Rules
The Part 107 400-foot rule near structures, and the 87-knot limit
Under 14 CFR 107.51(b) a small drone may not exceed 400 ft AGL, unless it stays within a 400 ft radius of a structure and no more than 400 ft above that structure's uppermost limit. Under 107.51(a) the limit on speed is 87 knots (100 mph) groundspeed, not airspeed.
What does 107.51(b) say about altitude?
The aircraft may not fly higher than the higher of two limits. The first (b)(1) is 400 feet above ground level. The second, in (b)(2), applies when the aircraft is within a 400-foot radius of a structure: then it may fly no higher than 400 feet above that structure's immediate uppermost limit. Both use the number 400, but each is measured from something different. That difference is where test questions live.
Ceiling in the open: 400 ft AGL, measured from the ground directly below the aircraft. Ceiling near a structure: top of the structure plus 400 ft, but only inside a 400 ft radius of it.
Sources (checked 2026-11-14): 14 CFR 107.51 (eCFR); FAA: Part 107 Waivers.
How do I apply the 400-foot radius exception?
Think of an upright cylinder around the structure. The radius is 400 feet, measured horizontally from the structure. Inside the cylinder, the ceiling rises to 400 feet over the structure's highest point. Outside, it drops back to 400 feet AGL. You cannot fly outward at the elevated altitude: leaving the radius means being at or below 400 ft AGL at the edge.
Worked example
A 60-foot building
Say a building is 60 ft tall (AGL). Inside a 400 ft radius of it, you may fly up to 60 + 400 = 460 ft AGL. At 401 ft horizontally from the building, you are outside the radius and your ceiling is 400 ft AGL. A drone at 450 ft AGL that drifts out past the radius is now 50 ft too high.
Worked example
A 949-foot radio tower
Say a tower is 949 ft AGL. Inside a 400 ft radius the ceiling is 949 + 400 = 1,349 ft AGL. If a pilot inspects the top of the tower at 1,200 ft AGL from 150 ft away, that is allowed under 107.51(b)(2). At 500 ft away from the tower at 1,200 ft AGL, the flight is out of compliance, because the aircraft is outside the radius and above 400 ft AGL.
The exception raises only the altitude limit. It does not authorize flying in controlled airspace without permission, and it does not relax the cloud clearance, flight visibility or visual line of sight rules. See airspace authorization.
Sources (checked 2026-11-14): 14 CFR 107.51 (eCFR); 14 CFR 107.41 (eCFR).
How do I convert an MSL altitude from a sectional chart to AGL?
AGL equals MSL minus the ground elevation below you. On a sectional, the obstruction symbol is labeled with the top of the obstruction in MSL, and the height above ground (AGL) is in parentheses underneath. Ground elevation at the site is the first number minus the second.
Worked example
Reading a tower label
Say a tower on a sectional is labeled 1,249 over (949). The top is 1,249 ft MSL and the tower stands 949 ft AGL. So the ground there is 1,249 - 949 = 300 ft MSL. Inside the 400 ft radius, the maximum legal altitude is 949 + 400 = 1,349 ft AGL, which is 1,349 + 300 = 1,649 ft MSL.
Check by working from the MSL top: 1,249 + 400 = 1,649 ft MSL. Both routes give the same answer, which is a good habit on test day.
Study the symbols in sectional chart symbols and the unit conventions in MSL vs AGL.
Sources (checked 2026-11-14): FAA Aeronautical Chart Users' Guide; 14 CFR 107.51 (eCFR).
What happens when the ground slopes?
The 400 ft AGL rule follows the terrain under the aircraft, not the takeoff point. If the ground drops away, your legal altitude in MSL drops with it.
Worked example
Flying off a ridge
Say you launch from a ridge at 800 ft MSL and fly out over a valley floor at 600 ft MSL. Over the launch point, 400 ft AGL equals 1,200 ft MSL. Over the valley, the same limit is 600 + 400 = 1,000 ft MSL. If you hold 1,150 ft MSL as you cross the edge, you are within the limit over the ridge but 150 ft too high over the valley (that is 550 ft AGL).
Altitude displayed on the controller is often relative to the takeoff point. Do not assume it equals AGL after a terrain change.
Sources (checked 2026-11-14): 14 CFR 107.51 (eCFR); FAA Remote Pilot sUAS Study Guide (FAA-G-8082-22).
What is the groundspeed limit, and how does wind affect it?
107.51(a) caps the aircraft at 87 knots, which the rule states as 100 miles per hour. The limit applies to groundspeed, the speed over the earth, not to airspeed through the air. A tailwind adds to groundspeed, and a headwind subtracts from it. A knot is a nautical mile per hour, about 1.15 statute miles per hour, so 87 knots comes to roughly 100 mph.
| Airspeed | Wind relative to the drone | Groundspeed | Within 87 kt? |
|---|---|---|---|
| 70 kt | 10 kt tailwind | 70 + 10 = 80 kt | Yes |
| 70 kt | 20 kt tailwind | 70 + 20 = 90 kt | No, 3 kt over |
| 80 kt | 25 kt headwind | 80 - 25 = 55 kt | Yes |
| 60 kt | 30 kt tailwind | 60 + 30 = 90 kt | No, 3 kt over |
| 50 kt | Crosswind only | About 50 kt | Yes, with a small effect |
Say a pilot is flying a fast fixed-wing drone at 75 knots of airspeed and a 15-knot wind comes in from behind. Groundspeed is 90 knots, about 103 mph: over the limit even though the airspeed was legal. The fix is to turn upwind, slow down or fly a different leg so that groundspeed stays at or under 87 knots.
Sources (checked 2026-11-14): 14 CFR 107.51 (eCFR); FAA Remote Pilot sUAS Study Guide (FAA-G-8082-22).
How do I answer a test question on 107.51?
- Identify which limit the question is testing: altitude (b), speed (a), visibility (c) or cloud clearance (d).
- For altitude, find what the altitude is measured from. "400 ft above the structure" is not the same as "400 ft above ground."
- For a chart question, subtract the AGL figure from the MSL figure to get ground elevation, then convert.
- For speed, add or subtract wind to get groundspeed before comparing with 87 knots.
- Check the horizontal distance: the 400 ft radius is the gate to the structure rule.
Common traps: treating the exception as a blanket 400 ft above any structure within a mile; using airspeed for the speed limit; and mixing up MSL and AGL after reading a label. The rest of the numbers in this rule, 3 statute miles of visibility and 500 ft below and 2,000 ft horizontally from clouds, are in Part 107 operating rules.
If you need more altitude than 107.51 allows, the section is on the waiver list in 107.205. See waivers and authorizations.
Sources (checked 2026-11-14): 14 CFR 107.51 (eCFR); 14 CFR 107.205 (eCFR).
How do the altitude and speed rules combine in one flight plan?
Real jobs hit both limits at once. A tower inspection flown from a ridge on a windy day needs the structure exception, a terrain check and a groundspeed check on the long downwind leg.
Worked example
Cell tower job with wind
Say the tower is 180 ft AGL, the site is at 500 ft MSL, and the sectional label reads 680 over (180). Inside a 400 ft radius the ceiling is 180 + 400 = 580 ft AGL, or 1,080 ft MSL. You plan an orbit at 150 ft from the tower at 450 ft AGL. That works. Your transit leg to a second tower 900 ft away has no structure within 400 ft, so there the ceiling is 400 ft AGL; do not carry 450 ft AGL across the gap. If the downwind leg has a 20-knot tailwind and the drone cruises at 70 knots airspeed, groundspeed is 90 knots and over the limit, so you slow the drone or fly the leg upwind.
The rule allows the structure exception only when you are within the radius, which is why you plan altitude by segment, not for the whole flight. Write the limits in your preflight notes: ceiling by segment, the MSL equivalent for each one, and a groundspeed cap you will not exceed.
Sources (checked 2026-11-14): 14 CFR 107.51 (eCFR); 14 CFR 107.49 (eCFR).
What are the most common 107.51 test traps in one list?
Watch for four swaps. Airspeed for groundspeed. MSL for AGL. "Above the structure" for "above the ground." And a distance from the structure that is beyond 400 feet. A question that gives you a tower height and a drone altitude almost always hinges on which of those four it wants you to notice. Redo the arithmetic with the 400 ft added to the top of the structure and the radius checked before you pick an answer, and treat a result that sits just over the limit as a violation, since the rule has no tolerance band.
Sources (checked 2026-11-14): 14 CFR 107.51 (eCFR).
Related guides
- Part 107 operating rules: altitude, speed, visibility and clouds
- MSL vs AGL: altitude math on a sectional chart
- Sectional chart symbols a remote pilot must know
- Part 107 waivers and airspace authorizations
- Part 107 visual line of sight and visual observers explained
- All Part 107 guides
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Sources
- 14 CFR 107.51 (eCFR), checked 2026-11-14
- 14 CFR 107.205 (eCFR), checked 2026-11-14
- FAA Aeronautical Chart Users' Guide, checked 2026-11-14
- FAA Remote Pilot sUAS Study Guide (FAA-G-8082-22), checked 2026-11-14
How this page was made: written by Certloom Learning from the sources above, with the rules checked against the current regulation text. Practice items are original questions from the app's reviewed bank. Spot an error? certloomlearning@gmail.com.