FIGURE 9-48. Wobble plate position.
makeup pressure pump forces oil through the center of this block and the pressure relief valve.
From this valve, the oil flows into the bottom of the transmission case. A scavenger pump removes the oil from the transmission case and circulates it through the oil cooler and filter before returning it to the reservoir. At the start of the cycle, oil is drawn from the reservoir, passed through a filter, and forced into the cylinder block by the makeup pressure pump.
The clutch, located in the output gear and clutch assembly, is an overrunning one-way, sprag-type device. Its purpose is to ratchet if the alternator becomes motorized; otherwise, the alternator might turn the engine. Furthermore, the clutch provides a positive connection when the transmission is driving the alternator.
There is another unit of the drive which must be covered, the governor system. The governor system, which consists of a hydraulic cylinder with a piston, is electrically controlled. Its duty is to regulate oil pressure flowing to the control cylinder assembly, as shown in figure 9-49.
The center of the system's hydraulic cylinder is FIGURE 9-49. Control cylinder.
speed drive Constant 1Estiat Estat snan Generator Oil out Oil in FIGURE 9-50. Electrical hydraulic control circuit.
slotted so the arm of the pump wobble plate can be connected to the piston. As oil pressure moves the piston, the pump wobble plate is placed in either overspeed, underspeed, or straight drive.
Figure 9-50 shows the electrical circuit used to govern the speed of the transmission. First, the main points of the complete electrical control circuit will be discussed (figure 9-50 and 9-51).
Then, for simplification, two portions, the overspeed circuit and the load division circuit, will be considered as individual circuits.
Note, then, in figures 9-50, that the circuit has a valve and solenoid assembly O and a control cylinder E, and that it contains such units as the tachometer generator D, the rectifier C, and adjustable resistor B, rheostat A, and the control coil Q.
Since it is driven by a drive gear in the transmission, the tachometer (often called tach) generator, a three-phase unit, has a voltage proportional to the speed of the output drive. Its voltage is changed from a.c. to d.c. by the rectifier. After rectification, the current flows through the resistor, rheostat, and valve and solenoid. Each of these units is connected in series, as shown in figure 9-51.
Under normal operating conditions, the output of the tach generator causes just enough current to enter the valve and solenoid coil to set up a magnetic field of sufficient strength to balance the spring force in the valve. When the alternator speed increases as the result of a decrease in load, the tach generator output increases also. Because of the greater output, the coil in the solenoid is sufficiently strengthened to overcome the spring force. Thus, the valve moves and, as a result, oil pressure enters the reduced speed side of the control cylinder.
In turn, the pressure moves the piston, causing the angle of the pump wobble plate to be reduced.
The oil on the other side of the piston is forced back through the valve into the system return.
Since the angle of the pump wobble plate is smaller, there is less pumping action in the transmission. The result is decreased output speed.
To complete the cycle, the procedure is reversed.
With the output speed reduction, tach generator output decreases; consequently, the flow of current 427
Constant speed drive Generator To load • Oil out Oil in FIGURE 9-51. Speed control circuit.
to the solenoid diminishes. Therefore, the magnetic field of the solenoid becomes so weak that the spring is able to overcome it and reposition the valve.
If a heavy load is put on the a.c. generator, its speed decreases. The generator is not driven directly by the engine; the hydraulic drive will allow slippage. This decrease will cause the output of the tach generator to taper off and, as a result, weaken the magnetic field of the solenoid coil. The spring in the solenoid will move the valve and allow oil pressure to enter the increase side of the control cylinder and the output speed of the transmission will be raised.
There are still two important circuits which must be discussed: the overspeed circuit and the load division circuit. The generator is prevented from overspeeding by a centrifugal switch (S in figure 9-52) and the overspeed solenoid coil R, which is located in the solenoid and valve assembly. The centrifugal switch is on the transmission and is driven through the same gear arrangement as the tach generator.
The aircraft d.c. system furnishes the power to operate the overspeed coil in the solenoid and coil assembly. If the output speed of the transmission reaches a speed of 7,000 to 7,500 r.p.m., the centrifugal switch closes the d.c. circuit and energizes the overspeed solenoid. This component then moves the valve and engages the latch which holds the valve in the underdrive position. To release the latch, energize the underdrive release solenoid.
The load division circuit's function is to equalize the loads placed on each of the alternators, which is necessary to assure that each alternator assumes its share; otherwise, one alternator might be overloaded while another would be carrying only a small load.
In figure 9-53, one phase of the alternator provides power for the primary in transformer G, whose secondary supplies power to the primaries of two other transformers, J and J2. Rectifiers K then change the output of the transformer secondaries from a.c. to d.c. The function of the two capacitors, L, is to smooth out the d.c.
pulsations.
The output of the current transformer F depends 428
CHAPTER 10
INSPECTION FUNDAMENTALS
GENERAL Inspections are visual examinations and manual checks to determine the condition of an aircraft or component. An aircraft inspection can range from a casual walkaround to a detailed inspection involving complete disassembly and the use of An inspection system consists of several processes, including: (1) Reports made by mechanics or by the pilot or crew flying an aircraft and (2) regularly scheduled inspections of an aircraft. An inspection system is designed to maintain an aircraft in the best possible condition.
Thorough and repeated inspections must be considered the backbone of a good maintenance program. Irregular and haphazard inspection will invariably result in gradual and certain deterioration of an aircraft. The time which must eventually be spent in repairing an aircraft thus abused often totals far more than any time saved in hurrying through routine inspections and maintenance.
It has been proven that regularly scheduled inspections and preventive maintenance assure airworthiness. Operating failures and malfunctions of equipment are appreciably reduced if excessive wear or minor defects are detected and corrected early. The importance of inspections and the proper use of records concerning these inspections cannot be overemphasized.
Airframe and engine inspections may range from preflight inspections to detailed inspections. The time intervals for the inspection periods vary with the models of aircraft involved and the types of operations being conducted. The airframe and engine manufacturer's instructions should be consulted when establishing inspection intervals.
Aircraft may be inspected using flight hours as a basis for scheduling, or on a calendar inspection system. Under the calendar inspection system, the appropriate inspection is performed on the expiration of a specified number of calendar weeks. The calendar inspection system is an efficient system from a maintenance management standpoint.
Scheduled replacement of components with stated hourly operating limitations is normally accomplished during the calendar inspection falling nearest the hourly limitation.
In some instances, a flight-hour limitation is established to limit the number of hours that may be flown during the calendar interval.
Aircraft operating under the flight-hour system are inspected when a specified number of flight hours are accumulated. Components with stated hourly operating limitations are normally replaced during the inspection that falls nearest the hourly limitation.
REQUIRED INSPECTIONS Federal Aviation Regulations (FAR) provide for the inspection of all civil aircraft at specific intervals, depending generally upon the type of operations in which they are engaged, for the purpose of determining their overall condition. Some aircraft must be inspected at least once each 12 calendar months, while inspection is required for others after each 100 hours of flight. In other instances, an aircraft may be inspected in accordance with an inspection system set up to provide for total inspection of the aircraft over a calendar or flight-time period In order to determine the specific inspection requirements and rules for the preformance of inspections, reference should be made to the Federal Aviation Regulations which prescribe the requirements for the inspection and maintenance of aircraft in various types of operations.
INSPECTION TECHNIQUES Before starting an inspection, be certain all plates, access doors, fairings, and cowling have been opened or removed and the structure cleaned. When opening inspection plates and 461
cowling, and before cleaning the area take note of any oil or other evidence of fluid leakage.
CHECKLIST Always use a checklist when performing the inspection. The checklist may be of your own design, one provided by the manufacturer of the equipment being inspected, or one obtained from some other source. The checklist should include the following:
1. Fuselage and hull group.
a. Fabric and skin-for deterioration, distortion, other evidence of failure, and defective or insecure attachment of fittings.
b. Systems and components-for proper installation, apparent defects, and satisfactory operation.
c. Envelope gas bags, ballast tanks, and related parts- for condition.
2. Cabin and cockpit group.
a. Generally-for cleanness and loose and security.
proper operation.
e.
charge.
e.
f. Engine controls-for defects proper travel, and proper safetying.
g. Lines, hoses, and clamps-for leaks, condition, and looseness.
h. Exhaust stacks-for cracks, defects, and proper attachment.
i. Accessories-for apparent defects in security of mounting.
j- All systems-for proper installation, general condition defects, and secure attachment.
k. Cowling-for cracks and defects.
Ground runup and functional checkcheck all powerplant controls and systems for correct response, all instruments for proper operation and indication.
4. Landing gear group.
a. All units-for condition and security of attachment.
b. Shock absorbing devices for proper oleo fluid level.
c. Linkage, trusses, and members-for undue or excessive wear, fatigue, and distortion.
d. Retracting and locking mechanismfor proper operation.
e. Hydraulic lines-for leakage.
f. Electrical system--for chafing and proper operation of switches.
g. Wheels-for cracks defects, and condition of bearings.
h. Tires-for wear and cuts.
i. Brakes-for proper adjustment.
j. Floats and skis-for security of attachment and obvious defects.
Wing and center section.
a. All components-for condition and security.
b. Fabric and skin-for deterioration, distortion, other evidence of failure, and security of attachment.
c. Internal structure (spars, ribs compression members) -for cracks, bends, and security.
d. Movable surfaces-for damage or obvious defects, unsatisfactory fabric or skin attachment and proper travel.
e. Control mechanism-for freedom of movement, alignment, and security.
f. Control cables- for proper tension, fraying, wear and proper routing through fairleads and pulleys,
6. Empennage group.
a. Fixed surfaces--for damge or obvious defects, loose fasteners, and security of
b. Movable control surfaces-for damage or obvious defects, loose fasteners, loose fabric, or skin distortion.
c. Fabric or skin-for abrasion, tears, cuts or defects, distortion, and deterioration.
7. Propeller group.
a. Propeller assembly-for cracks, nicks, bends, and oil leakage.
b. Bolts-for proper torquing, and safety-
tions and obvious defects.
d. Control mechanisms-for proper operation, secure mounting, and travel.
Communication and navigation group.
a. Radio and electronic equipment-for proper installation and secure mounting,
b. Wiring and conduits-for proper routing, secure mounting, and obvious defects.
c. Bonding and shielding for proper installation and condition.
d. Antennas-for condition, secure mounting and proper operation.
9. Miscellaneous.
a. Emergency and first-aid equipmentfor general condition and proper stowage.
b. Parachutes, life rafts, flares, etc.-inspect in accordance with the manufacturer's recommendations.
c. Autopilot system-for general condition, security of attachment, and proper operation.
AIRCRAFT LOGS "Aircraft logs" as used in this handbook is an inclusive term which applies to the aircraft logbook and all supplemental records concerned with the aircraft. The logs and records provide a history of maintenance and operation, control of maintanance schedules, and data for time replacements of components or accessories.
The aircraft logbook is the record in which all data concerning the aircraft is recorded. Information gathered in this log is used to determine the aircraft condition, date of inspections, time on airframe and engines. It reflects a history of all significant events occurring to the aircraft, its components, and accessories, and provides a place for indicating compliance with FAA Airworthiness Directives or manufacturers' serv.
ice bulletins.
SPECIAL INSPECTIONS raft. occasions atlined on the ature and are mechanic with AS such, they aing any one of follow the + maintenance manual.
Hard or Overweight Landing Inspection The structural stress induced by a landing depends not only upon the gross weight at the time but also upon the severity of impact. However, because of the difficulty in estimating vertical velocity at the time of contact, it is hard to judge whether or not a landing has been sufficiently severe to cause structural damage. For this reason, a special inspection should be performed after a landing is made at a weight known to exceed the design landing weight or after a rough landing, even though the latter may have occurred when the aircraft did not exceed the design landing weight.
Wrinkled wing skin is the most easily detected sign of an excessive load having been imposed during a landing. Another indication which can be detected easily is fuel leaks along riveted seams. Other possible locations of damage are spar webs, bulkheads, nacelle skin and attachments, firewall skin, and wing and fuselage stringers.
If none of these areas show adverse effects, it is reasonable to assume that no serious damage has occurred. If damage is detected, a more extensive inspection and alignment check may be necessary.
Severe Turbulence Inspection When an aircraft encounters a gust condition, the airload on the wings exceeds the normal wingload supporting the aircraft weight. The gust tends to accelerate the aircraft while its inertia acts to resist this change. If the combination of gust velocity and airspeed is too severe, the induced stress can cause structural damage.
A special inspection should be performed after a flight through severe turbulence. Emphasis should be placed upon inspecting the upper and lower wing surfaces for excessive buckles or wrinkles with permanent set. Where wrinkles have occurred, remove a few rivets and examine the rivet shanks to determine if the rivets have sheared or were highly loaded in shear.
Inspect all spar webs from the fuselage to the tip, through the inspection doors and other accessible openings. Check for buckling, wrinkles, and sheared attachments. Inspect for buckling in the area around the nacelles and in the nacelle skin, particularly at the wing leading edge.
Check for fuel leaks. Any sizeable fuel leak is an indication that an area may have received overloads which have broken the sealant and opened the seams.
If the landing gear was lowered during a period of severe turbulence, inspect the surrounding surfaces carefully for loose rivets, cracks, or buckling. The interior of the wheel well may give further indications of excessive gust conditions.
Inspect the top and bottom fuselage skin. An excessive bending moment may have left wrinkles of a diagonal nature in these areas.
Inspect the surface of the empennage for wrinkles, buckling, or sheared attachments, Also, inspect the area of attachment of the empennage to the fuselage.
The above inspections cover the critical areas. If excessive damage is noted in any of the areas mentioned, the inspection should be continued until all damage is detected.
PUBLICATIONS Aeronautical publications are the sources of information for guiding aviation mechanics in the operation and maintenance of aircraft and related equipment. The proper use of these publications will greatly aid in the efficient operation and maintenance of all aircraft. These include manufacturers' service bulletins, manuals, and catalogs, as well as FAA regulations, airworthiness directives, advisory circulars, and aircraft, engine and propeller specifications.
Bulletins Service bulletins are one of several types of publications issued by airframe, engine, and component manufacturers.
The bulletins may include: (1) The purpose for issuing the publication; (2) the name of the applicable airframe, engine, or component; (3) detailed instructions for service, adjustment, modification or inspection, and source of parts, if required; and (4) the estimated number of manhours required to accomplish the job.
Maintenance Manual The aircraft maintenance manual provided by the manufacturer contains complete instructions for maintenance of all systems and components installed in the aircraft. It contains information for the mechanic who normally works on units, assemblies, and systems, while they are installed in the aircraft, and not for the overhaul mechanic.
A typical aircraft maintenance manual contains:
(1) A description of the systems such as electrical, hydraulic, fuel, control, etc.; (2) lubrication instructions setting forth the frequency and the lubricants and fluids which are to be used in the various systems; (3) pressures and electrical loads applicable to the various systems; (4) tolerances and adjustments necessary to proper functioning of the airplane; (5) methods of leveling, raising, and towing; (6) methods of balancing control surfaces;
(7) identification of primary and secondary structures; (8) frequency and extent of inspections necessary to the proper operation of the airplane;
(9) special repair methods applicable to the airplane; (10) special inspection techniques requiring X-ray, ultrasonic, or magnetic particle inspection; and (11) a list of special tools.
Overhaul Manual The manufacturer's overhaul manual contains brief descriptive information and detailed step-bystep instructions covering work normally performed on a unit away from the aircraft. Simple, inexpensive items, such as switches and relays, on which overhaul is uneconomical, are not covered in the overhaul manual.
Structural Repair Manual This manual contains information and specific instructions from the manufacturer for repairing primary and secondary structure. Typical skin, frame, rib, and stringer repairs are covered in this manual. Also included are material and fastener substitutions and special repair techniques.
Illustrated Parts Catalog This catalog presents component breakdowns of structure and equipment in disassembly sequence.
Also included are exploded views or cutaway illustrations for all parts and equipment manufactured by the aircraft manufacturer.
Federal Aviation Regulations (FAR) Federal Aviation Regulations were established by law to provide for the safe and orderly conduct of flight operations and to prescribe airmen privileges and limitations. A knowledge of the FARs is necessary during the performance of maintenance, since all work done on aircraft must comply with FAR provisions.
Airworthiness Directives A primary safety function of the Federal Aviation Administration is to require correction of unsafe conditions found in an aircraft, aircraft engine, propeller, or appliance when such conditions exist and are likely to exist or develop in other products of the same design. The unsafe condition may exist because of a design defect, maintenance, or other causes. FAR Part 39, Airworthiness Directives, defines the authority and responsibility of the administrator for requiring the necessary corrective action. The Airworthiness Directives (AD) are the media used to notify aircraft owners and other interested persons of unsafe conditions and to prescribe the conditions under which the product may continue to be Airworthiness Directives are Federal Aviation Regulations and must be complied with, unless specific exemption is granted.
Airworthiness Directives may be divided into two categories: (1) Those of an emergency nature requiring immediate compliance upon receipt and
(2) those of a less urgent nature requiring compliance within a relatively longer period of time.
The contents of ADs include the aircraft, engine, propeller, or appliance model and serial numbers affected. Also included are the compliance time or period, a description of the difficulty experienced, and the necessary corrective action.
Type Certificate Data Sheets The type certificate data sheet describes the type design and sets forth the limitations prescribed by the applicable Federal Aviation Regulations. It also includes any other limitations and information found necessary for type certification of a particular model aircraft.
Type certificate data sheets are numbered in the upper right-hand corner of each page. This number is the same as the type certificate number. The name of the type certificate holder, together with all of the approved models, appears immediately below the type certificate number.
The issue date completes this group, which is enclosed in a box to set it off.
The data sheet is separated into one or more sections. Each section is identified by a Roman numeral followed by the model designation of the aircraft to which the section pertains. The category or categories in which the aircraft can be certificated are shown in parentheses following the model number. Also included is the approval date shown on the type certificate.
The data sheet contains information regarding:
1. Model designation for all engines for which the aircraft manufacturer obtained approval for use with this model aircraft.
2. Minimum fuel grade to be used.
3. Maximum continuous and takeoff ratings of the approved engines, including manifold pressure (when used), r.p.m., and horsepower (hp.).
4. Name of the manufacturer and model designation for each propeller for which the aircraft manufacturer obtained approval will be shown together with the propeller limits and any operating restrictions peculiar to the propeller or propeller-engine combination.
5. Airspeed limits in both m.p.h. and knots.
6. Center of gravity range for the extreme loading conditions of the aircraft is given in inches from the datum. The range may also be stated in percent of MAC (Mean Aerodynamic Chord) for transport category aircraft.
7. Empty weight c.g. range (when established) will be given as fore and aft limits in inches from the datum. If no range exists, the word "none" will be shown following the heading on the data sheet.
8. Location of the datum.
9. Means provided for leveling the aircraft.
10. All pertinent maximum weights.
I1. Number of seats and their moment arms.
12. Oil and fuel capacity.
13. Control surface movements.
14. Required equipment.
15. Additional or special equipment found necessary for certification.
16. Information concerning required placards.
It is not within the scope of this handbook to list all the items that can be shown on the type certificate data sheets. Those items listed above are merely to acquaint aviation mechanics with the type of information generally included on the data sheets.
A.T.A. Specification No. 100 The Air Transport Association of America (A.T.A.) issued the specifications for Manufacturers Technical Data June 1, 1956.
Quote: "This specification establishes a standard for the presentation of technical data, by an aircraft, aircraft accessory, or component manufacturer required for their respective products."
Quote: "In order to standardize the treatment of subject matter and to simplify the user's problem in locating instructions, a uniform method of arranging material in all publications has been developed."
The A.T.A. Specification 100 has the aircraft divided into systems such as electrical which covers the basic electrical system (ATA 2400). Numbering in each major system provides an arrangement for breaking the system down into several subsys- Late model aircraft, both over and under the 12,500 dividing line, have their parts manuals and maintenance manuals arranged according to The following table of A.T.A. System, Subsystem, and Titles is included for familiarization purposes.
ATA SPEC. 100-Systems Sys. Sub Title Sys. Sub Title 21 NING 25 EQUIPMENT/FURNISHINGS 00 00 General 10 10 Flight Compartment 20 20 Passenger Compartment 30 on trol 30 Buffet/Galley 40 40 Lavatories 50 Cargo Compartments/AG Spray Apparatus 60 ntrol 60 Emergency ntaminate Control 70 Accessory Compartments 22 26 FIRE PROTECTION 00 00 General 10 10 Detection 20 Correction 20 Extinguishing 30 30 Explosion Suppression 27 FLIGHT CONTROLS 23 ONS 00 General 00 10 Aileron and Tab 10 High Frequency (HF) 20 Rudder/Ruddervator and Tab 20 VHF/UHE 30 Elevator and Tab 30 Passenger Address and Entertainment 40 Horizontal Stabilizers/Stabilator 40 Interphone 50 Flaps 50 Audio Integrating 60 Spoiler, Drag Devices & Variable Static Discharging Aerodynamic Fairings 70 Audio and Video Monitoring 70 Gust Lock and Dampener 24 ELECTRICAL POWER 80 Lift Augmenting 00 General 28 FUEL 10 Generator Drive 00 General 20 AC Generation 10 Storage 30 DC Generation 20 Distribution/Drain Valves 40 External Power 30 Dump 50 Electrical Load Distribution 40 Indicating 466
ATA SPEC. 100-Systems-Continued Sys. Sub Sya. Sub Title 29 37 00 General 00 General 10 Main 10 Distribution 20 Auxiliary 20 Indicating 30 Indicating 38 WATER/WASTE 30 00 General 00 General 10 Potable 20 Wash 20 10 30 Airfoil Air Intakes 40 30 Waste Disposal Air Supply 40 39 AND 50 60 00 General 70 Water Lines 10 80 Detection 20 30 31 TEMS 40 00 General 50 10 Unassigned 60 20 Unassigned 30 Recorders 49 40 00 General 10 Power Plant 20 Engine 32 30 00 General 40 Ignition/Starting 10 Main Gear 50 Air 60 Engine Controls 40 witch 70 80 Indicating Exhaust 50 Steering 90 Oil 80 ety Switch 51 STRUCTURES 00 General 33 LIGHTS 52 DOORS 00 General 10 tor Panels 00 10 General Passenger/Crew 30 20 Emergency Exit 40 30 Cargo 50 40 Service 50 Fixed Interior 34 NAVIGATION 60 70 Door Warning Entrance Stairs 00 General 80 Landing Gear 20 30 53 FUSELAGE 40 00 General 50 10 Main Frame 60 20 30 Plates/Skin 35 OXYGEN 50 40 Attach Fittings 00 General 10 Crew 20 Passenger 54 30 Portable 00 General 10 Main Frame 36 PNEUMATIC 20 00 General 30 Plates/Skin 10 Distribution 40 Attach Fittings 20 Indicating 50 Fillets/Fairings 467
AT SPE 00-Syst Sys. Sub Sub Title 55 TROL 00 General 10 bilator 20 30 40 50 56 WINDOWS 00 General 20 10 20 Cabin 30 Door 40 57 WINGS 00 General 20 30 Plates/Skin 76 40 50 61 00 General 20 Controlling 30 Braking 40 Indicating 65 ROTORS 78 00 General 20 30 40 Controlling 50 Braking 60 Indicating 79 71 00 General 00 General 10 10 Cowling 20 Mounts 30 30 40 50 80 60 70 10 72 OP 81 CATING ENG 00 General 00 General ction 10 20 20 30 82 50 60 00 General 70 10 Storage 72 ATING 30 00 General 40 10 20 83 (EN DR ) 30 00 General 40 10 50 Lubrication 20 468
MAGNETIC PARTICLE INSPECTION detecting invisible cracks and other defects in ferromagnetic materials, such as iron and steel.
This method of inspection is a nondestructive test, which means it is performed on the actual part without damage to the part. It is not applicable to nonmagnetic materials.
In rapidly rotating, reciprocating, vibrating, and other highly stressed aircraft parts, small defects often develop to the point that they cause complete failure of the part. Magnetic particle inspection has proved extremely reliable for the rapid detection of such defects located on or near the surface. In using this method of inspection, the location of the defect is indicated approximate size and shape are outlined.
The inspection process consists of magnetizing the part and then applying ferromagnetic particles to the surface area to be inspected. The ferromagnetic particles (indicating medium) may be held in suspension in a liquid that is flushed over the part; the part may be immersed in the suspension liquid; or the particles, in dry powder form, may be dusted over the surface of the part.
The wet process is more commonly used in the inspection of aircraft parts.
If a discontinuity is present, the magnetic lines of force will be disturbed and opposite poles will exist on either side of the discontinuity. The magnetized particles thus form a pattern in the magnetic field between the opposite poles. This pattern, known as an "indication," assumes the approximate shape of the surface projection of the discontinuity. A discontinuity may be defined as an interruption in the normal physical structure or configuration of a part such as a crack, forging lap, seam, inclusion, porosity, and the like. A discontinuity may or may not affect the usefulness of a part.
can be established in a given magnetic circuit.)
Because of the magnetism in the part and the adherence of the magnetic particles to each other, the indication remains on the surface of the part in the form of an approximate outline of the discontinuity that is immediately below it.
The same action takes place when the discontinuity is not open to the surface, but since the amount of flux leakage is less, fewer particles are held in place and a fainter and less sharply defined indication is obtained.
If the discontinuity is very far below the surface, there may be no flux leakage and, therefore, no indication on the surface. The flux leakage at a transverse discontinuity is shown in figure 10-1. The flux leakage at a longitudinal discontinuity is shown in figure 10-2.
Types of Discontinuities Disclosed The following types of discontinuities are normally detected by the magnetic particle test:
cracks, laps, seams, cold shuts, inclusions, splits, tears, pipes, and voids. All these may affect the reliability of parts in service.
Cracks, splits, bursts, tears, seams, voids, and pipes are formed by an actual parting or rupture of the solid metal. Cold shuts and laps are folds that have been formed in the metal, interrupting its continuity.
Inclusions are foreign material formed by impurities in the metal during the metal processing stages. They may consist, for example, of bits of turnace lining picked up during the melting of the basic metal or of other foreign constituents.
Inclusions interrupt the continuity of the metal because they prevent the joining or welding of adjacent faces of the metal.
Development of indications When a discontinuity in a magnetized material is open to the surface and a magnetic substance in the form of an indicating medium is available on the surface, the flux leakage at the discontinuity tends to form the indicating medium into a path of higher permeability. (Permeability is a term FIGURE 10-1. Flux leakage at transverse disused to refer to the ease with which a magnetic flux continuity.