How often do the news articles say that “there’s a rule”, or that flying over 400 ft is illegal. Both statements are false, but surprisingly many people believe it.
The 400 ft limit is based on two sources. First is the FAA itself when the agency issued an Advisory Circular, AC 91-57, on june 9, 1981 (and later reissued as AC 91-57a on September 2, 2015). In the AC, the FAA said “Do not fly model aircraft higher than 400 feet above the surface”. But there’s two problems with using this as evidence that 400 ft is a rule. First, an Advisory Circular is not a rule. The FAA has never prosecuted a pilot for not following advice, though it is often cited as a reason for pursuing charges under 14 CFR §91.13 – “Careless or reckless operation”. Second is the first line in the AC: “This advisory circular outlines, and encourages voluntary compliance with, safety standards for model aircraft operators”.
The updated AC 91-57a doesn’t mention altitude, but it does say that a model aircraft operation “is determined with reference to Section 336 of Public Law 112-95 [The FMRA]”. Section 336 of the FMRA does not define an altitude, but it does say :”… the aircraft is operated in accordance with a community- based set of safety guidelines and within the programming of a nationwide community-based organization”. Why didn’t they just say “AMA“?
400 ft above – what?
The FAA said in the original AC 91-57 “above the surface”, so it’s pretty clear what they mean. They don’t expect you to fly as if you have a terrain following RADAR on your 2-pound drone, but use common sense. If you take off from a hilltop climb to 400 ft AGL then fly over a valley, you are more than 400 ft “above the surface”. The rule for manned aircraft (14 CFR §91.119) says “an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the aircraft”. So, considering that AC 91-57 is designed to keep hobby aircraft away from manned aircraft, if you stay within a 2,000 ft radius of your takeoff point, you are unlikely to meet a manned aircraft.
Despite the lack of clear laws on how high we can fly our small UAS, there can still be some pretty serious consequences to the choices we make. The advice is to stay below 400 ft, and it is good advice.