Q

Jet/Gas Turbine Engines

Describe how a jet/gas turbine engine works.

A

The jet engine or thermodynamic 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.

The mechanical arrangement of the gas turbine, i.e., compressor, combustion, turbine, exhaust, is in series so that the combustion cycle occurs continuously at a constant pressure.

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.

40
Q

Fuel Management

What is a fuel injection system, and what are its advantages and disadvantages?

A

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.
Advantages:
.. Freedom from vaporization ice (fuel ice)
.. More uniformed delivery of the fuel-air mixture around the combustion chamber section or each cylinder.
.. Improved control of fuel-air ratio
.. Fewer maintenance problems
.. Instant acceleration of the engine after idling, i.e., instant response
.. Increased engine efficiency

Disadvantages:
.. Starting an already hot fuel injection engine may be difficult due to vapor locking in the fuel lines .
.. Having very fine fuel lines, fuel injection engines are more susceptible to contamination (i.e., dirt or water) in the fuel.
.. Surplus fuel provided by the fuel injection system will pass through a return line, which is usually routed to only one of the fuel tanks.

This may result in either the fuel being vented overboard(thus reducing fuel available) or asymmetric (uneven) fuel loading.

41
Q

Jet/Gas Turbine Engines

What are thrust reverses, and how do they work?

A

Thrust reverses on jet/gas turbine engine reverse the airflow forward, there by creating a breaking action.

There are two types of thrust
reverses:
( 1) blockers or bucket design and
(2) reverse flow through the cascade vane.

42
Q

Jet/Gas Turbine Engines

Describe maximum takeoff thrust and its limitations.

A

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.

43
Q

Jet/Gas Turbine Engines

Describe maximum continuous thrust.

A

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.

44
Q

Jet/Gas Turbine Engines

What is the compression ratio of a gas turbine engine?

A

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:
First compressor stage: 
             Inlet pressure - 15 psi
             Outlet pressure - 60 psi 
             Compression Ratio - 4:1
45
Q

Jet/Gas Turbine Engines

What is the principle of the bypass engine?

A

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 X 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).

46
Q

Jet/Gas Turbine Engines

What is bypass ratio?

A

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.

Bypass ratio in an early models 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.

A high bypass ratio, i.e., 5:1, is usually common with ducted fan engines. (See Figure 2.8, “Triple-spool turbofan engine,” page 68.)

47
Q

Jet/Gas Turbine Engines

Describe the fan engine and its advantages.

A

The fan engine can be regarded as an extension of the bypass engine principle with the difference, that it discharges its cold bypass airflow and hot engine core
airflow separately.

The following are some of the main advantages of a fan engine:

  1. Smaller engine size.
  2. Better propulsive efficiency.
  3. Better specific fuel consumption.
  4. Reduced engine noise.
  5. Contamination (i.e., bird strikes, heavy water)
48
Q

Jet/Gas Turbine Engines

What are the advantages of a wide-chord fan engine?

A

The advantage of a wide-chord fan engine is its ability to optimize the
separate thrust properties of the bypass air mass and the engine-core air
mass from a single rotating blade row.

It does this by using the different
speeds between the blade tip and center without using inlet guide
vanes, thus keeping engine weight and mechanical complexity to an
acceptable level.

The chord is the length of the fan’s blade from its center mounting to its tip.

It can be said, therefore, that as a basic principle, the wider the fan chord, the higher is the blade tip speed, the greater is the generated airflow speed and mass, and the greater is the discharged air pressure, resulting in an increased engine thrust.

49
Q

Jet/Gas Turbine Engines

Describe a triple-spool turbofan engine, and its advantages.

A

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.

  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 airflow delivered to the atmosphere via the engine’s bypass ducts behind the fan.

The one part of air that flows through the engine N2 and N3 compressors becomes highly compressed, of which one-third is used for combustion and two-thirds is used for internal engine cooling.

50
Q

Jet/Gas Turbine Engines

Why is a fan engine flat rated?

A

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 at a fixed
temperature.

51
Q

Jet/Gas Turbine Engines

When and where is a jet/gas turbine (bypass) engine at its most efficient,
and why?

A

At high altitudes and high rpm speeds.

52
Q

Jet/Gas Turbine Engines

Why does a jet aircraft climb as high as possible?

A

Jet aircraft climb as high as possible
because the gas turbine (bypass) engines are most efficient when their
compressors are operating at high rpms, approximately 90 to 95 percent.

This high rpm speed results in the engine’s optimal gas flow condition that achieves its best specific fuel consumption (SFC).

53
Q

Jet/Gas Turbine Engines

What advantages does a jet-engined aircraft gain from flying at a high
altitude?

A

The advantages a jet engine gains from flying at high altitudes are

  1. Best specific fuel consumption (SFC)/increased (maximum) endurance.
  2. Higher true airspeed (TAS) for a constant indicated airspeed (lAS),
    providing an increased (maximum) attainable range
54
Q

Jet/Gas Turbine Engines

Explain the jet/gas turbine engine’s thrust-to-thrust lever position.

A

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.)

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 ofthe thrust lever could produce only 600 Ib
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.

55
Q

Jet/Gas Turbine Engines

What are the main engine instruments?

A

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
56
Q

Jet/Gas Turbine Engines

What is engine pressure ratio (EPR)?

A

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.

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.

The EPR reading is the primary engine thrust instrument, with the temperature of the turbine stage governing the engine’s maximum attainable thrust.

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.

57
Q

Jet/Gas Turbine Engines

What is exhaust gas temperature (EGT), and why is it an important engine parameter?

A

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.

58
Q

Jet/Gas Turbine Engines

Describe an engine wet start and its causes, indications, and actions.

A

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 (rpm) 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 blowout the fuel (approximately 60 seconds).

59
Q

Jet/Gas Turbine Engines

Describe an engine hung start and its causes, indications, and actions.

A

A hung start occurs when the engine ignites but does not reach its self sustaining rpm. (Self-sustaining speed is an rpm engine speed at and
above which the engine can accelerate on its own without the aid ofthe
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:

  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 rpms 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 blowout 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 decrease and EGT increase
Decreasing fuel = rpm increase and EGT decrease

60
Q

Jet/Gas Turbine Engines

Describe an engine hot start and its causes, indications, and actions.

A

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. Over-fueling (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.

  1. When the engine rpms have slowed to the reengagement speed, motor
    over the engine to blowout the fuel (approximately 60 seconds).
61
Q

Jet/Gas Turbine Engines

What is a variable/reduced-thrust takeoff?

A

A 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.

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.

62
Q

Jet/Gas Turbine Engines

A

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.

63
Q

Jet/Gas Turbine Engines

Why do you use reduced derated thrust takeoffs in a jet aircraft?

A

There are two main reasons for using a reduced derated takeoff.

  1. To protect engine life and to improve engine reliability.

Derated thrust takeoffs reduce the stress and attrition of the engine during
the takeoff period when the highest loads are placed on the engine.

  1. 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.
64
Q

Jet/Gas Turbine Engines

Why is the risk per flight decreased with a reduced-thrust takeoff?

A

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.
65
Q

Jet/Gas Turbine Engines

What happens to engine pressure ratio (EPR) on the takeoff roll?

A

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 ofthe engine’s slow response rate or lag is its
effect on the EPR reading because the reading of the jet pipe
exhaust pressure is taken from the rear of the engine and the compressor inlet pressure reading is taken from the front of the engine.

Therefore, as the engine throttles are opened up, initially the compressor inlet air pressure (psi) will increase before the jet pipe exhaust air pressure (psi) increases proportionally, thus creating an
increased or initially quickly increasing (inlet pressure) value for a constant or
slowly increasing ( Exaust pressure) value.

This results in an initial decrease in the
EPR reading.

Then, as the engine accelerates along its entire length,
the engine turbine and compressor rpm speeds increase, and the EPR
reading increases steadily to its takeoff setting.

66
Q

Jet/Gas Turbine Engines

Why does engine pressure ratio (EPR) need to be set by 40 to 80 knots on
the takeoff role?

A

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.
67
Q

Jet/Gas Turbine Engines

What is an engine windmill start, and when is it used?

A

A windmill start occurs when the engine is started ‘Without the aid ofthe
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.
(See Q:
What is the purpose of engine relight boundaries? page 79.)

68
Q

Jet/Gas Turbine Engines

What is the purpose of engine relight boundaries?

A

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.

69
Q

Jet/Gas Turbine Engines

What causes a jet/gas turbine upset, and how do you correct it?

A

Disturbed or turbulent airflow will cause ajetJgas 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 attack 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,
(3) rpm fluctuations.
70
Q

Jet/Gas Turbine Engines

What is a jet engine surge, what causes it, and what are the indications?

A

A surge is the reversal of airflow through an engine, where the highpressure
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.

  1. 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.

  1. Adjust the aircraft’s attitude to un stall the engines, which lead to
    the surge.
  2. Slowly and smoothly reopen the throttles.
71
Q

Jet/Gas Turbine Engines

Why are bleed valves fitted to gas turbine engines?

A

Bleed valves are fitted on 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.
72
Q

Jet/Gas Turbine Engines

Why do gas turbine engines have auto igniters, and how do they work?

A

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 angle 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 angle of attack of the
aircraft, via the AOA-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.

73
Q

Jet/Gas Turbine Engines

What is FADEC?

A

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.

74
Q

Jet/Gas Turbine Engines

What fuels are used commonly for civil jet aircraft?

A

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.
    (For fuel questions, see Chapter 6, “Aircraft Instruments and System,”
    pages 172-174.)
75
Q

Jet/Gas Turbine Engines

How do jet/gas turbine engines generate noise?

A

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.

76
Q

Jet/Gas Turbine Engines

How is jet/gas turbine engine noise controlled or reduced?

A

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.
77
Q

Jet/Gas Turbine Engines

Is there a critical engine on a jet/gas turbine aircraft?

A

There is no critical gas turbine engine because the engines are positioned
symmetrically with opposing revolution direction.

However, there is a
governor engine, i.e., an engine that is the master that sets the rpm
speed for the others.

(See Q: How does a crosswind affect the critical
engine? page 60.)