Powerplant O&P

Powerplant oral questions

What is the purpose of dynamic dampers on crankshafts?

They reduce engine vibration.

What types of bearings are generally found in reciprocating engines?

Plain, ball, and roller bearings.

Are the connecting rods used in each cylinder of a radial engine the same?

No. Radial engines use master and articulating rod assemblies.

Name the different types of piston rings.

Oil control, compression, and scraper rings.

Where should piston ring gaps be installed relative to each other?

Compression ring gaps should be staggered to prevent blow-by.

What could result from incorrectly installed piston rings?

Incorrect installation can lead to increased blow-by, reduced engine performance, and excessive oil consumption.

What purpose do oil control rings serve?

They regulate the thickness of the film on the cylinder walls.

What is used to help prevent valve surge or floating in an aircraft engine?

Two or more valve springs are used to help prevent valve surge or floating in an aircraft engine.

On engines equipped with hydraulic valve lifters, what should the running valve clearance be?

generally set to zero

What is the purpose of valve ovelap?

It allows better volumetric efficiency and lowers cylinder operating temperatures.

What indication are given in the event of a leaking or open primer while the engine is running?

The engine will not idle properly.

A cold cylinder is found when troublshooting a running engine. What does this indiacate?

No combustion in that cylinder.

When pulling a propeller through, a hissing sound is heard, indicating vlavle blow-by. What procedure should be performed nexst?

A compression check should be performed to identify the faulty cylinder.

What is the purpose of performing a compression test?

To determine if the valves, pistions and rings are sealing properly.

When an engine is overhauled, how is its total time affected? When an engine is rebuilt, how is its operating history affected?

The total time on the engine must be continued and the time since major overhaul is entered into the engine log. If the manufacturer rebuilds the engine, it is granted a zero-time status and is considered to have no previous operating history.

What ispection should be performed after a propeller strike without a sudden stopage?

1. Inspect engine mounts, the crankcase, and the nose section for damage.

2. Inspect the oil and oil filters/screens for metal particles.

3. Inspect the crankshaft or driveshaft or cracks and misalignment.

Name some of the precautions that should be performed prior to engine removal?

Disconnect the battery, turn off the fuel valve, chock the wheels, and if necessary, install a tail stand.

When inspecting a piston, where are the cracks most likely to be found?

In those areas that are highly stressed; usually the base of the piston bosses, inside at the junction of the bore and the walls, and the ring lands.

What does it mean if an engine part is within “serviceable limits”?

It means that the part is within the manufacturer’s limits and can be used in an engine. (Does not require replacement).

What tools or instruments are used to inspect a cylinder barrel for out-of-roundness?

A dial indicator or an inside micrometer is usually used to measure the top the the cylinder and the skirt. A telescopic gauge and micrometer can also be used. Two reading should be taken 90# to each other.

How can a loose stud in an engine crankcase be repaired?

Remove the loose stud and. inspect the hole for size and thread condition. It may be necessary to use an oversized stud or Heli-Coil insert.

What publication is used to check an engine for normal operation?

Manufacturer’s maintenance manual.

A weak cylinder is found during compression check. What must be inspected?

Inspections should focus on the valves, piston rings, cylinder walls, and the cylinder head gasket.

What purpose does an oil analysis serve?

Metal particles in the oil show normal and abnormal wear of the engine. This helps in evaluating the engine’s internal condition.

What inspection should be performed on an engine that has been in storage?

Perform an inspection to determine if there is any corrosion damage.

Where would a carbureator air temperature bulb be located?

In the ram air intake duct.

What is the fuel pressure range on float-type carbureators?

3 to 5 psi.

Name the basic components of a fuel indicator system.

The transmitter and the indicator.

What unit of measure is generally used to indicate fuel flow?

Pounds or gallons per hour.

Why is fuel flow monitored?

To determine fuel consumption and engine performance.

What does the manifold gauge indicate and how is it calibrated?

It measures the absolute pressure in the engine manifold and is calibrated in inches of mercury.

What directly controls manifold pressure?

Throttle opening and engine r.p.m.

During engine operation, where should the propeller control be set when checking the manifold pressure?

Low pitch, high r.p.m.

Is the aircraft electrical system required to power a cylinder head temperature gauge?

No. A thermocouple is used as the electrical source.

What are the possible indications of an oil pressure transmitter or indicator malfunction?

A severe or sudden drop in oil pressure while the oil temperature remains normal and the oil supply remains full.

What does a tachometer indicate on a reciprocating engine?

Engine r.p.m.

Where should the thermocouple be installed on a reciprocating engine using a single probe cylinder head temperature system?

On the hottest cylinder.

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

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

Name the major components in a gas turbine engine.

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

What determines the amount of airflow through a turbine engine?

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

Name the two types of compressors used in turbine engines.

Axial and centrifugal flow compressors.

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

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

What is a split compressor system?

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

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

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

What prevents the combustion chambers from burning?

Cooling air along the inside of the liner.

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

For flame propagation during star.

What function do axial flow turbine nozzles perform?

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

How is thermal stress relieved on a turbine disk?

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

What are carbon seals used for in turbine engines?

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

What is shaft horsepower?

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

Name the typical components of a typical APU.

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

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.

Where does the APU get its fuel supply?

From one of the aircraft’s main fuel tanks.

At what speed does a gass 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.

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.

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 specific amount of time to allow the EGTs 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.

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

Fuel pressure.

What is fan blade shingling?

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

When a turbofan or turbojet engine is shut down, what should a mechanic listen for during coast-down?

Any rubbing sound or other unusual noises from the engine.

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 wash are commonly used.

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.

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

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

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

Layout dye, felt tip marker, chalk.

How does relative humidity affect turbine engines?

Negligible effect.

What criteria determines replacement of life limited turbine engine components?

cycles, hours and/or calendar time.

if turbine blades are removed from a turbine disk, why should they be reinstalled in the same location?

To maintain the balance of the turbine wheel.

How are gas turbine engine tachometers calibrated?

They are calibrated in percent r.p.m.

On a turbine engine, what is the engine pressure ratio and what does it indicate?

It is the ratio between the total inlet pressure and total turbine exhaust pressure and is used to indicate the thrust developed by the engine.

On a turbine engine, what does exhaust gas temperature (EGT) indicate and how is it obtained?

EGT is the average temperature of the turbine discharge gasses and is obtained by thermocouples placed near the turbine exit.

Can the EGT system on a turbine engine be checked without running the engine?

Yes, by checking the resistance of the thermocouples and circuits.H

How can the remaining service life of starter-generator brushes be determined?

By visually inspecting the amount of wear groove remaining on the brushes.

What components of the starter-generator require periodic inspection?

Both communtator and brushes should be inspected for wear beyond operational limits.

Between field and armature windings in starter-generators, which ordinarily receive current for operation in the start mode?

Generally, both field and armature receive current for operation in the start mode.

what are possible sources of low pressure compressed air used for starting jet transport aircraft equipped with air turbine starters?

A ground power unit (GPU), and onboard auxiliary power unit (APU) compressed bleed air from an already running engine.

When should ignition ideally occur?

A specific number of degrees (as determined by the manufacturer) before the piston reaches top dead center on the compression stroke.

Why are dual magnetos used in the engine ignition system?

To improve combustion efficiency, and provide redundancy for safety.

Why is magneto timing so important?

For an engine to run properly, the spark plug in a cylinder has to fire at a specific time. To produce the required energy for the spark at the required time, the magneto must be in the E-gap position, the breaker points must start to open, and the distributor must be electrically aligned with the particular cylinder. If any of those requirements are not correctly adjusted, the engine will run rough or may not run at all. When preparing a magneto for installation, first the internal timing must be correctly adjusted, then the magneto-to-engine timing is adjusted.

What function does a magneto perform?

It produces a high voltage that forces a spark to arc across a spark plug gap.

Name the components of a high tension magneto system.

A permanent multipole rotating magnet, soft iron core, and pole shoes.

Name the three main circuits of a high tension magneto system.

Magnetic, primary, and secondary.

What is an E-gap angles?

It is a point a few degrees beyond the neutral position of a rotating magnet where maximum magnetic field stress exists.

What reduces arcing in the points and also aids the collapsing of the magnetic field in a magneto?

A capacitor.

How does a magneto produce the high voltage required to fire a spark?

In the rotation cycle, when the magnet rotor is in the E-gaps position, the primary points open, which interupts the current flow in the primary circuit causing a high rate of flux xhange in the core, and inducing a pulse of high voltage in the secondary coil.

What is the P-lead and how does it function?

The P-lead connects the ignition switch to the primary circuit of the magneto. When the ignition switch is turned off, the P-leads on both magnetos are grounded. When the ignition switch is turned to the “LEFT” position, the right magneto is grounded, so that only the left magneto operates, and visa versa.

What is a magneto timing light used for, and what does it indicate?

It is used for both internal amagneto timing and magneto-to-engine timing, and indicates the exact instant the magneto points open.

Why are turbine engine igniters generally not susceptible to carbon fouling?

Because the high energy sparks they produce clean off any deposits on the firing end.

What is the function of a duty cycle in relation to a turbine engine ignition system?

A duty cycle allows the operation of the ignition system for a given amount of time, and is then followed by a minimum specified cooling down period.

What are the three main components of a turbine engine ignition system?

Ignition exciters, high tension leads, igniters.

When the engine is operating, what does an ammeter / loadmeter indicate when connected to:

1. Battery positive lead?

2. Generator output lead?

When connected to the battery positive lead, the ammeter indicates whether the battery is charging or discharging. When connected to the generator output lead, it indicates the current produced by the generator, or electrical system load.

What is done to ensure that each generator shares the load in a multiple generator system?

The generators are parralleled.

Where would you find generator rating and performance data?

On the data plate attached to the generator.

How is the voltage of a DC generator controlled?

By varying the field current strength.

What is the most common type of alternator used in most aircraft AC aystems?

The three phase alternator.

How does a voltage regulator control the voltage of an alternator?

By regulating the voltage output of the DC exciter.

What does the speed of rotation and number of poles of an alternator determine?

The frequency of the alternator output.

What are some of the methods used to maintain 400 Hertz alternator output frequency on large turbojet or turbofan engines?

Constant speed drives, integrated drive generators, and variable-speed constant frequency (VSCF) power systems.

Name the major parts of a DC motor.

The armature, field, brushes and frame assembly.

Name the components of a direct cranking electrical starter system.

An electric motor, reduction gears, and an automatic engaging and disengaging mechanism.

Name three types of DC motors.

Series, shunt and compound.