If you are further than three miles from an airport the 400 ft limit does not apply and if the airport operator gives permission you can fly higher than 400 ft right on the airport.
But what does any of this have to do with the original thread?
ORIGINAL: KidEpoxy
Sammy
yes, to be fully compliant with AC91-57 you must obey part 3b about airworthiness.
Unfortunately, nobody is actually required to obey AC91-57.... there is no teeth behind the AC to mandate compliance.
If you want to look for the teeth, read up on AFS400 / Guidence0801 and how they use AC91-57.
To consider AC91-57 to apply in a 'must abide' way
one would first have to look at the 400' alt limit everywhere in the AC,
and how AMA is simply choosing to not abide it.