On one of the blogs I follow, a poster asked about installing a non-TSO’d part on an older aircraft. It is an interesting question, so I decided to write about it.
Certified aircraft meet either the rules of CAR 3 or more recently CFR 14 Part 23. This is called the Type Certificate. In general, you cannot add or remove anything from a certificated airplane without approval by the FAA. That approval comes in lots of flavors. (Note: I’m simplifying this. It gets really complicated.)
You can replace any part with a part that is listed in the original type certificate. Those part numbers are found in the original Aircraft Flight Manual or the parts manual for the aircraft. Many of the parts are also covered by TSOs, PMAs, or Standard Parts as described below.
Technical Standards Orders
A TSO is a minimum performance standard for specified materials, parts, and appliances used on civil aircraft. When authorized to manufacture a material, part, or appliances to a TSO standard, this is referred to as TSO authorization. Receiving a TSO authorization is both design and production approval.
An altimeter is common to all aircraft and the FAA has issued TSO-C10b that tells manufacturers the standards that altimeters must meet. If it meets the standards, and the manufacturer hasn’t specified something different when it certified the airplane, then it can be installed on the aircraft by a licensed A&P. When he makes a logbook entry for it, it is legal to fly the airplane. You can buy non-TSO’d altimeters for much less than TSO’d ones, and they might even be “better” but you can only install them on experimental airplanes.
Parts Manufacturer Approval (PMA)
Is a combined design and production approval for modification and replacement articles. It allows a manufacturer to produce and sell these articles for installation on type certificated products.
This is another way you can replace parts on your airplane. These cover generic type items. Oil filters, spark plugs, tires, etc. It also covers things that are made by reference to the original manufacturers specifications. Things like muffler shrouds and engine mounts come to mind.
The parts manual for an aircraft will refer to things like screws and gaskets by industry standard nomenclature. e.g. Cad Plated MS24694-S1 screws, MS24665 cotter pins.
You can replace the carpet in your plane with new carpet, provided it meets the flame resistant standards.
Often however, you can’t substitute a part from the hardware store that is as good or better than the original and you end up paying $150 for a 50 cent gasket. I once bought 25 feet of gasket for the landing gear doors from Cessna for $250. The exact same product is sold at Home Depot for $18. But it doesn’t have a PMA certificate.
Supplemental Type Certificates
A supplemental type certificate (STC) is a type certificate (TC) issued when an applicant has received FAA approval to modify an aeronautical product from its original design. The STC, which incorporates by reference the related TC, approves not only the modification but also how that modification affects the original design.
My airplane came with navigation and communication radios that we replaced with a Garmin GPS 430. Garmin conforms to TSO−C146() so we can use it as an approved primary navigation i.e. to fly en route, terminal, and WAAS approaches. There is an STC that details Limitations, Emergency Procedures, and Normal Procedures. As long as it was installed according to the STC we’re legal. That STC required adding pages to our Aircraft Flight Manual.
The autopilot on my plane was installed on the basis of an STC. This STC must be carried in the plane and there are a bunch of pages added to the AFM. STCs are filed with the FAA with a Form 337.
Some examples are: Ski holder for Cessna 182, Mirrors for checking if your wheels are down on Cessna 210, Gross weight increase for certain model Cessnas.
This gets complicated, but a simple example would be if you lost an inspection plate. You could have a local machine shop cut out a new piece from the same gauge aluminum and use it on your plane. The A&P would make a logbook entry stating that they replaced the missing plate with an owner-produced plate.
AC 23-27 – Parts and Materials Substitution for Vintage Aircraft
This advisory circular (AC) provides guidance for substantiating parts or materials substitutions to maintain the safety of old or out-of-production general aviation (GA) aircraft, or other GA aircraft where the parts or materials are either difficult or impossible to obtain.
This gives the owner some flexibility in keeping our older aircraft flying.
If you want to make a modification to an aircraft that is not covered by any of the above, you can ask the local FSDO to approve it.
There was some discussion about whether LED landing lights were PMA’d or required a field approval. To be on the safe side, some people got field approval before installing them. Field approval is often used to install a TSO’d part on an aircraft that was not originally installed on. i.e. bigger brakes and wheels on a bush plane.
I think that covers everything. Let me know if I left out a category.
The original poster, who is not familiar with airplane maintenance, posted a follow-up question.
“Now what about aircraft certified before TSO standards were issued?”
If the part you want to replace has no TSO or PMA standard, then you could replace it with a part from another aircraft of the same type or the same part that is called out in the type certificate. For example, the Airplane Flight Manual that came with my Cherokee specifies a Harrison #C-8526250 Oil Cooler. If I can find the same oil cooler at a junkyard, I can install it—provided that it is in serviceable condition. Otherwise, I’d need to comply with one of the other items on the list.
In your altimeter example, there is a TSO for altimeters. Altimeters are one of the items required to be in an aircraft and working. Even if the TSO didn’t exist when the aircraft was manufactured, it does now, so you cannot go to Aircraft Spruce and buy a non-TSO’d altimeter and install it in the aircraft.
You can do an end-run around the TSO requirement. For example, Dynon has been making a poor-man’s glass-panel for experimental aircraft. Dynon’s EFIS-D10A. It displays all of the six-pack instruments. The EAA obtained an STC that allows you to replace the attitude indicator on many certificated airplanes. You buy the EFIS from a vendor and pay EAA $100 and you can replace your TSO’d attitude indicator.
If you are going to replace a part on a certificated airplane, you need some legal basis for doing so. TSO’s, PMA’d, and Original Equipment Manufacture (OEM) parts meet the legal test. STCs are slightly more difficult to use, but they also easily meet the test. The other items on the list require judgement calls by your A&P or the FSDO.