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.
Because it is more strongly magnetic, nickel plating is more effective than chromium plating in preventing the formation of indications.
FIGURE 10-2. Flux leakage at longitudinal discontinuity.
Preparation of Parts for Testing Grease, oil, and dirt must be cleaned from all parts before they are tested. Cleaning is very important, since any grease or other foreign material present can produce nonrelevant indications due to magnetic particles adhering to the foreign material as the suspension drains from the part.
Grease or foreign material in sufficient amount over a discontinuity may also prevent the formation of a pattern at the discontinuity. It is not advisable to depend upon the magnetic particle suspension to clean the part. Cleaning by suspension is not thorough, and any foreign materials so removed from the part will contaminate the suspension, thereby reducing its effectiveness.
In the dry procedure, thorough cleaning is absolutely necessary. Grease or other foreign ing in nonrelevant indications and making it impossible to distribute the indicating medium evenly over the part's surface.
Effect of Flux Direction In order to locate a defect in a part, it is essential that the magnetic lines of force pass approximately perpendicular to the defect. It is, therefore, necessary to induce magnetic flux in more than one direction, since defects are likely to Longitudinal magnetization Attraction of particles at defects Circular magnetization сили томи terial.
Coatings of cadmium, copper, tin, and zinc do not interfere with the satisfactory performance of magnetic particle inspection, unless the coatings are unusually heavy or the discontinuities to be detected are unusually small.
Chromium and nickel plating generally will not interfere with indications of cracks open to the surface of the base metal but will prevent indications of fine discontinuities, such as inclusions.
Attraction of particles at defects B FIGURE 10-3. Effect of flux direction on strength of indication.
FIGURE 10-4. Circular magnetization of crankshaft.
exist at any angle to the major axis of the part.
This requires two separate magnetizing operations, referred to as circular magnetization and longitudinal magnetization. The effect of flux direction is illustrated in figure 10-3.
Circular magnetization is the inducing of a magnetic field, consisting of concentric circles of force about and within the part, by passing electric current through the part. This type of magnetization will locate defects running approximately parallel to the axis of the part.
Circular magnetization of a part of solid cross section is illustrated in figure 10-4. Each head of the magnetizing unit is electrically connected to a pushbutton control, so that when contact is made the magnetizing current passes from one head to the other through the part.
Figure 10-5 illustrates circular magnetization of a hollow part by passing the magnetizing current FIGURE 10-5. Circular magnetization of piston pin with conductor bar.
through a conductor bar located on the axis of the part.
In longitudinal magnetization the magnetic field is produced in a direction parallel to the long axis of the part. This is accomplished by placing the part in a solenoid excited by electric current.
The metal part then becomes the core of an electromagnet and is magnetized by induction from the magnetic field created in the solenoid.
In longitudinal magnetization of long parts, the solenoid must be moved along the part in order to magnetize it. (See figure 10-6.) This is necessary to ensure adequate field strength throughout the entire length of the part.
Solenoids produce effective magnetization for approximately 12 inches from each end of the coil, thus accommodating parts or sections approximately 30 inches in length..
Longitudinal magnetization equivalent to that FIGURE 10-6. Longitudinal magnetization of crankshaft (solenoid method).
FICURE 10-7. Longitudinal magnetization of steel propeller blade (cable method).
obtained by a solenoid may be accomplished by wrapping a flexible electrical conductor around the part, as shown in figure 10-7. Although this method is not as convenient, it has the advantage that the coils conform more closely to the shape of the part, thus producing somewhat more uniform magnetization.
The flexible coil method is also useful for large or irregularly shaped parts for which standard solenoids are not available.
Effect of Flux Density The effectiveness of the magnetic particle inspection also depends on the flux density or field strength at the surface of the part when the indicating medium is applied. As the flux density in the part is increased, the sensitivity of the test increases because of the greater flux leakages at discontinuities and the resulting improved formation of magnetic particle patterns.
Excessively high flux densities, however, may form nonrelevant indications; for example, patterns of the grain flow in the material. These indications will interfere with the detection of patterns resulting from significant discontinuities.
It is therefore necessary to use a field strength high enough to reveal all possible harmful discontinuities, but not strong enough to produce confusing nonrelevant indications.
Magnetizing Methods When a part is magnetized, the field strength in the part increases to a maximum for the particular magnetizing force and remains at this maximum as long as the magnetizing force is maintained.
When the magnetizing force is removed, the field strength decreases to a lower residual value depending on the magnetic properties of the material and the shape of the part. These magnetic characteristics determine whether the continuous or residual method is used in magnetizing the part.
In the continuous inspection method, the part is magnetized and the indicating medium applied while the magnetizing force is maintained. The available flux density in the part is thus at a maximum. The maximum value of flux depends directly upon the magnetizing force and the permeability of the material of which the part is The continuous method may be used in practically all circular and longitudinal magnetization procedures. The continuous procedure provides greater sensitivity than the residual procedure, particularly in locating subsurface discontinuities. The highly critical nature of aircraft parts and assemblies and the necessity for subsurface inspection in many applications have resulted in the continuous method being more widely used.
Inasmuch as the continuous procedure will reveal more nonsignificant discontinuities than the residual procedure, careful and intelligent interpretation and evaluation of discontinuities revealed by this procedure are necessary.
The residual inspection procedure involves magnetization of the part and application of the indicating medium after the magnetizing force has been removed. This procedure relies on the residual or permanent magnetism in the part and is more practical than the continuous procedure when magnetization is accomplished by flexible coils wrapped around the part.
In general, the residual procedure is used only with steels which have been heat treated for stressed applications.
Identification of Indications The correct evaluation of the character of indications is extremely important but is sometimes difficult to make from observation of the indications alone. The principal distinguishing features of indications are shape, buildup, width, and sharpness of outline. These characteristics, in general, are more valuable in distinguishing between types of discontinuities than in determining their severity. However, careful observation of the character of the magnetic particle pattern should always be included in the complete evaluation of the significance of an indicated discontinuity.
The most readily distinguished indications are those produced by cracks open to the suriace.
These discontinuities include fatigue cracks, heat- 472
Main gear outer cylinder Fatigue crack Torsion link lugs FIGURE 10-8. Fatigue crack in a landing gear.
treat cracks, shrink cracks in welds and castings, and grinding cracks.
Fatigue cracks give sharp, clear patterns, generally uniform and unbroken throughout their length and with good buildup. They are often jagged in appearance, as compared with the straight indications of a seam, and may also change direction slightly in localized areas. Figure 10-8 illustrates a fatigue crack.
Fatigue cracks are found in parts that have been in service but are never found in new parts. They are usually in highly stressed areas of the part or where a stress concentration exists for some reason. It is important to recognize that even a small fatigue crack indicates positively that failure of the involved parts is in progress.
Heat-treat cracks have a smooth outline but are usually less clear and have less buildup than fatigue cracks. On thin sections, such as cylinder barrel walls, heat-treat cracks may give very heavy patterns (figure 10-9). These heat-treat cracks have a characteristic form, consisting of short jagged lines grouped together.
Shrink cracks give a sharp, clear pattern and the line is usually very jagged. Since the walls of shrink cracks are close together, their indications generally build up to less extent than do indications of fatigue cracks.
Grinding cracks are fine and sharp but seldom have a buildup because of their limited depth.
Grinding cracks vary from single line indications to a heavy network of lines. Grinding cracks are generally related to the direction of grinding. For example, the crack usually begins and continues at right angles to the motion of a grinding wheel, giving a rather symmetrical pattern. Indications of grinding cracks can frequently be identified by means of this relation.
Indications of seams are usually straight, sharp, and fine. They are often intermittent and sometimes have very little buildup.
Hairlines are very fine seams in which the faces of the seam have been forced very close together during fabrication. Hairline indications are very fine and sharp, with very little buildup. Discontinuities of this type are normally considered detrimental only in highly stressed parts.
Inclusions are nonmetallic materials, such as slag materials and chemical compounds, that have been trapped in the solidifying ingot. They are usually elongated and strung out as the ingot is worked in subsequent processing operations.
FIGURE 10-9. Heat-treat cracks on cylinder barrel wall.
Inclusions appear in parts in varying sizes and shapes, from stringers easily visible to the eye to particles only visible under magnification. In a finished part they may occur as either surface or subsurface discontinuities.
Indications of subsurface inclusions are usually broad and fuzzy. They are seldom continuous or of even width and density throughout their length.
Larger inclusions, particularly those near or open to the surface, appear more clearly defined. Close examination, however, will generally reveal their lack of definition and the fact that the indication consists of several parallel lines rather than a single line. These characteristics will usually distinguish a heavy inclusion from a crack.
When cavities are located considerably below the surface of a part, the magnetic particle test is not a reliable method of detecting them. If any indication is obtained, it is likely to be an indistinct and inexact outline of the cavity, with the magnetic substance tending to distribute over the whole area rather than to outline clearly the boundary of the discontinuity. Defects of this type are detected more easily by radiographic pro- Laps may be identified by their form and location. They tend to occur at the ends or flash line of a forging. The indications are usually heavy and irregular. Islands and short branch indications usually break a lap indication of any length, and the scale included in the lap invariably gives fuzzy or small fernlike patterns stemming from the main indication.
When an ingot solidifies, the distribution of the various elements or compounds, generally, is not uniform throughout the mass of the ingot. Marked segregations of some constituents may thus occur.
As the ingot is forged and then rolled, these segregations are elongated and reduced in cross section. Upon subsequent processing, they may appear as very thin parallel lines or bands, known as banding.
Segregation in the form of banding is sometimes revealed by magnetic particle inspection, particularly when high field strengths are used.
Banding is not normally considered significant.
The most serious forms of segregation probably occur in castings. Here the basic condition of the metal remains unaltered in the finished part, and any segregations occur as they were originally formed. They may vary in size and will normally be irregular in shape. They may occur on or below Magnaglo Inspection Magnaglo inspection is similar to the preceding method, except that a fluorescent particle solution is used and the inspection is made under black fight. Efficiency of inspection is increased by the neon-like glow of defects, and smaller flaw indications are more readily seen. This is an excellent method for use on gears, threaded parts, and aircraft engine components. The reddish brown liquid spray or bath that is used consists of Magnaglo paste mixed with a light oil at the ratio of .10 to .25 ounce of paste per gallon of oil.
After inspection, the part must be demagnetized and rinsed with a cleaning solvent.
MAGNETIZING EQUIPMENT Fixed Unit (Nonportable) A fixed general-purpose unit is shown in figure
10-10. This unit provides direct current for wet continuous or residual magnetization procedures.
Either circular or longitudinal magnetization may be used, and it may be powered with rectified
a.c. as well as d.c.
The contact heads provide the electrical terminals for circular magnetization. One head is fixed in position. Its contact plate is mounted on a shaft surrounded by a pressure spring, in order that the plate may be moved longitudinally. The plate is maintained in the extended position by the spring until pressure transmitted through the work from the movable head forces it back.
The movable head slides horizontally in longitudinal guides and is motor driven. It is controlled by a switch. The spring allows sufficient overrun of the motor-driven head to avoid jamming it and also provides pressure on the ends of the work to ensure good electrical contact.
A plunger-operated switch in the fixed head cuts out the forward motion circuit of the movable head motor when the spring has been properly In some units the movable head is hand operated, and the contact plate is sometimes arranged for operation by an air ram. Both contact plates are fitted with various fixtures for supporting the work.
The magnetizing circuit is closed by depressing a pushbutton on the front of the unit. It is set to open automatically, usually after about one-half second.
Ammeter Pressure Contact Solenoid spring plate Nozzle Contact Movable head plate
• Fixed head Movable head switch Push button Pump switch Rheostat Short-circuiting- Circulating switch pump Solenoid* switch FIGURE 10-10. Fixed general-purpose magnetizing unit.
The strength of the magnetizing current may be set manually to the desired value by means of the rheostat or increased to the capacity of the unit by the rheostat short-circuiting switch. The current utilized is indicated on the ammeter.
Longitudinal magnetization is produced by the solenoid, which moves in the same guide rail as the movable head and is connected in the electrical circuit by means of a switch.
The suspension liquid is contained in a sump tank and is agitated and circulated by a pump.
The suspension is applied to the work through a nozzle. The suspension drains from the work through the wooden grill into a collecting pan that leads back to the sump. The circulating pump is operated by a pushbutton switch.
General-Purpose Portable Unit It is often necessary to perform the magnetic particle inspection at locations where fixed generalpurpose equipment is not available or to perform an inspection on members of aircraft structures without removing them from the aircraft. This has occurred, particularly on landing gears and engine mounts suspected of having developed cracks in service. Equipment suitable for this purpose, supplying both alternating-current and directcurrent magnetization, is available. A typical example is shown in figure 10-11.
This unit is only a source of magnetizing and demagnetizing current and does not provide a means for supporting the work or applying the susperision. It operates on 200-volt, 60-cycle, alternating current and contains a rectifier for producing direct current when required.
The magnetizing current is supplied through the flexible cables. The cable terminals may be fitted with prods, as shown in the illustration, or with contact clamps. Circular magnetization may be developed by using either the prods or clamps.