Based on our vast experience with EASA PART 66 examination questions and question databanks, we have prepared a practice example of questions with explanations to practice for your next exam.
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Module 15: Gas turbine engine
What is a defining characteristic of a free-turbine engine?
- A. There is a direct mechanical connection between the compressor and the output shaft
- B. There is no mechanical link between the gas generator turbine and the power turbine
- C. A clutch engages the compressor with the power output shaft
Correct Answer: B. There is no mechanical link between the gas generator turbine and the power turbine
Explanation: In a free-turbine engine, the turbine that drives the compressor (the gas generator) is mechanically separate from the power turbine that drives the propeller or output shaft. They are linked only by the flow of hot gas.
How does a jet engine generate thrust?
- A. By the reaction of the high-velocity gases leaving the exhaust
- B. By the propelling gases pushing against the surrounding air
- C. By the force of the air being drawn into the compressor
Correct Answer: A. By the reaction of the high-velocity gases leaving the exhaust
Explanation: A jet engine operates on Newton’s Third Law of Motion. It generates thrust by accelerating a mass of gas and expelling it at high velocity from the exhaust nozzle. The reaction to this action is an equal and opposite force (thrust) pushing the engine forward.
The basic gas turbine engine is split into a cold section and a hot section. Which components are part of the hot section?
- A. The combustor, diffuser, and exhaust
- B. The combustor, turbine, and exhaust
- C. The engine inlet, compressor, and turbine
Correct Answer: B. The combustor, turbine, and exhaust
Explanation: The hot section is where fuel is burned and energy is extracted from the hot gases. This section begins at the combustion chamber and includes the turbine stages and the exhaust section.
Which statement best describes the operating principle of a pure turbojet engine?
- A. It imparts a large acceleration to a small mass of air
- B. It imparts a small acceleration to a large mass of air
- C. It imparts a large acceleration to a large mass of air
Correct Answer: A. It imparts a large acceleration to a small mass of air
Explanation: A pure turbojet engine takes in a relatively small mass of air, compresses it, heats it, and then accelerates it to a very high velocity through the exhaust nozzle to produce thrust.
On a twin-spool axial flow turbofan engine, the fan is directly connected to and driven by which component?
- A. The high pressure compressor
- B. The low pressure turbine
- C. The forward turbine wheel
Correct Answer: B. The low pressure turbine
Explanation: The fan (N1) and low-pressure compressor are on the same shaft as the low-pressure (LP) turbine. The LP turbine extracts energy from the gas stream specifically to drive the fan and LP compressor.
According to Bernoulli’s Theorem as it applies to a flowing gas, what principle holds true?
- A. The total energy of the gas stream remains constant
- B. The static pressure is always less than the dynamic pressure
- C. The static pressure and dynamic pressure are always equal
Correct Answer: A. The total energy of the gas stream remains constant
Explanation: Bernoulli’s Theorem states that for a steady flow, the sum of all forms of energy in a fluid is constant. This means the total energy, which is the sum of static pressure, dynamic pressure, and potential energy, does not change along its path.
What is the working fluid used in a gas turbine engine?
- A. Kerosene
- B. Air
- C. Gasoline
Correct Answer: B. Air
Explanation: A gas turbine engine is an air-breathing engine. It ingests large quantities of air, which acts as the working fluid that is compressed, heated by the combustion of fuel, and then expanded to produce power.
When subsonic air flows through a nozzle that has a convergent shape, what happens to its velocity?
- A. It increases
- B. It decreases
- C. It remains constant
Correct Answer: A. It increases
Explanation: For subsonic flow, a convergent nozzle constricts the airflow, forcing it to speed up to maintain the mass flow rate. In this process, pressure energy is converted into kinetic energy.
In a twin-spool engine configuration, which component is driven by the first-stage turbine?
- A. The N1 compressor
- B. The N2 compressor
- C. Both N1 and N2 compressors
Correct Answer: B. The N2 compressor
Explanation: The first turbine stage immediately downstream of the combustor is the High-Pressure (HP) Turbine. It operates in the hottest, highest-pressure gas flow and drives the High-Pressure (N2) compressor.
At which location in an axial flow turbojet engine is the gas pressure at its highest?
- A. At the outlet of the compressor section
- B. At the entrance to the turbine section
- C. Within the combustion section
Correct Answer: A. At the outlet of the compressor section
Explanation: The compressor’s function is to raise the pressure of the incoming air. The highest pressure in the entire engine cycle is achieved at the discharge point of the final compressor stage, just before the air enters the diffuser and combustion chamber.
Where is the diffuser section in a gas turbine engine situated?
- A. Between the compressor section and the burner section
- B. Between the burner section and the turbine section
- C. Between station No. 7 and station No. 8
Correct Answer: A. Between the compressor section and the burner section
Explanation: The diffuser is located immediately after the compressor. Its purpose is to slow down the high-velocity compressor discharge air, converting its kinetic energy into a pressure increase before it enters the combustion chamber.
What would be the result of a shear failure in the Low Pressure (LP) shaft of a turbofan engine?
- A. Compressor overspeed
- B. Turbine runaway
- C. Compressor underspeed
Correct Answer: B. Turbine runaway
Explanation: If the LP shaft shears, the LP turbine is no longer connected to its load (the fan/LP compressor). With no load to resist its rotation, the flow of hot gas from the engine core will cause the disconnected LP turbine to accelerate uncontrollably, a condition known as turbine runaway.
What does the designation Pt7 signify in engine terminology?
- A. The total pressure at station No. 7
- B. The pressure and temperature at station No. 7
- C. The total inlet pressure
Correct Answer: A. The total pressure at station No. 7
Explanation: In engine station identification, P denotes pressure and t denotes total or stagnation properties. The number 7 refers to the station at the turbine exhaust. Therefore, Pt7 is the total pressure at the turbine exhaust.
Which of these engine types would be the most likely candidate for having a noise suppression unit fitted?
- A. Turbojet
- B. Turboprop
- C. Turboshaft
Correct Answer: A. Turbojet
Explanation: Pure turbojet engines produce a very high-velocity exhaust stream, which is the primary source of jet noise. Therefore, they are the most likely to require noise suppressors (or hush kits) to meet noise regulations.
What is the standard designation for the rotational speed of the low pressure (LP) compressor in a twin-spool engine?
- A. N1
- B. NG
- C. N
Correct Answer: A. N1
Explanation: N1 represents the rotational speed of the low-pressure spool, which includes the fan and the LP compressor. N2 represents the speed of the high-pressure spool.
Why does a turbojet engine operate more smoothly than a piston engine?
- A. It has no reciprocating parts
- B. The lubrication is better
- C. It runs at a lower temperature
Correct Answer: A. It has no reciprocating parts
Explanation: A gas turbine engine’s major components rotate continuously in one direction. It lacks the reciprocating pistons, connecting rods, and crankshaft of a piston engine, which are constantly starting, stopping, and changing direction, creating inherent vibration.
According to the gas laws, what happens when a volume of air is compressed?
- A. Heat is gained
- B. Heat is lost
- C. No heat is lost or gained
Correct Answer: A. Heat is gained
Explanation: Compressing a gas forces its molecules closer together, which increases their kinetic energy. This increase in molecular energy manifests as a rise in the temperature and heat content of the gas.
What is the consequence of having a high bypass ratio in a turbofan engine?
- A. It results in greater propulsive efficiency
- B. It results in a faster overall airflow
- C. It results in a slower airflow and greater propulsive efficiency
Correct Answer: C. It results in a slower airflow and greater propulsive efficiency
Explanation: A high bypass ratio engine moves a very large mass of air (cold stream) at a lower velocity. This is more efficient for generating thrust at subsonic speeds compared to moving a small mass of air at very high velocity.
If the electrical anti-ice system on an engine inlet is activated, what is the effect on the engine’s thrust output?
- A. It will decrease
- B. It will increase
- C. It will remain constant
Correct Answer: A. It will decrease
Explanation: Anti-ice systems use energy from the engine, typically in the form of bleed air or electrical power from an engine-driven generator. This diversion of energy reduces the overall energy available for producing thrust, resulting in a slight decrease.
On which application would you typically find a bellmouth compressor inlet?
- A. Helicopters
- B. Supersonic aircraft
- C. Aircraft with low ground clearance
Correct Answer: A. Helicopters
Explanation: Bellmouth inlets are highly efficient at low airspeeds and high angles of attack, which are common in helicopter operations and during ground testing of engines. They are not suitable for high-speed flight.
