General Operating Rules: Speed, Altitude and Right-of-Way
14 CFR 107.51, "Operating limitations for small unmanned aircraft," sets the four core numeric limits that apply to nearly every Part 107 flight: how fast you can fly, how high, how far you must be able to see, and how far you must stay from clouds. This lesson covers all four in full, with a survey of right-of-way as well ? two topics with enough depth to warrant their own dedicated lessons follow immediately after this one (Lesson D on the altitude/structure exception, Lesson J on right-of-way).
Groundspeed limit: 87 knots (100 mph)
A small unmanned aircraft may not exceed a groundspeed of 87 knots, which the regulation equates to 100 miles per hour. Groundspeed is speed relative to the ground, not airspeed ? a distinction that matters more for manned aircraft in windy conditions, but is worth knowing precisely because ACS test questions sometimes test unit conversion between knots and mph directly. Most consumer and commercial sUAS are nowhere near this limit in normal operations, but high-speed racing-style drones and some fixed-wing mapping platforms can approach it, making this a real operational limit rather than a purely theoretical one.
Altitude limit: 400 feet AGL (overview)
The small unmanned aircraft's altitude cannot exceed 400 feet above ground level (AGL) ? meaning height measured from the ground directly beneath the aircraft, not a fixed altitude above sea level. There is an important exception allowing flight higher than 400 feet AGL near a tall structure, covered in full detail with worked examples in Lesson D immediately following this one, since it has enough nuance to deserve its own dedicated treatment.
Minimum flight visibility: 3 statute miles
Flight visibility ? as observed from the location of the control station, not from the aircraft itself ? must be no less than 3 statute miles. This means the remote pilot (or visual observer) standing on the ground must actually be able to see that far in the prevailing conditions before and during the flight. Haze, fog, heavy precipitation, or smoke can all reduce visibility below this minimum even on a technically "daytime" flight, and the remote pilot is responsible for confirming visibility meets this standard before launching, not just assuming it based on a general weather forecast.
Cloud clearance: 500 feet below, 2,000 feet horizontal
The aircraft must stay at least 500 feet below any cloud and at least 2,000 feet horizontally away from any cloud. This requirement exists for the same fundamental reason cloud clearance rules exist throughout aviation: manned aircraft may be operating in or near clouds under instrument flight rules, and the sUAS must stay clear of that airspace to avoid a collision hazard, since a remote pilot cannot see into or through a cloud to spot an approaching aircraft.
Right-of-way (overview)
Small unmanned aircraft must always yield the right-of-way to other aircraft and airborne vehicles ? sUAS never has priority over manned aviation. The full right-of-way rule, including exactly what "yielding" means operationally and the collision-hazard prohibition, is covered in depth in Lesson J of this module.
Real-world scenario: checking all four limits before a flight
A pilot is hired to inspect a cell tower on a moderately hazy afternoon. Before launching, they confirm groundspeed will stay well under 87 knots for a slow, careful inspection pass; they check that the planned altitude will stay within 400 feet AGL of the ground at the tower's base (with the structure exception from Lesson D potentially relevant if inspecting near the tower's top); they visually confirm they can see roughly 3 miles in the current haze, not just guess based on the morning forecast; and they note scattered clouds are well above and to the side of the planned flight path, comfortably clearing the 500-foot/2,000-foot cloud clearance minimums. All four checks pass, and the flight proceeds legally.
The four numeric operating limitations of 14 CFR 107.51 shown together around a small unmanned aircraft.
View Larger| Limitation | Exact value | Measured from | Regulation |
|---|---|---|---|
| Groundspeed | 87 knots (100 mph) | The aircraft relative to the ground | 107.51(a) |
| Altitude | 400 feet AGL (structure exception applies ? see Lesson D) | Ground directly below the aircraft | 107.51(b) |
| Flight visibility | No less than 3 statute miles | The control station's location | 107.51(c) |
| Cloud clearance (vertical) | No less than 500 feet below | The nearest cloud | 107.51(c) |
| Cloud clearance (horizontal) | No less than 2,000 feet | The nearest cloud | 107.51(c) |
Checking the four 107.51 limits before every flight
Build this into a standard preflight habit, not a one-time memorization exercise.
Plan your maximum groundspeed for the mission
Confirm the planned flight profile will stay under 87 knots throughout, especially for fixed-wing or racing-style aircraft.
Confirm your planned altitude stays within 400 feet AGL, accounting for terrain
Remember AGL is measured from the ground directly below the aircraft, which can change as the aircraft moves over sloped or uneven terrain ? covered further in Module 4's MSL vs. AGL lesson.
Visually confirm flight visibility from the control station location
Don't rely solely on a general forecast ? assess actual visibility at the site immediately before flight.
Check cloud position relative to the planned flight path
Confirm at least 500 feet vertical and 2,000 feet horizontal clearance from any cloud along the route.
| Mistake | Why it happens | Correct understanding | Regulation / source |
|---|---|---|---|
| "87 mph is the speed limit." | Mixing up knots and mph, or misremembering which number pairs with which unit | The limit is 87 knots, which equals 100 mph ? not 87 mph. | 14 CFR 107.51(a) |
| "400 feet is measured from sea level (MSL)." | Confusing AGL and MSL altitude references | The 400-foot limit is AGL ? measured from the ground directly below the aircraft, not from sea level. | 14 CFR 107.51(b) |
| "Visibility is judged from wherever the aircraft happens to be flying." | Assuming visibility is an aircraft-side measurement | Flight visibility is observed from the location of the control station, on the ground. | 14 CFR 107.51(c) |
Is 87 knots the same as 87 mph?
No. 87 knots equals 100 mph ? the regulation states both figures together specifically because they are not numerically the same unit.
Can I ever fly higher than 400 feet AGL?
Yes, under the structure exception, covered fully with worked examples in Lesson D of this module.
Who determines whether visibility meets the 3-statute-mile minimum?
The remote pilot in command is responsible for confirming visibility at the control station's location meets the minimum before and during flight ? it is not a passive assumption based on a general forecast.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.