AC 43.13-1B

Par 12-37 Page 12-612 master compass. Using the same procedures, correct any error in the compass r eading using the check for errors on the East/West heading using the same procedures for the Nort h-South check, except the corrections should be made using the East-West correction brass screw.

Check the compass reading on all cardinal headings. Record the last reading and prepare a compass correction card. The maximum deviation (plus or minus) is 10 degrees on any one heading.

If the compass cannot be adjusted to meet the requirements, install another one.

NOTE: A common error that affects the compass’s accuracy is the mounting of a compass on or in the instrument panel using steel machine screws/nuts rather than brass hardware.

If the aircraft has an electrical system it is recommended that two complete compass checks be performed, one with minimum electrical equipment operating and the other with all electrical accessories on (e.g. radios, navigation radar, and lights). If the compass readings are not identical, then the mechanic should make up two separate compass correction cards. One with all the equipment on and one with the equipment off.

PNEUMATIC GYROS.

Venturi Systems. The early gyro instruments were all operated by air flowing out of a jet over buckets cut into the periphery of the gyro rotor. A venturi was mounted on the outside of the aircraft to produce a low pressure, or vacuum, which evacuated the instrument case, and air flowed into the instrument through a paper filter and then through a nozzle onto the rotor.

Venturi systems have the advantage of being extremely simple and requiring no power from the engine, nor from any of the other aircraft systems; but they do have the disadvantage of being susceptible to ice, and when they are most needed, they may become unusable.

There are two sizes of venturi tubes: those which produce four inches of suction are used to drive the attitude gyros, and smaller tubes, which produce two inches of suction, are used for the turn and slip indicator. Some installations use two of the larger venturi tubes connected in parallel to the two attitude gyros, and the turn and slip indicator is connected to one of these instruments with a needle valve between them. A suction gage is temporarily connected to the turn and slip indicator, and the aircraft is flown so the needle valve can be adjusted to the required suction at the instrument when the aircraft is operated at its cruise speed. (See figure 12-1.)

Vacuum Pump Systems. In order to overcome the major drawback of the venturi tube, that is, its susceptibility to ice, aircraft were equipped with engine driven vacuum pumps and the gyro instruments were driven by air pulled through the instrument by the suction produced by these pumps. A suction relief valve maintained the desired pressure (usually about four inches of mercury) on the attitude gyro instruments, and a needle valve between one of the attitude indicators and the turn and slip indicator restricted the airflow to maintain the desired 2 inches of suction in its case. Most of the early instruments used only paper filters in each of the instrument cases, but in some installations a central air filter was used to remove contaminants from the cabin air before it entered the instrument case.

The early vacuum pumps were vane-type pumps of what is called the wet

Par 12-38 Page 12-613

FIGURE 12-1. Venturi system for providing airflow through gyro instruments.

type-one with a cast iron housing and steel vanes.

Engine oil was metered into the pump to provide sealing, lubrication, and cooling, and then this oil, along with the air, was blown through an oil separator where the oil collected on baffles and was returned to the engine crankcase. The air was then exhausted overboard. Aircraft equipped with rubber deicer boots used this discharge air to inflate the boots. But before it could be used, this air was passed through a second stage of oil separation and then to the distributor valve and finally to the boots. (See figure 12-2.)

The airflow through the instruments is controlled by maintaining the suction in the instrument case at the desired level with a suction relief valve mounted between the pump and the instruments. This valve has a spring-loaded poppet that offsets to allow cabin air to enter the pump and maintain the correct negative pressure inside the instrument case.

The more modern vacuum pumps are of the dry type. These pumps use carbon vanes and do not require any lubrication, as the vanes provide their own lubrication as they wear away at a carefully predetermined rate. Other than the fact that they do not require an oil separator, systems using dry air pumps are quite similar to those using a wet pump. One slight difference, however, is in the need for keeping the inside of the pump perfectly clean. Any solid particles drawn into the system through the suction relief valve can damage one of the carbon vanes, and this can lead to destruction of the pump, as the particles

Par 12-38 Page 12-614

FIGURE 12-2. Instrument vacuum system using a wet-type vacuum pump.

broken off of one vane will damage all of the other vanes. To prevent particles entering the relief valve, its air inlet is covered with a filter, and this must be cleaned or replaced at the interval recommended by the aircraft manufacturer.

Positive Pressure Systems. Above about 18,000 feet there is not enough mass to the air drawn through the instruments to provide sufficient rotor speed, and, to remedy this problem, many aircraft that fly at high altitude use positive pressure systems to drive the gyros.

These systems use the same type of air pump as is used for vacuum systems, but th e discharged air from the pump is filtered and directed into the instrument case through the same fitting that receives the filtered air when the vacuum system is used. A filter is installed on the inlet of the pump, and then, before the air is directed into the instrument case, it is again filtered. A pressure regulator is located between the pump and the in-line filter to control the air pressure so only the correct amount is directed into the instrument case.

System Filters. The life of an air-driven gyro instrument is determined to a great extent by the cleanliness of the air that flows over the rotor. In vacuum systems, this air is taken from the cabin where there is usually a good deal of dust and very often tobacco

Par 12-38 Page 12-615 (and 12-18) smoke. Unless all of the solid contaminants are removed from the air before it enters the instrument, they will accumulate, usually in the rotor bearings, and slow the rotor. This cause s an inaccurate indication of the instrument and will definitely shorten its service life.

Dry air pumps are also subject to damage from ingested contaminants, and all of the filters in the system must be replaced on the schedule recommended by the aircraft manufacturer, and more often if the aircraft is operated under pa rticularly dusty conditions, especially if the occupants of the aircraft regularly smoke while flying. (See figures 12-3 and 12-4.)

FIGURE 12-3. Instrument vacuum system using a dry-type air pump.

FIGURE 12-4. Instrument pressure system using a dry-type air pump.

12-39.12-50. [RESERVED.]

Par 12-51 Page 12-616 (and 12-20)

SECTION 4. AVIONICS TEST EQUIPMENT

GENERAL. Certificated individuals who maintain airborne avionics equipment, must have test equipment suitable to perform that maintenance.

TEST EQUIPMENT CALIBRATION STANDARDS.

The test equipment calibration standards must be derived from and traceable to one of the following:

The National Institute of Standards and Technology.

Standards established by the test equipment manufacturer.

If foreign-manufactured test equipment, the standards of the country, where it was manufactured, if approved by the Administrator.

The technician must make sure that the test equipment used for such maintenance is the equipment called for by the manufacturer or equivalent.

Before acceptance, a comparison should be made between the specifications of the test equipment recommended by the manufacturer and those proposed by the repair facility.

The test equipment must be capable of performing all normal tests and checking all parameters of the equipment under test. The level of accuracy should be equal to or better than that recommended by the manufacturer.

For a description of avionics test equipment used for troubleshooting, refer to the equipment or aircraft manufacturing instruction manual.

TEST EQUIPMENT CALIBRATION. Test equipment such as meters, torque wrenches, static, and transponder test equipment should be checked at least once a year.

National Institute of Standards and Technology traceability can be verified by reviewing test equipment calibration records for references to National Institute of Standards and Technology test report numbers. These numbers certify traceability of the equipment used in calibration.

If the repair station uses a standard for performing calibration, that calibration standard cannot be used to perform maintenance.

The calibration intervals for test equipment will vary with the type of equipment, environment, and use. The accepted industry practice for calibration intervals is usually one year. Considerations for acceptance of the intervals include the following:

Manufacturer’s recommendation for the type of equipment.

Repair facility’s past calibration history, as applicable.

If the manufacturer’s manual does not describe a test procedure, the repair station must coordinate with the manufacturer to develop the necessary procedures, prior to any use of the equipment.

12-54.12-64. [RESERVED.]

Appendix 1

APPENDIX 1. GLOSSARY

The following words and terms represent some of those that are often encountered in the field of aviation. For a more complete list of definitions, a mechanic or technician should consult an aviation dictionary.

abrasion resistant PTFE a solid insulation wall of PTFE with hard, nonconductive grit positioned midway in the wall thickness, and significantly improves the resistance of the PTFE material to damage from wear.

acetylene—gas composed of two parts of carbon and two parts of hydrogen. When burned in the atmosphere of oxygen, it produces one of the highest flame temperatures obtainable.

acetylene regulator— manually adjustable device used to reduce cylinder pressure to torch pressure and to keep the pressure constant. They are never to be used as oxygen regulators.

adherend—one of the members being bonded together by adhesive.

Airworthiness Directive a regulation issued by the FAA that applies to aircraft, aircraft engines, propellers, or appliances, when an unsafe condition exists and that c ondition is likely to exist or develop in other products of the same type design.

airworthy—is when an aircraft or one of its component parts meets its type design and is in a condition for safe operation.

ambient light—the visible light level measured at the surface of the part.

ampere (A)the basic unit of current flow. One A is the amount of current that flows when a difference of potential of 1 V is applied to a circuit with a resistance of 1 Ω. One coulomb per second.

antennaa device designed to radiate or intercept electromagnetic waves.

anti-tear strips— strips of fabric of the same material as the airplane is covered with, laid over the wing rib under the reinforcing tape.

apparent power— the product of volts and amperes in AC circuits where the current and voltage are out of phase.

applianceany instrument, mechanism, equipment, part, apparatus, appurtena nce, or accessory, including communications equipment, that is used or intended to be used in operating or controlling an aircraft in flight, is installed in or attached to the aircraft, and is not part of an airframe, engine or propeller.

arma measurement of distance, in inches, feet, etc., used in weight and balance calculations. Normally only the longitudinal arm is of practical importance. The three axial arms are longitudinal arm, lateral arm, and vertical arm.

automatic direction finder (ADF) a radio receiver utilizing a directional loop antenna that enables the receiver to indicate the di rection from which a radio signal is being received; also called a radio compass.

automatic flight co ntrol system (AFCS) a flight control system incorporating an automatic pilot with additional systems such as a VOR coupler, an ILS approach coupler, and an internal navigation system that is fully automatic, so the aircraft can be flown in a completely automatic mode.

avionicsthe science and technology of electronics as applied to aviation.

azimuthangular distance measured on a horizontal circle in a clockwise direction from either north or south.

balancethe condition of stability which exists in an aircraft when all weight and forces are acting in such a way as to prevent rotation about an axis or pivot point.

base metal— the metal to be welded, brazed, soldered, or cut.

black light—electromagnetic radiation in the near ultraviolet range of wavelength.

Appendix 1

618 blade station—is a reference position on a blade that is a specified distanc e from the center of the hub.

bond—the adhesion of one surface to another, with or without the use of an adhesive as a bonding agent.

bonding—a general term applied to the process of electrically connecting tw o or more conductive objects. In aircraft, the purpose of bonding (except as applied to individual connections in the wiring and grounding systems) is to provide conductive paths for electric currents. This is accomplished by providing suitable low-impedance connections joining conductive aircraft components and the aircraft structure. Another purpose of bonding is to ensure the safe passage of current caused by lightning or static electricity through the aircraft structure.

borescope—a long, tubular optical instrument designed for remote visual inspection of surfaces.

brashness—a condition of wood characterized by low resistance to shock and by an abrupt failure across the grain without splintering.

braze welding— a welding process variation in which a filler metal, having a liquidus above 450 °C (840 °F) and below the solidus of the base metal is used. Unlike brazing, in braze welding the filler metal is not distributed in the joint by capillary action.

brazing—the joining of two pieces of metal by wetting their surface with molten alloy of copper, zinc, or tin.

bus or bus bar— solid copper strips to carry current between primary and secondary circuits;

also used as jumpers.

butt joint— a joint between two members aligned approximately in the same plane.

butyrate dope— a finish for aircraft fabric consisting of a film base of cellulose fibers dissolved in solvents with the necessary plasticizers, solvent, and thinners.

cable—(electrical)assembly of one or more conductors within an enveloping protective sheath so constructed as to per mit use of conductors separately or in a group.

center of gravity that point about which the aircraft would balance if suspended. For field weight and balance purposes/control, the center of gravity is normally calculated only along its longitudinal axis (nose to tail), disregarding both the lateral and vertical location.

certification—implies that a certificate is in existence which certifies or states a qualification.

check—a lengthwise separation of the wood, the greater part of which occurs across the rings of annual growth.

chemical conversion co ating (Specification MIL-C-81706)—is a chemical surface treatment used on aluminum alloys to inhibit corrosion and to provide a proper surface for paint finishing.

circuit—a closed path or mesh of closed paths usually including a source of EMF.

circuit breakera protective device for opening a circuit automatically when excessive current is flowing through it.

close-grained wood —wood with narrow and inconspicuous annual rings. The term is sometimes used to designate wood having small and closely-spaced pores, but in this sense the term “fine-textured” is more often used.

coil shot— production of longitudinal magnetization accomplished by passing current through a coil encircling the part being inspected.

compassa device used to determine direction on the Earth’s surface. A magnetic compass utilizes the Earth’s magnetic field to establish direction.

compression wood —identified by its relatively wide annual rings, usually eccentric, and its relatively large amount of summer wood, usually more than 50 percent of the width of the annual rings in which it occurs. Compression wood shrinks excessively lengthwise as compared with normal wood.

conductora wire or other material suitable for conducting electricity.

Appendix 1

619 conduita rigid metallic or nonmetallic casing, or a flexible metallic casing covered with a woven braid or synthetic rubber used to encase electrical cables.

contactelectrical connectors in a switch, solenoid or relay that controls the flow of current.

control panel an upright panel, open or closed, where switches, rheosta ts, meters, etc., are installed for the control and protection of electrical machinery.

chordan imaginary straight line joining the leading and trailing edges of an airfoil.

corrosion—the electrochemical deterioration of a metal resulting from chemical reaction to the surrounding environment.

creepage—is the conducting of electrical current along a surface between two points at different potentials. The current’s ability to pass between two points increases with higher voltage and when deposits of moisture or other conductive materials exist on the surfaces.

cross grain —grain not parallel with the axis of a piece. It may be either diagonal or spiral grain or a combination of the two.

cross coat—a double coat of dope or paint. It is sprayed on in one direction, and then immediately after the solvent flash-off, it is sprayed at right angles to the first coat.

cureto change the properties of a thermosetting resin irreversibly by vulcanization or chemical reaction. May be accomplished by the addition of curing (cross-linking) agents, with or without a catalyst, and with or without heat or pressure.

curing temperaturetemperature to which a resin or an assembly is subjected in order to cure the resin.

cutting torch —a device used in gas cutting of metals.

dampinglimiting the duration of vibration by either electrical or mechanical means.

datainformation that supports and/or describes the original aircraft design, alteration or repair including the following: (1) drawings, sketches, and or photographs; (2) engineering analysis;

(3) engineering orders; and (4) operating limitations.

datumimaginary vertical plane from which all horizontal measurements are made or indicated when the aircraft is in level flight attitude.

derating—is a technique whereby a part is stressed in actual usage at values well below the manufacturer’s rating for the part. By decreasing mechanical, thermal, and el ectrical stresses, the probability of degradation or catastrophic failure is lessened.

direct current electrode negative— the arrangement of direct current arc welding leads in which the work is the positive pole and the electrode is the negative pole of the welding arc.

direct current electrode positive —the arrangement of direct current arc welding leads in which the work is the negative pole and the electrode is the positive pole of the welding arc.

discontinuity—an interruption in the normal physical structure or conf iguration of a part, such as a crack, lap, seam, inclusion, or porosity.

distal tip—the tip, lens end, of a borescope.

dope—liquid applied to fabric to tauten it by shrinking, strengthen it, and render it airtight by acting as a filler.

dopeproofing—protecting a surface from the chemicals and chafing qualities of dope and doped fabrics.

drape—the ability of tape and broad goods to conform to a contoured shape.

drip loop a bundle installation method used to prevent water or other fluid contaminants from running down the wiring into a connector.

dry rot—a term loosely applied to many types of wood decay but especially to that which, when in an advanced stage, permits the wood to be easily crushed to a dry powder. The term is actually a misnomer for any decay, since all fungi require considerable moisture for growth.

Appendix 1

620 dwell time—the total time that a penetrant, emulsifier (or remover), or developer remains on the surface of the test part.

dye penetrant inspection— an inspection method for surface cracks in which a pene trating dye is allowed to enter any cracks present and is pulled out of the crack by an absorbent developer. A crack appears as a line on the surface of the developer.

edge grain —edge-grain lumber has been sawed parallel with the pith of the log and approximately at right angles to the growth rings; that is, the rings form an angle of 45 degrees or more with the surface of the piece.

electricity—one of the fundamental quantities in nature consisting of elementary particles, electrons and protons, which are manifested as a force of attraction or repulsion, and also in work that can be performed when electrons are caused to move; a material agency which, when in motion, exhibits magnetic, chemical, and thermal effects, and when at rest is accompanied by an interplay of forces between associated localities in which it is present.

electromagnet—temporary magnet which is magnetized by sending current through a coil of wire wound around an iron core.

Electromagnetic/Radio Frequency Interference (EMI/RFI)—frequency spectrum of electromagnetic radiation extending from subsonic frequency to X-rays. This term should not be used in place of the term Radio Frequency Interference (RFI).

(See radio frequency interference.) Shielding materials for the entire EMI spectrum are not readily available.

electromotive force (EMF) —difference of electrical potential measured in volts.

electron—a negative charge that revolves around the nucleus of an atom; a unit of a negative electrical charge.

electronics—general term that describes the branch of electrical science and technology that treats the behavior and effects of electron emission and transmission.

electron Volt (eV)— a unit of energy equal to the energy aquired by an electron falling though potential differences of one volt, aproximately

1.602X 10-19 joule.

emulsion-type cleaner— a chemical cleaner which mixes with water or petroleum solvent to form an emulsion (a mixture which will separate if allowed to stand). It is us ed to loosen dirt, soot, or oxide films from the surface of an aircraft.

epoxy—one of various usually thermosetting resins capable of forming tight cross-linked polymer structures ma rked by toughness, strong adhesion, high corrosion, and chemical resistance, used especially in adhesives and surface coating.

epoxy primer—a two-part catalyzed material used to provide a good bond between a surface and a surface coating.

epoxy resin— a common thermosetting resin which exhibits exceptionally good adhesion, low cure shrinkage, and low water-absorption properties.

erosion—loss of metal from metal surfaces by the action of small particles such as sand or water.

ETFE(Frequently referred to by the trade name, TEFZEL) a copolymer of PTFE and polyethylene.

exciter—small generator for supplying direct current to the alternator’s field windings.

exfoliation corrosion— a form of intergranular corrosion that attacks extruded metals along their layer-like grain structure.

expandable sleeving— open-weave braided sleeving used to protect wire and cables from abrasion and other hazards (commonly known by trade name EXPANDO).

FEP—fluorinated ethylene propylene (commonly known by the trade name, TEFLON). A melt extrudable fluorocarbon resin, very similar in appearance and performance to PTFE, but with a maximum temperature rating of 200 °C.

ferrous metal—iron, or any alloy containing iron.

fiberglassthe most common material used to reinforce structures in home-built and experimental aircraft.

Available as mat, roving, fabric, etc. It is

Appendix 1

621 incorporated into both thermoset and thermoplastic resins.

The glass fibers increase mechanical strength, impact resistance, stiffness, and dimensional stability of the matrix.

fillthreads in a fabric that run crosswise of the woven material.

filiform corrosion— a thread, or filament-like corrosion which forms on aluminum skins beneath the finish.

finish—external coating or covering of an aircraft or part.

flat grain —lumber has been sawed parallel with the pith of the log and approximately tangent to the growth rings; that is, the rings form an angle of less than 45 degrees with the surface of the piece.

fluorescent—a substance is said to be fluorescent when it will glow or fluoresce when excited by ultraviolet light. Some types of dye-penetrant material use fluorescent dyes which are pulled from the cracks by a developer and observed under “black” ultraviolet light.

flux—materials used to prevent, dissolve, or facilitate removal of oxides and other undesirable surface substances. Also, the name for magnetic fields.

fretting corrosion— corrosion damage between close-fitting parts which are allowed to rub together. The rubbing prev ents the formation of protective oxide films and allows the metals to corrode.

fuse—a protective device containing a special wire that melts when current exceeds the rated value for a definite period.

functional checkthis test may require the use of appropriate test equipment.

galvanic corrosion— corrosion due to the presence of dissimilar metals in contact with each other.

gas cylinder— a portable container used for transportation and storage of a compressed gas.

gas tungsten arc welding—(GTAW) an arc welding process which produces coalescence of metals by heating them with an arc between a tungsten (nonconsumable) electrode and the work.

Shielding is obtained from a gas or gas mixture.

Pressure may or may not be used and filler metal may or may not be used.

generator—a device for converting mechanical energy into electrical energy.

global positioning system (GPS)a navigation system that employs satellite transmitted signals to determine the aircraft’s location.

grain—the direction, size, arrangement, appearance, or quality of the fibers in wood or metal.

grain - diagonal —annual rings in wood at an angle with the axis of a piece as a result of sawing at an angle with the bark of the tree.

grommet—an insulating washer that protects the sides of holes through which wires must pass/or a metal or plastic drain attached to fabric on aircraft.

gross weight the total weight of the aircraft including its contents.

grounding—the term is usually applied to a particular form of bonding that is the process of electrically connecting conduc tive objects to either conductive structure or some other conductive return path for the purpose of safely completing either a normal or fault circuit.

harness—a cable harness is a group of cables or wires securely tied as a unit.

honeycombmanufactured product consisting of a resin-impregnated sheet or metal material which has been corrugated or expanded into hexagon-shaped and other structural-shaped cells. Primarily used as core material for sandwich constructions.

inductance (L) the ability of a coil or conductor to oppose a change in current flow.

insulatora material that will not conduct current to an appreciable degree.

Appendix 1

622 integrated circuitsmall, complete circuit built up by vacuum deposition and other techniques, usually on a silicon chip, and mounted in a suitable package.

intergranular corrosion— the formation of corrosion along the grain boundaries within a metal alloy.

interlocked-grained wood —wood in which the fibers are inclined in one direction in a number of rings of annual growth, then gradually reverse and are inclined in an opposite direction in succeeding growth rings, then reverse again.

laminatea product obtained by bonding two or more laminae of the same material or of different materials.

laminated wood—a piece of wood built up of plies or laminations that have been joined either with glue or with mechanical fastenings. The term is most frequently applied where the plies are too thick to be classified as veneer and when the grain of all plies is parallel.

leakage field— the magnetic field forced out into the air by the distortion of the field within a part, caused by the presence of a discontinuity or change in section configuration.

linterthe short fiber left on the cotton seed after ginning.

localizerthat section of an ILS that produces the directional reference beam.

LORAN (Long-Range Navigation) a radio navigation system utilizing master and slave stations transmitting timed pulses. The time difference in reception of pulses from several stations establishes a hyperbolic line of position that may be identified on a LORAN chart. By utilizing signals from two pairs of stations, a fix in position is obtained.

magnetic field— the space around a source of magnetic flux in which the effects of magnetism can be determined.

marker beacon a radio navigation aid used in an instrument approach to iden tify distance to the runway.

As the aircraft crosses over the marker-beacon transmitter, the pilot receives an accurate indication of the airplane’s distance from the runway through the medium of a flashing light and an aural signal.

master switcha switch designed to control all electric power to all circuits in a system.

moisture content of wood— weight of the water contained in the wood usually expressed in percentage of the weight of the kiln-dry wood.

multiconductor cable— consists of two or more cables or wires, all of which are encased in an outer covering composed of synthetic rubber, fabric, or other material.

nick—a sharp notch-like displacement of metal surface.

nomex braidNOMEX is the trade name for a hightemperature polyamide thread that is braided over the larger sizes (# 8 gage and larger) of many of the military specification wires. It can be encountered in either an offwhite or black/green color.

normalizing—reforming of the grain structure of a metal or alloy by proper heat treatment to relieve internal stresses.

open circuit an incomplete or broken electrical circuit.

open-grained wood —common classification of painters for woods with large pores, such as oak, ash, chestnut, and walnut. Also known as “coarsetextured.”

operational check this is an operational test to determine whether a system or component is functioning properly in all aspects in conformance with minimum acceptable manufacture design specifications.

optical fiber any filament or fiber made of dielectric materials that guides light whether or not it is used to transmit signals.

orifice—opening through which gas or air flows. It is usually the final opening controlled by a valve.

oxidizing—combining oxygen with any other substance. For example, a metal is oxidized when the metal is burned, i.e., oxygen is combined with all the metal or parts of it.

Appendix 1

623 oxidizing flame—an oxy-fuel gas flame having an oxidizing effect due to excess oxygen.

oxygen cutting— cutting metal using the oxygen jet which is added to an oxygen-acetylene flame.

oxygen regulator— manually-adjustable device used to reduce cylinder pressure to torch pressure and to keep the pressure constant. They are never to be used as fuel gas regulators.

peel ply— a layer of resin-free material used to protect a laminate for later secondary bonding (sometimes referred to as a release film).

pickling—the treatment of a metal surface by an acid to remove surface corrosion.

pitch—is the distance, in inches, that a propeller section will move forward in one revolution, or the distance a nut will advance in one revolution of the screw in a single thread.

pitch distribution— is the gradual twist in the propeller blade from shank to tip.

pitted—small irregular shaped cavities in the surface of the parent material usually caused by corrosion, chipping, or heavy electrical discharge.

pitting—the formation of pockets of corrosion products on the surface of a metal.

plastican organic substance of large molecular weight which is solid in its finished state and, at some stage during its manufacture or its processing into a finished article, can be shaped by flow.

polyester braida plastic braiding thread, when used as the outer surface of a wire, provides a cloth-like appearance.

polyimide tapea plastic film (commonly referred to by the trade name, KAPTON). The tape has a dark brown color, and is frequently coated with a polyimide varnish that has a very distinct mustard yellow color. At times, the spiral edge of the outermost tape is apparent under the varnish topcoat. It may be used for wire insulation. Total polyimide tape insulated wire constructions are inactive for new design on military aircraft and are subject to the procedures defined in FAA Advisory Circular AC 29-2A Change 2 Paragraph 29.1359 in Civil Aircraft.

polyimide varnisha liquid form of polyimide that is applied to the outer surface of a wire through the process of repeated dipping through the varnish bath with subsequent heat curing. The successive layers rarely reach a total buildup of 1 mil.

polymerization—basic processes for making large (high-polymer) molecules from small ones, normally without chemical change; can be by addition, condensation, rearrangement, or other methods.

porosity—cavity-type discontinuities in metal formed by gas entrapment during solidification.

prepreg—a mat, a fabric, or covering impregnated with resin that is ready for lay up and curing.

propeller—is a rotating airfoil that consists of two or more blades attached to a central hub which is mounted on the engine crankshaft.

protractor—is a device for measuring angles.

PTFE Tape (Insulation) polytetrafluoroethylene tape (commonly known by the trade name, TEFLON), wrapped around a conductor and then centered with heat, fusing the layers into a virtually homogeneous mass. It is used both as a primary insulation against the conductor, and as an outer layer or jacket over a shield. Maximum temperature rating is 260 °C.

PVF2 Polyvinylidine Fluoridea fluorocarbon plastic, that when used in aircraft wire, is invariably radiation cross-linked and employed as the outer layer.

radar (radio detecting and ranging) radio equipment that utilizes reflected pulse signals to locate and determine the distance to any reflecting object within its range.

radomea nonmetallic cover used to protect the antenna assembly of a radar system.

reinforcing tape— a narrow woven cotton or polyester tape used over aircraft fabric to reinforce it at the stitching attachments.

relayan electrically-operated remote-control switch.