Aircraft fastener holes may require repair after corrosion removal, fastener replacement, or minor damage. This section explains approved hole repair methods, the use of Acres fastener sleeves, and the basic construction and hardware of aircraft control cable and push-pull linkage systems.
Many of the blind fasteners are manufactured in oversized diameters to accommodate slightly enlarged holes resulting from drilling out the original fastener. When installing rivets or bolts, ensure that the hole is neither elongated nor drilled at an angle.
To reduce the chances of an incorrectly drilled rivet or bolt hole, first use a slightly smaller drill bit, then enlarge to the correct diameter. The last step to prepare the hole for the fastener is to deburr the hole using a larger drill bit by hand or a special deburring tool. This practice also works well when drilling out a previously attached fastener.
If the drill bit is not perfectly centered of the rivet, bolt, or screw, the hole can easily be elongated, but when using a smaller drill bit, drill the head only off the fastener. The remaining head ring and stem can then be pushed out using a pin punch of the appropriate diameter. If an improperly drilled hole is discovered, the options are to re-drill the hole to the next larger diameter for an acceptable fastener or repair the hole using an Acres fastener sleeve.
Repair of Damaged Holes with Acres Fastener Sleeves
Acres fastener sleeves are thin-wall tubular elements with a flared end. The sleeves are installed in holes to accept standard bolts and rivet-type fasteners. The existing fastener holes are drilled 1⁄64 inch oversize for installation of the sleeves. The sleeves are manufactured in 1-inch increments. Along their length, grooves provide a place to break or cut off excess length to match fastener grip range. The grooves also provide a place to hold adhesive or sealing agents when bonding the sleeve into the hole.
Advantages & Limitations
The sleeves are used in holes that must be drilled 1⁄64 inch oversize to clean up corrosion or other damage. The oversize hole with the sleeve installed allows the use of the original diameter fastener in the repaired hole. The sleeves can be used in areas of high galvanic corrosion where the corrosion must be confined to a readily replaceable part. Oversizing of holes reduces the net cross-sectional area of a part and should not be done unless absolutely required.
Consult the aircraft, engine, or component manufacturer before repairing damaged holes with Acres fastener sleeves.
Identification
The sleeve is identified by a standard code number that represents the type and style of sleeve, a material code, the fastener shank diameter, and surface finish code letter, and grip length of the sleeve. [Figure 1]
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| Figure 1. Acres sleeve identification |
The basic code number represents the type and material of the sleeve. The first dash number represents the diameter of the sleeve for the fastener installed, and the second dash represents the grip length of the sleeve. The required sleeve length is determined during installation and the excess is broken off the sleeve. A JK5512A-05N-10 is a 100° low profile head sleeve of aluminum alloy. The diameter is for a 5⁄32-inch fastener with no surface finish and is 5⁄8 inch in length.
Hole Preparation
Refer to Figure 2 for the correct drill numbers for standard- or close-fit holes.
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| Figure 2. Acres sleeve identification, installation, and breakoff procedure |
Inspect the hole after drilling to ensure all corrosion has been removed before installing the sleeve. The hole must also be the correct shape and free from burrs. The countersink must be enlarged to receive the flare of the sleeve, so the sleeve is flush with the surrounding surface.
Sleeve Installation
After selecting the correct type and diameter sleeve, use the 6501 sleeve breakoff tool for final installation length. Refer to Figure 2 for the sleeve breakoff procedure. The sleeve may be installed with or without being bonded in the hole. When bonding the sleeve in a hole, use MIL-S-8802A Class B 1/2 sealant. Reinstall the original-size fastener and torque as required.
Sleeve Removal
Sleeves not bonded in the hole may be removed by either driving them out with a drift pin of the same diameter as the outside diameter of the sleeve, or they may be deformed and removed with a pointed tool. Bonded sleeves may be removed by this method, but care should be used not to damage the structural hole. If this method cannot be used, drill out the sleeves with a drill 0.004 to 0.008 inch smaller than the installation drill size. After drilling, remove the remaining sleeve material can be removed using a pointed tool and applying an adhesive solvent to the sealant.
Control Cables & Terminals
Cables are the most widely used type of linkage in primary flight control systems. Cable-type linkage is also used in engine controls, emergency extension systems for the landing gear, and various other systems throughout the aircraft.
Cable-type linkage has several advantages over the other types. It is strong and lightweight, and its flexibility makes it easy to route through the aircraft. An aircraft cable has a high mechanical efficiency and can be set up without backlash, which is very important for precise control.
Cable linkage also has some disadvantages. Tension must be adjusted frequently due to stretching and temperature changes.
Aircraft control cables are fabricated from carbon steel or stainless steel.
Cable Construction
The basic component of a cable is a wire. The diameter of the wire determines the total diameter of the cable. Several wires are preformed into a helical or spiral shape and then formed into a strand. These preformed strands are laid around a straight center strand to form a cable.
Cable designations are based on the number of strands and the number of wires in each strand. The most common aircraft cables are the 7 × 7 and 7 × 19.
The 7 × 7 cable consists of seven strands of seven wires each. Six of these strands are laid around the center strand. [Figure 3] This is a cable of medium flexibility and is used for trim tab controls, engine controls, and indicator controls.
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| Figure 3. Cable cross-sections |
The 7 × 19 cable is made up of seven strands of 19 wires each. Six of these strands are laid around the center strand. [Figure 3] This cable is extra flexible and is used in primary control systems and in other places where operation over pulleys is frequent.
Aircraft control cables vary in diameter, ranging from 1⁄16 to 3⁄8 inch. The diameter is measured as shown in Figure 3.
Cable Fittings
Cables may be equipped with several different types of fittings, such as terminals, thimbles, bushings, and shackles.
Terminal fittings are generally of the swaged type. They are available in the threaded end, fork end, eye end, single shank ball end, and double shank ball end. The threaded end, fork end, and eye end terminals are used to connect the cable to a turnbuckle, bell crank, or other linkage in the system. The ball end terminals are used for attaching cables to quadrants and special connections where space is limited. Figure 4 illustrates the various types of terminal fittings.
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| Figure 4. Types of terminal fittings |
The thimble, bushing, and shackle fittings may be used in place of some types of terminal fittings when facilities and supplies are limited and immediate replacement of the cable is necessary.
Turnbuckles
A turnbuckle assembly is a mechanical adjustment device consisting of two threaded terminals and a threaded barrel. [Figure 5]
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| Figure 5. Typical turnbuckle assembly |
Turnbuckles are fitted in the cable assembly for making minor adjustments in cable length and for adjusting cable tension. One of the terminals has right-hand threads and the other has left-hand threads. The barrel has matching right- and left-hand internal threads. The end of the barrel with the left-hand threads can usually be identified by a groove or knurl around that end of the barrel.
When installing a turnbuckle in a control system, it is necessary to screw both terminals an equal number of turns into the barrel. It is also essential that all turnbuckle terminals be screwed into the barrel until not more than three threads are exposed on either side of the turnbuckle barrel.
After a turnbuckle is properly adjusted, it must be safetied. The methods of safetying turnbuckles are discussed in the Aircraft Safety Wiring and Fastener Safetying section.
Push-Pull Tube Linkage
Push-pull tubes are used as linkage in various types of mechanically-operated systems. This type of linkage eliminates the problem of varying tension and permits the transmission of both compression and tension loads through a single tube.
A push-pull tube assembly consists of a hollow aluminum alloy or steel tube with adjustable end fittings and check nuts at both ends. [Figure 6]
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| Figure 6. Push-pull tube assembly |
The check nuts secure the end fittings after the tube assembly has been adjusted to its correct length. Push-pull tubes are generally made in short lengths to prevent vibration and bending under compression loads.
Quick Review: Hole Repair & Hole Repair Hardware
When should an oversized fastener be used during an aircraft hole repair?
Why should damaged or corroded fastener holes be inspected before repair?
What are the advantages of aircraft control cables in mechanical control systems?
Why are turnbuckles regularly inspected during aircraft maintenance?
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