RF pulser Amplifier Transducer Rate Flaw generator Specimen Timing circuit Cathode ray oscilloscope FIGURE 10-13. Block diagram of basic pulse-echo system.
transmitting and receiving, the picture on the oscilloscope appears to be stationary.
A few microseconds after the sweep is initiated, the rate generator electrically excites the pulser and the pulser in turn emits an electrical pulse.
The transducer converts this pulse into a short train of ultrasonic sound waves. If the interfaces of the transducer and the specimen are properly orientated, the ultrasound will be reflected back to the transducer when it reaches the internal flaw and the opposite surface of the specimen. The time interval between the transmission of the initial impulse and the reception of the signals from within the specimen is measured by the timing circuits. The reflected pulse received by the transducer is amplified, then transmitted to the oscilloscope, where the pulse received from the flaw is displayed on the CRT screen. The pulse is displayed in the same relationship to the front and back pulses as the flaw is in relation to the front and back surfaces of the specimen. (See figure
10-14.)
The Reflectoscope is a pulse-echo type instrument. The Reflectoscope can be used for the detection of defects such as cracks, folds, inclusions, delaminations, partial welds, voids, shrinks, porosity, flaking, and other subsurface defects.
The principle of operation is pictured in figure 10-15, where electrical pulses are transformed by the crystal into ultrasonic vibrations which are transmitted into the material. The portion of the electrical pulse delivered to the cathode-ray tube causes an initial pulse indication, as shown in figure 10-15, view A. The back reflection has formed in view B, the vibrations having traveled to the bottom of the part and reflected back to the searching unit, which transforms them back into electrical pulses. The screen's vertical indication of their return is known as the "first back-reflection indication." If a defect is present (figure 10-15, view C), a portion of the vibrations traveling through the material is reflected from the defect, causing an additional indication on the screen. The horizontal-sweep travel indicates the time elapsed since the vibrations left the crystal.
This type of operation, referred to as straightbeam testing, is suitable for the detection of flaws whose planes are parallel to the plane of the part.
By means of angle-beam testing, also referred to as shear-wave testing, the usefulness of the Reflectoscope includes the following:
1. Flaws whose planes lie at an angle to the plane of the part.
2. Discontinuities in areas that cannot be reached with the standard straight-beam technique.
3. Some internal defects in plate and sheet stock.
4. Some types of internal defects in tubing, pipe and bar stock, such as inclusions and small cracks near the surface.
5. Cracks in parent metal resulting from welding.
6. Some defects in welds.
Transducer Flaw
- Flaw Speciman Cathode ray tube FICURE 10-14. Oscilloscope display in relationship to flaw location.
Coaxial cable Quartz crystal - Material.
material face Echo from Echo from material face and back face material face, defect, Echo from - Defect and back face FIGURE 10-15. Reflectoscope operation-straight-beam testing.
Angle-beam testing differs from straight-beam testing only in the manner in which the ultrasonic waves pass through the material being tested. As shown in figure 10-16, the beam is projected into the material at an acute angle to the surface by means of a crystal cut at an angle and mounted in plastic. The beam or a portion thereof reflects successively from the surfaces of the material or any other discontinuity, including the edge of the piece. In straight-beam testing, the horizontal distance on the screen between the initial pulse and the first back reflection represents the thickness of the piece; while in angle-beam testing, this distance represents the width of the material between the searching unit and the opposite edge of the piece.
Resonance System This system differs from the pulse method in that the frequency of transmission is, or can be, continuously varied. The resonance method is principally used for thickness measurements when the two sides of the material being tested are smooth and parallel. The point at which the frequency matches the resonance point of the Coaxial cable
- Quartz crystal Material Defect FICURE 10-16. Reflectoscope operation-angle-beam testing.
material being tested is the thickness-determining factor. It is necessary that the frequency of the ultrasonic waves, corresponding to a particular dial setting, be accurately known. Checks should be made with standard test blocks to guard against possible drift of frequency.
If the frequency of an ultrasonic wave is such that its wavelength is twice the thickness of a specimen (fundamental frequency), then the reflected wave will arrive back at the transducer in the same phase as the original transmission so that strengthening of the signal, or a resonance, will occur. If the frequency is increased so that three times the wavelength equals four times the thickness, then the reflected signal will return completely out of phase with the transmitted signal and cancellation will occur. Further increase of the frequency, so that the wavelength is equal to the thickness again, gives a reflected signal in phase with the transmitted signal and resonance occurs once more.
Transducer incident wave Reflected wave 1 Reflecting
• surface Wavelength T = 2 F = F, (Fundamental frequency) T = WF = 2F, (2nd Harmonic) Material under test I = 1½W F = 3F, (3rd Harmonic) T = 2W F = 4F, (4th Harmonic) FICURE 10-17. Conditions of ultrasonic resonance in a metal plate.
Pulse H.F.
amplifier Oscillator
I CRT I Motor - Tuning capacitor Transducer Horizontal Material time-base Contacts generator FIGURE 10-18. Block diagram of resonance thickness measuring system.
By starting at the fundamental frequency and gradually increasing the frequency, the successive cancellations and resonances can be noted and the readings used to check the fundamental frequency reading. (See figure 10-17.)
In some instruments, the oscillator circuit contains a motor-driven capacitor which changes the frequency of the oscillator. (See figure 10-18.)
In other instruments, the frequency is changed by electronic means.
The change in frequency is synchronized with the horizontal sweep of a CRT. The horizontal axis thus represents a frequency range. If the frequency range contains resonances, the circuitry is arranged to present these vertically. Calibrated transparent scales are then placed in front of the tube, and the thickness can be read directly. The instruments normally operate between 0.25 mc.
and 10 mc. in four or five bands.
The resonant thickness instrument can be used to test the thickness of such metals as steel, cast iron, brass, nickel, copper, silver, lead, aluminum, and magnesium. In addition, areas of corrosion or wear on tanks, tubing, airplane wing skins, and other structures or products can be located and evaluated.
Direct-reading, dial-operated units are available that measure thickness between .025 inch and 3 inches with an accuracy of better than I1 percent.
Ultrasonic inspection requires a skilled operator who is familiar with the equipment being used as well as the inspection method to be used for the many different parts being tested.
EDDY CURRENT TESTING Electromagnetic analysis is a term which describes the broad spectrum of electronic test methods involving the intersection of magnetic fields and circulatory currents. The most widely used technique is the eddy current.
Eddy currents are composed of free electrons, which are made to "drift" through metal, under the influence of an induced electromagnetic field.
Eddy current is used in aircraft maintenance to inspect jet engine-turbine shaft and veins, wing skins, wheels, bolt holes, and spark plug bores for cracks, heat or frame damage. In aircraft manufacturing plants, eddy current is used to inspect castings, stampings, machine parts, forgings, and extrusions.
Basic Principles When an alternating current is passed through a coil it develops a magnetic field around the coil which in turn induces a voltage of opposite polarity in the coil and opposes the flow of original current.
If this coil is placed so that the magnetic field passes through an electrically conducting specimen, eddy currents will be induced into the specimen.
The eddy currents create their own field which varies the original field's opposition to the flow of original current. Thus the specimen's susceptibility to eddy currents determine the current flow through the coil (see figure 10-19).
Oscillator Amplifier The magnitude and phase of this counter field is dependent primarily upon the resistivity and permeability of the specimen under consideration, and it is this fact that enables us to make a qualitative determination of various physical properties of the test material. The interaction of the eddy current field with the original field results is a power change that can be measured by utilizing electronic circuitry similar to a wheatstone bridge.
The specimen is either placed in or passed through the field of an electromagnetic induction coil, and its effect on the impedance of the coil or on the voltage output of one or more test coils is observed. The process--whereby electric fields are made to explore a test piece for various conditionsinvolves the transmission of energy through the specimen much like the transmission of X-rays, heat, or ultrasound. In the transmission of X-rays, heat, or ultrasound, the energy flows in beams having a recognizable direction and intensity and obeys the laws of absorption, reflection, diffraction, and diffusion. Receiver elements can be placed into the beams and a direct measurement of energy flow is possible. However, in electromagnetic tests.
the energy distributes itself in a vaguely known manner and undergoes a transformation in the process, from magnetic to electric energy, and subsequently, back to magnetic energy. Since the induced currents flow in closed circuits, it is neither convenient nor generally possible, to intercept them at the specimen boundaries.
Meter Probe Sample part FIGURE 10-19. Eddy current inspection circuit.
FIGURE 10-20. Examples of good welds.
VISUAL INSPECTION Nondestructive testing by visual means is the oldest method of inspection. Defects which would escape the naked eye can be magnified so they will be visible. Telescopes, borescopes, and magnifying glasses aid in performing visual inspection.
A discussion of visual inspection in this chapter will be confined to judging the quality of completed welds by visual means. Although the appearance of the completed weld is not a positive indication of quality, it gives a good clue to the care used in making it.
A properly designed joint weld is stronger than the base metal which it joins. The characteristics of a properly welded joint are discussed in the following paragraphs.
A good weld is uniform in width; the ripples are even and well feathered into the base metal, which shows no burn due to overheating. (See figure
10-20.) The weld has good penetration and is free of gas pockets, porosity, or inclusions. The edges of the bead illustrated in figure 10-20 (B) are not in a straight line, yet the weld is good, since penetration is excellent.
Penetration is the depth of fusion in a weld.
Thorough fusion is the most important characteristic which contributes to a sound weld. Penetration is affected by the thickness of the material to be joined, the size of the filler rod, and how it is added. In a butt weld the penetration should be 100 percent of the thickness of the base metal. On a fillet weld the penetration requirements are 25 to 50 percent of the thickness of the base metal. The width and depth of bead for a butt weld and fillet weld are shown in figure 10-21.
To assist further in determining the quality of a welded joint, several examples of incorrect welds are discussed in the following paragraphs.
The weld shown in figure 10-22 (A) was made too rapidly. The long and pointed appearance of the ripples was caused by an excessive amount of heat or an oxidizing flame. If the weld were Reinforcement Bead width ¼ to ½ T Throat 3 to 5 T 2 to 3T Leg 1 1/3 to 1 1/2 T
• Approx. ½ T 25 to 50% T 100% Penetration (A) (B) FIGURE 10-21. (A) Butt weld and (B) fillet weld, showing width and depth of bead.
D FIGURE 10-22. Examples of poor welds.
cross-sectioned, it probably would disclose gas pockets, porosity, and slag inclusions.
Figure 10-22 (B) illustrates a weld that has improper penetration and cold laps caused by insufficient heat. It appears rough and irregular and its edges are not feathered into the base metal.
The puddle has a tendency to boil during the welding operation if an excessive amount of acetylene is used. This often leaves slight bumps along the center and craters at the finish of the weld. Cross-checks will be apparent if the body of the weld is sound. If the weld were cross-sectioned, pockets and porosity would be visible. Such a condition is shown in figure 10-22 (C).
A bad weld with irregular edges and considerable variation in the depth of penetration is shown in D of figure 10-22. It often has the appearance of a cold weld.
CHAPTER 11
GROUND HANDLING, SAFETY, AND SUPPORT EQUIPMENT
INTRODUCTION Aircraft maintenance technicians devote a portion of their aviation career working with ground support equipment and ground handling i aircraft. The complexity oi support equipmen nd the hazards involved in ground handing o expensive aircraft require that maintenance technicians possess a detailed knowledge of safe procedures used in aircraft servicing, taxing, runup, and in the use of ground support equipment. The information provided in this chapter is intended as a general guide for working on all types of aircraft.
GENERAL The following instructions cover the starting procedures for reciprocating, turboprop, and turbojet engines. These procedures are presented only as a general guide for familiarization with typical procedures and methods. Detailed instructions for starting a specific type of engine can be found in the manufacturer's instruction book.
Before starting an aircraft engine:
1. Position the aircraft to head into the prevailing wind to insure adequate air flow over the engine for cooling purposes.
2. Make sure that no property damage or personal injury will occur from the propeller or pro-
5. If the aircraft is jet engine powered, the area in front of the jet inlet must be kept clear of personnel, property and/or debris. Also, the exhaust area must be kept clear.
6. These "before starting" procedures apply to reciprocating, turbo-propeller and turbo-jet powerplants.
STARTING ENGINES Reciprocating Engines The following procedures are typical of those used to start reciprocating engines. There are, however, wide variations in the procedures for the many reciprocating engines. No attempt should be made to use the methods presented here for actually starting an engine. Instead, always refer to the procedures contained in the applicable manufacturer's instructions.
Reciprocating engines are capable of starting in fairly low temperatures without the use of engine heating or oil dilution, depending on the grade of The various covers (wing, tail, cockpit, wheel, etc.) protecting the aircraft must be removed before attempting to turn the engine. External sources of electrical power should be used when starting engines equipped with electric starters.
This eliminates an excessive burden on the aircraft battery. All unnecessary electrical equipment should be left off until the generators are furnishing electrical power to the aircraft power
3. If external electrical power is used for starting, make sure that it can be removed safely.
4. During any and all starting procedures a "fire guard" equipped with a suitable fire extinguisher shall be stationed in an appropriate place. ("Fire guard"-someone familiar with aircraft starting "Fire extinguisher"-a CO2 extinguisher at least of 5-lb. capacity. Appropriate place"-adjacent to the outboard side of the engine, in view of the pilot, and also where he can observe the engine/aircraft for indication of starting prob- Before starting a radial engine that has been shut down for more than 30 minutes, check the ignition switch for off; turn the propeller three or four complete revolutions with the starter, or it may be pulled through by hand to detect a hy.
draulic lock if one is present.
Any liquid present in a cylinder is indicated by the abnormal effort required to rotate the propeller, or by the propeller stopping abruptly during rotation. Never use force to turn the propeller when a hydraulic lock is detected.
Sufficient force can be exerted on the crankshaft to bend or break a connecting rod if a lock is present.
To eliminate a lock, remove either the front or rear spark plug from the lower cylinders and pull the propeller through. Never attempt to clear the hydraulic lock by pulling the propeller through in the opposite direction to normal rotation. This tends to inject the liquid from the cylinder into the intake pipe. The liquid will be drawn back into the cylinder with the possibility of complete or partial lock occurring on the subsequent start.
To start the engine, proceed as follows:
1. Turn the auxiliary fuel pump on, if aircraft is so equipped.
2. Place the mixture control to the position recommended for the engine and carburetor combination being started. As a general rule, the mixture control should be in the "idle cutoff" position for pressure type carburetors and in the "full rich" position for float type carburetors.
Many light aircraft are equipped with a mixture control pull rod which has no detented intermediate positions. When such controls are pushed in flush with the instrument panel, the mixture is set in the "full rich" position. Conversely, when the control rod is pulled all the way out, the carburetor is in the "idle cutoff" or "full lean" position. Unmarked intermediate positions between these two extremes can be selected by the operator to achieve any desired mixture setting.
3. Open the throttle to a position that will provide 1,000 to 1,200 r.p.m. (approximately ⅛ to ½ inch from the "closed" position).
4. Leave the preheat or (carburetor air) control in the "cold" position to prevent damage and fire in case of backfire. These auxiliary heating devices should be used after the engine warms up.
They improve fuel vaporization, prevent fouling of the spark plugs, ice formation, and eliminate icing in the induction
5. Energize the starter after the propeller has made at least two complete revolutions, and turn the ignition switch on. On engines equipped with an induction vibrator, turn switch to the "both" position.
When starting an engine that uses an impulse coupling magneto, ignition switch to the "left" position.
Place the ignition switch to "start" when the magneto incorporates a retard breaker assembly. Do not crank the engine continuously with the starter for more than 1 minute. Allow a 3- to 5-minute period for cooling the starter between successive attempts. Otherwise the starter may be burned out due to overheating.
6. Move the primer switch to "on" intermittently, or prime with one to three strokes of priming pump, depending on how the aircraft is equipped. When the engine begins to fire, hold the primer on while gradually opening throttle to obtain smooth operation.
After the engine is operating smoothly on the primer, move the mixture control to the "full rich" position. Release the primer as soon as a drop in r.p.m. indicates the engine is receiving additional fuel from the carburetor.
Hand Cranking If the aircraft has no self-starter, the engine must be started by swinging the propeller. The person who is turning the propeller calls,ifuel on, switch off, throttle closed, brakes on." The person operating the engine will check these items and repeat the phrase. The switch and throttle must not be touched again until the person swinging the prop calls "contact." The operator will repeat "contact" and then turn on the switch. Never turn on the switch and then call "contaet."
When swinging the prop, a few simple precautions will help to avoid accidents. When touching a propeller, always assume that the ignition is on. The switches which control the magnetos operate on the principle of short-circuiting the current to turn the ignition off. If the switch is faulty, it can be in the "off" position and still permit current to flow in the magneto primary circuit.
Be sure the ground is firm. Slippery grass, mud, grease, or loose gravel can lead to a fall into or under the propeller. Never allow any portion of your body to get in the way of the propeller. This applies even though the engine is not being Stand close enough to the propeller to be able to step away as it is pulled down. Stepping away after cranking is a safeguard in case the brakes fail. Do 490
not stand in a position that requires leaning toward the propeller to reach it. This throws the body off balance and could cause you to fall into the blades when the engine starts.
In swinging the prop, always move the blade downward by pushing with the palms of the hand.
Do not grip the blade with the fingers curled over the edge, since "kickback" may break them or draw your body in the blade path.
Excessive throttle opening and intermittent priming after the engine has fired are the principal causes of backfiring during starting. Gradual opening of the throttle while priming continuously will reduce the initial "over rich" mixture to a smooth running, best power mixture as the engine picks up speed. An engine operating on an "over rich" mixture is sluggish but will not backfire.
When starting an engine using a priming pump, move the mixture control into "full rich" position, if not previously placed there, when the engine begins to fire. If the engine fails to start immedi-.
ately, return the mixture control to "idle cutoff" position. Failure to do so will create an excessive amount of fuel in the carburetor air scoop, constituting a fire hazard.
Avoid priming the engine before it is turned over by the starter. This can result in fires, scored or scuffed cylinders and pistons, and, in some cases, engine failures due to hydraulic lock. If the engine is inadvertently flooded or overprimed, turn the ignition switch off and move the throttle to the "full open" position. To rid the engine of the excess fuel, turn it over by hand or by the starter.
If excessive force is needed to turn over the engine, stop immediately. Do not force rotation of the engine. If in doubt, remove the lower cylinder spark plugs. If very serious overloading has occurred, it may be necessary to remove the lower cylinder intake pipes. To reduce the likelihood of damage to the engine due to overpriming on some medium and large aircraft, the engine blower drain valves should be checked frequently for fouling or Immediately after the engine starts, check the oil pressure indicator. If oil pressure does not show within 30 seconds, stop the engine and determine the trouble. If oil pressure is indicated, adjust the throttle to the aircraft manufacturer's specified
r.p.m. for engine warm-up. Warm-up r.p.m. will usually be in the 1,000 to 1,300 r.p.m. range.
Most aircraft reciprocating engines are air cooled and depend on the forward speed of the aircraft to maintain proper cooling. Therefore, particular care is necessary when operating these engines on the ground.
During all ground running, operate the engine with the propeller in full low pitch and headed into the wind with the cowling installed to provide the best degree of engine cooling. The engine instruments should be monitored closely at all times. Do not close the cowl flaps for engine warm-up; closing of the cowl flaps may cause the ignition harness to overheat. When warming up the engine, make sure that personnel, ground installations, equipment that may be damaged, or other aircraft are not in the propeller wash.
Extinguishing Engine Fires In all cases a fireguard should stand by with a CO2 fire extinguisher while the aircraft engine is being started. This is a necessary precaution against fire during the starting procedure. He should be familiar with the induction system of the engine so that in case of fire he can direct the CO, into the air intake of the engine to extinguish it. A fire could also occur in the exhaust system of the engine from liquid fuel being ignited in the cylinder and expelled during the normal rotation of the engine.
If an engine fire develops during the starting procedure, continue cranking to start the engine and blow out the fire. If the engine does not start and the fire continues to burn, discontinue the start attempt. The fireguard should extinguish the fire using the available equipment. The fireguard must observe all safety practices at all times while standing by during the starting procedure.
TURBOPROP ENGINES Prestart Procedures The various covers protecting the aircraft must be removed. Engine tailpipes should be carefully inspected for the presence of fuel or oil. A close visual inspection of all accessible parts of the engines and engine controls should be made, followed by an inspection of all nacelle areas to determine that all inspection and access plates are secured. Sumps should be checked for water. Air inlet areas should be inspected for general condition and foreign material. The compressor should be checked for free rotation, when the installation permits, by reaching in and turning the blades by hand.
The following procedures are typical of those used to start turboprop engines. There are,
however, wide variations in the procedures applicable to the many turboprop engines, and no attempt should be made to use these procedures in the actual starting of a turboprop engine. These procedures are presented only as a general guide for familiarization with typical procedures and methods. For starting of all turboprop engines, refer to the detailed procedures contained in the applicable manufacturer's instructions or their approved equivalent.
The first step in starting a turbine engine is to provide an adequate source of power for the starter. Where an air turbine starter is used, the starting air supply may be obtained from a gas-turbine compressor (GTC), an external source, or an engine cross-bleed operation. To start the first engine, use a GTC or low-pressure, large-volume tank. Start the remaining engine(s) using bleed air from the running engine.
While starting an engine, always observe the following:
1. Never energize the starter while the engine is rotating.
2. Do not move the power lever of any engine while it is being bled for cross-bleed
3. Do not perform a ground start if turbine inlet temperature is above that specified by the manufacturer.
4. Do not use bleed air from an engine that is accelerating.
Starting Procedures To start an engine on the ground, perform the following operations:
1. Place the start selector switch to the desired engine and the start-arming switch (if so equipped) to the "start" position.
2. Turn the aircraft boost pumps on.
3. Place the fuel and ignition switch on.
4. Position the low-r.p.m. switch in low or normal (high).
5. Make sure that the power lever is in the "start" position. If the propeller is not at the "start" position, difficulty may be encountered in making a start.
6. Depress the start switch and, if priming is necessary, depress the primer button.
7. Make sure the fuel pump parallel light comes on at, or above, 2,200 r.p.m. and remains on up to 9,000 r.p.m.
8. Check the oil pressure and temperature.
Maintain the power lever at the "start" position until the specified minimum oil temperature is reached.
9. Disconnect the ground power supply.
If any of the following conditions occur during the starting sequence, turn off the fuel and ignition switch, discontinue the start immediately, make an investigation and record the findings.
1. Turbine inlet temperature exceeds the specified maximum. Record the observed peak temperature.
2. Acceleration time from start of propeller rotation to stabilized r.p.m. exceeds the specified time.
3. There is no oil pressure indication at 5,000 r.p.m. for either the reduction gear or the power unit.
4. Torching (visible burning in the exhaust nozzle other than normal enrichment) or excessive smoke is observed during initial fire-up.
5. The engine fails to ignite by 4,500 r.p.m.
or maximum motoring r.p.m. (whichever is first), and r.p.m. stagnates or begins to decay-
6. Abnormal vibration is noted or compressor surge occurs (indicated by backfiring).
7. There is fuel spewing from the nacelle drain, indicating that the drip valve did not close.
8. Fire warning bell rings. (This may be due to either an engine fire or failure of an anti-icing shutoff valve to close.)
TURBOJET ENGINES Preflight Operations Unlike reciprocating engine aircraft, the turbojet-powered aircraft does not require a preflight runup unless it is necessary to investigate a suspected malfunction.
Before starting, all protective covers and airinlet duct covers should be removed. If possible, the aircraft should be headed into the wind to obtain better cooling, faster starting, and smoother engine performance. It is especially important that the aircraft be headed into the wind if the engine is to be trimmed.
The runup area around the aircraft should be cleared of both personnel and loose equipment.
Distance in feet → 30 Ft.
- 200 100° 35K
- 150 Velocity in knots - K 125° 60K Temperature in °F.
Exhaust
- 100 → 12 ft. 100° 150° 100K 200° 200K 150° BOK 300K 300° 200° 100K 500K 500° 300° 200K 900K 700° 25 ft.
Air intake Air intake Idle Takeoff FIGURE 11-1. Engine intake and exhaust hazard areas.
The turbojet engine intake and exhaust hazard areas are illustrated in figure 11-1. Care should also be taken to ensure that the runup area is clear of all items such as nuts, bolts, rocks, rags, or other loose debris.
A great number of very serious accidents occur involving personnel in the vicinity of turbojet engine air inlets. Extreme caution should be exercised when starting turbojet aircraft.
The aircraft fuel sumps should be checked for water or ice, and the engine air inlet should be inspected for general condition and the presence of foreign objects. The forward compressor blades and the compressor inlet guide vanes should be visually inspected for nicks and other damage.
If possible, the compressor should be checked for free rotation by turning the compressor blades by hand.
All engine controls should be operated, and engine instruments and warning lights should be checked for proper operation.
Starting a Turbojet Engine The following procedures are typical of those used to start many turbojet engines. There are, however, wide variations in the starting procedures used for turbojet engines, and no attempt should be made to use these procedures in the actual starting of an engine. These procedures are presented only as a general guide for familiarization with typical procedures and methods. In the starting of all turbojet engines, refer to the detailed procedures contained in the applicable manufacturer's instructions or their approved Most turbojet engines can be started by either air turbine or combustion-type starters. Airturbine starters use compressed air from an external source. This source may be a ground cart unit or air bled from another engine on the aircraft that is in operation. Combustion starters are small gas turbine engines that obtain power from expanding gases generated in the starter's combustion chamber. These hot gases are produced by the burning of fuel and air or, in some cases, a slow-burning solid or liquid monopropellant specially compounded for such starter units.
Fuel is turned on either by moving the powerlever to "idle" position or by opening a fuel shutoff valve. If an air-turbine starter is used, the engine should start or "light up" within approximately 20 seconds after the fuel is turned on. This is an arbitrarily chosen time interval that, if exceeded, indicates a malfunction has occurred and the start should be discontinued. After the cause of the trouble has been removed, another start may be made. If a combustion starter is used, the 20-second interval need not be observed, since starter operation will discontinue automatically after a predetermined time interval. The following procedures are useful only as a general guide, and are included to show the sequence of events in starting a turbojet engine.
1. Move power lever to "off" position unless the engine is equipped with thrust reverser.
If the engine is so equipped, place the power lever in the "idle" position.
2. Turn on electrical power to engine.
3. Turn fuel system shutoff switch to fuel on" position.
4. Turn fuel boost pump switch on.
5. A fuel inlet pressure indicator reading of 5 p.s.i. ensures fuel is being delivered to engine fuel pump inlet.
6. Turn engine starter switch on; when engine begins to rotate, check for oll pressure rise.
7. Turn ignition switch on after engine begins to rotate.
8. Move throttle to idle (if engine is not equipped with thrust reverser).
9. Engine start (light up) is indicated by a rise in exhaust gas temperature.
10. After engine stabilizes at idle, ensure that none of the engine limits are exceeded.
11. Turn engine starter switch off after start.
12. Turn ignition switch off.
Unsatisfactory Turbojet Starts
1. Hot Starts.
A hot start occurs when the engine starts, but the exhaust gas temperature exceeds specified limits. This is usually caused by an excessively rich fuel/air mixture entering the combustion chamber. The fuel to the engine should be shut off immediately.
2. False or Hung Start.
False or hung starts occur when the engine starts normally but the r.p.m. remains at some low value rather than increasing to the normal starting r.p.m. This is often the result of insufficient power to the starter, or the starter cutting off before the engine starts self-accelerating. In this case, the engine should be shut down.