The 400-foot AGL ceiling is the most widely cited rule in drone operations. Ask any pilot about altitude limits and they will say 400 feet. But the actual regulation -- 14 CFR 107.51(b) -- is more nuanced than the shorthand version suggests.
There are situations where you can legally fly above 400 feet. There are also situations where the effective limit is lower than 400 feet. Understanding both matters for legal and safe operations.
What the Rule Actually Says
14 CFR 107.51(b) states that a remote pilot may not operate a small unmanned aircraft at an altitude above 400 feet above ground level, unless the small unmanned aircraft 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.
The full text creates two separate altitude limits:
- Default: 400 feet AGL -- applies everywhere
- Structure exception: 400 feet above the top of the structure -- available when flying within 400 feet of a structure
The Structure Exception
The structure exception is significant for utility inspection, telecommunications tower inspection, building inspection, and any other operations involving tall structures.
If you are inspecting a 500-foot communications tower, you can fly up to 900 feet AGL (500 feet to the top of the structure + 400 feet above it) as long as you remain within 400 feet horizontally of the tower.
A few important notes about the structure exception:
"Structure" is broadly defined. It includes buildings, towers, antennas, bridges, and any constructed object. Natural objects like trees and hills do not qualify.
The 400-foot horizontal radius is strictly measured. Once you move more than 400 feet from the structure, the 400-foot AGL ceiling applies regardless of the structure height.
Other rules still apply. Flying above 400 feet AGL near a controlled airport still requires the appropriate airspace authorization, even if the structure exception permits the altitude.
Class B airspace has different rules. In Class B airspace, flying above 400 feet AGL requires coordination with ATC regardless of the structure exception.
Where the Limit Is Effectively Lower Than 400 Feet
Several factors can reduce the effective legal altitude below 400 feet:
Airspace floors. Inside Class B, C, D, or E surface airspace, your authorization specifies the approved altitude. LAANC authorizations commonly cap at 100, 200, or 300 feet depending on your location in the UAS Facility Map. Your authorization ceiling, not 400 feet, is the binding limit.
TFR floors. Active TFRs specify altitude ranges. A TFR from the surface to 1,000 feet prohibits flight at any altitude in that range, including below 400 feet.
Instrument approach corridors. While not a hard regulatory floor for drones, the base leg and final approach of an instrument approach at an uncontrolled airport may bring manned aircraft to 300-400 feet AGL. Flying below the legal ceiling near an approach corridor still creates collision risk.
Manufacturer airspace restrictions. Some drones (notably DJI aircraft) have geofencing or firmware-enforced altitude limits in certain areas. These are not FAA regulations but can practically restrict your operation.
Altitude in Controlled vs. Uncontrolled Airspace
In uncontrolled Class G airspace, the 400-foot ceiling (or structure exception) is the only altitude limit from the FAA for UAS.
In controlled airspace (B, C, D, or E surface), your authorized altitude ceiling from LAANC or a DroneZone waiver is the binding limit. This ceiling is commonly lower than 400 feet at many controlled airspace locations.
The UAS Facility Map shows the maximum altitude at which LAANC can automatically approve a flight at your specific grid location. At many locations near runways, that ceiling is 0 feet -- meaning LAANC cannot approve the flight at any altitude and a DroneZone waiver is required.
Altitude and the Safety Score
Altitude is a direct input into preflight scoring because flying higher carries different risks than flying lower:
- Higher altitude increases visibility to manned aircraft (reduces collision risk) but also increases your exposure in busy approach corridors
- At lower altitudes in controlled airspace, you may be at or above the LAANC ceiling for your location
- Altitude affects the scoring of certain restricted zones -- some hospital helipad buffers have altitude-specific rules
UAS SkyCheck accepts your planned altitude as an input and calculates the safety score against the actual authorized ceiling for your location. Free, no account required.
Practical Summary
| Situation | Effective Altitude Limit | |-----------|------------------------| | Uncontrolled airspace, no structure | 400 ft AGL | | Within 400 ft of a 500-ft tower | 900 ft AGL (500 + 400) | | LAANC authorization with 200 ft ceiling | 200 ft AGL | | Active TFR from surface to 1,000 ft | 0 ft (no flight) | | Class B inner core | Per ATC authorization |
Always check your authorization ceiling against your planned altitude before launching.