Turbine Engines

What is meant by creep of the turbine blades?

A condition of permanent elongation of the turbine blades. Creep is caused by the high temperatures and the high centrifugal loading imposed on the blades.

What is used to prevent a heavily loaded APU from exceeding its maximum EGT?

A load control valve modulates the pneumatic load to maintain EGTs within limits.

Name the main components of a typical APU.

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

What is meant by a hung start in a turbine engine?

A start in which ignition occurs, but the engine does not accelerate to a self-sustaining speed.

What is meant by a hot start in a turbine engine?

A start in which ignition occurs, but the internal temperatures go high enough that they can damage the engine.

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

A turbofan engine uses a fan to bypass some air around the engine core and to provide thrust. The advantage is that this provides additional thrust without increasing fuel flow.

What is meant by a free-turbine turboshaft engine?

A turboshaft engine that has a turbine wheel, or stage of turbine wheels, that is not used to drive the compressor of the gas generator section of the engine. This free turbine drives the propeller in a turbprop engine or the transmission and rotor of a helicopter.

Why do some axial-flow turbine engines have more than one set of turbines and compressors?

A two-spool turbine engine has a low-pressure and a high-pressure compressor, each driven by its own turbine. The two independent systems operate at the speed which they are most efficient.

What is meant by trimming a turbojet engine?

Adjusting the fuel control to get the correct idling and maximum-thrust RPM.

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

After the APU is unloaded by closing the bleed air valve, it is run for a specified amount of time to allow the EGT to cool and stabilize. This cool down period is typically three minutes. If a heavily loaded APU is abruptly shut down without any cooling down period, damage could occur as a result of thermal shock.

Name the major components in a gas turbine engine.

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

What is a split compressor system?

Also called dual or twin spool compressors, these are connected to the turbine section with two rotor shafts, one inside the other.

What is meants by a turbofan engine?

An axial-flow turbine engine in which the first stage of compressor blades are lengthened, so they can force air around the outside of the gas generator portion of the engine.

How is the compressor of a turbine engine cleaned?

An emulsion-type cleaner is sprayed through the engine while it is being motored by the starter or operated at a low speed. The wash is followed by a clean water rinse.More vigorous cleaning is done by injection a mild abrasive, such a ground-up apricot pits or walnut shells, into the engine while it is operating at a low speed.

What kind of bearings are used to support the rotor shaft of an aircraft turbine engine?

Anti-friction bearings, such as ball bearings or roller bearings.

When a turbofan or turbojet engine is shutdown, what should a mechanic [be] listening for during coastdown?

Any rubbing sound or other unusual noise from the engine.

At what speed does a gas turbine APU operate and how is this speed maintained?

At or near its rated speed regardless of electrical or pneumatic loads imposed. The APU fuel control automatically adjusts the fuel flow to maintain the rated speed.

At what point in a turbine engine is the temperature the highest?

At the inlet to the high-pressure turbine.

Name two types of compressors commonly used in turbine engines.

Axial and contrifugal flow compressors.

What are the two basic types of compressors that are used in aircraft turbine engines?

Axial-flow and centrifugal.

How is thermal stress relieved on a turbine disc?

By directing bleed air onto the face of the disk, or by grooves being cut in the disk.

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

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

What are three types of combustors used on aircraft turbine engines?

Can-type, annular-type, and can-annular type.

What prevents the combustion chamber from burning?

Cooling air along the inside of the liner.

What criteria determines replacement of life limited turbine engine components?

Cycles, hours and/or calender time.

Name the different types of compressor blade damage that may be found during inspection.

Dents, cracks, galling, pitting, scratches, burrs, burns, gouges.

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

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

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

For flame propagation during start.

Where does the APU get its fuel supply?

From one of the aircraft's main fuel tanks.

What powers the variable inlet guide vanes used on some APUs to regulate compressor intake airflow?

Fuel pressure.

Why is it important that a turbojet engine be allowed to cool before it is shut down after it has been operated at a high power setting?

If the engine is shut down while it is hot, there is a possibility that the shroud will contact around the turbine wheel and seize the rotor.

Where can you find the limits of repair allowed for the compressors blades of an aircraft turbine engine?

In the FAA-approved service manual issued by the manufacturer of the engine.

Where is water injected into a turbine engine for cooling purposes?

Into the compressor inlet and into the engine diffuser case.

What is shaft horsepower?

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

What is fan blade shingling?

It is the overlapping of the midspan shrouds of the fan blade.

What kind of markers can be used to mark the parts in hot and cold sections during repairs?

Layout dye, felt tip marker, chalk.

What are two types of thrust reversers that are used on turbojet engines?

Mechanical blockage-type and the aerodynamic blockage-type.

How does relative humidity affect turbine engines?

Negligible effect.

What determines the amount of airflow through a turbine engine?

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

What are the two basic sections of a turbine engine?

The hot section and the cold section.

What is the purpose of the stators in an axial-flow compressor?

The stators convert some velocity energy into pressure energy and change the direction of the air so it is proper for the next stage of rotors.

What is the main purpose of the turbine nozzle in an aircraft turbine engine?

The turbine nozzle directs the hot gases as they leave the combustors so they will turn the turbine wheel with maximum efficiency.

How can you tell if a turbofan or turbojet engine is out of trim?

There will be a high exhaust gas temperature at the target engine pressure ratio for takeoff power.

What is the function of the interconnect tubes between the cans of a turbine engine that uses can-type combustors?

These tubes allow the flame to travel from the cans that contain the igniters to all the other cans when the engine is being started.

What are carbon seals used for in turbine engines?

They are used as oil seals for the rotor shaft bearings.

What functions do axial flow turbine nozzles perform?

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

If turbine blades are removed from a turbine disc, why should they be re-installed in the same location?

To maintain the balance of the turbine wheel.

Why are compressor washes performed and what methods are commonly used?

To remove any contaminants from the compressor section and improve engine performance. The fluid wash and abrasive grit wash are commonly used.

How many igniters are there normally in an aircraft turbine engine?

Two.

When is the greatest demand placed on an APU?

When supplying bleed air.

How is an APU generally started?

With its own electrical starter and battery power.