Par 6-193 Page 6-285 (and 6-40) SECTION 12. PLATED PARTS
CHROMIUM AND NICKEL-PLATED PARTS.
Nickel and chromium platings are used extensively as protective and wear-resistant coatings over high-strength steel parts (landing gear journals, shock strut pistons, etc.).
Chromium and nickel plate provide protection by forming a somewhat impervious physical coat over the underlying base metal. When breaks occur in the surface, the protection is destroyed.
The amount of reworking that can be performed on chromium and nickel-plated components is limited.
The rework should consist of light buffing to remove corrosion products and produce the required smoothness. The buffing should not take the plating below the minimum allowable thickness.
Whenever a chromium or nickel-plated component requires buffing, coat the area with a corrosionpreventive compound, if possible.
When buffing exceeds the minimum thickness of the plating, or the base metal has sustained corrosive attack, the component should be removed and replaced.
The removed component can be restored to serviceable condition by having the old plating completely stripped and re plated in accordance with acceptable methods and specifications.
CADMIUM AND ZINC-PLATED PARTS.
Cadmium plating is used extensively in aircraft construction as a protective finish over both steel and copper alloys. Protection is provided by a sacrificial process in which the cadmium is attacked rather than the underlying base material. Properly functioning cadmium surface coatings may show mottling, ranging from white to brown to black spots on their surfaces. These show the sacrificial protection being offered by the cadmium coat, and under no condition should such spotting be removed merely for appearance sake. In fact, cadmium will continue to protect even when actual breaks in the coating develop and bare steel or exposed copper surfaces appear.
When the breakdown of the cadmium plating occurs and the initial appearance of corrosion products on the base metal develops, some mechanical cleaning of the area may be necessary but shall be limited to removal of the corrosion products from the underlying base material.
Under no condition should such a coating be cleaned with a wire brush. If protection is needed, a touch-up with primer or a temporary preservative coating should be applied. Restoration of the plate coating cannot be done in the field.
Zinc coatings offer protection in an identical manner to cadmium, and the corrective treatment for failure is generally the same as for cadmium-plated parts.
However, the amount of zinc on aircraft structures is very limited and usually does not present a maintenance problem.
6-195.6-205. [RESERVED.]
Par 6-206 Page 6-286
SECTION 13. CORROSION PROOFING OF LAND PLANES
CONVERTED TO SEA PLANES
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 fresh water 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.
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.
Unless already protected , treat exposed fittings or fittings that can be reached through inspection openings with two coats of zinc chromate primer, paralketone, nonwater-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.
Coat non-stainless control cables with grease or paralketone or other comparable protective coating, if not replaced with corrosion-resistant cables.
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.
RECOMMENDED PRECAUTIONS.
Recommended precautions are those which are suggested as a means of maintaining such aircraft in condition for safe operation over extended periods.
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.
Incorporate additional provisions for free drainage and ventilation of all interiors to prevent collection of moisture (scoop-type marine drain grommets).
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.
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 by a coat of dope-proof paint or wrap with cellophane tape and rejoin the fabric. This precaution is advisable within a few months after start of operation as a seaplane.
Spray the interior of metal-covered wings and fuselages with an adherent corrosion inhibitor.
Par 6-220 Page 6-287 Place bags of potassium or sodium dichromate in the bottom of floats and boat hulls to inhibit corrosion.
Prevent the entry of water by sealing, as completely as po ssible, all openings in wi ngs, fuselage, control-surface members, openings for control cables, tail-wheel wells, etc.
6-209.6-219. [RESERVED.]
Par 6-220 Page 6-288 SECTION 14. HANDLING AND CARE OF AIRCRAFT RECOVERED FROM WATER IMMERSION.
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, it removes lubricants, deteriorates aircraft materials, and destroys electrical and avionics components.
Sea water, because of salt content, is more corrosive than fresh water. However, fresh water may also contain varying amounts of salt and, as drying occurs, the salt concentration is incr eased and corrosive attack accelerated.
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, radi os, and radar should be disassembled, as necessary, and the contaminants completely removed.
INITIAL FRESH WATER 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:
Mix liquid detergent (MIL-D-16791, type I) and isopropyl alcohol (TT-I-735) in ratio of eight parts detergent, to 20 parts of alcohol. Add the detergent/alcohol mixture to 72 parts of tap water and mix thoroughly. For use, a dd one part of the preceding concentrate to nine parts of tap water (warm water if available) and mix thoroughly.
If the above specified detergent/alcohol materials are not available, use water-emulsion cleaning compound (MIL-C-43616). Add one part compound to nine parts water. If the MIL cleaning compound is not available, use any available mild household detergent solution with fresh tap water.
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 fresh water, preferably hot.
Major accessories, engine parts, etc., should be removed and all surfaces flushed with fresh water, 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 heatdrying facility is available, wipe the cleaned parts with suitable drying cloths.
The constant-speed propeller mechanism should be disassembled, as required, to permit complete decontamination. Clean parts with steam or fresh water, 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.
AIRFRAME. The salvable components of the fuselage, wings, empennage, seaplane and amphibian hulls and floats, and movable surfaces should be processed as follows:
Par 7-1 Page 7-289 The fabric from fabric-covered surfaces should be removed and replaced.
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 cl eaning electrical equipment, such as terminal boards and relays.
Areas that have been steam cleaned should be rinsed immediately with either hot or cold fresh water.
Touch up all scratches and scars on painted surfaces using zinc chromate primer or preservative.
Undrained hollow spaces or fluid entrapment areas should be provided temporary draini ng facilities by drilling out rivets at the lowest point. Install new rivets after drainage.
Remove and replace all leather, fabric upholstery, and insulation. Plastic or rubber foam that cannot be cleaned of all corrosive water must be replaced.
Par 7-1 Page 7-290 All drain plugs or drive screws in tubular structures should be removed and the structure blown out with compressed air. If water has reached the tubular inte riors, carefully flush with hot fresh water 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 drain plugs or drive screws.
Clean sealed wood, metallic, and other non-metallic areas, excluding acrylic plastics, with warm water. Replace wood, metalite, and other porous materials expos ed to water-immersion unless surfaces are adequately sealed to prevent penetration by water. Virtually all solvents and phenolic ty pe cleaning agents are detrimental to acrylics and will either soften the plastic or cause crazing.
Remove instruments and radios and applicable cables and plumbing, and repair and inspect as necessary.
6-224.6-234. [RESERVED.]
Par 7-1 Page 7-291 CHAPTER 7. AIRCRAFT HARDWARE, CONTROL CABLES,
AND TURNBUCKLES
SECTION 1. RIVETS
GENERAL.
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.
Roundhead rivets (AN430) are used in the interior of aircraft except where clearan ce is required for adjacent members.
Flathead rivets (AN442) are used in the interior of the aircraft where interferen ce of adjacent members does not permit the use of roundhead rivets.
Brazierhead rivets (AN455 and AN456) are used on the exterior surfaces of airc raft where flush riveting is not essential.
Countersunk head rivets MS20426 (supersedes AN426 100-degree) are used on the exterior surfaces of aircraft to provide a sm ooth 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-HD BK5 Metallic Materials and Elements for Fight Vehicle Structures, and U.S.A.F./Navy T./O. 1-1A-8, Structural Hardware.”
Typical rivet types are shown in table 7-10.
MATERIAL APPLICATIONS.
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.
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. See Metallic Materials and Elements for Flight Vehicle Structures (MIL-HDBK-5) for differences between the types of rivets specified here.
The 1100 rivets of pure aluminum are used for riveting nonstructural parts fabricated from the softer aluminum alloys, such as 1100, 3003, and 5052.
When riveting magnesium alloy structures, 5056 rivets are used exclusively due to their corrosionresistant qualities in combination with the magnesium alloys.
Mild steel rivets are used primarily in riveting steel parts. Do not use galvanized rivets on steel parts subjected to high heat.
Corrosion-resistant steel rivets are used primarily in riveting corrosion-resistant steel parts such as firewalls, exhaust stack bracket attachments, and similar structures.
Par 7-14 Page 7-292 Monel rivets are used in special cases for rive ting 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.
Copper rivets are used for riveting copper alloys, leather, and other nonmetallic materi als. This rivet has only limited usage in aircraft.
Hi-Shear rivets are sometimes used in connections where the shearing loads are the pr imary 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 a ssembly, even where dissimilar metals
Par 7-14 Page 7-293 are being joined. However, it is advisa ble to touch up each end of the driven rivet with primer to allow the later application of the general airplane finish.
Blind rivets in the MS20600 through MS20603 series rivets and the mechanically-locked stem NAS 1398, 1399, 1738, and 1739 rivets sometimes may be substituted for solid rivets. They should not be used wh ere the looseness or failure of a few rivets will impair the airworthiness of the aircraft. Design allowables for blind rivets are specified in MIL-HDBK-5. Specific structural applicati ons are outlined in MS33522. Nonstruc tural applications for such blind rivets as MS20604 and MS20605 are contained in MS33557.
7-3.7-13. [RESERVED.]
Par 7-14 Page 7-294 SECTION 2. SCREWS
GENERAL. In general, screws differ from bolts by the following characteristics.
Screws usually have lower material strength, a looser thread fit, head shapes form ed 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 his 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.
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, or U.S.A.F./Navy T.O.1-1A-8/NAVAIR 01-1A-8, Structural Hardware.
Typical screw types are shown in table 7-11.
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.
MACHINE SCREWS. These screws are available in four basic head styles: flathead (countersunk), roundhead, fillister, and socket head.
Flathead machine screws (AN505, AN510, AN507, NAS200, NAS514, NAS517, and NAS662) are used in countersunk holes where a flush surface is desired.
Roundhead machine screws (AN515 and AN520) are general-purpose screws for use in nonstructural applications.
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.
Socket head machine screws (NAS608 and NAS609) are designed to be driven into tapped holes by means of internal wrenches. They ma y be used in applications requiring high strength, compactness of assembled parts, or sinking of heads below surfaces into fitted holes.
PANHEAD SCREWS (NAS600 THROUGH NAS606, NAS610 THROUGH NAS616, NAS623, AND NAS1402 THROUGH NAS1406). Flathead screws (MS35188 through MS35203), panhead machine screws (MS35024 through MS35219), and truss-head screws (AN526) are generalpurpose screws used where head height is not important.
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. Selftapping machine screws (AN504 and AN530), may be used to attach minor
Par 7-34 Page 7-295 nonstructural parts. Self-tapping sheet metal screws (AN504, AN530, AN531 and NAS548) may be used in blind applications for the temporary attachment of sheet meta l 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.
Par 7-34 Page 7-296 CAUTION: Self-tapping screws should never be used as a replacement for standard screws, nuts, bolts, or rivets in any aircraft structure.
WOOD SCREWS AN545 and AN550, MS35492 and MS35493 are screws used in wood structures of aircraft.
7-20.7-33. [RESERVED.]