Powerplant

How are conventional reciprocating engines classified?

They are classified according to cylinder arrangement with respect to the crankshaft (in-line, V-type, radial and opposed), or according to method of cooling (liquid cooled or air-cooled).

What is the mäin advantage of a horizontally opposed engine over a radial engine for powering modern aircraft?

The horizontally opposed engine has a much smaller frontal area and is easier to streamline than a radial engine.

Why is valve clearance adjusted on radial engines, but not on most horizontally opposed engines?

Most horizontally opposed engines have hydraulic valve lifters that keep all the clearance out of the valve train. Radial engines have solid lifters.

What are the different types of piston rings?

Compression rings, oil control rings, and Scraper rings.

What is the purpose of the oil control rings?

They are used to control the thickness of the oil film on the-cylinder walls.

What may be the result of installing piston rings incorrectly?

Excessive oil consumption.

What type of piston rods are commonly found in radial engines?

A master and articulating rod assembly.

What is the reason for using hydraulic valve lifters in an aircraft engine?

Hydraulic valve lifters keep all the clearance out of the valve operating mechanism. This decreases the wear of the valve train components.

At what speed does the camshaft turn, relative to the crankshaft speed in a horizontally opposed engine?

The camshaft turns at one half of the crankshaft speed.

Where is the piston in a reciprocating engine when the ignition spark occurs?

About 30 degrees of crankshaft rotation before the piston reaches the top center on the compression stroke.

What type of bearings are in general use in reciprocating engines?

Plain bearings, which are generally used for crankshaft, cam ring, camshaft, connecting rods, and accessory drive shaft bearings. Roller bearings, which are used primarily as crankshaft main bearings, but have other applications as well. Ball bearings, which are used for supercharger impeller shaft bearings, rocker arm bearings in some engines, and as propeller thrust bearings.

What is the indication of valve blow-by?

Valve blow-by is indicated by a hissing or whistle when pulling the propeller through prior to starting the engine. A cylinder compression check should be made to identify the faulty cylinder.

What is the purpose of using more than one spring for valve closing?

If only one Spring is used to close the valve, it will vibrate or surge at certain speeds. To eliminate this difficulty, two or more springs, one inside the other, are installed on each valve.
Each spring will vibrate at a different engine speed, and the spring surge vibrations will be dampened. Two or more springs also reduce the danger of weakness and possible failure by breakage due to heat and metal fatigue.

What is the purpose of valve overlap?

Valve overlap permits better volumetric efficiency and lowers the cylinder operating temperatures.

Describe the inspection you would give the valve springs during engine overhaul.

They should be cleaned thoroughly, and then visually inspected for evidence of overheating, cracks, broken ends, and for compression Strength.

What is the purpose of using valves with sodium filled stems?

Some intake and exhaust valve stems are hollow and partially filled with metallic sodium.
Sodium is used because it is an excellent heat conductor. The sodium melts at about 208°F, and the movement of the valves circulates the liquid sodium which enables it to carry the heat from the valve head to the stem, where it is dissipated through the valve guide to the cylinder head and cooling fins.

How is hydraulic lock removed from an aircraft engine?

Remove a spark plug from the locked cylinder and drain all the oil out.

What causes engine sudden stoppage?

Striking an object or engine seizure due to internal damage.

What type of inspection must be made after sudden stoppage of an engine due to striking and object?

The propeller drive shaft must be checked for misalignment, and the propeller checked for track.

What is the purpose of crankshaft dynamic dampers used in aircraft engines?

Dampers are used to overcome forces which cause deflection of the crankshaft and torsional vibration. These forces are generated by the power impulses of the pistons. Crankshaft vibrations are reduced by placing floating dampers (weights) in the counterweight assembly, particularly in a single throw type crankshaft.

How would you make a "runout" check on a crankshaft that is still installed in the engine?

Remove the propeller from the shaft and attach a dial indicator gage to the front of the crankcase.
Adjust the position of the gage needle So that it is touching the shaft. Turn the engine through with the starter and note any changes in the gage reading.

What causes detonation in an aircraft engine?

Excessive heat and pressure in the engine cylinder causes the air-fuel mixture to reach its critical pressure and temperature. Under these conditions, the mixture explodes rather than burns. This explosion is called detonation.

What can be learned about the condition of an engine by studying the results of a compression check?

From the cylinder compression check you can determine if the valves, piston rings, and pistons are adequately sealing the combustion chamber.

How can a cold cylinder be located on a double row radial engine?

A cold cylinder check can be made with a cold cylinder indicator, sometimes called a "Magic Wand", to locate any cylinder that has a temperature lower than the normal operating temperature of the other cylinders.

What instrument can be used to check a cylinder bore for out of roundness?

A cylinder bore can be checked with dial indicator, a telescopic gage and micrometer, or an inside micrometer.

Why is it important to preserve engines that will not be operated for an extended period of time?

Moisture will accumulate in the engine, initiating the corrosion process which can ruin the engine.

What are the major components of a typical gas turbine engine?

A typical gas turbine engine consists of an air inlet, a compression section, a combustion section, a turbine section, an exhaust section, and the accessory section.

What are the two principal types of compressors used in turbojet aircraft?

The two most common compressors in use today are the centrifugal flow and the axial flow types.

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 are the three types of combustion chamber systems?

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

What four types of gas turbine engines are used in aircraft?

Turbofan, turboprop, turboshaft, and turbojet.

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 contract around the turbine wheel and seize the rotor.

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.

What is the purpose of the interconnector tubes between can type combustion chambers?

To spread the flame to the combustion chambers that are not equipped with igniter plugs during engine starting.

What prevents burning of the liner walls in can type combustion chambers?

Louvers are provided along the axial length of the liners to direct a cooling layer of air along the inside wall of the liner.

What types of damage may be found when inspecting compressor blades?

Dents, scratches, gouges, galling, bumps, burrs, pitting and cracks.

What is the location and function of the diffuser section in a turbine engine?

The diffuser is the divergent section of the engine located between the compressor section and the burner cans. Its function is to change the high velocity compressor discharge air to static pressure.

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.

How are compressor blades attached to a disk-type rotor?

The compressor blades are fitted into the disks by either bulb-type or fir-tree type roots. The blades are then locked by means of Screws, peening, locking wires, pins, or keys.

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.

What is the major difference between a turboprop and a turbojet engine?

A turboprop engine usually has more turbine stages than a turbojet engine. In addition to operating the compressor and accessories, the turboprop turbine must also drive a propeller.

What is 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.

What is the effect of air density on the thrust of a turbine engine?

Air density determines the mass of air that is used by a jet engine. The factors that affect density are the temperature and pressure of the air. If density increases, thrust increases, and vice versa.

What is a split compressor system in a turbine engine?

The split compressor system requires two concentric shafts joining the turbine stages to their respective compressors.

What are two methods of relieving thermal stress on a turbine disk?

Bleed air and grooves allow cooling air onto the face of the disk.

What may be the indications if a turbojet engine is out of trim?

The engine has a high exhaust gas temperature (EGT) at target engine pressure ratio (EPR) for takeoff.

What prevents burning of combustion chambers?

Cooling air along the inside of the liner.

Name an advantage of a turbofan over a turbojet.

Additional thrust without increasing fuel flow.

What is meant by trimming a turbojet engine?

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

Where are most APUs located in modern jet aircraft?

In the tail cone of the fuselage.

What is the function of an APU?

An APU provides electric power and compressed air when the main engines are not operating.

What are two sources of compressed air from an APU?

From bleed air from the APU turbine compressor, or from a load compressor driven by a free turbine in the engine.

How is most of the troubleshooting done for a modern APU?

By the fault codes generated by the FADEC.

What type of engine is an APU?

A turboshaft engine.

What places the greatest demand on a gas turbine APU?

Bleed air loads.

What may be used as a guide for a 100-hour inspection on an aircraft engine?

FAR Part 43, Appendix D.

What additional inspection must be performed if the cylinder compression is weak?

You must make an internal cylinder inspection for improper internal tolerances. Either by borescope or disassembly.

Where are the engine operating limitations found?

In the Aircraft Specifications or Type Certificate Data Sheet.

Where can a mechanic look to identify an aircraft engine?

On the engine identification plate that is affixed to the engine at an accessible location.

What publication can a mechanic find the operating limitations for an aircraft engine?

The Engine Specifications or Engine Type Certificate Data Sheet.

What type of FAA approval is required when a change to an engine type design is not enough to require a new Type Certificate?

A Supplemental Type Certificate is required.

What type of FAA approval is required when a change is made to the engine principle of operation?

A new Type Certificate must be obtained.

Name at least one place where a list of life limited parts may be found.

The Type Certificate Data Sheet or The Airworthiness Limitations Section of the Instructions for Continued Airworthiness.

When removing turbojet or turbofan turbine blades, why is it important that they be installed in the same location?

To maintain the balance of the turbine wheel.

What items are checked in a hot section inspection of a turbine engine?

The combustion section is checked for cracks or distortion. The turbine wheel, the turbine case and the exhaust section are checked for cracks, indications of overheating, and any indication of warpage, erosion, or burning.

What inspection must be performed on an aircraft reciprocating engine if it has been operated on jet fuel?

The inside of the cylinders must be inspected with a borescope. The oil must be changed, and the filters examined, and the engine given a careful run-up and operation check.

What items must be included in the operational check of a reciprocating engine prior to approval for return to service after a 100-hour inspection?

Power output (static and idle RPM); Magnetos; Fuel and Oil pressure; Cylinder and Oil temperature.

How are reciprocating engine tachometers calibrated?

In hundreds of RPM.

How are turbine engine tachometers calibrated?

In percent of the engine's rated takeoff RPM.

Where do reciprocating engine manifold pressure gauges read the pressure?

From the intake manifold of the engine.

Why do most engine oil pressure gages have a restrictor in the line between the engine and the instrument?

The restrictor damps out pressure pulsations in the oil to keep the needle from oscillating.

How does the pressure gauge measure the fuel flow of a horizontally opposed, fuel-injected reciprocating engine?

By the pressure drop across injector nozzles.

What is indicated if the needle of a ratiometer-type oil temperature indicator pegs to the high side of the dial as soon as the aircraft master switch is turned on?

There is an open in the bulb circuit that causes the instrument to see an infinite resistance. The higher the resistance in the bulb circuit, the higher the temperature indication.

What type of indicating system is used to measure the exhaust gas temperature of a turbine engine?

A set of thermocouples arranged in an averaging circuit. These thermocouples are installed in the tail pipe of the engine.

What should a thermocouple-type cylinder head temperature indicator read when the engine is not operating?

The temperature of the outside air.

How does a torquemeter measure the torque produced by an engine?

The pressure it measures is produced in a torque sensor through oil and is proportional to the amount of strain in the torsional shaft that drives the reduction gears of the turboprop engine.

What two pressures are measured to get the engine pressure ratio of a turbojet engine?

The turbine discharge total pressure and the compressor inlet total pressure.

What are the two materials that the thermocouples are made of that are used in a turbine engine exhaust gas temperature system?

Chromel and alumel.

What kind of mechanism is normally used to measure oil pressure in a reciprocating engine?

A bourdon tube mechanism.

What is measured by the tachometers used on a two-spool gas turbine engine?

The N1 tachometer shows the RPM of the low-pressure compressor, and the N2 tachometer shows the RPM of the high-pressure compressor.

What is the range of pressure needed for a fuel pressure gauge used with a float carburetor?

Normally from 1 to 25 psi.

Does the tachometer of a geared reciprocating engine indicate the speed of the crankshaft or of the propeller shaft?

The speed of the crankshaft.

What is turbojet EPR?

It is a ratio between total turbine discharge pressure to total inlet pressure and is an indication of thrust being developed by the engine.

What are the colors used in annunciator indicating systems, and what do they represent?

Green indicates a safe condition, yellow indicates a caution alert, and red indicates a warning alert.

What are four types of engine fire detection systems?

Thermoswitch system, thermocouple system, thermistor-type continuous-loop system, and pneumatic-type continuous-loop system.

Can a thermal switch-type fire detection system indicate a general overheat condition?

No, this is a spot-type fire detection system, and it cannot indicate an overheat condition.

Can a thermocouple-type fire detection system indicate a general overheat condition?

No, this is a rate-of-temperature-rise-type of system, and it cannot indicate a general overheat condition.

How is a thermocouple-type fire detection system checked for operation?

Current is sent through the heater in the thermal test unit. This heats the thermocouple, which produces enough current to close the sensitive relay. Current through the contacts of the sensitive relay closes the slave relay and turns on the fire warning light.

Does a break in a continuous-loop fire detection system keep the system from detecting a fire?

No, the unit will show a fault when it is tested, but it will still warn of a fire.

Does the pneumatic-type continuous-loop fire detector system detect a general overheat condition as well as a fire?

Yes, a high temperature on a small portion of the sensor or a lower temperature over the entire sensor will cause it to release enough gas to close the diaphragm switch and warn of a fire.

What are the two most used fire extinguishing agents used for engine fires?

Halogenated hydrocarbon, such as Halon 1301, and carbon dioxide.

What extinguishing agent is used in the high-rate discharge fire extinguishing systems installed in most turbojet engine installations?

Halon 1301.

What happens when the pilot pulls the fire-pull T-handle in the flight deck of a jet aircraft?

The bottle discharge switch is uncovered and armed, the generator field relay is tripped, fuel is shut off to the engine, hydraulic fluid is shut off to the pump, the engine bleed air is shut off, and the hydraulic pump low-pressure lights are deactivated.

How can a mechanic determine the state of charge of the HRD bottles for the aircraft fire extinguishing system?

A pressure gage on the bottle shows the pressure of the contents of the bottle.

What method is used to discharge an HRD bottle in a turbine engine aircraft?

An electrically ignited powder charge blows a knife through the seal on the bottle and discharges the contents.

Which type of fire detection system operates on the rate of temperature rise?

The thermocouple system.

What is indicated if the yellow blow-out plug of the fire extinguisher system on the side of an engine nacelle is blown out?

The built-in fire extinguishing system has been discharged by normal operation.

What is indicated if the red blow-out plug of the fire extinguisher system on the side of an engine nacelle is blown out?

The built-in fire extinguishing system has been discharged by a high temperature condition.

How is a pneumatic-type continuous-element loop detector system checked for operation?

Low voltage alternating current is sent through the sheath of the detector element. This heats the element, and it releases enough gas to close the diaphragm switch.