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"Trust the Tuck Method" Sport Aviation / Experimenter "Technically Speaking" Article October 2015

“Cotter Pins”

Sport Aviation / Experimenter magazine “Technically Speaking” October 2015 Article

Our monthly column in Sport Aviation/Experimenter  Magazine for the month of October was an article on Cotter pins

We discuss The “Tuck Method” of installing a Cotter pin

Trust The Tuck Method

Cotter pins have been commonplace in aviation as far back as the Wright brothers. This also appears to have been the same time frame that the last update was made in regards to the subject of cotter pins. Even the * FAA advisory circular (AC) 43.13-1B is unchanged from its original version and dedicates only two paragraphs to the subject: “Cotter pins are used to secure such items as bolts, screws, pins, and shafts. Their use is favored because they can be removed and installed quickly. The diameter of the cotter pins selected for any application should be the largest size that will fit consistent with the diameter of the cotter pin hole and/or the slots in the nut. Cotter pins should not be reused on aircraft.” (Par. 7-127, a.) This information is followed by a brief, one sentence paragraph which reads: “To prevent injury during and after pin installation, the end of the cotter pin can be rolled and tucked.” (Par. 7-127, b.) In this article, we will reference Par. b to justify a different method for installation of the venerable cotter pin.

Even as far back as the 1970’s, a dilemma concerning cotter pins was encountered by the hang gliding community. For the hang glider pilots, nearly every day of flying involved “bagging” and “un-bagging” the wing. This was a process by which the hang glider wing, fabric, tubing, cables, and every other part was folded up and tucked together inside of a long tubular fabric bag which was zipped together. The purpose, of course, was for protecting and transporting the hang glider on top of a vehicle to and from the gliding site. One of the most disheartening, yet common occurrences, happened during the “un-bagging” or “set-up” process. As the forward wing spars were unfolded and spread out into the typical wing configuration, the sharp, cut-off end of a cotter key would catch on the fabric causing a tear. Over the years, and out of necessity, many different methods were developed and used to protect the fabric from these nasty little cut-off cotter pin ends. But one method rose to the top and became commonplace in the hang glider community and subsequently has become standard practice in the light sport industry today. We refer to this as the “Tuck Method” of installing a cotter pin. Figure: 1

In AC 43.13-1B Chapter 7 Par.127, there are two methods which are referred to as acceptable methods. The first is the standard method Figure: 2 and the second is the alternate method. Figure: 3 The “Tuck Method” is simply a variation on the alternate method and paragraph b provides justification for the “Tuck Method” as acceptable practice.

If you have never used this method before there are a couple of tricks that will make the installation simple, easy and clean. Step 1: Figure: 4 Insert the cotter pin horizontally and pull snug with a pair of needle nose pliers and wrap the cotter pin legs around the perimeter of the castle nut in the same method that you would use in accomplishing the alternate method.

Step 2: Then, using a pair of diagonal cutters, cut the legs just slightly past the adjacent slot in the castle nut, say, .010”- .080” past the far side of the slot in the castle nut. Step 3: Next, grab the end of the leg with a pair of needle nose pliers and bend the leg slightly more than 90°, while simultaneously pulling the leg away from the edge of the castle nut. Step 4: Now reposition your pliers and “Tuck” the bent end of each leg into the adjacent slot on the castle nut. Step 5: After tucking each of the two legs into the adjacent slots, you will be left with a very professional looking “tuck method” cotter pin installation. The two legs, which are tucked into each slot, will prevent rotation of the cotter pin and make it virtually impossible for the cut off ends to come in contact with any other object.
Several areas where the “Tuck Method” cotter pin installation may be preferred include: Control cable attachments around the rudder pedal assembly where your pant legs, socks, or shoes could become engaged with an open cotter pin. Any cotter pin installations around upholstery or insulation which may become entangled with a cotter pin end. And, in general, any place where passengers or pilot could be exposed to the cotter pin. Nothing worse than having that sharp cotter pin end stick into your skin.
Standard practices for the use and installation of cotter pins:
· Use a castle nut and cotter pin any time the nut and bolt are subject to rotation. Example, flight control hinges. Figure: 5

· Use a castle nut and cotter pin where the part is intended to be removed and reinstalled on a regular basis. An example of this would be your landing gear axle nut.
· Never reuse a cotter pin.
· Use the largest diameter cotter pin which will fit consistent with the diameter of the cotter pin hole.
· A finished cotter pin installation should be tight. Loose installations can cause premature wear and failure.
· (AN380) MS24665 cadmium plated carbon steel cotter pins can be used with ambient temperatures up to 450 degrees F. (232 C.) Use in non-corrosive environments. Cadmium plated cotter pins should be used with cadmium plated bolts or nuts.
· (AN381) MS24665 corrosion resistant steel (stainless) cotter pins can be used with ambient temperatures up to 800 degrees F. (427 C.). Use corrosion resistant steel cotter pins when using corrosion resistant steel bolts or nuts. Stainless cotter pins are often used in corrosive environments. And used where there is a requirement for the nonmagnetic properties.
· Prevent FOD, (foreign object damage) keep track of the cut off ends of your cotter pins.
· The cut off ends of a standard installation should not extend beyond the bolt diameter on top leg and should not extend beyond the bottom of the nut on the lower leg. Figure: 6

In some respects, this “Tuck Method,” drawn from the hang gliding community, is analogous to the basics of aviation safety and quality. If you choose to adopt this method, it will provide a clean finish to your cotter pin installation, while providing additional protection from possible, while likely minor, harm to your aircraft, passengers, or even yourself.

AC 43.13-1B chapter 7 Par.127
Hints for homebuilder Video

*Advisory Circulars are informational documents produced by the Federal Aviation Administration to inform and guide institutions, operations, and individuals within the aviation industry, as well as the general public.



As with many of the articles that we write, we also try and incorporate a companion video. You can click here or on the video player below to go to the EAA website and the hints for homebuilder section to see a video on how the tuck method is accomplished in real time.

“Twist Welding” Sport Aviation / Experimenter “Technically Speaking” Article August 2015

About Brian Carpenter (255 Articles)
CEO Rainbow Aviation / Adventure Aircraft

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