The 400-Foot AGL Rule and the Structure Exception
The 400-foot AGL altitude limit introduced in Lesson C has a genuinely useful built-in exception that lets remote pilots legally fly higher than 400 feet when working near tall structures ? no waiver required. This lesson works through exactly how that exception is measured, with worked numeric examples, since this is one of the most frequently tested and most frequently misunderstood parts of 14 CFR 107.51.
The base rule: 400 feet AGL
As covered in Lesson C, a small unmanned aircraft's altitude cannot exceed 400 feet above ground level, measured from the ground directly beneath the aircraft. Without any exception, this would make it illegal to inspect the top of any structure taller than 400 feet ? radio and cell towers, wind turbines, tall smokestacks, and some skyscrapers all commonly exceed that height.
The structure exception: how it actually works
14 CFR 107.51(b) allows a small unmanned aircraft to fly higher than 400 feet AGL if two conditions are both met: the aircraft remains within a 400-foot radius of a structure, and it does not fly higher than 400 feet above the structure's immediate uppermost point. In plain terms ? you get an extra 400 feet of altitude "on top of" the structure's own height, but only while staying laterally close to that structure (within 400 feet horizontally).
This is not a waiver and does not require any special FAA application ? it is a built-in part of the base regulation itself, available to every remote pilot without additional paperwork, as long as the two conditions above are actually met on that specific flight.
Worked example 1: a 300-foot radio tower
A radio tower stands 300 feet tall. Under the structure exception, a remote pilot inspecting the antenna array near the top can fly up to 300 + 400 = 700 feet AGL, as long as the aircraft stays within a 400-foot lateral radius of the tower throughout that portion of the flight. Once the aircraft moves more than 400 feet away from the tower, it must come back down to the standard 400-foot AGL ceiling (unless it's now within range of a different qualifying structure).
Worked example 2: a 600-foot broadcast tower
A taller broadcast tower stands 600 feet ? already above the standard 400-foot ceiling on its own. Using the structure exception, a remote pilot can fly up to 600 + 400 = 1,000 feet AGL while staying within 400 feet laterally of the tower, enough altitude to inspect equipment mounted at or near the very top of the structure. Note the exception is measured from the structure's own height, not from the base 400-foot ceiling ? the "extra 400 feet" always stacks on top of wherever the structure's uppermost point actually is.
Real-world scenario: a cell tower antenna swap inspection
A telecom contractor hires a Part 107 pilot to verify a new antenna installation at the top of a 450-foot cell tower. The pilot plans the flight to stay within 400 feet laterally of the tower structure throughout, and confirms the planned inspection altitude of 500 feet AGL is well within the 450 + 400 = 850-foot ceiling the structure exception allows for this tower ? no waiver needed. If the same pilot later needed to fly a separate transect a quarter mile away to photograph the surrounding property for the same job, that portion of the flight would need to drop back down to the standard 400-foot AGL limit, since it would no longer be within 400 feet of the tower.
The structure exception: 400 feet of extra altitude "stacked" on top of a structure's own height, within a 400-foot lateral radius.
View Larger| Structure height | Max altitude under exception | Lateral radius allowed | Regulation |
|---|---|---|---|
| 150 ft (typical cell tower) | 550 ft AGL | 400 ft | 14 CFR 107.51(b) |
| 300 ft (radio tower) | 700 ft AGL | 400 ft | 14 CFR 107.51(b) |
| 450 ft (tall cell/broadcast tower) | 850 ft AGL | 400 ft | 14 CFR 107.51(b) |
| 600 ft (major broadcast tower) | 1,000 ft AGL | 400 ft | 14 CFR 107.51(b) |
| No qualifying structure nearby | 400 ft AGL (standard limit) | N/A | 14 CFR 107.51(b) |
Planning a flight using the structure exception
Work through this before any flight that will exceed 400 feet AGL near a tall structure.
Confirm the structure's actual height
Use reliable documentation (site plans, tower registration data, or manufacturer specs) rather than a visual estimate.
Calculate the maximum allowed altitude
Add 400 feet to the structure's height to get your ceiling under the exception.
Plan the flight path to stay within 400 feet laterally of the structure
Any portion of the flight moving outside that radius must drop back to the standard 400-foot AGL ceiling.
Check other airspace requirements independently
The structure exception addresses altitude only ? airspace authorization requirements (Module 3) still apply in full and are not waived by this exception.
| Mistake | Why it happens | Correct understanding | Regulation / source |
|---|---|---|---|
| "I get 400 extra feet above the base 400-foot limit anywhere near a tall building." | Misreading the exception as adding to the standard ceiling instead of the structure's own height | The extra 400 feet stacks on top of the structure's own height, not on top of the standard 400-foot AGL limit. | 14 CFR 107.51(b) |
| "I can fly at the exception altitude anywhere in the general area of the structure." | Overlooking the 400-foot lateral radius requirement | The aircraft must remain within a 400-foot horizontal radius of the structure to use the exception. | 14 CFR 107.51(b) |
| "The structure exception requires a waiver application." | Assuming any deviation from the standard 400-foot limit needs FAA approval | The structure exception is built into the base regulation and requires no separate waiver. | 14 CFR 107.51(b) |
Do I need a waiver to use the structure exception?
No. It is a built-in part of 14 CFR 107.51(b) itself, available to any Part 107 flight that meets both the lateral-radius and above-structure altitude conditions ? no separate FAA application is needed.
Is the extra altitude added to 400 feet AGL or to the structure's height?
It's added to the structure's own height. A 600-foot tower allows up to 1,000 feet AGL under the exception (600 + 400), not 800 feet (400 + 400).
What happens if I fly farther than 400 feet from the structure while still above 400 feet AGL?
At that point the exception no longer applies, and the flight must comply with the standard 400-foot AGL ceiling unless another qualifying structure is nearby.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.