Almost any airplane more than a handful of years out of the factory has been modified to some extent. These pages have, on several occasions, discussed the difference between minor modifications requiring only a logbook entry, major mods accomplished as a field approval with an FAA Form 337 and more complicated additions necessitating a supplemental type certificate (STC). For my own airplane, which has been in service coming up on 50 years, I keep a three-ring binder documenting and preserving the Forms 337 and STC paperwork. The binder isn't full, but it won't take much before I'll need to find another place to keep all those important forms.
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