CENTER LINE THIN DIMENSION → THIN
LEADER THIN
BREAK (LONG) THIN BREAK (SHORT) • THICK
PHANTOM THIN
SECTIONING THIN
HIDDEN MEDIUM
STITCH LINE MEDIUM
OUTLINE OR THICK
VISIBLE LINE
DATUM LINE THICK
CUTTING PLANE EXTRA THICK
VIEMING PLANE EXTRA THICK
CUTTING PLANE FOR
COMPLEX OR OFFSET EXTRA THICK VIEW'S FIGURE 2-15. The meaning of lines.
- Extension line Dimension line - Phantom line Leader line
•Center line Sectioning fine -Break line
SECTION AA Outline Hidden line Cutting plane line FIGURE 2-16. Correct uses of lines.
Hidden Lines Hidden lines indicate invisible edges or contours. Hidden lines consist of short dashes evenly spaced and are frequently referred to as dash lines.
Outline or Visible Lines The outline or visible line is used for all lines 3/16 DRILL .3125 DRILL on the drawing representing visible lines on the 3 HOLES .3217 REAM object.
EQUALLY SPACED 3/16 DRILL 80° C'Sk Stifch Lines 7/16 C' BORE 1/4 DRILL TO 5/16 DIA Stitch lines indicate stitching or sewing lines 1/8 DEEP and consist of a series of evenly spaced dashes.
2 HOLES 80°_ Cutting Plane and Viewing Plane Lines Cutting plane lines indicate the plane in which a sectional view of the object is taken. In figure 2-16, plane line A-A indicates the plane in which section A-A is taken.
.2560 DRI 1/4 DRILL 3/8 C'BORE Viewing plane lines indicate the plane from 1/8 DEEP, 3 HOLES which a surface is viewed.
FIGURE 2-17. Dimensioning holes.
Leader Lines Leaders are solid lines with one arrowhead and indicate a part or portion to which a note, number, or other reference applies.
Break Lines Break lines indicate that a portion of the object is not shown on the drawing. Short breaks are made by solid, freehand lines. For long breaks, solid ruled lines with zigzags are used. Shafts.
rods, tubes, and other such parts which have a portion of their length broken out, have the ends of the break drawn as indicated in figure 2-16.
Phantom Lines Phantom lines indicate the alternate position of parts of the object or the relative position of a missing part. Phantom lines are composed of one long and two short evenly spaced dashes.
Sectioning Lines Sectioning lines indicate the exposed surfaces of an object in sectional view. They are generally thin, full lines, but may vary with the kind of material shown in section.
READING DRAWINGS A drawing cannot be read all at once any more than a whole page of print can be read at a glance. Both must be read a line at a time. To read a drawing effectively, follow a systematic procedure.
Upon opening a drawing, read the drawing number and the description of the article. Next, check the model affected, the latest change letter, and the next assembly listed. Having determined that the drawing is the correct one, proceed to read the illustration(s).
In reading a multiview drawing, first get a general idea of the shape of the object by scanning all the views; then select one view for a more careful study. By referring back and forth to the adjacent view, it will be possible to determine what each line represents.
Each line on a view represents a change in the direction of a surface, but another view must be consulted to determine what the change is. For example, a circle on one view may mean either a hole or a protruding boss, as in the top view of the object in figure 2-18. Looking at the top view, we see two circles; however, the other view must be consulted to determine what each circle represents. A glance at the other view tells us that the smaller circle represents a hole, and the larger 45
ance of 0.01, the drawing would show the total dimensions as: 199. A print tolerance (usually found in the title block) is a general tolerance that can be applied to parts where the dimensions are noncritical. Where a tolerance is not shown on a dimension line, the print tolerance applies.
To complete the reading of a drawing, read the general notes and the contents of the material block, check and find the various changes incorporated, and read the special information given in or near views and sections.
FIGERE 2-18. Reading Views.
circle represents a protruding boss. In the same way, the top view must be consulted to determine the shape of the hole and the protruding boss.
It can be seen from this example that one cannot read a print by looking at a single view, chen more than one view is given. Two views will not always describe an object, and when three views are given. all three must be consulted to be sure the shape has been read correctly.
After determining the shape of an object, determine its size. Information on dimensions and tolerances is given so that certain design requirements may be met. Dimensions are indicated by figures either with or without the inch mark. If no inch mark is used, the dimension is in inches.
It is customary to give part dimensions and an overall dimension that gives the greatest length of the part. If the overall dimension is missing, it can be determined by adding the separate part dimensions.
Drawings may be dimensioned in decimals or fractions. This is especially true in reference to tolerances. Many firms, instead of using plus and minus signs for tolerances, give the complete dimension for both tolerances. for cxample. if a dimension is 2 inches with a plus or minus toler-
DIAGRAMS A diagram may be defined as a graphic representation of an assembly or system, indicating the various parts and expressing the methods or principles of operation.
There are manv types of diagrams; however, those with which the aviation mechanic will be concerned during the performance of his job may be grouped into two classes or types— installation diagrams and schematic diagrams.
Instellation Diagrams Figure 2-19 is an example of an installation diagram. This is a magram of the gust lock systems of an aircraft. It identifes each of the components in the systems and shows their location in the aircraft. Each letter (A, B, C, etc.) on the principal view refers to a detail view located elsewhere on the diagram. Bach detail view is an enlarged drawing of a portion of a system. The numbers on the various views are referred to as call outs, and serve to identify each component.
installation diagrams are used extensively in aircraft maintenance and repair manuals and are invaluable in identifying and locating components and understanding the operation of varimis systens.
Schematic Dicgram Schematic diagrams do not indicate the location of individual components in the aircraft, but do locate components with respect to each other within the system. Figure 2-20 illustrates a schematic diagram of an aircraft hydraulic system.
The hydraulic pressure gage is not necessarily located above the landing gear selector valve in the aircraft; however, it is connected to the pressure line that lead: to the selector valve.
Schematic diagrams of this type are used mainly in table shooting. Note that rach line is coded 46
1. Gust lock handle 7. Elevator latch
2. Right aileron latch 8. Turnbuckles
3. Retracting springs (3) 9. Seals
4. Spring bungee 10. Left aileron
5. Push Rod 11. Tie cable
6. Rudder latch 12. Lock cable
13. Unlock cable FIGURE 2-19. Example of installation diagram (gust lock system).
RH ENGINE PUMP
SYSTEM PRESSURE
ENGINE PUMP SUCTION
IDLING CIRCUIT PUMP PRESSURE
RETURN FLOW
HAND PUMP PRESSURE
HAND PUMP SUCTION hate CHECK VALVE ACCUM. AIR GAGES GEN. SYSTEM ACCUMULATOR BRAKE ACCUMULATOR •
TO BRAKE SYSTEM
RESTRICTOR
THERMAL RELIEF VALVE
FROM EMERGENCY SEL. VALVE (BRAKE)
UNLOADING AND RELIEF VALVE
SNUBBER AND VENT ENG
VL FLAPS RESERVOIR SEL. VAL.
NORMAL LAND. CEAF SEL. VALVE EMER.
SELECTOR VALVE
XXXXXXX
LH ENGINE PUMP FIGURE 2-20. Aircraft hydraulic system schematic.
for ease of reading and tracing the flow. Each component is identified by name, and its location within the system can be ascertained by noting the lines that lead into and out of the unit.
In tracing the flow of fluid through the system, it can be seen that the engine-driven pumps receive a supply of fluid from the reservoir. One-way check valves are installed in both left and right pump pressure lines so that failure of one pump will not render the pressure from the other pump ineffective. Fluid flows to the relief side of the unloading and relief valve, and through the check valve, which will hold pressure built up beyond this point. Pressure is then directed through all lines leading to each selector valve, where it is checked if no units are being operated.
Pressure builds up in the line routed to the control port of the unloading valve and begins to 48
BLOCK IN ADD DETAIL
DARKEN VIEWS ADD DIMENSIONS ¼" →T 9-13-67 WEDGE .WOK FIGURE 2-21. Steps in sketching.
charge the system accumulator. Pressure to charge the brake accumulator is routed through a check valve incorporated in the thermal relief valve; this prevents the pressure from returning to the general system.
Although the general system accumulator starts charging at the same time, it will not charge as fast, because the fluid passes through a restrictor valve. The general system pressure will bleed into the brake system whenever the brake pressure drops below system pressure.
As soon as the pressure reaches the relief valve setting, the valve will open slightly. General system pressure increases until it reaches the value established as the system operating pressure. At this point, through the line leading to the control part of the unloading valve, the pressure will force the unloading and relief valve completely open. The pressure trapped in the system by the one-way check valve holds the value open to create an idling circuit, which prevails until some unit of the hydraulic system is operated.
Schematic diagrams, like installation diagrams, are used extensively in aircraft manuals.
DRAWING SKETCHES A sketch is a simple, rough drawing that is made rapidly and without much detail. Sketches may take many forms-from a simple pictorial presentation to a multiview orthographic projection.
A sketch is frequently drawn for use in manufacturing a replacement part. Such a sketch must provide all necessary information to those persons who must manufacture the part.
A mechanic need not be an accomplished artist.
However, in many situations, he will need to prepare a drawing to present an idea for a new design, a modification, or a repair method. The medium of sketching is an excellent way of accomplishing this.
The rules and conventional practices for making mechanical drawings are followed to the extent that all views needed to portray an object accurately are shown in their proper relationship. It is also necessary to observe the rules for correct line use (figures 2-15 and 2-16) and dimensioning.
To make a sketch, first determine what views are necessary to portray the object; then block in the views, using light construction lines. Next, complete the details, darken the object outline, and sketch extension and dimension lines. Complete the drawing by adding notes, dimensions, title, date and, when necessary, the sketcher's name. The steps in making a sketch of an object are illustrated in figure 2-21.
where on the drawing. In this case, the more easily drawn symbol for cast iron is used for the sectioning, and the material specification is listed in the bill of materials or indicated in a note.
CAST IRON MAGNESIUM, ALUMINUM AND Figure 2-22 illustrates a few standard material ALUMINUM ALLOTS symbols.
Shape Symbols Symbols can be used to excellent advantage, when it is desired to show the shape of an object.
STEEL RUBBER, PLASTIC ELECTRICAL INSULATION Typical shape symbols used on aircraft drawings are shown in figure 2-23. Shape symbols are usually shown on a drawing as a revolved or removed section.
Electrical Symbols BRASS, BRONZE, AND COPPER BABBITT, LEAD, ZINC AND ALLOYS Electrical syn bols (figure 2-24) represent various electrical de ices rather than an actual drawing of the units. Having learned what the various symbols indicate, it becomes relatively simple to look at an electrical diagram and determine what each unit is, what function it serves, and how it is WOOD ACROSS GRAIN WOOD WITH GRAIN connected in the system.
CARE OF DRAFTING INSTRUMENTS
CORK, FELT, FABRIC
ASBESTOS, LEATHER
AND FIBRE FIGURE 2-22. Material symbols.
The degree to which a sketch is complete will depend on its intended use. Obviously, a sketch not be dimensioned. If a part is to be manunecessary construction details.
DRAWING SYMBOLS The drawings for a component are composed largely of symbols and conventions representing its shape and material. Symbols are the shorthand of drawing. They graphically portray the characteristics of a component, with a minimum amount of drawing.
Material Symbois Section-line symbols show the kind of material from which the part is to be constructed. The material may not be indicated symbolically when Good drawing instruments are expensive precision tools. Reasonable care given to them during their use and storage will prolong their service life.
T-squares, triangles, and scales should not be used, or placed, where their surfaces or edges may be damaged. Use a drawing board only for its intended purpose, and not in a manner that will mar the working surface.
Compasses, dividers, and pens will provide better results with less annoyance, if they are correctly shaped and sharpened, and they are not damaged by careless handling.
Store drawing instruments in a place where they are not likely to be damaged by contact with other tools or equipment. Protect compass and divider points by inserting them into a piece of soft rubber or similar material. Never store ink pens without first cleaning and drying them thoroughly.
MICROFILM The practice of recording drawings, parts catalogs, and maintenance and overhaul manuals on microfilms was introduced in recent years. Microfilm is regular 16-mm. or 35-mm. film. Since 35-mm. film is larger, it provides a'better reproduction of drawings. Depending on the size of 50
SQUARE SECTION SQUARE SECTION (METAL) (WOOD)
ROUND SECTION ROUND SECTION (SOLID) (TUBULAR)
ANGLE SECTION CHANNEL SECTION (METAL) (METAL)
I BEAM SQUARE SECTION (METAL) (TABULAR) FIGURE 2-23. Shape symbols.
the drawing to be reproduced, a varying number of drawings can be photographed on one reel of 35-mm. film. To view or read drawings or manuals on a reel of film, you need either a portable 35-mm.
film projector or a microfilm reader or viewer.
The advantage of microfilm is that several reels, which represent perhaps hundreds of drawings, require only a small amount of storage space.
Too, a person working on an aircraft may need to refer to a specific dimension. He can place the reel of microfilm in a projector, locate the drawing or desired information, and read the dimension.
If he has to study a detail of the drawing, or work with the drawing for a long period of time, an enlarged photographic reproduction can be made, using the microfilm as a negative.
Microfilm of drawings has many other uses and advantages. However, microfilm is not intended to replace the need for original drawings, especially where the originals are modified and kept current over a long period of time.
When drawings are filmed on continuous reels, corrections can be made by cutting out superseded drawings and splicing in the revised ones.
When these corrections become numerous, the procedure becomes impractical and is discarded in favor of again filming all the related drawings.
A method that allows corrections to be made easily is to photograph the drawings and then cut up the film into individual slides. This has one disadvantage; it requires considerable time to convert the film into slides, insert them into transparent protective envelopes, and arrange them in sequence so that desired drawings can be located quickly.
A 70-mm. microfilm has become available very recently. With it, larger size drawings can be reproduced as individual frames or slides, and these can be inserted in regular paper envelopes and kept in an ordinary file. When held to the light, this large microfilm can be read with the naked eye.
CONDENSERS LAMPS - Red, Green, White
CONDUCTORS TOR →
SINGLE INTERSECTING CROSSING OVER TRANSFORMER SOLENOID
ELECTRICALLY EACH OTHER
MOTORS - GENERATORS 5 OR OR D.C. А.C D.C. A.C.
METERS
VOLT AMMETER RESISTOR RHEOSTAT
RELAYS CONNECTORS P S
REMOVABLE FIXED B
NOT ALL ALL C
SPST MOMENTARY NORMAL OR SPDT MPDT PINS PINS D
MOMENTARY NORMAL OR SHOWN SHOW
ON MOMENTARY
CONTACTS CONTACTS P DESIGNATED PINS
S DESIGNATED SOCKETS
CIRCUIT BREAKERS
AUTOMATIC PUSH PUSH RESET SWITCH MOMENTARY RESET RESET PULL OFF TYPE SWITCH TYPE p
CURRENT BATTERY BUS
LIMITER
SWITCHES - SPST MOMENTARY SPST CENTER SPŮT SPDT MOMEN- ao SPDT CENTER OFF SPOT NORMAL OR PRES- IC THERM- SPDT A BIMETAL-
ON OFF TARY ON NORMAL OR MOMENTARY SURE AL CUT-
MOMENTARY OUT
CONTACTS 10 CONTACTS: O NORMAL POSITION - PUSH-BUTTON TYPE - A MOMENTARY POSITION ала 2,0
MPST MPDT O CENTER OFF MECHANICAL LINKAGE MOMENTARY NORMAL NORMAL OR
ON ON MOMENTARY
FUSE SPLICE GROUND CONTACTS
POLARITY BASIC DC + 10A ELECTRICAL DISCONNECT POSITIVE NEGATIVE FIGURE 2-24. Electrical symbols.
CHAPTER 3
AIRCRAFT WEIGHT AND BALANCE
PURPOSE The primary purpose of aircraft weight and balance control is safety. A secondary purpose is to achieve the utmost in efficiency during flight.
Improper loading reduces the efficiency of an aircraft from the standpoint of ceiling, maneuverability, rate of climb, speed, and fuel consumption.
It can be the cause of failure to complete a flight, or even to start it. Possible loss of life and destruction of valuable equipment may result from overstressed structures or from a sudden shift in cargo and consequent change in flight characteristics.
The empty weight and the corresponding c.g.
(center of gravity) of all civil aircraft must be determined at the time of certification. The manufacturer can weigh the aircraft, or he can compute the weight and balance report. A manufacturer is permitted to weigh one aircraft out of each 10 produced. The remaining nine aircraft are issued a computed weight and balance report based on the averaged figures of aircraft that are actually weighed. The condition of the aircraft at the time of determining empty weight must be one that is well defined and can be easily repeated.
NEED FOR REWEIGHING Aircraft have a tendency to gain weight because of the accumulation of dirt, greases, etc., in areas not readily accessible for washing and cleaning.
The weight gained in any given period of time will depend on the function of the aircraft, its hours in flight, atmospheric conditions, and the type landing field from which it is operating. For this reason, periodic aircraft weighings are desirable and, in the case of air carrier and air taxi aircraft, are required by Federal Aviation Regulations.
Privately owned and operated aircraft are not required by regulation to be weighed periodically.
They are usually weighed when originally certificated, or after making major alterations that can affect the weight and balance. Even though the aircraft need not be weighed, it must be loaded so that the maximum weight and c.g. limits are not exceeded during operation.
Airline aircraft (scheduled and nonscheduled) carrying passengers or cargo are subject to certain rules that require owners to show that the aircraft is properly loaded and will not exceed the authorized weight and balance limitations during operation.
THEORY OF WEIGHT AND BALANCE The theory of weight and balance is extremely simple. It is that of the familiar lever that is in equilibrium or balance when it rests on the fulcrum in a level position. The influence of weight is directly dependent upon its distance from the fulcrum. To balance the lever the weight must be distributed so that the turning effect is the same on one side of the fulcrum as on the other. In general, a lighter weight far out on the lever has the same effect as a heavy weight near the fulcrum. The distance of any object from the fulcrum is called the lever arm. The lever arm multiplied by the weight of the object is its turning effect about the fulcrum. This turning effect is known as the moment.
Similarly, an aircraft is balanced it it remains level when suspended from an imaginary point.
This point is the location of its ideal c.g. An aircraft in balance does not have to be perfectly level, but it must be reasonably close to it. Obtaining this balance is simply a matter of placing loads so that the average arm of the loaded aircraft falls FIGURE 3-1. An airplane suspended from its center of gravity (c.g.).
within the c.g. range. The exact location of the range is specified for each type of airplane.
MATHEMATICAL PROOF Weight and balance control consists of mathematical proof of the correct weight, balance, and loading within specified limits. These limits are set forth in the specifications for a particular aircraft. The removal or addition of equipment changes the aircraft empty weight and the c.g.
The useful load is affected accordingly. The effects these changes produce on the balance of an aircraft must be investigated to determine the effect on the flight characteristics of the aircraft.
WEIGHT AND BALANCE DATA Weight and balance data can be obtained from the following sources:
a. The aircraft specifications.
b. The aircraft operating limitations.
c. The aircraft flight manual.
d. The aircraft weight and balance report.
When weight and balance records have been lost and cannot be duplicated from any source, the aircraft must be re-weighed. A new set of weight and balance records must be computed and compiled.
TERMINOLOGY In the study of weight and balance principles, computation, and control, it is necessary to know the meaning of the terms used. The following terminology is used in the practical application of weight and balance control, and should be thoroughly studied.
The Datum The datum is an imaginary vertical plane from which all horizontal measurements are taken for balance purposes, with the aircraft in level flight attitude. It is a plane at right angles to the longitudinal axis of the aircraft. For each aircraft make and model, all locations of equipment, tanks, baggage compartments, seats, engines, propellers, etc., are listed in the Aircraft Specification or Type Certificate Data Sheets as being so many inches from the datum. There is no fixed rule for the location of the datum. In most cases it is located on the nose of the aircraft or some point on the aircraft structure itself. In a few cases, it is located a certain distance forward of the nose section of the aircraft. The manufacturer has the choice of locating the datum where it is most convenient for measurement, locating equipment, and weight-and-balance computation.
The datum location is indicated on most aircraft specifications. On some of the older aircraft, where the datum is not indicated, any convenient datum may be selected. However, once the datum is selected, it must be properly identified so that anyone who reads the figures will have no doubt about the exact datum location. Figure 3-2 shows some datum locations used by manufacturers.
The Arm The arm is the horizontal distance that an item of equipment is located from the datum. The arm's distance is always given or measured in inches, and, except for a location which might.
be exactly on the datum (0), it is preceded by the algebraic sign for plus (+) or minus (-). The plus (+) sign indicates a distance aft of the datum and the minus (-) sign indicates a distance forward of the datum. If the manufacturer chooses a datum that is at the most forward location on an aircraft (or some distance forward of the aircraft), all the arms will be plus (+) arms. Location of the datum at any other point on the aircraft will result in some arms being plus (+), or aft of the datum, and some arms minus (-), or forward of The arm of each item is usually included in parentheses immediately after the item's name or weight in the specifications for the aircraft,
e.g., seat (+23). When such information is not given, it must be obtained by actual measurement.
Datum, arm, c.g., and the forward and aft c.g.
limits are illustrated in figure 3-3.
The Moment A moment is the product of a weight multiplied by its arm. The moment of an item about the datum is obtained by multiplying the weight of the item by its horizontal distance from the datum. Likewise, the moment of an item about the c.g. can be computed by multiplying its weight by the horizontal distance from the c.g.
A 20-pound weight located 30 inches from the datum would have a moment of 20 X 30 or 600 Ib.-in. Whether the value of 600 Ib.-in. is preceded by a plus (+) or minus (-) sign depends on 54
Datum
(a) Firewall location
(b) Nose location
(c) Nose location
(d) Center line of rotation location FIGURE 3-2. Various datum locations.
whether the moment is the result of a weight Any weight item removed is a minus weight.
being removed or added and its location in relation When multiplying a weight by an arm, the reto the datum. Any item of weight added to the sulting moment is plus if the signs are alike and aircraft either side of the datum is plus weight. minus if the signs are unlike.