The collar should be positioned so that the shearing edge of the pin groove is just below the top of the collar. It is permissible to add a 0.032-in. (approximately) steel washer between the collar and the material to bring the collar to the desired location.
The washer may be positioned on the rivet head side of the material when using a flathead rivet.
Hi-Shear rivets are installed with standard bucking bars and pneumatic riveting hammers. They require the use of a special gun set that incorporates collar swaging and trimming and a discharge port through which excess collar material is discharged. A separate size set is required for each shank diameter.
Prepare holes for pin rivets with the same care as for other close tolerance rivets or bolts. At times, it may be necessary to spot-face the area under the head of the pin so that the head of the rivet can fit tightly against the material. The spot-faced area should be 1e in. larger in diameter than the head diameter.
Pin rivets may be driven from either end. Procedures for driving a pin rivet from the collar end are:
(1) Insert the rivet in the hole.
(2) Place a bucking bar against the rivet head.
(3) Slip the collar over the protruding rivet end.
(4) Place previously selected rivet set and gun over the collar. Align the gun so that it is perpendicular to the material.
(5) Depress the trigger on the gun, applying pressure to the rivet collar. This action will cause the rivet collar to swage into the groove on the rivet end.
(6) Continue the driving action until the collar is properly formed and excess collar material is trimmed off. (See figure 5-71.)
Procedures for driving a pin rivet from the head end are:
(1) Insert the rivet in the hole.
(2) Slip the collar over the protruding end of rivet.
(3) Insert the correct size gun rivet set in a bucking bar and place the set against the collar of the rivet.
(4) Apply pressure against the rivet head with a flush rivet set and pneumatic riveting hammer.
(5) Continue applying pressure until the Hi-Shear rivet set insert stud and place bar against head.
Slip collar over stud and place gun set against collar.
Apply pressure (collar begins to deform).
Apply pressure until collar is trimmed.
Remove bar and set.
FIcURE 5-71. Using pin rivet set.
collar is formed in the groove and excess collar material is trimmed off.
inspection Pin rivets should be inspected on both sides of the material. The head of the rivet should not be marred and should fit tightly against the material.
Figure 5-72 illustrates acceptable and unacceptable rivets.
Removal of Pin Rivets The conventional method of removing rivets by drilling off the head may be utilized on either end of the pin rivet (figure 5-73). Center punching is recommended prior to applying drilling pressure. In some cases alternate methods may be more desirable for particular instances.
Grind a chisel edge on a small pin punch to a blade width of ⅛ in. Place this tool at right angles to the collar and drive with a hammer to split the collar down one side. Repeat the operation on the opposite side. Then, with the chisel blade, pry the collar from the rivet. Tap the rivet out of the hole.
Use a special hollow punch having one or more 188
Acceptable rivets Protective shoulder Broken edge 1/32-inch maximum resisting overdrive Normal Slightly overdriven Occurs in maximum grip Unacceptable rivets Drive until collar trims More than 1/32-inch Underdriven collar - Do not remove, but continue driving until collar trims, Overdriven collar - Remove.
Pin too long - Remove Pin too short - Remove.
FICURE 5-72. Pin rivet inspection.
blades placed to split the collar. Pry the collar from the groove and tap out the rivet.
Grind a pair of nippers so that cutting blades will cut the collar in two pieces, or use nippers at right angles to the rivet and cut through the small neck.
A hollow-mill collar cutter can be used in a power hand drill to cut away enough collar material to permit the rivet to be tapped out of the work.
SPECIFIC REPAIR TYPES Before discussing any type of a specific repair that could be made on an aircraft, remember that the methods, procedures, and materials mentioned in the following paragraphs are only typical and should not be used as the authority for the repair.
When repairing a damaged component or part, consult the applicable section of the manufacturer's structural repair manual for the aircraft, Normally, a similar repair will be illustrated, and the types of material, rivets, and rivet spacing and the methods and procedures to be used will be listed. Any addi- 189
* Line of stress 1 Drill off head or collar.
•- Trimmed hole 2 Tap rivet out with punch.
Removing from head end Draw parallel lines 1 Split coLar with 1/8-1nch chisel and pry collar off.
2-1/2 D
2. Tap rivet out with hammer Locate lines Effective Effective area area B. Removing from collar end FICURE 5-73. Removal of pin rivets.
tional knowledge needed to make a repair will also - P be detailed.
If the necessary information is not found in the structural repair manual, attempt to find a similar repair or assembly installed by the manufacturer of Locate rivets the aircraft.
2-1/2 D Smooth Skin Repair Added rivet radius Minor damage to the outside skin of an aircraft can be repaired by applying a patch to the inside of the damaged sheet. A filler plug must be installed in the hole made by the removal of the damaged skin area. It plugs the hole and forms a smooth outside surface necessary for aerodynamic smoothness of modern day aircraft.
The size and shape of the patch is determined in general by the number of rivets required in the repair. If not otherwise specified, calculate the required number of rivets by using the rivet formula.
Make the patch plate of the same material as the Draw outline of patch original skin and of the same thickness or of the next greater thickness. FIGURE 5-74. Elongated patch.
Elongated Octagonal Patch Whenever possible, use an elongated octagonal patch for repairing the smooth skin. This type of patch provides a good concentration of rivets within the critical stress area, eliminates dangerous stress concentrations, and is very simple to lay out.
This patch may vary in length according to the condition of the repair.
Follow the steps shown in the paper layout of this patch (figure 5-74). First, draw the outline of the trimmed-out damage. Then, using a spacing of three to four diameters of the rivet to be used, draw lines running parallel to the line of stress. Locate the lines for perpendicular rows two and one-half rivet diameters from each side of the cutout, and space the remaining lines three-fourths of the rivet pitch apart.
Locate the rivet spots on alternate lines perpendicular to the stress lines to produce a stagger between the rows and to establish a distance between rivets (in the same row) of about six to eight rivet diameters. After locating the proper number of rivets on each side of the cutout, add a few more if necessary so that the rivet distribution will be uniform. At each of the eight corners, swing an arc of two and one-half rivet diameters from each corner rivet. This locates the edge of the patch. Using straight lines, connect these arcs to complete the layout.
Round Patch Use the round patch for flush repairs of small holes in smooth sheet sections. The uniform distribution of rivets around its circumference makes it an ideal patch for places where the direction of the stress is unknown or where it is known to change frequently.
If a two-row round patch is used (figure 5-75), first draw the outline of the trimmed area on paper.
Draw two circles, one with a radius equal to the radius of the trimmed area plus the edge distance, and the other with a radius ¾-in. larger. Determine the number of rivets to be used and space twothirds of them equally along the outer row. Using any two adjacent rivet marks as centers, draw intersecting arcs; then draw a line from the point of intersection of the arcs to the center of the patch.
Do the same with each of the other pairs of rivet marks. This will give half as many lines as there are rivets in the outer row. Locate rivets where these ines intersect the inner circle. Then transfer the layout to the patch material, adding regular outer Approx 3/4" transverse -2-1/2 D pitch FIGURE 5-75, Layout of a two-row round patch.
edge material of two and one-half rivet diameters to the patch.
Use a three-row round patch (figure 5-76) if the total number of rivets is large enough to cause a pitch distance smaller than the minimum for a tworow patch. Draw the outline of the area on paper;
then draw a circle with a radius equal to that of the trimmed area plus the edge distance. Equally space one-third of the required number of rivets in this row. Using each of these rivet locations as a center, draw arcs having a ¾-in. radius. Where they intersect, locate the second row rivets. Locate the third row in a similar manner. Then allow extra material of two and one-half rivet diameters around the outside rivet row. Transfer the layout to the patch material.
-Approx 3/4" transverse pitch
• 2-1/2 D FIGURE 5-76. Layout of a three-row round patch.
After laying out and cutting the patch, remove the burrs from all edges. Chamfer the edges of all external patches to a 45° angle and turn them 191
slightly downward so that they will fit close to the of a panel between adjacent stringers and bulksurface (figure 5-77). heads. Where a section of skin is damaged to such an extent that it is impossible to install a standard skin repair, a special type of repair is necessary.
The particular type of repair required depends on whether the damage is reparable outside the mem- Approx 1/64" ber, inside the member, or to the edges of the panel.
Damage which, after being trimmed, has less than eight and one-half manufacturer's rivet diameters of material inside the members requires a patch which extends over the members, plus an extra row of rivets along the outside of the members. For dam- 1-1/4 D age which, after being trimmed, has eight and onehalf rivet diameters or more of material, extend the patch to include the manufacturer's row of rivets 2-1/2 D and add an extra row inside the members. Damage FIGURE 5-77. Chamfering and turning edge. which extends to the edge of a panel requires only one row of rivets along the panel edge, unless the Panel Repair manufacturer used more than one row, The repair In aircraft construction, a panel is any single procedure for the other edges of the damage follows sheet of metal covering. A panel section is the part the previously explained methods.
Repair seam same as strongest parallel adjacent seam Damage Repair seam same as strongest parallel adjacent seam.
Use original holes & add as needed. Trimmed Hole Additional Rivets Radiused Corners Rivet Symbols
• 3/16"
• 5/32"
• 1/8" FIcURE 5-78. Panel skin patch.
The procedures for making all three types of panel repairs are similar. Trim out the damaged portion to the allowances mentioned in the preceding paragraph. For relief of stresses at the corners of the trim-out, round them to a minimum radius of ½ in. Lay out the new rivet row with a transverse pitch of approximately five rivet diameters and stagger the rivets with those put in by the manufacturer. See figure 5-78.
Cut the patch plate from material of the same thickness as the original or the next greater thickness, allowing an edge distance of two and one-half rivet diameters. At the corners, strike arcs having the radius equal to the edge distance. Chamfer the edges of the patch plate for a 45° angle and form the plate to fit the contour of the original structure.
Turn the edges downward slightly so that the edges fit closely.
Place the patch plate in its correct position, drill one rivet hole, and temporarily fasten the plate in place with a fastener. Using a hole finder, locate the position of a second hole, drill it, and insert a second fastener. Then, from the back side and through the original holes, locate and drill the remaining holes. Remove the burrs from the rivet holes and apply corrosion protective material to the contacting surfaces before riveting the patch into place.
Stringer Repair The fuselage stringers extend from the nose of the aircraft to the tail, and the wing stringers extend from the fuselage to the wing tip. Surface control stringers usually extend the length of the control surface. The skin of the fuselage, wing, or control surface is riveted to stringers.
Stringers may be damaged by vibration, corrosion, or collision. Damages are classified as negligible, damage reparable by patching, and damage necessitating replacement of parts. Usually the damage involves the skin and sometimes the bulkhead or formers. Such damage requires a combination of repairs involving each damaged member.
Because stringers are made in many different shapes repair procedures differ. The repair may require the use of preformed or extruded repair material, or it may require material formed by the airframe mechanic. Some repairs may need both kinds of repair material.
When repairing a stringer, first determine the extent of the damage and remove the rivets from the surrounding area. Then remove the damaged area by using a hacksaw, keyhole saw, drill, or file.
In most cases, a stringer repair will require the use of an insert and splice angle. When locating the splice angle on the stringer during repair, be sure to consult the applicable structural repair manual for the repair piece's position. Some stringers are repaired by placing the splice angle on the inside, whereas others are repaired by placing it on the outside.
Extrusions and preformed materials are commonly used to repair angles and insertions or fillers.
If repair angles and fillers must be formed from flat sheet stock, use the brake. It may be necessary to use bend allowance and sight lines when making the layout and bends for these formed parts. For repairs to curved stringers, make the repair parts so that they will fit the original contour.
When calculating the number of rivets to be used in the repair, first determine the length of the break. In bulb-angle stringers, the length of the break is equal to the cross sectional length plus three times the thickness of the material in the standing leg (to allow for the bulb), plus the actual cross sectional length for the formed stringers and straight angles.
Substitute the value obtained, using the procedure above as the length of the break in the rivet formula, and calculate the number of rivets required. The rivet pitch should be the same as that used by the manufacturer for attaching the skin to the stringer. In case this pitch exceeds the maximum of 10 rivet diameters, locate additional rivets between the original rivets. Never make the spacing less than four rivet diameters.
When laying out this spacing, allow two and one-half rivet diameters for edge distance on each side of the break until all required rivets are located. At least five rivets must be inserted on each end of the splice section. If the stringer damage requires the use of an insertion or filler of a length great enough to justify more than 10 rivets, two splice angles should usually be used.
If the stringer damage occurs close to a bulkhead, cut the damaged stringer so that only the filler extends through the opening in the bulkhead The bulkhead is weakened if the opening is enlarged to accommodate both the stringer and the splice angle. Two splice angles must be used to make such a repair.
Because the skin is fastened to the stringers, it is often impossible to drill the rivet holes for the repair splices with the common air drill. These holes can be drilled with an angle drill. When riveting a stringer, it may be necessary to use an offset rivet set and various shaped bucking bars.
Former or Bulkhead Repairs Bulkheads are the oval-shaped members of the fuselage which give form to and maintain the shape of the structure. Bulkheads or formers are often called forming rings, body frames, circumferential rings, belt frames, and other similar names. They are designed to carry concentrated stressed loads.
There are various types of bulkheads. The most common type is a curved channel formed from sheet stock with stifleners added. Others have a web made from sheet stock with extruded angles riveted in place as stiffeners and flanges. Most of these members are made from aluminum alloy. Corrosion-registant steel formers are used in areas which are exposed to high temperatures.
Bulkhead damages are classified in the same manner as other damages. Specifications for each type of damage are established by the manufacturer and specific information is given in the maintenance manual or structural repair manual for the aircraft.
Bulkheads are identifed with station numbers, which are very helpful in locating repair information.
Repairs to these members are generally placed in one of two categories: (1) One-third or less of the cross sectional area damaged, or (2) more than one-third of the cross sectional area damaged. If one-third or less of the cross sectional area has been damaged, a patch plate, reinforcing angle, or both, may be used. First, clean out the damage and then use the rivet formula to determine the number of rivets required in order to establish the size of the patch plate. For the length of the break, use the depth of the cutout area plus the length of the flange.
If more than one-third of the cross sectional area is damaged, remove the entire section and make a splice repair (figure 5-79). When removing the damaged section, be careful not to damage the surrounding equipment, such as electrical lines, plumbing, instruments, and so forth. Use a hand file, rotary file, snips, or a drill to remove larger damages. To remove a complete section, use a hacksaw, keyhole saw, drill, or snips.
Measure the length of break as shown in figure 5-79 and determine the number of rivets required by substituting this value in the rivet formula. Use the double shear value of the rivet in the calculations. The result represents the number of rivets to be used in each end of the splice plate.
Most repairs to bulkheads are made from flat sheet stock if spare parts are not available. When fabricating the repair from flat sheet, remember that the substitute material must provide cross sectional tensile, compressive, shear, and bearing strength equal to the original material. Never substitute material which is thinner or has a cross sectional area less than the original material.
Curved repair parts made from flat sheet must be in the "O" condition before forming, and then must be heat treated before installation.
Longeron Repair Generally, longerons are comparatively heavy members which serve approximately the same function as stringers. Consequently, longeron repair is similar to stringer repair. Because the longeron is a heavy member and more strength is needed than with a stringer, heavy rivets will be used in the L = F+ W+F or = 2F + W
• Cross section of stringer FIGURE 5-79. Deterinining length of break.
repair. Sometimes bolts are used to install a longeron repair; but, because of the greater accuracy required, they are not as suitable as rívets. Also, bolts require more time for installation.
If the longeron consists of a formed section and an extruded angle section, consider each section separately. Make the longeron repair as you would a stringer repair. However, keep the rivet pitch between four- and six-rivet diameters. If bolts are used, drill the bolt holes for a light drive fit.
Spar Repair The spar is the main supporting member of the wing. Other components may also have supporting members called spars which serve the same function as the spar does in the wing. Think of spars as the "hub" or "base" of the section in which they are located, even though they are not in the center. The spar is usually the first member located during the construction of the section, and the other components are fastened directly or indirectly to it.
Because of the load the spar carries, it is very important that particular care be taken when repairing this member to ensure that the original strength of the structure is not impaired. The spar is so constructed that two general classes of repairs, web repairs and cap strip repairs, are usually necessary.
For a spar web butt splice, first clean out the damage; then measure the full width of the web section. Determine the number of rivets to be placed in each side of the splice plate by substituting this value for the length of break in the rivet formula.
Prepare an insert section of the same type material and thickness as that used in the original web.
Make a paper pattern of the rivet layout for the splice plate using the same pitch as that used in the attachment of the web to the cap strip. Cut the splice plates from sheet stock having the same weight as that in the web, or one thickness heavier, and transfer the rivet layout from the paper pattern to the splice plates.
Give all contacting surfaces a corrosion-resistant treatment and rivet the component parts of the repair into place. The rivets used in attaching the insert section to the cap strips are in addition to those calculated for attaching the splice plates. Replace all web stiffeners removed during the repair.
An exploded view of a spar web butt splice is shown in figure 5-80.
Top view of butt splice Cap strip Insert Splice plate FIGURE 5-80. Spar web butt splice.
When making a spar web joggle splice, no splice plates are needed. Instead, form the web repair section so that it overlaps the original web suffciently to accommodate the required number of rivets. Make a joggle in each end of the repair section so that the repair piece contacts the cap strips to which it is riveted. Rivet calculation for this repair is similar to that described for butt splicing.
Many forms of cap strips are used in aircraft manufacturing, and each equires a distinct type of repair. In calculating the number of rivets required in an extruded T-spar cap strip repair, take the width of the base of the T, plus the length of the leg as the length of the break, and use double shear values.
Place one-fourth of the required number of rivets in each row of original rivets in the base of the T-section. Locate them midway between each pair of the original rivets. Locate the remainder of the rivets along the leg of the T-section in two rows.
Consider all original rivets within the area of the splice as part of the required rivets.
Make the filler piece of a similar piece of T-section extrusion or of two pieces of flat stock. It is possible to make the splice pieces of extruded angle material or to form them from sheet stock; in either case, they must be the same thickness as the cap strip. Figure 5-81 shows an exploded view of a Insert T-spar cap strip repair. The rivets used in the leg of the cap strip may be either the round-, flat-, or brazier-head type; but the rivets used in the base must be the same type as those used in the skin.
The repair of milled cap strips is limited to damages occurring to flanges. Damages beyond flange areas require replacement of the entire cap strip. To make a typical flange repair, substitute the depth of the trimmed-out area as the length of break in the rivet formula and calculate the number of rivets required. Form a splice plate of the required length and drill it to match the original rivet layout. Cut an insert to fit the trimmed-out area and rivet the repair in place. If the trimmed-out area is more than 4 in. in length, use an angle splice plate to provide added strength.
Rib and Web Repair Web repairs can be generally classified into two types: (1) Those made to web sections considered critical, such as those in the wing ribs, and (2) those considered less critical, such as those in elevators, rudders, flaps, and the like. Web sections must be repaired in such a way that the original strength of the member is restored.
In the construction of a member using a web (figure 5-82), the web member is usually a lightgage aluminum alloy sheet forming the principal depth of the member. The web is bounded by heavy aluminum alloy extrusions known as cap strips.
These extrusions carry the loads caused by bending Cap strip stiffeners Depth Web Splice piece Spar FIGURE 5-81. T-spar cap strip repair. FIGURE 5-82. Construction of a web member.
Stamped beads. Damage to ribs and webs which require a repair larger than a simple plate will probably need a JOIC patch plate, splice plates, or angles and an inser- tion. To repair such a damage by forming the necessary parts may take a great deal of time; therefore, if damaged parts which have the necessary areas intact are available from salvage, use them.
For example, if an identical rib can be located in salvage and it has a cracked web but the area in question is intact, clean out the damaged area; then cut the repair piece from the rib obtained from salvage. Be sure to allow plenty of material for correct rivet installation. Using a part from salvage will eliminate a great deal of hard work plus the heat-treating operation needed by a new repair piece.
FIGURE 5-83. Stamped beads in a web.
and also provide a foundation for attaching the skin. The web may be stiffened by stamped beads, formed angles, or extruded sections riveted at regular intervals along the web.
The stamped beads (figure 5-83) are a part of the web itself and are stamped in when the web is made. Stiffeners help to withstand the compressive loads exerted upon the critically stressed web members.
Often ribs are formed by stamping the entire piece from sheet stock. That is, the rib lacks a cap strip, but does have a flange around the entire piece, plus lightening holes in the web of the rib.
Ribs may be formed with stamped beads for stifieners, or they may have extruded angles riveted on the web for stiffeners.
Most damages involve two or more members;
however, it may be that only one member is damaged and needs repairing. Generally, if the web is damaged, all that is required is cleaning out the damaged area and installing a patch plate.
The patch plate should be of sufficient size to ensure room for at least two rows of rivets around the perimeter of the damage; this will include proper edge distance, pitch, and transverse pitch for the rivets. The patch plate should be of material having the same thickness and composition as the original member. If any forming is necessary when making the patch plate, such as fitting the contour of a lightening hole, use material in the "O" condition and then heat treat it after forming.
Leading Edge Repair The leading edge is the front section of a wing, stabilizer, or other airfoil. The purpose of the leading edge is to streamline the forward section of the wings or control surfaces so that the airflow is effective. The space within the leading edge is sometimes used to store fuel. This space may also house extra equipment such as landing lights, plumbing lines, or thermal anti-icing systems.
The construction of the leading edge section varies with the type of aircraft. Generally, it will consist of cap strips, nose ribs, stringers, and skin. The cap strips are the main lengthwise extrusions, and they stiflen the leading edges and furnish a base for the nose ribs and skin. They also fasten the leading edge to the front spar.
The nose ribs are stamped from aluminum alloy sheet. These ribs are U-shaped and may have their web sections stiffened. Regardless of their design, their purpose is to give contour to the leading edge.
Stiffeners are used to stiffen the leading edge and supply a base for fastening the nose skin. When fastening the nose skin, use only flush rivets.
Leading edges constructed with thermal anti-icing systems consist of two layers of skin separated by a thin air space. The inner skin, sometimes corrugated for strength, is perforated to conduct the hot air to the nose skin for anti-icing purposes.
Damage to leading edges are also classified in the same manner as other damages. Damage can be caused by contact with other objects, namely, pebbles, birds in flight, and hail. However, the major 197
cause of damage is carelessness while the aircraft countersunk rivets and forming countersunk shop is on the ground. heads to get a smooth contour.
A damaged leading edge will usually involve several structural parts. Flying-object damage will probably involve the nose skin, nose ribs, stringers, and possibly the cap strip. Damage involving all of these members will necessitate installing an access Insert door to make the repair possible. First, the damaged area will have to be removed and repair procedures established. The repair will need insertions and splice pieces. If the damage is serious enough, it may require repair of the cap strip and stringer, a new nose rib, and a skin panel. When repairing a leading edge, follow the procedures prescribed in the appropriate repair manual for this type of repair. FiLer Trailing Edge Repair A trailing edge is the rearmost part of an airfoil, found on the wings, ailerons, rudders, elevators, and stabilizers. It is usually a metal strip which forms the shape of the edge by tying the ends of a rib section together and joining the upper and lower skins. Trailing edges are not structural members, but they are considered to be highly stressed in all cases.
Damage to a trailing edge may be limited to one point or extended over the entire length between two or more rib sections. Besides damage resulting from collision and careless handling, corrosion damage is often present. Trailing edges are particularly subject to corrosion because moisture collects or is trapped in them.
Thoroughly inspect the damaged area before starting repairs, and determine the extent of damage, the type of repair required, and the manner in which the repair should be performed. When making trailing edge repairs, remember that the repaired area must have the same contour and be made of material with the same composition and temper as the original section. The repair must also be made to retain the design characteristics of the airfoil.
Damage occurring in the trailing edge section between the ribs can be repaired as shown in figure
5-84. Cut out the damaged area and make a filler of either hardwood, fiber, or cast aluminum alloy to fit snugly inside the trailing edge. Then make an insert piece of the same material as the damaged section and shape it to match the trailing edge, Assemble the pieces as shown and rivet them into place using FIcURE 5-84. Trailing edge repair between ribs.
To repair damage occurring at or near a rib, first remove sufficient trailing edge material to allow a complete splice to fall between the ribs. This usually requires two splices joined by an insert piece of similar trailing edge material or of formed sheet stock. The repair procedure is similar to that for damage between ribs. Figure 5-85 shows this type of repair.
STRUCTURAL SEALING Various areas of airframe structures are sealed compartments where fuels or air must be confined Some of these areas contain fuel tanks; others consist of pressurized compartments such as the cabin.
Because it is impossible to seal these areas completely airtight with a riveted joint alone, a sealing compound or sealant must be used. Sealants are also used to add aerodynamic smoothness to exposed surfaces such as seams and joints in the wings and fuselage.
Three types of seals are ordinarily used. Rubber seals are installed at all points where frequent breaking of the seal is necessary, such as emergency exits and entrance doors. Sealing compounds are used at points where the seal is seldom broken 198
be detected by the change in sound when the instrument passes over it. After completing the test, rensert move the soapless bubble solution from the outside of the aircraft by washing with clear water to prevent corrosion.
Here are a few precautionary measures to follow during the testing procedure just discussed. With personnel inside, the area should never be pressurized to a pressure higher than has previously been established during testing with the section empty.
No person who has a cold or who has recently had one, or whose sinuses are impaired in any way, should work in the pressurized section. A qualified operator should be present at the pressurization equipment control panel at all times while the sec- Slug tion is being pressurized.
Pressurization may not always be necessary to FIcURE 5-85. Trailing edge repair near a rib. determine defectively sealed areas. Sealants should be repaired when:
except for structural maintenance or part replacement, as with riveted lap and butt seams. Special seals are required for passing cables, tubing, mechanical linkages, or wires out of the pressurized or sealed areas.
Wires and tubes are passed through pressure bulkheads by using bulkhead fittings such as cannon plugs for wiring and couplings for tubing.
These fittings are sealed to the bulkhead and the wires and tubes are fastened to them from each side. All seals of moving components such as flight controls are subject to wear and utmost care must be used when they are installed. Also, they must be checked regularly.
(1) Sealants have peeled away from the structure.
(2) Seams are exposed through the sealant fillet.
(3) Fillet- or hole-filling sealant is exposed through the smooth overcoating.
(4) Sealant is damaged by the removal and re-installation of fasteners, access doors, or other sealed parts.
(5) Cracks or abrasions exist in the sealant.
Determining Sealant Defects Pressure tightness of an area or section is checked before and after a repair is made. Ground pressurization is accomplished by filling the section with air from an external source through ground pressure test fittings.
With the sections pressurized to a given pressure, locate leaks on the outside of the aircraft by applying a soapless bubble solution to all seams and joints in the suspected area. Air bubbles will locate the general area of leakage. A specific leak is then isolated on the inside of the aircraft by passing the free end of a stethoscope or similar listening device along the seams in the leakage area. The leak can Sealant Repair All surfaces which are to be sealed must be cleaned to ensure maximum adhesion between the sealant and the surface. Loose foreign material can be removed by using a vacuum cleaner on the affected area. Scrape all the old sealant from the repair area with a sharp plastic, phenolic, or hardwood block to prevent scratches, and apply a stripper and a cleaner.
The cleaner should not be allowed to dry on a metal surface, but should be wiped dry with clean rags. Do not remove the cleaner with soiled rags since the metal surface must be free of all dirt, grease, powder, and so forth. The surface can be checked for cleanliness by pouring water over it after being wiped dry of the cleanser. If the surface 199