Par 6-206 Page 6-46 SECTION 13. CORROSION PROOFING OF LAND PLANES CONVERTED TO
SEAPLANES
6-206. GENERAL. A special problem is encountered in the conversion of land planes to seaplanes. In general, land planes do not receive corrosion proofing to the same extent as do seaplanes. Corrosion-proofing standards for land planes converted to seaplanes are divided into two classes: necessary minimum precautions and recommended precautions. Regardless of such precautions, it is imperative that the exterior surfaces of seaplanes be washed with clear freshwater immediately following extended water operation, or at least once a day when operated in salty or brackish water. Wash interior surfaces of seaplanes exposed to spray, taking care to prevent damage to electrical circuits or other items subject to injury.
6-207. NECESSARY MINIMUM PRECAUTIONS. The following procedures are considered the minimum to safeguard the airworthiness of converted aircraft and are not in themselves intended to maintain airworthiness for an indefinite period.
a. Unless already protected, treat exposed fittings or fittings that can be reached through inspection openings with two coats of zinc-chromate primer, paralketone, non-water-soluble heavy grease, or comparable materials. This applies to items such as wing-root fittings, wing-strut fittings, control-surface hinges, horns, mating edges of fittings, and attached bolts.
b. Coat nonstainless control cables with grease, paralketone, or other comparable protective coating, if not replaced with corrosion-resistant cables.
c. Inspect all accessible sections of aircraft structure. Clean structural parts showing corrosion and refinish if corrosion attack is superficial. If a part is severely corroded, replace with an adequately corrosion-proofed part.
6-208. RECOMMENDED PRECAUTIONS. Recommended precautions are those which are suggested as a means of maintaining such aircraft in condition for safe operation over extended periods.
a. Provide additional inspection openings to assist in detecting corrosion. Experience has shown openings to allow inspection of the lower and rearward portion of the fuselage to be particularly desirable.
b. Incorporate additional provisions for free drainage and ventilation of all interiors to prevent collection of moisture (scoop-type marine drain grommets).
c. Protect the interior of structural steel tubing. This may be done by air and watertight sealing or by flushing with hot linseed oil and plugging the openings. Inspect tubing for missing sealing screws, presence of entrapped water, local corrosion around sealing screws, welded clusters, and bolted fittings, which may be indicative of entrapped moisture.
d. Slit the fabric of fabric-covered aircraft longitudinally on the bottom of the fuselage and tail structure for access to these sections. Coat the lower structural members with zinc-chromate primer (two coats); follow with a coat of dope-proof paint or wrap with cellophane tape and
Par 6-208 Page 6-47 rejoin the fabric. This precaution is advisable within a few months after start of operation as a seaplane.
e. Spray the interior of metal-covered wings and fuselages with an adherent corrosion inhibitor.
f. Place bags of potassium or sodium dichromate in the bottom of floats and boat hulls to inhibit corrosion.
g. Prevent the entry of water by sealing, as completely as possible, all openings in wings, fuselage, control-surface members, openings for control cables, tail-wheel wells, etc.
6-209.–6-219. [RESERVED.]
Par 6-220 Page 6-48 SECTION 14. HANDLING AND CARE OF AIRCRAFT RECOVERED FROM WATER
IMMERSION
6-220. GENERAL. Aircraft recovered from partial or total immersion in standing water or flash floods require an in-depth inspection and cleaning of both the exterior and interior areas. Water immersion increases the probability of corrosive attack, removes lubricants, deteriorates aircraft materials, and destroys electrical and avionics components.
a. Seawater, because of salt content, is more corrosive than freshwater. However, freshwater may also contain varying amounts of salt, and, as drying occurs, the salt concentration is increased and corrosive attack accelerated.
b. Prompt action is the most important factor following recovery of an aircraft from water immersion. Components of the aircraft which have been immersed, such as the powerplant, accessories, airframe sections, actuating mechanisms, screws, bearings, working surfaces, fuel and oil systems, wiring, radios, and radar, should be disassembled, as necessary, and the contaminants completely removed.
6-221. INITIAL FRESHWATER OR DETERGENT WASH. As soon as possible after the aircraft is recovered from water immersion, thoroughly wash all internal and external areas of the aircraft using a water/detergent solution as follows:
a. Mix liquid detergent (MIL-D-16791, type I) and isopropyl alcohol (TT-I-735) in ratio of 8 parts detergent to 20 parts alcohol. Add the detergent/alcohol mixture to 72 parts tapwater and mix thoroughly. For use, add one part of the preceding concentrate to nine parts tapwater (warm water if available) and mix thoroughly.
b. If the above specified detergent/alcohol materials are not available, use water-emulsion cleaning compound (A-A-59921). Add one part compound to nine parts water. If the A-A-59921 cleaning compound is not available, use any available mild household detergent solution with fresh tapwater.
6-222. RECIPROCATING ENGINES AND PROPELLERS. Remove the propeller from the engine and the engine from the aircraft. The exterior of the engine and propeller should be washed with steam, or freshwater, preferably hot.
a. Major accessories, engine parts, etc., should be removed and all surfaces flushed with freshwater, preferably hot. If facilities are available, immerse the removed parts, time permitting, in hot water or hot oil, 180 °F, for a short time. Soft water is preferred. Change the water frequently. All parts must be completely dried by air blast or other means. If no heat-drying facility is available, wipe the cleaned parts with suitable drying cloths.
b. The constant-speed propeller mechanism should be disassembled, as required, to permit complete decontamination. Clean parts with steam or freshwater, preferably hot. Dry the cleaned parts in an oven, but if a heat-drying facility is not available, wipe the cleaned parts with suitable drying cloths.
Par 6-223 Page 6-49
6-223. AIRFRAME. The salvable components of the fuselage, wings, empennage, seaplane and amphibian hulls and floats, and movable surfaces should be processed as follows:
a. The fabric from fabric-covered surfaces should be removed and replaced.
b. Clean the aircraft interior and exterior using steam under pressure with steam cleaning compound. Direct the steam into all seams and crevices where corrosive water may have penetrated. Avoid steam cleaning electrical equipment, such as terminal boards and relays.
c. Areas that have been steam cleaned should be rinsed immediately with either hot or cold freshwater.
d. Touch up all scratches and scars on painted surfaces using zinc-chromate primer or preservative.
e. Undrained hollow spaces or fluid entrapment areas should be provided temporary draining facilities by drilling out rivets at the lowest point. Install new rivets after drainage.
f. Remove and replace all leather, fabric upholstery, and insulation. Plastic or rubber foam that cannot be cleaned of all corrosive water must be replaced.
g. All drainplugs or drivescrews in tubular structures should be removed and the structure blown out with compressed air. If water has reached the tubular interiors, carefully flush with hot freshwater and blow out water with compressed air. Roll the structure as necessary to remove water from pockets. Fill the tubes with hot linseed oil, approximately 180 °F, drain oil, and replace drainplugs or drivescrews.
h. Clean sealed wood, metallic, and other nonmetallic areas, excluding acrylic plastics, with warm water. Replace wood, metalite, and other porous materials exposed to water immersion, unless surfaces are adequately sealed to prevent penetration by water. Virtually all solvents and phenolic-type cleaning agents are detrimental to acrylics and will either soften the plastic or cause crazing.
i. Remove instruments and radios and applicable cables and plumbing, and repair and inspect as necessary.
6-224.–6-234. [RESERVED.]
Par 7-1 Page 7-1 CHAPTER 7. AIRCRAFT HARDWARE, CONTROL CABLES, AND TURNBUCKLES SECTION 1. RIVETS
7-1. GENERAL.
a. Standard solid-shank rivets and the universal head rivets (AN470) are used in aircraft construction in both interior and exterior locations. All protruding head rivets may be replaced by MS20470 (supersedes AN470) rivets. This has been adopted as the standard for protruding head rivets in the United States.
b. Roundhead rivets (AN430) are used in the interior of aircraft except where clearance is required for adjacent members.
c. Flathead rivets (AN442) are used in the interior of the aircraft where interference of adjacent members does not permit the use of roundhead rivets.
d. Brazier head rivets (AN455 and AN456) are used on the exterior surfaces of aircraft where flush riveting is not essential.
e. Countersunk head rivets MS20426 (supersedes AN426 100-degree) are used on the exterior surfaces of aircraft to provide a smooth aerodynamic surface, and in other applications where a smooth finish is desired. The 100-degree countersunk head has been adopted as the standard in the United States. Refer to MIL-HDBK-5, Department of Defense Handbook:
Metallic Materials and Elements for Aerospace Vehicle Structures, or MMPDS-10, and USAF/Navy T.O. 1-1A-8, Structural Hardware.
f. Typical rivet types are shown in Table 7-10.
7-2. MATERIAL APPLICATIONS.
a. Rivets made with 2117-T4 are the most commonly used rivets in aluminum alloy structures. The main advantage of 2117-T4 is that it may be used in the condition received without further treatment.
b. The 2017-T3, 2017-T31, and 2024-T4 rivets are used in aluminum alloy structures where strength higher than that of the 2117-T4 rivet is needed. Refer to MIL-HDBK-5 or MMPDS-10 for differences between the types of rivets specified here.
c. The 1100 rivets of pure aluminum are used for riveting nonstructural parts fabricated from the softer aluminum alloys, such as 1100, 3003, and 5052.
d. When riveting magnesium alloy structures, 5056 rivets are used exclusively due to their corrosion-resistant qualities in combination with the magnesium alloys.
e. Mild steel rivets are used primarily in riveting steel parts. Do not use galvanized rivets on steel parts subjected to high heat.
Par 7-2 Page 7-2
f. Corrosion-resistant steel rivets are used primarily in riveting corrosion-resistant steel parts such as firewalls, exhaust stack bracket attachments, and similar structures.
g. Monel rivets are used in special cases for riveting high-nickel steel alloys and nickel alloys. They may be used interchangeably with stainless steel rivets as they are more easily driven. However, it is preferable to use stainless steel rivets in stainless steel parts.
h. Copper rivets are used for riveting copper alloys, leather, and other nonmetallic materials. This rivet has only limited usage in aircraft.
i. Hi-Shear rivets are sometimes used in connections where the shearing loads are the primary design consideration. Its use is restricted to such connections. It should be noted that Hi-Shear rivets are not to be used for the installation of control surface hinges and hinge brackets. Do not paint the rivets before assembly, even where dissimilar metals are being joined.
However, it is advisable to touch up each end of the driven rivet with primer to allow the later application of the general airplane finish.
j. Blind rivets in the NASM20600 through NASM20603 series rivets and the mechanically-locked stem NAS1398, 1399, 1738, and 1739 rivets sometimes may be substituted for solid rivets. They should not be used where the looseness or failure of a few rivets will impair the airworthiness of the aircraft. Design allowable for blind rivets are specified in MIL-HDBK-5 or MMPDS-10. Specific structural applications are outlined in NASM33522. Nonstructural applications for such blind rivets as NASM20604 and NASM20605 are contained in NASM33557.
CAUTION: For sheet metal repairs to airframe, the use of blind rivets must be authorized by the airframe manufacturer or approved by a representative of the Federal Aviation Administration (FAA).
NOTE: For more information on blind rivets, see Chapter 4, subparagraph 4-57f.
7-3.–7-13. [RESERVED.]
Par 7-14 Page 7-3 SECTION 2. SCREWS
7-14. GENERAL. In general, screws differ from bolts by the following characteristics.
a. Screws usually have lower material strength, a looser thread fit, head shapes formed to engage a screwdriver, and the shank may be threaded along its entire length without a clearly defined grip. Screws may be divided into three basic groups: structural screws, machine screws, and self-tapping screws. Screws are marked as required by the applicable Army/Navy (AN), National Aerospace Standard (NAS), or Military Standard (MS) drawing. Normally a manufacturer places their trademark on the head of the screw. Several types of structural screws are available that differ from the standard structural bolts only in the type of head.
b. It would be impossible to cover all screws that are available to the aviation market;
therefore, only the most frequently used screws will be discussed in this text. Design specifications are available in MIL-HDBK-5, MMPDS-10, or USAF/Navy T.O. 1-1A-8/ NAVAIR 01-1A-8, Structural Hardware.
c. Typical screw types are shown in Table 7-11.
7-15. STRUCTURAL SCREWS. NAS502, NAS503, AN509, NAS220 through NAS227, and NAS583 through NAS590 may be used for structural applications, similar to structural bolts or rivets. These screws are fabricated from a material with a high-tensile strength and differ from structural bolts only in the type of head.
7-16. MACHINE SCREWS. These screws are available in four basic head styles: flathead (countersunk), roundhead, fillister, and socket head.
a. Flathead machine screws (AN505, AN510, AN507, NAS200, NAS514, NAS517, and NAS662) are used in countersunk holes where a flush surface is desired.
b. Roundhead machine screws (AN515 and AN520) are general-purpose screws for use in nonstructural applications.
c. Fillister head machine screws (AN500 through AN503, AN116901 through AN116912, AN116913 through AN116924, AN116962 through AN116990, AN117002 through AN117030, and AN117042 through AN117070) are general-purpose screws that may be used as capscrews in light mechanical applications and are usually drilled for safety wire.
d. Socket head machine screws (NAS608 and NAS609) are designed to be driven into tapped holes by means of internal wrenches. They may be used in applications requiring high strength, compactness of assembled parts, or sinking of heads below surfaces into fitted holes.
7-17. PAN HEAD SCREWS (NAS600 THROUGH NAS606, NAS610 THROUGH NAS616, NAS623, AND NAS1402 THROUGH NAS1406). Flathead screws (MS35188 through MS35203), pan head machine screws (MS35024 through MS35219), and truss-head screws (AN526) are general-purpose screws used where head height is not important.
Par 7-18 Page 7-4
7-18. SELF-TAPPING SCREWS. The self-tapping screw taps their own mating thread when driven into untapped or punched holes slightly smaller than the diameter of the screw.
Self-tapping machine screws (AN504 and AN530) may be used to attach minor nonstructural parts. Self-tapping sheet metal screws (AN504, AN530, AN531, and NAS548) may be used in blind applications for the temporary attachment of sheet metal for riveting and the permanent assembly of nonstructural assemblies. The MS21318 is a roundhead drive screw used in the attachment of nameplates or in sealing drain holes, and is not intended to be removed after installation. They are normally installed by driving the screw into a drilled hole with a hammer.
CAUTION: Self-tapping screws should never be used as a replacement for standard screws, nuts, bolts, or rivets in any aircraft structure.
7-19. WOOD SCREWS. AN545, AN550, MS35492, and MS35493 are screws used in wood structures of aircraft.
7-20.–7-33. [RESERVED.]
Par 7-34 Page 7-5 SECTION 3. BOLTS
7-34. GENERAL. “Hardware” is the term used to describe the various types of fasteners and small items used to assemble and repair aircraft structures and components. Only hardware with traceability to an approved manufacturing process or source should be used. This traceability will ensure that the hardware is at least equal to the original or properly-altered condition. Hardware that is not traceable or is improperly altered may be substandard or counterfeit, since their physical properties cannot be substantiated. Selection and use of fasteners are as varied as the types of aircraft; therefore, care should be taken to ensure fasteners are approved by the FAA for the intended installation, repair, or replacement. Threaded fasteners (bolts/screws) and rivets are the most commonly used fasteners because they are designed to carry shear and/or tensile loads.
7-35. BOLTS. Most bolts used in aircraft structures are either general-purpose, internal wrenching, or close-tolerance AN, NAS, or MS bolts. In certain cases, fastener manufacturers produce bolts of different dimensions or greater strength than the standard types. Such bolts are made for a particular application, and it is of extreme importance to use like bolts in replacement.
Design specifications are available in MIL-HDBK-5, MMPDS-10, or USAF/Navy T.O. 1-1A-8/ NAVAIR 01-1A-8. References should be made to military specifications and industry design standards such as NAS, the Society of Automotive Engineers (SAE), and Aerospace Material Standards (AMS). Typical bolt types are shown in Table 7-12.
7-36. IDENTIFICATION. Aircraft bolts may be identified by code markings on the bolt heads.
These markings generally denote the material of which the bolt is made, whether the bolt is a standard AN-type or a special-purpose bolt, and sometimes include the manufacturer.
a. AN standard steel bolts are marked with either a raised dash or asterisk, corrosion-resistant steel is marked by a single dash, and AN aluminum alloy bolts are marked with two raised dashes.
b. Special-purpose bolts include high-strength, low-strength, and close-tolerance types.
These bolts are normally inspected by magnetic particle inspection methods. Typical markings include “SPEC” (usually heat-treated for strength and durability) and an aircraft manufacturer’s part number stamped on the head. Bolts with no markings are low strength. Close-tolerance NAS bolts are marked with either a raised or recessed triangle. The material markings for NAS bolts are the same as for AN bolts, except they may be either raised or recessed. Bolts requiring nondestructive inspection (NDI) by magnetic particle inspection are identified by means of colored lacquer or head markings of a distinctive type. (See Figure 7-1.)
FIGURE 7-1. TYPICAL AIRCRAFT BOLT MARKINGS Standard head bolt Drilled hex head bolt Countersunk head bolt Internal hex head bolt Eyebolt Clevis bolt 4037 * +
SO CIA COOPER AN Standard AN Standard AN Standard AN Standard AN Standard Steel Bolt Steel Bolt Steel Bolt Steel Bolt Steel Bolt (Corrosion Resistant) SPEC AN Standard AN Standard AN Standard AN Standard Special Bolt Steel Bolt Steel Bolt Steel Bolt Steel Bolt 63-59131 Special Bolt Drilled Head Special Bolt NAS Close Aluminum Bolt Tolerance Bolt Alloy (2024) Bolt Magnetically Orange-Dyed Clevis Bolt Reworked Bolt Low Strength Inspected Magnetic Ins. Material Bolt
7-37. GRIP LENGTH. In general, the bolt grip length of a fastener is the thickness of the material the fastener is designed to hold when two or more parts are being assembled. Bolts of slightly greater grip length may be used, provided washers are placed under the nut or bolt head.
The maximum combined height of washers that should be used is 1/8 inch. This limits the use of washers necessary to compensate for grip, up to the next standard grip size. Over the years, some fasteners' specifications have been changed. For this reason, it is recommended when making repairs to an aircraft, whose original hardware is being replaced, that you must first measure the bolt before ordering, rather than relying on the parts manual for identification. In the case of plate nuts, if proper bolt grip length is not available, add shims under the plate. All bolt installations which involve self-locking or plain nuts should have at least one thread of the bolt protruding through the nut.
7-38. LOCKING OR SAFETYING OF BOLTS. Lock or safety all bolts and/or nuts, except self-locking nuts. Do not reuse cotter pins or safety wire.
7-39. BOLT FIT. Bolt holes, particularly those of primary connecting elements, have close tolerances. Generally, it is permissible to use the first-lettered drill size larger than the nominal bolt diameter, except when the AN hexagon bolts are used in light-drive fit (reamed) applications Par 7-36 Page 7-6
Par 7-39 Page 7-7 and where NAS close-tolerance bolts or AN clevis bolts are used. A light-drive fit can be defined as an interference of 0.0006 inch for a 5/8-inch bolt. Bolt holes should be flush to the surface, and free of debris to provide full bearing surface for the bolt head and nut. In the event of oversized or elongated holes in structural members, reaming or drilling the hole to accept the next larger bolt size may be permissible. Care should be taken to ensure items, such as edge distance, clearance, and structural integrity are maintained. Consult the manufacturer’s structural repair manual, the manufacturer’s engineering department, or the FAA before drilling or reaming any bolt hole in a critical structural member.
7-40. TORQUES. The importance of correct torque application cannot be overemphasized.
Undertorque can result in unnecessary wear of nuts and bolts, as well as the parts they secure.
Overtorque can cause failure of a bolt or nut from overstressing the threaded areas. Uneven or additional loads that are applied to the assembly may result in wear or premature failure. The following are a few simple but important procedures that should be followed to ensure that correct torque is applied.
NOTE: Be sure that the torque applied is for the size of the bolt shank and not the wrench size.
a. Calibrate the torque wrench at least once a year, or immediately after it has been abused or dropped, to ensure continued accuracy.
b. Be sure the bolt and nut threads are clean and dry, unless otherwise specified by the manufacturer.
c. Run the nut down to near contact with the washer or bearing surface and check the friction drag torque required to turn the nut. Whenever possible, apply the torque to the nut and not the bolt. This will reduce rotation of the bolt in the hole and reduce wear.
d. Add the friction drag torque to the desired torque. This is referred to as “final torque,” which should register on the indicator or setting for a snap-over type torque wrench.
e. Apply a smooth even pull when applying torque pressure. If chattering or a jerking motion occurs during final torque, back off the nut and retorque.
NOTE: Many applications of bolts in aircraft/engines require stretch checks prior to reuse. This requirement is due primarily to bolt stretching caused by overtorquing.
f. When installing a castle nut, start alignment with the cotter pin hole at the minimum recommended torque plus friction drag torque.
NOTE: Do not exceed the maximum torque plus the friction drag. If the hole and nut castellation do not align, change the washer or nut and try again.
Exceeding the maximum recommended torque is not recommended.
g. When torque is applied to bolt heads or capscrews, apply the recommended torque plus friction drag torque.
h. If special adapters are used which will change the effective length of the torque wrench, the final torque indication or wrench setting must be adjusted accordingly. Determine the torque wrench indication or setting with adapter installed as shown in Figure 7-2.
FIGURE 7-2. TORQUE WRENCH WITH VARIOUS ADAPTERS _ NOTE
WHEN USING A TORQUE WRENCH ADAPTER WHICH
CHANGES THE DISTANCE FROM THE TORQUE
WRENCH DRIVE TO THE ADAPTER DRIVE, APPLY
THE FOLLOWING FORMULAS TO OBTAIN THE CORRECTED TORQUE READING:
Short Open End Adapter TORQUE
ADAPTER WRENCH
DRIVE HANDGRIP
CENTERLINE CENTERLINE
WRENCH (PREDETERMINED) DRIVE "CENTERLINE Set Screw Adapter FORMULA TxL. • Y L+E EXAMPLE: (WITH "E" AS PLUS DIMENSION) / - 104115 - 11.5 - 117.3 135 x 1 T = 135 LB. IN Y = UNKNOWN Hose Clamp Adapter L = 10.0 IN.
E = 1.5 IN. Y = 117 LB. IN.
•LEGEND T = ACTUAL (DESIRED) TORQUE Y = APPARENT (INDICATED) TORQUE L = EFFECTIVE LENGTH LEVER E = EFFECTIVE LENGTH OF EXTENSION
TORQUE WRENCH
WRENCH HANDGRIP
DRIVE CENTERLINE
CENTERLINE ADAPTER (PREDETERMINED) DRIVE
CENTERLINE Flare Nut Wrench Adapter FORMULA TXL - = Y EXAMPLE: (WITH "E" AS MINUS DIMENSION) T = 135 LB. IN. Y • 135.115 - 185 = 158.82 Y = UNKNOWN Spanner Wrench L = 10.0 IN.
Adapter E = 1.5 IN. Y = 159 LB. IN.
Par 7-40 Page 7-8