A remote pilot wants to inspect a 500-foot antenna tower with a small UA. Under § 107.51's structure exception to the 400-foot AGL altitude limit, how is the aircraft's maximum altitude determined?
Why: 14 CFR 107.51(b): the small UA cannot fly higher than 400 feet AGL unless it is flown within a 400-foot radius of a structure and does not fly higher than 400 feet above the structure's immediate uppermost limit.
Applies the general 400-feet-above-the-ground limit, ignoring that the structure exception in § 107.51(b) measures the 400 feet from the structure's own top, not from the ground at its base.
Invents an unlimited altitude; § 107.51(b) caps the structure exception at 400 feet above the structure's uppermost point and requires staying within a 400-foot lateral radius of it.