Aircraft Maintenance: Understanding 'standard parts'

For most repairs, we rely on the aircraft manufacturer’s illustrated parts catalog to identify the part number needed. From there, we typically choose from original equipment certified parts or alternative parts manufacturer approval (PMA) parts that arrive with a paper trail detailing their pedigree and suitability for installation on an aircraft. However, there are other times when we may be repairing a “part-of-a-part” or doing a minor alteration (such as adding a starter-engaged lamp on the panel). In these situations, debate often sets in about “What is an aircraft part?” and “Where do I have to buy it?”

Many mechanics consider this definition to be limited to hardware with an AN (Army/Navy) or MS (Military Standard) number acceptable for use on an aircraft. However, if we dive deeper into the document, we find that the FAA definition of a “standard part” is far broader than AN/MS nuts or bolts:

“A part manufactured in complete compliance with an established U.S. Government or industry-accepted specification, which includes design, manufacturing, and uniform identification requirements. The specification must include all information necessary to produce and conform to the part. The specification must be published so that any party may manufacture the part. Examples include, but are not limited to, National Aerospace Standard (NAS), Air Force/Navy (AN) Aeronautical Standard, Society of Automotive Engineers (SAE), Aerospace Standard (AS), Military Standard (MS), etc.”

Note that this section expands the definition to include any established industry-accepted specification, opening up a world of standard part options for aircraft maintenance. But, there’s still more to be found further on in the advisory circular under the heading “Discrete Electrical and Electronic Component Parts”:

“Electrical and electronic parts, such as resistors, capacitors, diodes, and transistors, if not specifically marked by the equipment manufacturer’s part number or marking scheme, may be substituted or used as replacement parts, provided that such parts are tested or it is determined that they meet their published performance specifications and do not adversely affect the performance of the equipment or article into or onto which they are installed. The performance of such equipment or article must be equal to its original or properly altered or repaired condition. Integrated circuits such as hybrids, large scale integrated circuits (LSIC), programmable logic devices, gate arrays, application specific integrated circuits (ASIC), memories, Central Processing Units (CPU), etc., are not included because their highly specialized functionality does not readily lend itself to substitution.”

The key words in this section are “such as.” Any discrete electrical or electronic component (including switches, lamps, relays, and other components) meeting the requirements quoted above can be considered acceptable to the FAA, as long as it meets an acceptable industry standard and has been obtained through a means that ensures it meets its performance specs.

So, what does this all mean to the average aircraft owner, mechanic, or avionics shop? It means that the FAA has provided us with a path to understanding what parts are acceptable for repairs and alterations when we need to go “off book” from the aircraft manufacturer’s parts catalog. It also demonstrates the responsibility incumbent on the mechanic to determine both the eligibility of a part (whether it is a suitable part) as well as the conformity of the part (its quality and traceability). Done properly, a mechanic can determine the requirements of the part and purchase it from a reputable supplier, along with specs/drawings and certificate of conformity documentation, recording the details in the aircraft logs.

The latest advisory circular is considered a recent publication on the FAA timeline, having had a major update in 2018. Therefore, many mechanics may not be familiar with its valuable content. The fact is that standard parts can save owners thousands of dollars by repairing components that would otherwise be considered unsalvageable. They can also help mechanics add safety and functionality to aircraft through minor alterations such as starter-engaged lamps, avionics master switches, and similar upgrades using commercially available components. I’d encourage you to give it a read and perhaps pass it on to your mechanic (along with a cup of coffee). Until next time, I hope you and your families remain safe and healthy, and I wish you blue skies.