Airframe and Powerplant

Aircraft-Quality Hardware

November 1, 2006

Does that bolt have a birth certificate?

"Because of the small size of most hardware items, their importance is often overlooked; however, the safe and efficient operation of any aircraft is greatly dependent upon correct selection and use of aircraft structural hardware." — Aviation Structural Mechanic S 3 & 2 Navy training course manual, NAVPERS 10308-A

Jerry, an airframe and powerplant technician, was yelling instructions to Larry, another A&P, in typical A&P language: "Get me an AN525-8-R9, two AN 960-8s, and an AN365-832."

Larry answered, "We don't have any AN365-832s but we have lots of NAS1021N-08s. Will that do?"

"Heck," said Jerry, "I don't know. It looks like I'll have to climb down and get out the book to see if they changed the part number again."

Jerry wasn't surprised when the book revealed that AN365-832 and NAS1021N-08 were the same part.

As Jerry climbed back up onto the nacelle of the Grumman Goose he had been working on, Larry heard more mumbling and deduced that Jerry was still ticked off about the rationale — or lack of it, from his viewpoint — of the seemingly arbitrary practice of changing part numbers.

Later, over a cup of coffee, Jerry ranted, "Why change a part number when the people that have been using the hardware have been calling a nylon self-locking nut by part number AN 365-832 for decades? What's the point?"

The AN standard

In the 1967 version of James L. McKinley and Ralph D. Bent's Maintenance and Repair of Aerospace Vehicles, a textbook for A&P technician schools, hardware identification is taught almost exclusively with reference to AN numbers. AN 3 through AN 20 part numbers refer to hex-head bolts; Clevis bolts are AN 21 through AN 36; eye bolts are AN 42 through 49; and so on.

If it looks as if the part-number jungle is becoming overgrown, imagine how much time it takes — and how much it costs the aircraft owner — when a professional A&P has to keep up with these changes. It also can be problematic for owners engaged in preventive maintenance.

Many years ago hardware was identified by Air Force/Navy (AN) and military standard (MS) part numbers. Later, national aerospace standard (NAS) part numbers appeared. About 10 years ago, SAE International introduced another new standard called the aerospace specification (AS), which added a fourth part-numbering system. SAE got into the standards business when the U.S. Department of Defense (DOD) turned over the responsibility for some 1,500 non-critical standards. Today, SAE International has the responsibility for implementing new standards and overseeing the maintenance of existing standards.

Frank Bokulich is an aerospace standards engineer at SAE International. When asked what causes part-number changes, he said, "Markets determine which part number is needed. After the conversion from the DOD system we have tried to keep the existing part numbers." This may be so, but there are still times when a part-number cross reference is needed.

The aircraft parts manual

The first resource for getting the correct part number when ordering hardware should always be the airplane parts manual or kit plans. In fact, aircraft owners are required to perform preventive tasks by the same standards as licensed A&P technicians. This means the owner bears the final responsibility not only for installing the correct piece of aircraft-quality hardware, but also for installing it correctly. So start with the parts manual or the kit plans.

Since many parts manuals date back two, three, four, or even five decades, the part numbers may not reflect time-related changes. Let's say, for this exercise, that an owner buys a 1960 Cessna 182 and decides to learn how to do preventive maintenance chores. To this end he eventually finds a parts book, published in 1959, on the Internet or off a table at an aviation swap meet. Fortunately, most of the hardware part numbers in the 47-year-old book are still valid. But there will always be part numbers that have been superseded. The part number in the Cessna book for the common cotter pin is AN380. That number is no longer listed by most aircraft hardware suppliers. It's been replaced by MS24665. Even if the diameter and grip of a bolt are known, there are other variables that must be detailed when ordering. For instance, the ubiquitous AN 3 bolt can be ordered as standard cadmium-plated, as corrosion-resistant, with a hole in the head for safety wiring, with a hole in the shank to accommodate a cotter pin, with no holes, or with both holes. These differences account for seven different part numbers for what looks like a common bolt to a person who doesn't know all the variables.

One resource is to get on the Internet. The Aircraft Spruce & Specialty, Genuine Aircraft Hardware, Freeman's Just Plane Hardware, and Wicks Aircraft Supply Web sites are all very helpful. The most comprehensive Web site is from Genuine Aircraft Hardware, of Paso Robles, California.

Genuine Aircraft Hardware

In 1977 at 18 years old, Tom Brink studied hard for months before passing the barrage of tests that permitted him to exercise the privileges of an A&P. Under the guidance of George Bedford and Dennis Kellerman, he soon advanced to the position of manager of a small FAA-approved general aviation repair station in Lompoc, California.

Frustration at job delays because of an inability to obtain aircraft hardware, and an increasing number of requests by local non-aviation businesses for quality hardware, led Brink and his wife to start Genuine Aircraft Hardware in 1984. At first they worked out of a 211-square-foot room, counting nuts, screws, and bolts into Ziploc bags and labeling the bags with Sharpie permanent markers. They worked out of that room until 1988 when Brink moved the operation to Paso Robles. Since then the company has again expanded with the main office on the Paso Robles airport and a satellite office in Newnan, Georgia.

In 1994 Brink published the company's first hardware reference book — sized for keeping in a toolbox — and the business took off. "I handed out 200 books at an airplane trade show, and business was up 30 percent in one month," said Brink. The latest version contains more than 330 pages and is an invaluable resource for technicians and owners who desire a comprehensive hardware reference book. The books are free with a $75 merchandise order, or can be purchased outright for $6.

With the growing acceptance of computers and the Internet as business tools, Brink seized the opportunity and opened the company Web store. The site is very close to fully incorporating all the information in the reference book. Other aviation Web sites such at the ones at Aircraft Spruce & Specialty, Freeman's Just Plane Hardware, and Wicks Aircraft Supply are also very useful when ordering hardware.

Traceability

"Only hardware with traceability to an approved manufacturing process or source should be used." — FAA Advisory Circular 43.13-1B/2A, "Acceptable Methods, Techniques and Practices — Aircraft Inspection and Repair"

To ensure that the nut, bolt, screw, or other piece of hardware that is installed on a certified airplane complies with modern aerospace standards, there must be traceability. Is traceability important for every piece of hardware? Probably not, but there's no question that fasteners used to maintain structural integrity such as engine mount bolts, wing attach bolts, and control surface attach bolts must meet aerospace strength and durability standards.

Reputable hardware sales facilities are able, if asked, to provide traceability paperwork. Genuine Aircraft Hardware always includes the manufacturer, the manufacturer's lot number, and company's purchase order number, as well as the receipt number and the date package on every hardware order.

In addition, Genuine can supply, upon request, a certificate of conformance certifying that it has possession of a certificate of conformance from its suppliers stating that their products were manufactured in compliance with all U.S. government standards applicable at the time of manufacture.

It's very rare for maintenance technicians to enter traceability information such as the fastener part number, manufacturer's name, and manufacturer's lot number when writing up their work in the aircraft records. Be that as it may, today's savvy technicians ask for and keep traceability paperwork, especially when replacing critical fasteners.

A small package

When Van's Aircraft was asked for the weight of the hardware package for one of its popular two-place kitbuilt airplanes, the company answered that there was typically 39 pounds of hardware in one of its 700-pound airframe-components kits. With the installation of engine, prop, wiring, radios, and paint, a typical weight is approximately 1,100 pounds, yielding a hardware weight of 3.5 percent of the total weight.

One of the reasons that hardware doesn't weigh much when compared to an airplane's total empty weight is because of standards, which ensure the quality and strength of what we know as aircraft-quality hardware. Going back again to our typical AN 3 aircraft bolt, which is only 3/16 of an inch in diameter, standards mandate that it be cadmium-plated for corrosion resistance and heat-treated to 125,000 pounds per square inch tensile strength.

To learn more about aircraft-quality hardware, get Genuine Aircraft Hardware's book or use the Internet. Always use aircraft-quality hardware and ask your A&P to obtain a certificate of conformance when high-stress or critical pieces of hardware are installed, and you'll be on the right track toward maintaining the airworthiness of your airplane.


E-mail the author at steve.ells@aopa.org.


Information in a Part Number

The first portion of a part number identifies the type of hardware.

  • AN 3 through AN 20 are "standard" hex-head bolts with a hole in the shank for the installation of cotter pins.
  • If a C follows the AN number, it designates a corrosion-resistant bolt.
  • If an H follows the AN number, it designates a hole for safety wire in the head of the bolt.
  • If an A follows the complete part number, it designates that the cotter pin hole is absent.
  • The number after AN designates the bolt diameter in 1/8-inch steps.
  • The number after the diameter designates the length of the bolt in 1/8-inch steps up to 7/8 inch; then the length numbers start at -10 and again increase in 1/8-inch steps to -17, and so on (e.g., AN4C-14A is a bolt with a 1/4-inch diameter, of corrosion-resistant steel that has a 1.063-inch grip, and a 1.531-inch overall shank length that does not have a cotter pin hole).
  • AN365, MS20365, and MS21044 all identify the same part — a nylon self-locking nut.
  • Dash numbers after the primary identifying number indicate the diameter and the threads per inch.
  • An AN 365-428 nut fits onto an AN 4 bolt and has 28 threads per inch.
  • The same nut identified by new part numbers is an MS21044N-4.

The key to understanding the evolving world of aircraft hardware part numbers is a good reference book. — SWE