3. Turbine Engines (Jeppesen)

Question 1

Explain the main difference and advantage of turbofan over turbojet engines.

Answer 1

A turbofan engine uses a fan to bypass some air around the engine core and to provide thrust. This provides additional thrust without increasing fuel flow.

Question 2

Name the major components in a gas turbine engine.

Answer 2

Air inlet, compressor section, combustion section, turbine section, exhaust section, gearbox and accessory section.

Question 3

What determines the amount of airflow through a turbine engine?

Answer 3

The forward speed of the aircraft, the compressor speed, and the air density.

Question 4

Name the two types of compressors commonly used in turbine engines.

Answer 4

Axial flow compressors and centrifugal flow compressors.

Question 5

Name two different methods of attaching turbine engine rotor blades to the rotor disks.

Answer 5

Dovetail-type root, Bulb-type root, or Fir tree-type root.

Question 6

What is a split compressor system?

Answer 6

It’s either a dual or twin spool compressor that’s connected to the turbine section with two rotor shafts, one inside the other.

Question 7

What are the different types of combustion chambers used in turbine engines?

Answer 7

Can, annular, can-annular, and reverse flow annular.

Question 8

What prevents the combustion chambers from burning?

Answer 8

Cooling air along the inside of the liner.

Question 9

What is the purpose of the interconnecting tubes attached between can-type combustion chambers?

Answer 9

For flame propagation during start.

Question 10

What functions do axial flow turbine nozzles perform?

Answer 10

They direct the mass airflow to drive the turbine rotor at a specific angle.

Question 11

How is thermal stress relieved on a turbine disc?

Answer 11

By directing bleed air onto the face of the disc, or by cutting grooves in the disc.

Question 12

What are carbon seals used for in turbine engines?

Answer 12

They’re used as oil seals for the turbine shaft bearings.

Question 13

What is shaft horsepower?

Answer 13

It is an indication of the torque developed by a turboprop or turboshaft engine.

Question 14

Name the main components of a typical A.P.U.

Answer 14

A small power turbine for power and bleed air, and an electrical generator.

Question 15

When is the greatest demand placed on an A.P.U.?

Answer 15

When supplying bleed air.

Question 16

How is an A.P.U. generally started?

Answer 16

With its own electrical starter and battery power.

Question 17

Where does the A.P.U. get its fuel supply?

Answer 17

From one of the aircraft’s main fuel tanks.

Question 18

At what speed does a gas turbine A.P.U. operate and how is this speed maintained?

Answer 18

At or near its rated speed regardless of electrical or pneumatic loads imposed. The speed is maintained by the A.P.U. fuel control, which automatically adjusts fuel flow to maintain its rated speed.

Question 19

What is used to prevent a heavily loaded A.P.U. from exceeding its maximum E.G.T.?

Answer 19

A load control valve that modulates the pneumatic load to maintain the E.G.T. within limits.

Question 20

How is an APU shut down and why is this procedure used?

Answer 20

You shut it down by unloading the APU, which means closing the bleed air valve. You let the APU run for a specified amount of time, usually 3 minutes, to let the EGT cool down and stabilize. The procedure is used because if a heavily loaded APU is abruptly shut down without any cooling down period, damage could occur as a result of thermal shock.

Question 21

What powers the variable inlet guide vanes used on some A.P.Us to regulate compressor intake airflow?

Answer 21

Fuel pressure.

Question 22

What is fan blade shingling?

Answer 22

It’s the overlapping of the midspan shrouds of the fan blades.

Question 23

When a turbofan or turbojet engine is shutdown, what should a mechanic listen for during coastdown?

Answer 23

Any rubbing sound or other unusual noises from the engine.