Ae2028 2 marks

JEPPIAAR ENGINEERING COLLEGE AE2028 AERO ENGINE MAINTENANCE & REPAIR

DEPT. OF AERONAUTICAL ENGINEERING STANLY WILSON D

Department of Aeronautical Engineering

"When once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for

there you have been, and there you will always long to return."

www.Vidyarthiplus.com




PART A

1. Name four common engine classifications by cylinder arrangement a) Inline engines b) ‘V’ type engines c) Opposed or flat type engines d) Radial type engines

2. What is the purpose of dynamic damper?

The purpose of the dynamic dampers is to relieve the whip and vibration caused by the rotation of the crank shaft. They are suspended from or installed in specified crank cheeks.

3. What should be done if there is no indication of oil pressure shortly after starting the engine?

After starting the engine, check for oil pressure immediately. If oil pressure does not register within the prescribed time (10-30 sec) shut down the engine and identify the problem.

4. What is the function of the dimensional inspection during overhaul?

Dimensional inspection is employed to determine the degree of wear for parts of the engine where moving surfaces are in contact with other surfaces. If the wear between the surfaces exceeds the table of limits the parts must be replaced or repaired.

5. What are the methods and purpose of the preliminary visual inspection?

Visual inspection is accomplished by direct examination and with the use of magnifying glass. In both case a strong light should be used to reveal the defects. Visual inspection reveals cracks, corrosion, nicks, scratches,





galling, scoring and other disturbances. Parts that are damaged beyond repair should be discarded and marked. So that they will not be reused.

6. How are aircraft piston engines classified?

Aircraft piston engines are classified by

a) Cylinder arrangement b) Cooling method c) Number of stroke per cycle

7. Write short notes on Master Articulated Rod

The master articulated rod assembly is used primarily for radial engines, although some ‘v’ type engines employed this type of rod assembly. It is made of alloy steel forging, machined, polished to final dimensions and heat treated to resist vibration and stresses. The master rod is similar to other connecting rods except that it is constructed to provide for the attachment of the articulated rods on the large end. Master rod bearings are of plain type and consist of a split or a sleeve. The articulated rods are hinged to the master rod flanges by means of knuckle pins.

8. Enumerate the basic steps of the overhaul process. 1. Receiving inspection 2. Disassembly 3. Visual inspection 4. Cleaning 5. Structural inspection 6. Dimensional inspection 7. Repair and replacement 8. Reassembly 9. Installation 10. Engine testing and run in





11. Preservation and storage

9. What are the methods adopted for structural inspection of aero engine parts? or State the methods followed in ‘Non Destructive testing’

Engine parts are structurally inspected by the following methods

1. Magnetic particle testing 2. Liquid penetrant inspection 3. Eddy current inspection 4. Ultrasonic inspection 5. Radiography inspection

10. Explain the term “Manifold Absolute Pressure” (MAP)

Manifold absolute pressure is the absolute pressure of the fuel air mixture immediately before it enters the intake port of the cylinder. Absolute pressure is the pressure above a complete vacuum and is indicated in pounds per square inch absolute or inches of mercury in metric system.

11. What is meant by Trouble shooting? Trouble shooting is the step by step procedure used to determine the cause of a given fault and then select the best and quickest solution. When trouble shooting, the technician must evaluate the performance of the engine by comparing data on how the engine should operate and how it is currently operating.

12. Why is ‘Compression Testing’ is carried out in piston engine?

The purpose of testing the cylinder compression is to determine the internal condition of the combustion chamber by ascertaining if any appreciable leakage is occurring.





13. What is valve overlap?

The angular distance through which both the valves are open is called valve over lap or valve lap.

14. What effect does exhaust back pressure have on engine performance?

Exhaust back pressure has a decided effect on engine performance because any pressure above atmospheric at the exhaust port of a cylinder will reduce volumetric efficiency. Exhaust back pressure begins with at the cylinder exhaust port. So both size and shape of the opening and passage will affect the pressure.

15. How is the term “overhauled engine” defined?

Overhauled engine, according to the FAA, (Federation of Aviation Authority) is one that has been disassembled, cleaned, inspected, repaired as necessary, reassembled, and tested according to the manufacturer instruction and specification.

16. Describe a Power check

The time during which an engine is operated at full power is referred to as a power check. The purpose of this check is to ensure satisfactory performance.

17. List the advantages of the opposed- type engine design.

1. The engine has a low weight power ratio.

2. Because of its flat shape it is very well adapted to streamlining and to horizontal installation in the nacelle.

3. It is reasonably free from vibration.





18. Name the four principle type of crankshaft.

1. Single- throw crank shaft 2. Double- throw crank shaft 3. Four- throw crank shaft 4. Six- throw crank shaft

19. What is the engine preheating procedure?

Preheating an engine consists of forcing heated air into the engine area to heat the engine, lubricants, and accessories. Preheating is required for most aircraft reciprocating engines when outside temperatures are +10ºF and below as stipulated in the manufacturer´s instruction.

20. Explain the term ‘Backfiring’

Backing firing occurs during starting of a cold engine because of poor combustion. The F/A mixture is still burning at the time the intake valve opens, and the flame burns back through the intake valve. This sometimes causes a fire in the induction system.

21. Differentiate between overhauled engine and rebuilt engine.

OVERHAULED ENGINE is one that has been disassembled, cleaned, inspected, repaired as necessary, reassembled, and tested as per manufacturer instructions and specifications.

REBUILT ENGINE is one that has been disassembled, cleaned, inspected, repaired as necessary, reassembled, and tested as per manufacturer instructions and specifications. In addition the parts that are reused must meet the limits and tolerances specified for new parts and also Rebuilt engine must be functionally tested to meet the requirements of a new engine.





22. Enumerate the precision measuring tools used in overhaul shop of a piston engine.

1. Depth gauges

2. Dial gauges

3. Height gauges

4. Micrometers

5. Small-hole gauges

6. Surface plate

7. Telescope gauges

8. V blocks

23. Define Fretting.

The surface erosion caused by very slight movement between two surfaces which are tightly pressed together.

24. Define ‘E’ gap

In a magneto, the number of degrees of rotating between the neutral position and the position where the contact points open is called the E-gap angle, usually called E gap or Efficiency gap

25. What is meant by choke bored cylinder?

In some cylinder, the cylinder is bored with a slight taper. The end of the bore nearest the head is made smaller than the skirt end to allow for the expansion caused by the greater operating temperature near the head. Such a cylinder is called chocked bore cylinder





26. Describe the two types of ‘antifriction’ bearings.

Roller bearings called ‘antifriction’ bearings. They are straight roller bearings, tapered roller bearings. Because the rollers eliminate friction to a large extent. Straight roller bearings are used for radial loads. Tapered roller bearings are used for radial and thrust loads. 27. Why are crankpins usually hollow?

Crankpins usually hollow for three reasons

1. It reduces the total weight of the crank shaft 2. It provides the passage for the lubricating oil 3. It serves as a chamber for collecting carbon deposits, sludge, and other

foreign substances.

28. What are the advantages of a dual-magneto ignition?

1. If one magneto or any part of one magneto system fails to operate, the other magneto system will furnish ignition.

2. When two spark, igniting the F/A mixture in each cylinder at different places, provide quicker and complete combustion than a single spark.

29. What is the spark plug rotation?

Excessive electrode erosion is caused by magneto constant polarity firing and capacitance after firing. Constant polarity occurs with even numbered cylinder magnetos. To equalize this wear, during overhaul spark plugs are removed and all odd number cylinder plugs are fitted in even number cylinder and all even number cylinder plugs are fitted in odd number cylinder. This is called spark plug rotation.

30. Why exhaust valve stem or head is made hollow?

Exhaust valves operate at high temperatures, to dissipate the heat

rapidly the hallow stem or head is filled partly with metallic sodium





which melts at 200°F and dissipate the heat through valve guide and

valve head to atmosphere.

PART B

1. Explain the inspection and maintenance of engine control carried out in piston engines.

Engine controls such as throttle, mixture control, propeller, and cowl flap controls need to be checked during 100hrs inspection.

CHECKS

1. Inspect push- pull controls for wear and smooth operation. 2. Operate the system slowly and watch for any strain on the rods and

tubing (may cause bending or twisting) 3. Examine each rod end i.e., threaded and observe whether the rod is

screwed into the socket body for enough to be seen through the inspection hole.

4. Eliminate any play by making certain that all the connections are tight.

5. Examine the guides to see if the rods bind too much on the guides, but do not mistake any binding for spring back. Replace guides that cause binding.

6. Adjust the length of screw end rods by screwing them into or out of the control end. Re-tighten the locknuts.

7. If any rod is removed, label it to show its location on reassembly. 8. Replace any ball bearing rod ends that cause lost motion.

2. (i) what is the purpose of testing the cylinder compression?





(ii) Name the two basic types of compression testers

(iii) How are both compression testers utilized in checking the compression of aircraft cylinders? Explain the procedures with neat sketches.

COMPRESSION TESTING





COMPRESSION TESTING OF AIRCRAFT ENGINE CYLINDERS

The purpose of testing the cylinder compression is to determine the internal condition of the combustion chamber by ascertaining any appreciable leakage is occurring.

TYPES OF COMPRESSION TESTERS

1. Direct compression tester 2. Differential- pressure tester

DIRECT COMPRESSION TESTER

Record all readings and compare to specifications. It indicates the actual pressure in the cylinder.

1. DRY COMPRESSION CHECK 2. WET COMPRESSION CHECK

Procedure:

1. Warm up the engine and shut it down. 2. Remove the spark plug as soon as possible. 3. Rotate the engine with a starter to expel any excess oil or loose carbons. 4. Install tester in one of the cylinder. 5. Using engine starter, rotate the engine three revolutions and note the

reading. (External power source e.g. battery can be used to run engine starter.

6. Compare the readings with each other. Any cylinder having approximately 15 psi lower than the other should be suspected defective.





WET COMPRESSION CHECK

1. A wet compression check should be done on defective cylinder to find out the problematic area.

2. Squeeze oil or throw oil into the cylinder having low pressure reading

3. With the compression tester gauge re check the pressure.

4. If the compression gauge reading goes up while oiling the cylinder indicating problem in piston rings or wall of the cylinder.

5. If the readings remain same then valve or cylinder head gasket having the problem.

DIFFERENTIAL- PRESSURE TESTER

To check the compression of a/c engine by measuring the leakage through the cylinder that is caused by worn or damaged components.

PRINCIPLE

The regulator air pressure is applied to one side of the orifice with the air valve closed and no leakage on the other side of the orifice, and then both the pr. gauge will read the same. When the air valve is opened the leakage through the cylinder increases. The cylinder pr. gauge will record a proportionally low reading.

PROCEDURE

1. Warm up the engine and shut it down. 2. Remove the spark plug as soon as possible. 3. With the air valve closed apply an external source of clean air approx.

100-120 psi 4. Install an adapter in the spark plug bushing and connect the compression

tester to one of the cylinder.





5. Adjust the pressure regulator gauge to 80 psi; at this the cylinder pr gauge should also register 80 psi.

6. Now turn the crankshaft by hand in the direction of rotation until the piston is coming up on its compression stroke.

7. Open air valve and pressurize the cylinder to approx. 20 psi 8. Continue rotating the engine against the pressure until the piston reaches

the TDC. 9. Open the air valve completely. Check the regulated pr and adjust, if

necessary to 80 psi.

10. Observe the pressure indication on cylinder pr gauge

11. Compare the cylinder pr. Gauge with regulator pr. Gauge

12. The difference between the cylinder pr. and pr. of regulator pressure gauge is the amount of leakage through the cylinder

13. If the leakage is more than 25% go for the wet test.

PROBLEMATIC AREA (when leakage occurs)

1. Air is leaking through crank case breather (leakage around the piston rings or hole in the piston)

2. Air leaking from the valve because of the carbon piece (can be hear air exiting from the exhaust stacks)

1. Define trouble shooting and explain the six step trouble shooting procedure.

TROUBLE SHOOTING

Trouble shooting is the step by step procedure used to determine the cause of a given fault and then select the best and quickest solution.





SIX STEP TROUBLE SHOOTING PROCEDURE

1. SYSTEM RECOGNITION:

The first step in trouble shooting – involves having knowledge of engine condition that is not normal and knowing to what extent the fault is affecting the engines performance.

2. SYMTEM ELABORATION:

It is the next logical step, once a fault or malfunction has been detected. Test equipment helps the technician to evaluate the performance of the engine and its components. The technicians should use these aids to assess the effects of the symptoms and to provide additional information to further define the symptoms.

3. LIST OF PROBABLE FAULTY FUNCTIONS

When the technician has located all the symptoms of malfunction or fault it is the third step to list the possible causes.

To aid this process, manufacturer manual list the ‘probable cause’ for the symptom and corrective action.

4. LOCALIZING THE FAULT

Localizing the fault means, to determine which functional system of the engine is creating problem. This trouble may be traced by trouble shooting charts.

5. ISOLATING THE FAULT TO A COMPONENT

Once the malfunction is isolated to one system, additional testing is done to isolate the fault to a specific component. The technicians use test equipments to measure the correct output for various components.





6. FAILURE ANALYSIS

Once the fault or malfunction traced to a specific component, attempt to be made to determine the cause of failure.

Without analyzing the reason, substituting a new component into the system will damage the new component.

If the component is the probable cause for all the abnormal symptoms noticed earlier steps, then it can be assumed that the component is at fault.

SPARK PLUG





2. Explain in sequence about servicing procedure of spark plug with neat diagram.

PURPOSE

Scheduled servicing intervals are determined by the individual aircraft operator. These intervals will vary according to the operating conditions, engine models, and spark plug types. The principal determining factor in the removal and servicing of spark plug is the width of the spark –gap that is, the distance between the electrodes where the spark is produced.

SERVICING PROCEDURE

1. Removal a. Shielded terminal connectors are removed by loosening the elbow nut

with the proper size crowfoot or open end wrench. (Care must be taken to avoid damage to the elbow and sleeve or the barrel insulator.

b. Remove the spark plug from the cylinder by using the proper size of deep-socket wrench.(recommended 6point wrench)

c. Removed spark plug should be placed in a tray with numbered holes so that the engine cylinder from which the spark plug has been removed can be identified.

d. Spark plug should not be dropped on hard surface because cracks may occur in ceramic insulation which is not apparent on visual inspection. Any plug which has been dropped should be rejected or returned

2. PRELIMINARY INSPECTION a. Immediately after removal, the spark plug should be given a careful

visual examination. All unserviceable plugs should be discarded.(spark plugs with cracked insulators, badly eroded electrodes, damaged shells or threads should be rejected)





3. DEGREASING a. All oil and grease should be removed from both interior and exterior of

the plug according to the approved degreasing method.(Either vapor degreasing)

4. DRYING a. After degreasing spark plugs should be dried inside and outside either

by compressed air or a drying oven. 5. CLEANING

a. Spark plug should be cleaned for lead and carbon deposits form on the ceramic core, the electrodes, and inside of the spark plug shell by abrasive blasting machine designed for cleaning spark plug.

b. Immediately after cleaning by the wet-blast method the plugs must be oven dried to prevent rusting and to ensure satisfactory electrical test.

6. REGAPPING

a. The tools and methods used to set spark plug gaps will vary with shape, type, and arrangement of electrodes.

b. The gap in any spark plug is measured by round wire gages.

c. To measure minimum and maximum widths for the gap, a spark plug gap gage will have two wires.(0.011’-0.015’)

d. Smaller dimension gage must pass through and larger dimension gage must be too large to pass through the gap.

e. If the gap is too large it is closed by means of a special gap setting tool.





7. INSPECTION AND TESTING

a. Visual inspection is done with a magnifying glass. Good lighting must be provided.

b. The following item are examined: threads, electrodes, shell hexagons, ceramic insulation, and the connector seat.

c. Spark plugs are tested by applying high voltage, equivalent to normal ignition voltage. While plug is under pressure.

d. Spark plug which fails to function properly during the pressure test should be baked in an oven for about 4hrs at 225ºF.

8. GASKET SERVICING

a. When spark plugs are installed either new or recondition gasket should be used.

b. Used gaskets should be annealed by heating to a cherry red and immediately quenched in light motor oil.

c. After quenching oil to be removed with a solvent.

d. Then immerse it in a solution of 50%water and 50% nitric acid to remove oxides.

e. After the acid bath gasket should be rinsed in running water and air dried.

9. PLUG ROTATION

a. Excessive electrode erosion is caused by magneto constant polarity firing and capacitance after firing.

b. To equalize this wear, keep the plugs in engine sets, placing them in the tray identified by cylinder locations. After the plugs have been serviced, rotate them as instructed.





10. INSTALLATIONS

a. Before installing, proper type of the plug to be selected.

b. Long and short reach should not be interchanged.

c. Before installing plug and cylinder threads are to be cleaned

d. Anti seize compound to be applied on the threads

e. Install the plug along with gasket and tighten with hand

f. At last tighten them with wrench.

3. Explain the safety practices followed in preparation of for overhaul an engine in an overhaul shop.

THE SAFETY PRACTICES FOLLOWED IN PREPARATION FOR OVERHAUL OF AN ENGINE IN AN OVERHAUL SHOP.

1. Arrangement of the shop should be such that crowding does not exist. 2. Materials and parts must be stored in proper racks and containers. 3. Walk ways should be painted on the floor and kept free of obstructions at

all times other than equipment movement. 4. Suitable engine overhaul stands must be provided. 5. Adequate work station and bench space should be provided. 6. All machinery must be in good condition and provide with safety guards. 7. Electric equipment and power outlets must be correctly identified for

voltage and power handling capability. 8. Electric outlets must be designed so that it is not possible to plug in

equipment other than designed for the voltage and the type of current supplied at the outlet.





9. Adequate lighting must be provided for all areas. 10. Cleaning areas must be adequately ventilated to remove all dangerous

fumes from solvents, degreasing agents, and volatile liquids. 11. Proper engine hoisting equipment must be available for engine lifting. 12. Operators should wear safety shoes to prevent injuries to their feet

incase heavy equipment or parts are dropped. 13. Operators of lathes, machine tools, drills, and grinders must wear safety

goggles. 14. Operators of cleaning equipment must wear protective clothing, rubber

gloves, and face shields. 15. The floor of the shop should have a surface which does not become

slippery. 16. Grease, oil and similar materials should be removed from the floor

immediately after they are spilled. 17. Proper type of fire extinguishers should be available to the shop

personnel. 18. Personnel should take care to stay clear of an engine which is being

hoisted except as necessary to prevent it from swaying or turning. 19. All areas must be kept free of sand, dust, grease, dirty rags, and other

types of contaminants. 20. Greasy or oily rags must be stored in closely covered metal cans. 21. Flammable liquids must be stored in an approved fire proof area. 22. The area where engines are run should be paved and kept free of debris

which could be picked up by air currents caused by a propeller or jet intake.





6. Name the auxiliary ignition devices and with the help of neat diagram describe the operation of an induction vibrator

AUXILIARY IGNITION DEVICE

When attempting to start an engine, often the engine will not rotate the crankshaft fast enough to produce the required coming in speed of the magneto. In these instances a source of external high-tension current is required for ignition purposes. These various devices are called IGNITION BOOSTERS OR AUXILIARY IGNITION DEVICES

1. Impulse coupling 2. Booster coil 3. Induction vibrator





INDUCTION VIBRATOR

The function of the induction of the induction vibrator is to supply low voltage (pulsating direct current) for the magneto primary coil, which induces a sufficiently high voltage in the secondary coil for starting.

OPERATION

When the starter switch is closed battery voltage is applied to the vibrator coil through the vibrator contact points and through the retard

Contact points in the left magneto. As the coil is energized, the breaker points open and interrupt the current flow, thus de energizing the coil, VC. Through spring action the contact points close and again energizes the coil, causing the points to open. Thus the contact points of the vibrator continue to make and break contact many times per second, sending an interrupted current through both the main and retard contact points of the magneto.

The vibrator sends an interrupted battery current through the primary winding of the magneto coil. The magneto coil then acts as a battery ignition coil and produces high tension impulses, which are distributed through the distributor rotor, distributor block, and cables to the spark plug.

These high tension impulses are produced during the entire time that both sets of magneto contact points are open. When the contact points are closed, spark cannot be generated. Although the vibrator continues to send interrupted current impulses through the magneto contact points, the interrupted current will flow through the contact points to ground.





7. With the help of a simplified diagram, explain the fundamental operation of a pressure injection carburetor.

PRESSURE INJECTION CARBURETOR

The basic principles of the pressure injection carburetor can be explained that mass air flow is utilized to regulate the pressure of fuel to a metering system which in turn governs the fuel flow. The carburetor increases the fuel flow in proportion to mass air flow and maintains a correct F/A ratio in accordance with the throttle and mixture settings of the carburetor





Pressure carburetors do have some advantages over float-type carburetors, they operate during all types of flight maneuvers (including aerobatics), and carburetor icing is less of a problem.

The main parts of a pressure injection carburetor system: 1. The throttle unit, 2. The regulator unit, 3. The fuel control unit.

When the carburetor is operating, the air flows the air flows through the throttle unit. At the entrance to the air passage are impact tubes which develop a pressure proportional to the velocity of the incoming air. This pressure is applied to chamber A in the regulator unit. As the air flows through the venturi, a reduced pressure is developed in accordance with the velocity of the air flow. The reduced pressure is applied to chamber B in the regulator unit. The high pressure in chamber A and the low pressure in chamber B will create a differential pressure across the diaphragm between the two chambers. The force of this pressure differential is called the Air Metering force, and as this force increases it opens the poppet valve and allows fuel under pressure from the fuel pump to flow into chamber D. This unmetered fuel exerts force on the diaphragm between chamber D and chamber C and thus tends to close the poppet valve. The fuel flows through one or more metering jets in the fuel control unit and then to the discharge nozzle. Chamber C of the regulator unit is connected to the output of the fuel control unit to provide metered fuel pressure to act against the diaphragm between chamber C and D. Thus, unmetered fuel pressure acts against the D side of the diaphragm, and metered fuel pressure acts against the chamber C side. The fuel pressure differential produces a force called the fuel metering force.

When the throttle opening is increased, the air flow through the carburetor is increased and the pressure in the venturi is increased. So the pressure in chamber B is lowered, the impact pressure to chamber is increased, and the





diaphragm between chambers A & B moves to the right because of the differential pressure. (Air metering force) This movement opens the poppet valve and allows more fuel to flow into chamber D. This increases pressure in chamber D and tends to move the diaphragm and poppet valve to the left against the air metering force, however this movement is modified by the pressure of metered fuel in chamber C. The pressure differential between chamber C & D (fuel metering force) is balanced against the air metering force at all times when the engine is operating at a given setting.

C The chamber pressure is established approximately 5psi by the spring- loaded, diaphragm-operated main discharge nozzle valve. This valve prevents leakage from the nozzle when the engine is not running.

8. Explain the various methods adopted for Non-Destructive Testing (NDT) techniques.

The various methods adopted for Non- Destructive testing are

1. Magnetic particle testing 2. Liquid penetrant inspection 3. Eddy current inspection 4. Ultrasonic inspection 5. Radiography

Magnetic particle testing

Magnetic particle testing is a nondestructive method for locating surface and subsurface discontinuities (cracks or defects) in ferromagnetic materials such as steel. The parts are magnetized by passing a strong electric current through or around them. An electric current is always accompanied by a magnetic field. The magnetic lines of flux travel through the part when it is magnetized. A





crack or discontinuity will create a flux leakage. A flux leakage occurs when the lines of flux leave the surface of the material, resulting in a concentration of magnetic strength at the discontinuity. Magnetic particles are applied to the magnetized part and concentrate in the areas of flux leakage, giving a sign of discontinuity. After the parts have been inspected, they must be demagnetized. If this is not done the parts will pick up and hold small steel particles, which can cause serious damage in the engine during operation.

Liquid penetrant inspection

Engine parts made of aluminum alloys, magnesium alloys, bronze, or any other metal which cannot be magnetized are inspected by means of a fluorescent penetrant, a dyepenetrant, ultrasonic equipment, or eddy current equipment. The parts are thoroughly cleaned and stripped of paint. The penetrant is applied to the part and allowed to enter any surface discontinuities. The excess penetrant is cleaned off, and then the part is given a coat of developer. The developer draws out any penetrant that may have entered surface discontinuities, making them visible either under white light in the case of visible penetrant or black light in the case of fluorescent penetrant.

Eddy current inspection

Eddy current inspection is to discover defects inside metal parts. The eddy current tester applies high frequency electromagnetic waves to the metal, and these waves generate eddy currents inside the metal. If the metal is uniform in its structure, the eddy current will flow in a uniform pattern and this will be shown by the indicator. If a discontinuity exists, the effect of the eddy currents will be changed and the indicator will produce a reading greater than normal for the particular test.

Ultrasonic inspection

Ultrasonic inspection utilizes high frequency sound waves to reveal flaws in metal parts. The element transmitting the waves is placed on the part, and a





reflected wave is received and registered on a oscilloscope. If there is a flaw in the part, the reflected wave will show a “blip “on the oscilloscope trace. The position of the blip indicates the depth of the flaw.

Radiography

X ray or radiographic inspection is to detect certain types of metal defects. The x-ray is effective in detecting discontinuities inside casting, forging and welds. A power full x-ray can penetrate metal or several inches and produce an image which will reveal defects within the metal.

9. Write short notes on

or

Piston engine testing procedure after overhaul

Answer

1. Pre-oiling: Before the engine is started for the first time, it should be pre-oiled to remove air trapped in oil passages and lines and to ensure that all bearing surfaces are lubricated. One method: One spark plug is removed from each cylinder. The crankcase or external oil tank to be filled with oil for run in, and the engine is cranked with the starter until an oil pressure indication is read on the oil pressure gage. Another method: oil is forced by means of a pressure oiler at a prescribed pressure, through the oil galleries until it comes out an oil outlet or the opposite end of an oil gallery.





Power check: Manufacturer’s overhaul manual provides instruction and run in schedule for a newly overhauled engine. The purpose of run in is to permit newly installed parts to burnish or wear in, piston rings to seat against cylinder wall, and valves to become seated. And also makes it possible to observe the engines operation under controlled conditions and to ensure proper operation from idle to 100 percent power. The time during which an engine is operated at full power is referred to as power check. The run in should be accomplished with the engine installed in a test cell equipped as specified in the manufacturer’s overhaul manual. Oil consumption run: An oil consumption run is made at the end of the test. Record the oil temperature. Stop the engine. Place a previously weighed container under the oil tank or oil sump and remove the drain plug. Allow the oil to drain for 15 mints. Replace the drain plug. Weigh the oil and the container. Record the weight of the oil (total weight less the weight of the container). Replace the oil in the tank or sump. Start the engine, warm up the engine to the specified rpm± 20rpm and operate at this speed for 1 hour. At the end of one hour of operation with the same oil temperature drain the oil as before. The difference in oil weights at the start and end of the run will give the amount of oil used in 1hour.





10. If the following defects occur in trouble shooting of a piston engine, what are the possible causes and remedial actions?

Defects cause remedy

1. High cylinder 1.Octane rating of 1. Drain fuel and Head temperature fuel too low fills with correct Grade 2. Improper manual leaning 2. Use leaning Procedure procedure set forth in the operators Manual 3. Bent or loose cylinder 3.Inspect for condition baffles and correct it 4. Dirt between cooling fins 4. Remove dirt

2. Engine will not 1.No fuel in the tank 1. Fill fuel in the tank

Start 2.Fuel valves turned off 2. Turn on fuel valve

3. Fuel line plugged 3. Starting at carburetor,

check fuel line back to

tank. Clear obstruction

4. Defective or stuck 4. Check for operation

mixture control of mixture control

5. Pressure discharge 5. Replace it





nozzle valve diaphragm

ruptured

6. Primer system in operative 6. Repair primer system

JET ENGINE MAINTENANCE AND TROUBLE SHOOTING

1) Name the events during which special inspections are carried out in gas turbine engine.

Events which may cause the engine to require special inspections are foreign object ingestion, bird ingestion, ice ingestion, over limit operation (temperature and rpm), excessive “G” loads and any other event that could cause internal and external engine damage.

2) What is twin spool axial compressor? A dual compressor jet engine utilizes two separate compressors, each with its own driving turbine. This type of engine is called a “twin spool” or “split compressor” engine

3) Write the two main methods of measuring and correcting any

unbalance of gas turbine engine. The two main methods of measuring and correcting unbalance are single plane (static) balancing and two plane (dynamic) balancing. 4) Explain the term “condition monitoring” Aviation maintenance and operations groups are continually striving to improve the service reliability of their gas turbine engines and, at the same time, reduce the operating costs. One tool which can aid both of these efforts is engine performance monitoring.





5) Define ‘online maintenance’

Maintenance work that is required to maintain an engine and its systems in an air worthy condition while it is installed in an aircraft is called on line maintenance.

6) List the different types of fuel spray nozzles. 1. Simplex nozzle 2. Variable port nozzle 3. Duplex nozzle 4. Spill type nozzle 5. Air spray nozzle

7) Describe gas-turbine starter.

Gas turbine starter is used for some jet engines and it is completely self contained. It has its own fuel and ignition system, starting system, (electric or hydraulic) a self contained oil system. This type of starter is economical to operate and provides high power output for a comparatively low weight.

8) State the methods of balancing of Gas-turbine components

The two main methods of measuring and correcting unbalance are single plane (static) balancing and two plane (dynamic) balancing.

9. Compare a gas-turbine igniter plug with a spark plug.

1. Design and configuration of igniter plug and spark plug are different.

2. Since igniter plug is designed to operate at a lower surrounding pressure than is a spark plug, the spark gaps in an igniter are greater.

3. Spark discharge of an igniter causes much more rapid erosion of the electrodes than the spark provided by the spark plug.





10. At what engine speed does the starter system disengage in a jet engine?

The starter is coupled to the engine through a reduction gear and ratchet mechanism or clutch, which automatically disengages after the engine has reached a self-sustaining speed.

11. What is the purpose of compressor wash?

The purpose of compressor wash is to remove the baked-on salt, dirt, or other types of contamination deposits. Because during engine operations the deposits accumulate on engine compressor and turbine blades and deteriorating the engine performance.

12. Describe the purpose of trend analysis.

Trend analysis involves the recording and analysis of gas-turbine engine performance and certain mechanical parameters over a period of time. The primary aim of trend analysis is to provide a means of detecting significant changes in the performance parameters resulting from changes in the mechanical condition of the engine.

13. What is “hung start” or “false” start?

If the engine fails to accelerate properly or does not reach the idle rpm position, the starting attempt is called a false start or a hung start.

14. Enumerate the operational checks to ensure that a gas turbine engine is in satisfactory operating condition.

To ensure that a gas turbine engine is in satisfactory operating condition, engine and aircraft manufacturers specify certain operational checks to be routinely performed. They are

1. Dry motoring check 2. Wet motoring check 3. Idle check 4. Power assurance check





15. What is the purpose of bell mouth air inlet being used in test stand?

The operation of an engine on a test stand is usually accomplished with a bell mouth air inlet. The purpose of this type of inlet is to eliminate any loss of air pressure at the compressor inlet.

16. What is scheduled maintenance?

Scheduled maintenance includes the periodic recurring inspections that must be carried out in accordance with the engine section of the aircraft maintenance schedule. Usually calculated in aircraft flying hours.

17. What is the function of the exhaust nozzle?

The function of the exhaust nozzle is to control the velocity and temperature of the exhaust gases. When a convergent nozzle is used, the velocity of the gas is increased and the thrust is in line with the engine.

18. Enumerate the techniques required for Non routine inspections.

The techniques required for Non routine inspections are

a. Visual inspection b. Inspection with lights c. Use of magnifying glass d. Applications of fluorescent or dyepenetrants e. Use of bores cope or video scope f. Use of radiography.

19. Define “trouble shooting.

Trouble shooting is the step by step procedure used to determine the cause of a given fault and then select the best and quickest solution. When trouble shooting, the technician must evaluate the performance of the engine by comparing data on how the engine should operate and how it is currently operating.





20. What are the three major sections of a gas turbine engine?

A gas turbine engine has three major sections, an air compressor, a combustion section, and a turbine section.

21. What is the purpose of turbine?

The purpose of the turbine is to extract kinetic energy from the high velocity gases leaving the combustion section of the engine. The energy is converted to shaft horse power for the purpose of driving the compressor.

22. What is the function of the diffuser in gas turbine engine?

The function of the diffuser is to reduce the velocity of the air. As the velocity of the air decreases, its static pressure increases. As the static pressure increases, the ram pressure decreases. The diffuser is the point of highest pressure within the engine.

23. Enumerate the instruments used for Non- routine inspection of gas- turbine engine.

a. Borescope or video scope

b. Fiberscope

c. Electronic imaging

24. What action to be taken when an engine has been operated with no oil pressure for more than two minutes?

When an engine has been operated with no oil pressure for more than two minutes the engine must be removed for overhaul.

25. What is “hot start”?





During the starting of a gas turbine engine, the EGT (exhaust gas temperature) exceeds the prescribed safe limit, the engine is said to have had a hot start.

ROUTINE OPERATIONAL INSPECTIONS

1. Explain the procedure of carrying out “A” check inspection performed on gas Turbine engine after 100 hrs operation.

A typical airline may designate standard service operations and inspections by such names as “No. 1 service, No. 2 service, “A” check, “B” check

“No. 1 service: It is performed by station personnel each time the airplane lands or after several landings. The service will include correction of critical log items as well as regular service (fuel and resupply), and a walk around inspection.

“No. 2 service: Review of the flight log and cabin log, check of engine oil quantity, visual inspection of the engine with cowls open.

“A” check: It is performed after approximately 100hrs of operation.

Inspection and service related to the engine are as follows.

1. Fill oil tanks and enter the quarts added for each engine in the inspection records.

2. Service the constant- speed drive as required. 3. Check engine inlet, cowling, and pylon for damage, irregularities, and

leakage. 4. Inspect the engine exhaust section for damage using a strong

inspection light and note the condition of rear turbine. 5. Check the thrust- reverser ejector and reverser buckets for security and

damage.





6. Check the reverser system, with ejectors extended, for cracks, buckling, and damage.

PERIODIC INSPECTIONS

Periodic inspections are required after a given number of operation hours, flight cycles, or a combination of both. These inspections are classified as routine, minor, or major. Scheduling of inspections is established by the operator of the aircraft.

OVER SPEED INSPECTION

2. Explain over speed inspection for a typical high- bypass fan engine

Over speed inspection for a typical high-bypass fan engine is primarily concerned with rotating assemblies. Manufacturer specifies the following inspections if the fan has been operated at speed from 116 to 120 percent rpm.

1. Check the fan rotor for freedom of rotation. 2. Check the first-stage fan shroud for excessive rub. 3. Inspect the low pressure compressor with a borescope. 4. Inspect the inlet and exhaust nozzles for particles. 5. Inspect all four stages of the LPT with a borescope for blade and vane

damage. Inspect the fourth stage blades through the exhaust nozzle. 6. If the fan speed has exceeded120percent, the fan rotor, fan mid shaft, and

LPT rotor must be removed, disassembled, and inspected in accordance with instructions.

7. If the core- engine rotor (high pressure compressor, high pressure turbine) has been operated at speeds from 107 to 108.5 percent, the following inspections are to be carried out. a. Inspect the exhaust nozzle for particles.





b. Inspect the core compressor with a borescope for blade and vane damage.

c. Inspect the HPT with a borescope for blade damage. 8. If the core engine rotor has been operated above 108.5 percent, the

engine must be removed, disassembled, and inspected according to instruction.

NON ROUTINE OPERATIONS

3. State and explain in detail about the instruments used during Non routine inspections.

During the operation of the gas turbine engine, various events may occur which cause the engine to require immediate special inspection to determine whether the engine has been damaged and what corrective actions must be taken. Among some of the events which require special inspections are foreign object injection, bird ingestion, ice ingestion, over limit operation (temperature and rpm), excessive “G” loads, and any other event that could cause internal and external engine damage.

Instruments used for Non routine inspections a. Borescope b. Fiberscope c. Electronic imaging

BORESCOPE The borescope was used for examining the insides of cylinder bores on reciprocating engine and is now extensively used on turbine engines. The borescope is a rigid instrument that may be compared with a small periscope. At the one end is an eye piece with one or more lenses attached to the light carrying tube. At the end of the tube are a mirror, a lens, and strong light.





The tube is inserted through engine borescope ports located in the engine case at points necessary to allow for examination of all critical areas inside the engine. The ports are normally closed. When the borescope inspections are to be performed, the technician should identify the plugs which are removed to be sure that they are reinstalled in the same ports. FIBERSCOPE A variation of the rigid borescope is the fiberscope. The flexible fiberscope has a controllable bending section near the tip so that the observer can direct the scope after it has been inserted into an engine inspection port. The bending action allows the fiberscope to scan the area inside the engine once inside the port. Many times it is necessary to inspect around the corners inside the engine when no inspection entry port is available to allow a direct line of sight.

ELECTRONIC IMAGING A new imaging technique, Electronic imaging, is able to produce sharp, true-color, magnified images that can be seen on a video monitor. One such system is the video- probe 2000. A video imaging system includes an inspection probe, a video processor, and a video monitor for displaying the image. The system uses a tiny charged-coupled device (CCD) sensor in the tip of the probe. The CCD sensor acts like a miniature TV camera to transmit the image electronically to a video monitor. First light is transmitted to the inspection area, either by light emitting diodes or by fiber –optic light guides. A fixed focus lens in the tip of the probe gathers reflected light from the area and directs it to the surface of the CCD sensor. The signal then travels down the length of the probe through amplifiers. The video processor receives the signal, digitizes it, assembles it, and outputs it directly to a video monitor, video tape recorder.





CONDITIONING MONITORING

4. Discuss about condition monitoring of the engine on ground and at altitude. Aviation maintenance and operations groups are continually striving to improve the reliability of the gas turbine engines and, at the same time reduce operating cost by monitoring engine performance through trend analysis. Trend analysis involves the recording and analysis of gas turbine engine performance and certain mechanical parameters over a period of time. The primary aim of trend analysis is to provide a means of detecting significant changes in the performance parameters resulting from changes in mechanical condition of the engine A gas turbine engine operates with various performance parameters at steady state condition. Once the initial relationships have been established for the various parameters, a specific change will not vary significantly from this calibration unless some external force affects it. Thus, abnormal performance of an engine will be indicated by parameter relationships deviating from the norm. Data collection methods will vary depending on whether the data are collected manually or by an onboard computer. Data should be collected at regular intervals. Variable loads extracted from the engine, such as generator, hydraulic, air conditioning, and bleed air, will have an effect on trend accuracy. To minimize these effects, each time a set of reading is taken, with regard to altitude and power. To reduce fluctuations in the data, ensure that the engine parameters are stabilized before taking the data readings. Condition monitoring devices are designed to give an indication of any engine deterioration at the earliest possible stage. This facilitates quick diagnosis which can be followed by either further monitoring or immediate action on the problem. Condition monitoring devices and equipment can be categorized into the areas of flight recorders and ground indicators.





GAS TURBINE ENGINE BALANCING

5. How is the balancing of gas turbine engine carried out? Explain.

Because of the high rotational speed, any unbalance in the main rotating assembly of a gas turbine engine is capable of producing vibrations and stresses which increase as the square of the rotational speed. Therefore very accurate balancing of the rotating assembly is necessary.

The two main methods of measuring and correcting unbalance are single plane (static) balancing and two plane (dynamic) balancing. Single plane balancing is used when the unbalance is in one plane only, that is, the unbalance goes centrally through the component at 90° to the axis. The single plane method is appropriate for component such as individual compressors and turbine discs. For compressor assemblies and turbine rotor assemblies possessing appreciable actual length, unbalance may be present at many positions along the axis. Therefore two plane balancing may be required.

OPERATIONAL CHECKS TO BE ROUTINELY PERFORMED BY MAINTENANCE PERSONNEL.

6. Explain in brief the routine operation checks performed to ensure the gas turbine engine is in satisfactory operating condition.

Or

Explain the following operational checks in gas turbine engine.

To ensure that a gas turbine engine is in satisfactory operating condition, engine and aircraft manufacturers specify certain operational checks to be routinely performed by maintenance personnel. The particular types of checks and the procedures to be followed vary, depending on the type of engine and aircraft involved.

DRY MOTORING CHECKS

The dry motoring check is required during or after inspection or maintenance to ensure that the engine rotates freely, that instrumentation function properly, starter operation meets speed requirements for successful starts. This check is





also used to prime and leak checks the lubrication system when maintenance has required replacement of system components.

PROCEDURE:

1. Ascertain that all conditions required prior to a normal start are met. 2. Position engine controls and switches as follows:

1. Ignition, OFF 2. Fuel shutoff lever, OFF 3. Throttle, Idle 4. Fuel Booster Pump, ON

3. Energize the starter and motor the engine as long as necessary to check instruments for positive indications of engine rotation and oil pressure.

4. De-energize the starter and make the following checks during coast down:

(a) Listen for unusual noises. Check for roughness. (b) Inspect the lubricating system lines, fittings and accessories

for leakage. (c) Check the oil level in the oil tank.

WET MOTORING CHECK

It is necessary to check the operation of fuel system components after removal and replacement or to perform a de preservation of the fuel system.

PROCEDURE:

1. Position engine controls and switches as follows:

1. Ignition, OFF 2. Fuel shutoff lever, OFF 3. Throttle, Idle 4. Fuel Booster Pump, ON

2. Energize the starter.





3. when core engine speed(N2) reaches 10 percent, move the fuel shutoff lever to ON and check for oil pressure indication.

4. Continue motoring the engine until the fuel flow is 500-600 lb/h or for a maximum of 60s. Observe the starter operating limits.

5. Move the fuel shutoff lever to OFF and continue motoring the engine for at least 30 s to clear the fuel from the combustion chamber . check to see that fuel flow drops to zero.

5. De-energize the starter and, during coast down, check for unusual noises.

6. Inspect the fuel system lines, fittings and accessories for leakage. 7. Check the concentric fuel shroud for leakage. No leakage is

permitted. 8. Inspect the lubrication system for leakage. 9. Check the oil level in the oil tank.

IDLE CHECK

The idle check consists of checking for proper engine operation as evidenced by leak free connections, normal operating noise, and correct indications on engine related instruments. Engine drain lines must be disconnected from drain cans to check for leakage.

PROCEDURE

1. Start the engine according to approved procedure. 2. Stabilize the engine at the ground idle. 3. Check fan speed (N1), Core engine speed (N2), Oil pressure, and

exhaust gas temperature (EGT) should be within the proper ranges according to the ground idle speed chart and engine specifications.

4. Visually inspect fuel, lubrication, and pneumatic lines, fittings and accessories for leakage.





5. De-energize flight idle solenoid. During operations above ground idle, do not exceed the open cowling limitations imposed by the airframe manufacturer.

6. Stabilize at flight idle and check the same parameters checked for ground idle. See that they are within the limitations set forth on the flight idle speed charts.

POWER ASSURANCE CHECK

The power assurance check is performed to make sure that the engine will achieve takeoff power on a hot day without exceeding rpm and temperature limitations. During the tests the engine is not used to supply power for any aircraft systems (Electric, Hydraulic or other). The engine is tested at 50% and, 75%, and maximum power.

During the power assurance check EGT must be observed constantly to avoid the possibility of over temperature. If the EGT approach maximum allowable, the throttle must be retarded sufficiently to hold the EGT within limits.

PROCEDURE

1. Set the engine power at nominal N2 speed as indicated on the appropriate chart for the total air temperature (TAT).

2. Four minutes after the throttle lever is set, record the average readings of TAT, N1 speed, N2 speed, EGT, EPR (Engine Pressure Ratio) and fuel flow (Wf)

3. Using N1 (where N1= Target N1- observed N1) as a correction factor, adjust readings according to the parameter adjustments set forth in the operations manual.

4. Before a hot engine is shut down it should be operated at ground idle speed for about 3 minutes to permit temperature reduction and stabilization. As soon as the engine is shut down the EGT gauge





should be observed to see that EGT start decrease. If the EGT does not decrease, an internal fire is indicated, and the engine should be dry motored at once to blow out the fire.

After the engine is shut down, technician should listen for unusual noises in the engine such as scraping, grinding, bumping and squealing.

FOREIGN OBJECT DAMAGE (FOD) & BLADE DAMAGE

7. Explain the special inspection carried out after the gas turbine engine experienced the snags of “Foreign object damage” and ‘Blade damage’

Foreign object damage to a gas-turbine engine may consist of anything from small nicks and scratches to complete disablement or destruction of the engine. The flight crew of an aircraft may or may not aware that FOD has occurred during a flight. If damage is substantial, however, it will be indicated by vibration and by changes in the engine’s operating parameters.

When FOD has occurred, the inspections required depend on the nature of the foreign objects. If an external inspection indicates substantial damage to the fan section or to the inlet guide vanes, the engine must be removed and overhauled. If the damage to the forward section of the engine is slight, a boroscope inspection of the interior of the engine is necessary. Damage to vanes, fan blades, compressor blades can be repaired if it does not exceed the limits specified by the manufacturer. If the engine operates normally after repairs are made, it can be placed back in service.

FAN BLADE SHINGLING

Fan blade shingling is the overlapping of the midspan shrouds of the fan blades. Shingling will take place, when the blades of a rotating fan encounter resistance which forces them sideways an appreciable distance.

Shingling can be caused by engine stall, bird strike, FOD, or engine over speed, in which case the fan must be inspected at both the upper and lower surfaces of the midspan shrouds for chafing, scoring, and other damage adjacent to the interlock surfaces. All blades that are overlapped or show indications of overlapping must be removed and





inspected according to manual. No cracks are permitted in the fan blades. Blade tips are examined for curl, and the lightening holes are checked for cracks and deformation.

Inspection of the abradable material for damage due to rubbing of fan blade tips and inspects the fan speed sensor head for damage due to blade contact.

CONNECTING RODS

8. What are the checks and inspections carried out on a connecting rod during overhaul?

The inspection and repair of connecting rods include

1. Visual inspection 2. Checking of alignment 3. Re- bushing 4. Replacement of bearings

VISUAL INSPECTION

Visual inspection should be done with magnifying glass or bench microscope. A rod which is bent or twisted should be rejected. Inspect all surfaces of the rod for cracks, corrosion, pitting, galling or other damages. Evidence of any galling is sufficient reason to reject the complete rod assembly.

CHECKING ALIGNMENT

Check bushings that have been replaced to determine if the bushing and rod bores are square and parallel to each other. The alignment can be checked several ways. One method which requires a push fit arbor, a surface plate, and two parallel blocks of equal height.

To measure the squareness, or twist insert the arbors into the rod bores. Place the parallel blocks on a surface plate. Place the ends of the arbors on the parallel blocks. Check the clearance at the points where the arbors rest on the blocks, using a thickness gauge. This clearance, divided by the separation of blocks in inches will give the twist per inch of the length.





To determine the bushing or bearing parallelism (convergence), insert the arbors in the rod bores. Measure the distance between arbors on each side of the connecting rod at points that are equidistance from the rod centre line. For exact parallelism distance checked on both the sides should be the same. Consult the manufacturer’s table of limits for the amount of misalignment permitted.

Other inspections and operations must be performed to be referred from the manufacturer’s overhaul manual.

LINE MAINTENANCE & HEAVY MAINTENANCE

9. What are line maintenance and heavy maintenance? Explain in detail about maintenance practices.

The scope of line maintenance consists of removal and installation of external components and engine accessories as well as hot section inspection. Much of the work considered to be line maintenance is removal and replacement of malfunctioning line replaceable units (LRU).

HEAVY MAINTENANCE

Heavy maintenance consists of removal, installation and repair of components considered beyond the capabilities of the line maintenance facility. Heavy maintenance procedures require considerable equipment and engine knowledge. Heavy maintenance normally performed at an overhaul facility.

MAINTENANCE PRECAUTIONS

1. The ignition system is potentially lethal, therefore any work done on the high energy ignition units, the igniter plugs or the harness must be disconnected and at least 1 min allow to elapse before the high tension lead is disconnected.

2. Before carrying out work on electrical system, make sure that the system is safe, by switching off the power or by tripping and tagging the appropriate circuit breakers.

3. When the oil system is replenished, care must be taken so that no oil is spilled. If any oil is accidentally spilled, clean it off immediately.





4. Before inspecting the air intake or the exhaust system, make sure that there is no possibility of the starter system being operated or the ignition system being energized.

5. After any repair, adjustment, or component change, ensure that no loose items have been left inside the air intake and exhaust system.

6. Observe fire safety precautions at all times when procedures involve the use of fuels or similar combustibles.

PRINCIPLE AND OPERATION OF JET ENGINE

10. Explain the working principle of operation of gas Turbine engine.

The basic operation of the gas turbine or turbo jet engine is relatively simple. Air is brought into the front of the turbine engine and compressed. Fuel is mixed with this air and burned and the heated exhausted gases rush out the back of the engine. The parts of the turbo jet engine work together to change fuel energy to energy of motion to cause the greatest thrust.

A turbine engine has major 3 sections such as:

1. Air compressor 2. Combustion section 3. Turbine section

The engine may also be divided into cold section and hot section. The front part of the engine contains the air compressor which is the cold section. The Combustion and the turbine sections make up the hot section of the engine.

The compressor packs several tons of air into the combustion chamber every minute and works somewhat like a series of fans. The fuel is forced into the combustion chamber through nozzles, a spark provides ignition, and the mixture burns, creating hot exhaust gases. These gases expand and are ejected from the rear of the engine. As the gases leave, they spin a turbine which is located behind the combustion chamber. By means of an interconnecting shaft, the rotating turbine is connected to and turns the compressor, completing the cycle.





After rushing by the turbine, the hot gases continue to expand and blast out through the exhaust nozzle with a high velocity, creating the force which propels a jet aircraft.

STARTING SYSTEMS FOR GAS TRUBINE ENGINES

11. Enumerate the various starters used for starting the gas turbine engines and explain in detail the operation of any one of them

Starters for gas turbine engines may be classified as Air turbine starters, Electric starters, and Fuel Air (F/A) combustion starters.

ELECTRIC STARTERS

The comparatively small gas turbine engines (6000lbs of thrust) are equipped with heavy duty electric starters or starter generators. These are simply electric motors or motor generator units which produce very high starting torque because of the large amounts of electric power they consume.

The electric starter is coupled to the engine through a reduction gear and a ratchet mechanism or clutch, which automatically disengages after the engine has reached a self sustaining speed.

The electric supply, which may be of a high or low voltage, is passed through a system of relays and resistances to allow the full voltage to be build up progressively as the starter gain speed. The electric supply also provides the power for the operation of the ignition system. The electric supply is automatically cancelled when the starter load is reduced, either the engine has started satisfactorily or the time cycle is completed.

AIR TURBINE STARTERS

This type of starters requires a high volume air supply, which is provided by a ground starter unit, a compressed air bottle on the airplane, an auxiliary power unit on the aircraft, or compressed bleed air from other engines on the aircraft.

The low pressure air turbine starter is designed to operate with a high volume, low pressure air supply, usually obtained from an external turbo compressor unit mounted on a ground service cart or from the airplanes low pressure air supply. The supply must produce a pressure of about 35 psig and a flow of more than 100 lbs/min.





The starter is light weight turbine air motor equipped with a rotating assembly, a reduction gear system, a splined output shaft, a cut out switch mechanism, an over speed switch scroll assembly and a gear housing.

The low pressure air introduced into the scroll through a 3” duct. From the scroll, air passes through nozzles vanes to the outer rim of the turbine wheel. Since this is an inward flow turbine design the air expands radially inward toward the centre of the wheel and is then expelled through the exducer. The exhausted air passes through the screen and out to the atmosphere. The expansion of air from a pressure about 35 psig to atmospheric pressure imparts energy to the turbine wheel, causing it to reach a speed of about 55,000 rpm. This low torque high speed is converted to a high torque low speed by means of the 23.2: 1 reduction gearing.

COMBUSTION STARTERS

The two types of combustion starters are the gas turbine starters, and the cartridge type starters.

A gas turbine starter is completely self-contained. It has its own fuel and ignition system, its own starting system (usually electric or hydraulic) and a self contained oil system. This type of starter provides a high power output for a low weight.

The starter consists of a small, compact gas turbine engine, having a turbine driven centrifugal compressor, a reverse flow combustion system and a mechanically independent free power turbine. The free power turbine connected to the main engine via a two stage reduction gear, an automatic clutch and output shaft.

When the starting cycle is initiated the gas turbine starter is rotated by its own starter motor until it reaches a self sustaining speed at which point the starting and ignition systems automatically switch off. Then the acceleration continues up to a control speed of 60,000 rpm. When it is accelerating, the exhaust gas is being directed via nozzle guide vanes, on to the free power turbine to drive the main engine. Once the main engine reaches a self sustaining speed, a cut out switch operates and shut down the gas turbine starter. As the starter runs down, the clutch automatically disengages from the output shaft and the main engine accelerates up to idling rpm under its own power.





CHECKS CARRIEDOUT ON CRANK SHAFT

12. Explain the various checks and alignment in crankshaft during overhaul

1. Inspect all surfaces of the shaft for cracks. 2. Check the bearing surfaces for galling, scoring or other damage. 3. Check oil transfer tubes for tightness. 4. Carry out visual inspection and Non destructive testing such as magnetic particle

testing or radiography. 5. Check the crankshaft for straightness.

PROCEDURE

a. Place the crank shaft in ‘v’ blocks supported at the locations specified in the

applicable engine overhaul manual.

b. Using a surface plate and a dial indicator to measure the run out.

c. Crank shaft run out or bending is checked by mounting the shaft on ‘v’ blocks

placed on a level surface plate and rotating the shaft while reading the run out on

the dial gauge.

d. Crank shaft run out should be checked at the centre main journals while the shaft is

supported at the thrust and rear journals.

e. It should also be checked at the propeller flange or at the front propeller bearing

seat.

f. If the total indicator reading exceeds the dimensions given in the manufacturer’s

table and limits, the shaft must not be reused.

g. A bent crankshaft should not be straightened. Any attempt to do so will result in

rupture of the nitride surface of the bearing journals.

h. Measure the outside diameter of the crankshaft main and rod bearing journals and

compare the reading with the table of limits.



Ae2028 2 marks

Engineering

engine testing

engine performance

dimensional inspection

current inspection

ultrasonic inspection

radiography inspection

common engine classifications

preliminary visual inspection