Maximum altitude within 400 feet of a tall structure

Remote Pilot (Part 107) practice question 22 of 34 on Regulations / Operating Rules. ACS element UA.I.B.K21d · difficulty: applied. Want to answer it in a timed quiz? Use the Remote Pilot (Part 107) practice mode.

Question bank updated 2026-09-22

QUESTIONA remote pilot in command is inspecting a communications tower whose uppermost point is 650 feet above ground level. While the small unmanned aircraft stays within a 400-foot radius of the tower, what is the maximum altitude above ground level permitted by § 107.51(b)?

A1,050 feet AGL, which is 400 feet above the tower's uppermost limit
B400 feet AGL, the base altitude limit that applies regardless of nearby structures
C650 feet AGL, level with the top of the tower

Show the answer and explanation

Correct answer: A. 1,050 feet AGL, which is 400 feet above the tower's uppermost limit

A — correct § 107.51(b) allows the aircraft to exceed 400 feet AGL when it is within a 400-foot radius of a structure and does not fly higher than 400 feet above the structure's immediate uppermost limit: 650 + 400 = 1,050 feet AGL.
B — incorrect The 400-foot AGL limit has an exception for flight within a 400-foot radius of a structure. Near the tower, the ceiling rises to 400 feet above the structure's uppermost limit.
C — incorrect The structure exception allows flight up to 400 feet above the top of the structure, not merely level with it, so the limit is 1,050 feet AGL.

BACKGROUND

Section 107.51(b) sets the altitude limit for a small unmanned aircraft at 400 feet above ground level (AGL). The limit is relaxed when the aircraft is flown within a 400-foot radius of a structure, provided it does not fly higher than 400 feet above the structure's immediate uppermost limit. Both conditions must hold at the same time: the horizontal radius of 400 feet and the vertical cap of 400 feet above the top of the structure. For a 650-foot tower, the permitted ceiling inside that radius is 650 + 400 = 1,050 feet AGL. Outside the 400-foot radius the ordinary 400-foot AGL limit applies again, so the descent must be planned before leaving the structure's vicinity.

STUDY TIP

For structure questions, add 400 feet to the structure's height and confirm the aircraft is within a 400-foot horizontal radius. Both numbers are 400.

KEY POINTS

Base limit: 400 feet AGL.
Near a structure: within a 400-foot radius, up to 400 feet above its uppermost limit.
650-foot tower gives a ceiling of 1,050 feet AGL inside the radius.

SOURCES

14 CFR 107.51(b)

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