Certified Aircraft Parts, When do STC, AML, PMA, 337, Minor/Major, Field Approval Apply???

Bartman

Bartman

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So you need to replace a part on your certified aircraft. Here are the options as far as I know but what do they each mean?

1. Replace the part with an exact same part from the original manufacturer/source, logbook entry signed by A&P.
2. Replace the part with different part that holds STC, is AML required? 337 required to comply with STC requirements (doesn't matter if major or minor repair).
3. Replace the part with part that has PMA, is STC also required?
4. Replace part with new part that has TSO, does major/minor apply or STC? Does TSO trump STC/AML?
5. Replace part with totally different part that isn't previously used this way, field approval by FAA with IA signature on 337 form.
6. Find that original part is no longer available and choose suitable replacement without specific approval for your airframe. For example, AeroFlash strobes are no longer made, must a replacement strobe unit be AML to be fitted or can a suitable alternative be installed? 337?

Thanks! Let's get this straightened out once and for all even if it isn't technically possible! 😵‍💫
 
First, sort out the difference between major and minor repairs and alterations. Anything that is classed as minor does not need any documentation other than a logbook entry.

Next, check AC 23-27 for approved data for parts substitution.
 
Bob Turner said:
sort out the difference between major and minor repairs and alterations. Anything that is classed as minor does not need any documentation other than a logbook entry.
Click to expand...

by what basis do you assess the suitability of a part to be used for a minor repair if not an equal to the original?
 
Bartman said:
by what basis do you assess the suitability of a part to be used for a minor repair if not an equal to the original?
Click to expand...
it's in the AC

(1) What should I consider before replacing a standard part?

(a) The substitute, replacement standard part must have properties and
performance at least equal to those of the original standard part in all applications and service
environments. The replacement parts must adhere to the design, manufacturing, test and
acceptance criteria, and uniform identification requirements established by an industry or U.S.
government specification. Before replacing any standard part, review the following
considerations:
l. Does replacing the original standard part with the replacement part affect
the interchangeability of the part for future repairs?
2. Does the part replacement affect the fit of the part or cause the part to
interfere with mating parts?
3. Will the safety of the aircraft be compromised by replacement of the
standard part?
4. Does the part replacement have a detrimental effect on the overall product
quality?
5. Is the part dissimilar to other installations (original equipment, approved
by field approval, approved by DER, etc.)?
6. Does the replacement part require special installation procedures, special
inspections, or different operating instructions from the original standard part?
NOTE: If you answer yes, to any of these questions, you may not be able to make the
substitutions as outlined in this AC.

(b) Replacement standard parts must be as structurally sound as the original
configuration. Any doubt regarding the acceptability of the standard part affecting any of the
above items should be eliminated by compliance to 14 CFRpart 21, § 21.303. The replacement
standard part must have no significant bearing on the effective use or operation of its intended
application and must not adversely affect the performance, durability, weight, interchangeability,
maintainability, reliability, or operation of the aircraft.
 
To answer #6 in the original post, if you can answer NO to all of the questions in the AC for the replacement part, the new strobe/nav light unit can be installed and it's a logbook entry.

If the installation in similar aircraft requires a 337 form to comply with STC requirements, wouldn't a 337 also be required for use according to the AC bulletin? It seems like a 337 wouldn't be required.
 
Minutia

AC 23-27
1. PURPOSE
a. This AC provides guidance for substantiating parts or materials substitutions to maintain
the safety of old or out-of-production GA aircraft, or other GA aircraft where the parts or
materials are either difficult or impossible to obtain.
 
Have you noticed that filament light bulbs are starting to get "difficult to obtain?"

If a light bulb has actually received a PMA or TSO, that means it is bright enough and rugged enough, and therefore will not affect safety or performance; thus one could easily assume this one is minor even without 23-27.

But to the question - on the Cubs some of us use automotive on the brake lines. The specs are incredible - 5000psi working pressure, 6000psi burst. The testing entity is like ASME - I think they call it JA something. Meets 23-27.

For the longest time we didn't care, because the factory used all automotive in the brake lines, and one couldn't tell whether it was 1940 Plymouth or 1965 Mustang by looking at it. But now we are legal!
 
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