What is the 400 ft AGL rule under Part 107?

Under 14 CFR Part 107.51, a remote pilot-in-command must keep a small unmanned aircraft at or below 400 feet above ground level during operations. This ceiling helps keep drone traffic separated from manned aircraft, which typically operate at higher altitudes. The limit applies to the unmanned aircraft's actual altitude above the ground directly beneath it, not above the pilot or a fixed reference point.

What is the exception to the 400 ft AGL limit under Part 107?

14 CFR Part 107.51 allows a small UAS to fly higher than 400 feet AGL when operating within a 400-foot lateral radius of a structure. In that case, the aircraft may fly up to 400 feet above the structure's immediate uppermost limit. This exception accommodates operations such as inspecting communication towers or tall buildings, where remaining within 400 feet of the structure keeps the unmanned aircraft in a volume of airspace already associated with that obstacle.

How do you measure altitude for the Part 107 400 ft AGL rule near uneven terrain?

The 400-foot AGL limit is measured relative to the ground directly below the unmanned aircraft at any given point along its flight path, not from the takeoff point. This means that as terrain rises or falls, the allowable altitude changes accordingly, and the remote pilot must continuously account for terrain elevation changes throughout the operation. The FAA discusses this concept in the Part 107 regulatory preamble, emphasizing that remote pilots must plan routes carefully when flying over sloped or hilly ground to avoid inadvertently exceeding the altitude limit.

Part 107 Maximum Altitude Limits (400 ft AGL Rule)

One of the most fundamental rules governing small unmanned aircraft system (sUAS) operations under 14 CFR Part 107 is the maximum altitude limit. Whether you are a hobbyist transitioning to commercial operations or a brand-new remote pilot certificate applicant, understanding exactly where the 400-foot ceiling applies — and where it does not — is critical both for safety and for passing the FAA knowledge test. The rule sounds simple at first, but its nuances catch many pilots off guard.

This article breaks down the 400-foot AGL rule in plain English, explains the engineering and airspace logic behind it, covers the one significant operational exception, and walks you through the edge cases the FAA loves to test. By the end, you will know not just the rule itself but why it exists and how to apply it in real-world flight scenarios.

The Core Rule: 400 Feet AGL

Under 14 CFR Part 107.51(b), a remote pilot in command must not operate a small unmanned aircraft above a maximum altitude of 400 feet above ground level unless the aircraft is flown within 400 feet of a structure (discussed below). The measurement baseline is ground level — meaning the terrain or surface directly beneath the aircraft — not mean sea level (MSL). This is an important distinction. If you are flying over a hilltop that sits 1,000 feet MSL, the rule still limits you to 1,000 + 400 = 1,400 feet MSL at that location, but your operational ceiling from a regulatory standpoint remains 400 feet above the ground beneath you.

This is fundamentally different from how manned aviation typically describes altitudes. Instrument flight rules and airspace ceilings are almost always expressed in MSL. For Part 107 remote pilots, however, the reference is always the ground directly below, which means the MSL equivalent of your ceiling changes constantly as terrain rises and falls. When conducting operations over rolling or mountainous terrain, remote pilots must account for this continuously shifting reference point to remain compliant.

Why the 400-Foot Limit Exists

The 400-foot AGL ceiling was not chosen arbitrarily. The FAA designed it to create a buffer zone that keeps sUAS traffic separated from the vast majority of low-altitude manned aircraft operations. Under 14 CFR 91.119, manned aircraft operating under visual flight rules (VFR) must generally remain at least 500 feet from any person, vessel, vehicle, or structure in non-congested areas, while over congested areas they must remain at least 1,000 feet above the highest obstacle within a 2,000-foot radius of the aircraft — a substantially stricter minimum, not simply a higher variant of the 500-foot rule. By capping drone operations at 400 feet AGL, the FAA creates a 100-foot buffer beneath the lowest point where manned VFR traffic is routinely expected to fly in uncongested areas.

Additionally, most controlled airspace structures — Class B, C, and D — begin at the surface or at specific MSL altitudes that translates to hundreds or thousands of feet above ground. The 400-foot rule keeps the majority of sUAS flights out of the busiest layers of controlled airspace (when operating in Class G airspace) without requiring individualized altitude negotiations for every flight. It is a pragmatic, safety-first design that has proven effective at limiting mid-air conflict risk between drones and manned aircraft.

The Structure Exception: Flying Higher Than 400 Feet AGL

Part 107.51(b) includes a critically important exception: a remote pilot may operate above 400 feet AGL when the aircraft is flown within 400 feet of a structure. Under this exception, the aircraft may fly up to 400 feet above the structure's immediate uppermost limit. This allows remote pilots to conduct legitimate inspections, maintenance surveys, and photography of tall objects such as communication towers, wind turbines, skyscrapers, and bridges without being artificially constrained by the flat 400-foot ceiling.

Here is how the math works in practice. Suppose you are contracted to inspect a 600-foot communications tower. The base of the tower sits at ground level. Without the exception, you could only fly to 400 feet AGL — nowhere near the top. With the structure exception, you may fly up to 400 feet above the top of the tower, which means you can legally reach 1,000 feet AGL (600 feet of tower + 400 feet above it), provided you remain within 400 feet laterally of the structure at all times. The moment your aircraft moves more than 400 feet horizontally away from the structure at an altitude above 400 feet AGL, you are in violation of Part 107.

This exception applies to any structure — natural or man-made. A cliff face, a large building, a transmission line tower, or an antenna all qualify. However, the remote pilot must exercise careful judgment. Tall structures near airports are frequently surrounded by controlled airspace, and simply qualifying for the structure exception does not waive any airspace authorization requirements. If the tower happens to sit within Class D airspace, for example, you still need authorization through the Low Altitude Authorization and Notification Capability (LAANC) system or a formal FAA waiver before flight.

Controlled Airspace and the Altitude Rule Working Together

The 400-foot AGL ceiling and airspace requirements are separate but interrelated constraints. Flying below 400 feet AGL does not automatically make an operation legal — you also need to be operating in Class G (uncontrolled) airspace, or you need prior authorization to fly in Class B, C, D, or the surface area of Class E airspace. Conversely, receiving authorization to fly in controlled airspace does not override the 400-foot ceiling unless your authorization explicitly permits a higher altitude.

Remote pilots must think of compliance as a two-part checklist: (1) Am I within the altitude limit? and (2) Am I authorized to fly in the airspace class beneath me? Both conditions must be satisfied simultaneously. This layered approach is why airspace evaluation tools like LAANC and the FAA's B4UFLY app are so valuable — they help remote pilots visualize both airspace class boundaries and any altitude limitations that apply to their specific location.

Key Numbers and Rules

400 feet AGL — The standard maximum altitude for Part 107 sUAS operations under 14 CFR 107.51(b).
400 feet above a structure — The maximum additional altitude permitted when operating within 400 feet laterally of a structure.
Within 400 feet laterally — The aircraft must remain within this horizontal distance of the structure to qualify for the altitude exception.
AGL, not MSL — The altitude reference is always above the ground directly beneath the drone, not above mean sea level.
Separate from airspace authorization — The altitude rule does not substitute for airspace class authorization requirements; both must be satisfied.
Waivers available — The FAA may issue a waiver under Part 107.200 permitting operations above 400 feet AGL when a remote pilot demonstrates the operation can be conducted safely.

Waivers for Higher Altitude Operations

If your operation genuinely requires flight above 400 feet AGL and no structure exception applies, the FAA provides a formal waiver process under 14 CFR Part 107.200. The remote pilot in command (or operator) must apply through the FAA DroneZone portal and demonstrate that the proposed operation can be conducted at the requested altitude without creating an undue hazard to other aircraft. The FAA evaluates factors such as geographic location, proximity to airports, the type of airspace involved, and proposed risk mitigations. Waivers are granted on a case-by-case basis and typically include specific conditions, operating limitations, and expiration dates.

Common Test Traps

Confusing AGL with MSL: The FAA knowledge test may describe a scenario where terrain elevation is given in MSL, then ask about the allowable drone altitude. Remember — your operational ceiling is 400 feet above the ground surface below you, not 400 feet above sea level.
Misapplying the structure exception: A common distractor question implies you can fly to any altitude near a tall building. The rule is 400 feet above the top of the structure, not 400 feet above ground near the structure, and you must remain within 400 feet laterally.
Assuming the exception removes airspace requirements: The structure exception only addresses altitude — it does not grant airspace authorization. A tower inside Class D airspace still requires authorization regardless of the exception.
Forgetting the lateral boundary: Students often memorize the vertical component of the structure exception but forget the 400-foot lateral limit. The instant your drone moves farther than 400 feet horizontally from the structure while above 400 feet AGL, it is a violation.
Thinking hobbyist rules are the same: Recreational flyers operating under FAA recreational rules (Section 44809) are subject to different statutory language. Part 107 rules apply to commercial and certificated remote pilots; mixing up the two frameworks on the test will cost points.