Chapter 3 - Turbine Engines FAA QUESTIONS Flashcards
Jet engine turbine blades removed for detailed inspection must be reinstalled in
the same slot
Turbine blades are individually weighed and coded for installation in the disks in such a way that they best distribute the weight evenly around the disk.
If a blade is removed from a disk for inspection, it must be reinstalled in the same slot from which it was removed.
The fan rotational speed of a dual axial compressor forward fan engine is the same as the
low-pressure compressor
The fan is a portion of the low-pressure compressor of a dual axial-flow compressor. The rotor blades in the fan section are long enough that the air they move passes around the outside of the gas generator portion of the engine.
The fan rotational speed is the same as that of the low-pressure compressor
When the leading edge of a first-stage turbine blade is found to have stress rupture cracks, which of the following should be suspected?
Overtemperature condition.
Stress rupture cracks or deformation of the leading edge of the first-stage turbine blades are usually caused by an overtemperature condition.
Overtemperature operation must be suspected when finding this type of damage
The stators in the turbine section of a gas turbine engine
increase the velocity of the gas flow.
The turbine in a gas turbine engine extracts energy from the burning gases as they pass through it. The stators in front of the rotating turbine wheels increase the velocity of the gases and direct them so they will strike the rotors at the correct angle.
What should be done initially if a turbine engine catches fire when starting?
Turn off the fuel and continue engine rotation with the starter.
If a turbine engine catches fire in the process of starting, turn the fuel off and continue rotating the engine with the starter to force enough air through the engine to blow the fire out.
The pressure of subsonic air as it flows through a convergent nozzle
decreases.
When subsonic air flows through a convergent nozzle, its velocity increases and its pressure decreases.
This action is in accordance with Bernoulli’s principle, which gives us the relationship between kinetic energy and potential energy in a column of moving fluid. The kinetic energy relates to the velocity of the fluid and the potential energy relates to the pressure of the fluid.
What are the two basic elements of the turbine section in a turbine engine?
Stator and rotor.
The turbine section of a gas turbine engine consists of two basic elements, the stator and the rotor.
In what section of a turbojet engine is the jet nozzle located?
Exhaust.
The rear opening of a turbine-engine exhaust duct is called the exhaust nozzle (jet nozzle).
The nozzle acts as an orifice, the size of which determines the density and velocity of the gases as they leave the engine.
What is one purpose of the stator blades in the compressor section of a turbine engine?
Control the direction of the airflow.
The stator blades in an axial-flow compressor convert the high-velocity energy of the air into pressure energy.
They also direct the airflow from each of the rotor stages to obtain the maximum possible blade efficiency.
Which of the following engine variables is the most critical during turbine engine operation?
Turbine inlet temperature.
Turbine-inlet temperature is the highest temperature inside a turbine engine. Therefore, it is the most critical variable of engine operation.
It is impractical to measure turbine-inlet temperature in most engines. So, the temperature-measuring thermocouples are usually installed at the turbine discharge. The turbine-outlet temperature gives a relative indication of the temperature at the turbine inlet.
Therefore, if the turbine-outlet temperatures are kept within range, it can be assumed that the turbine-inlet temperatures are also within range.
A cool-off period prior to shutdown of a turbine engine is accomplished in order to
allow the turbine wheel to cool before the case contracts around it.
A rule of thumb for turbine engine operation: when an engine has been operated above approximately 85% RPM for periods longer than one minute; during the last five minutes before shutdown, the engine should be operated below 85% RPM (preferably at idle) for a period of five minutes. This prevents the possibility of the engine case contracting around the turbine wheels before they have cooled and contracted to their normal size.
It is also important that all surfaces contacted with engine oil be cooled to their normal operating temperature, to prevent oil left on a hot surface from coking (turning into a hard carbon deposit).
What must be done after the fuel control unit has been replaced on an aircraft gas turbine engine?
Retrim the engine.
After a fuel control has been replaced on a turbine engine, the engine must be re-trimmed.
Trimming a gas turbine engine consists of adjusting the fuel control to give the engine the correct idle and maximum RPM.
When starting a turbine engine,
a hot start is indicated if the exhaust gas temperature exceeds specified limits.
A hot start is indicated if the engine starts but the exhaust gas temperature exceeds specified limits.
A turbine engine compressor which contains vanes on both sides of the impeller is a
double entry centrifugal compressor
A centrifugal compressor with vanes on both sides of the impeller is called a double-entry centrifugal compressor.
Hot spots in the combustion section of a turbojet engine are possible indicators of
malfunctioning fuel nozzles.
Hot spots are possible indicators of a serious condition, such as malfunctioning fuel nozzles or other fuel-system malfunctions.
The abbreviation Pt7 used in turbine engine terminology means
the total pressure at station No. 7.
Pressures in a turbine engine are identified according to their type and to the location at which they are measured.
P(t7) is the total pressure (the pressure a body of moving fluid has when its movement is stopped) measured at station 7, the turbine discharge.
One function of the nozzle diaphragm in a turbine engine is to
Direct the flow of gases to strike the turbine blades at the desired angle.
One of the functions of the nozzle diaphragm in a turbine engine is to deflect the gases to a specific angle in the direction of the turbine wheel rotation.
The purpose of a bleed valve, located in the beginning stages of the compressor, in an aircraft gas turbine engine is to
vent some of the air overboard to prevent a compressor stall.
Some aircraft gas turbine engines have automatic bleed valves that prevent compressor stall or surge by venting overboard some of the air from the compressor. This prevents the air from ‘piling up’ in the higher-pressure stages of the compressor and restricting the flow of air through the engine.
The pressure of supersonic air as it flows through a divergent nozzle
decreases.
When supersonic air flows through a divergent nozzle (a nozzle whose cross-sectional area increases in the direction of air flow), its velocity increases and its pressure decreases.
Reduced blade vibration and improved airflow characteristics in gas turbines are brought about by
shrouded turbine rotor blades.
Shrouded turbine blades are used to reduce blade vibration and improve the airflow characteristics through the turbine.
Who establishes the recommended operating time between overhauls (TBO) of a turbine engine used in general aviation?
The engine manufacturer.
The engine manufacturer establishes the recommended TBO of a turbine engine, and these times are approved by the FAA.
(1) Welding and straightening of turbine engine rotating airfoils does not require special equipment.
(2) Welding and straightening of turbine engine rotating airfoils is commonly recommended by the manufacturer.
Regarding the above statements,
neither No. 1 nor No. 2 is true.
Neither statement is true. Welding and straightening of rotating airfoils in a gas turbine engine require special equipment. Quite often, neither procedure is authorized by the engine manufacturer.
What type igniter plug is used in the low tension ignition system of an aircraft turbofan engine?
Self-ionizing or shunted-gap type plug.
Self-ionizing shunted-gap igniters are used in the low-tension ignition system of some aircraft turbofan engines. These igniters have a ceramic semiconductor material between the center electrode and the shell. The resistance of this semiconductor is low when it is relatively cool, but it increases as it heats up. When the storage capacitor discharges through the igniter, the current initially flows to ground through the semiconductor, which gets so hot it becomes incandescent and its resistance increases. The air gap between the electrodes becomes ionized, and its resistance drops below that of the semiconductor. The remainder of the current discharges across the air gap in a surge as a high-energy spark.
A weak fuel to air mixture along with normal airflow through a turbine engine may result in
a lean die-out.
If the fuel furnished to a turbine engine is decreased to the point that there is a weak fuel to air mixture with a normal flow of air through the engine, there is a danger of a lean die-out.
Between each row of rotating blades in a turbine engine compressor, there is a row of stationary blades which act to diffuse the air. These stationary blades are called
stators.
The stationary blades between each set of rotating blades in an axial-flow turbine-engine compressor are called stators.
The function of the stators is to receive the air from each stage of the compressor and deliver it to the next stage at the proper velocity, direction, and pressure.
Dirt particles in the air being introduced into the compressor of a turbine engine will form a coating on all but which of the following?
Turbine blades
Dirt particles introduced into the turbine engine will cause a coating to form on the casings, inlet guide vanes and compressor blades.
The extreme heat in the turbine section prevents the coating from forming on the turbine blades.
In which type of turbine engine combustion chamber is the case and liner removed and installed as one unit during routine maintenance?
Can.
The can-type combustion chamber of a turbojet engine consists of an outer case, or housing and inside it is a perforated stainless steel combustion chamber or liner.
The case and liner are removed as a unit for routine maintenance.
The non-rotating axial-flow compressor airfoils in an aircraft gas turbine engine, are called
stator vanes
The nonrotating airfoils in an aircraft gas turbine engine axial-flow compressor are called stator vanes.
If, during inspection at engine overhaul, ball or roller bearings are found to have magnetism but otherwise have no defects, they
must be degaussed before use.
Ball and roller bearings that are in good condition but are shown to have magnetism in them may have the magnetism removed with a suitable degausser. If the bearings are allowed to remain in their magnetized state they will be damaged by the foreign ferrous particles they attract.
If the RPM of an axial-flow compressor remains constant, the angle of attack of the rotor blades can be changed by
changing the velocity of the airflow.
Two factors affecting the angle of attack of an axial-flow compressor blade are the velocity of the air through the engine and the RPM of the compressor.
If the airflow entering the engine is restricted, reducing its velocity, the angle of attack of the compressor blade will increase to such a point that compressor stall can occur.
In a dual axial-flow compressor, the first stage turbine drives
N(2) compressor.
The first-stage turbine in a twin-spool turbojet engine drives the high-pressure compressor, the N(2) compressor.
Which of the following types of combustion sections are used in aircraft turbine engines?
Multiple-can, annular, and can-annular.
Three basic types of combustion sections used in gas turbine engines are the multiple-can type, the annular type, and the can-annular type.
A turbine engine compressor which contains vanes on both sides of the impeller is a
double entry centrifugal compressor.
A centrifugal compressor with vanes on both sides of the impeller is called a double-entry centrifugal compressor.
A turbine engine hot section is particularly susceptible to which kind of damage?
Cracking.
Cracking is one of the most widely found forms of damage in the hot section of a gas turbine engine.
Vibration and the extremes of temperature cause the thin metal of which hot-section components are made, to crack.
