The Space Between: Who Controls the Airspace Where Small Unmanned Aircraft Systems (UAS) Fly?

This article was originally published in UAS Vision.

 No one disputes that the federal government has exclusive jurisdiction over “navigable air space,” which has historically begun “above the minimum safe altitudes of flight,” including the airspace needed to ensure safety in the takeoff and landing of aircraft.  The FAA generally sets the minimum safe altitude as “500 feet above the surface [of non-congested areas], except over open water or sparsely populated areas.” 

This is a judgment call based, in part, on the technological capabilities of most manned aircraft.  The development of smaller unmanned aircraft—which, despite being unmanned, are still “aircraft,” in the FAA’s view—has expanded the practical envelope of the “minimum safe altitudes of flight.”  A small UAS can operate safely at a much lower altitude than anything large and heavy enough to carry a person.  Indeed, under the FAA’s new rules, a small UAS may generally be flown only below 400 feet.

But this raises a question that has lain dormant for decades:  If the minimum safe altitude for flight is essentially zero feet AGL, is there a private property right in the airspace directly above the land that might put a different lower limit on what is “navigable”?  And, if so, who has the authority to regulate that airspace?

Perhaps the most common argument is that states can regulate the area below the navigable air space.  Supporters of this view cite United States v. Causby, 328 U.S. 256 (1946), a case about an unfortunate farmer whose livelihood was threatened by low-level military flights, to argue that if the federal government is subject to takings claims for flights below the navigable air space, then its control ends there.  States, the argument goes, can therefore step in to regulate the remaining air space.  After all, flights in this airspace are unlikely to be interstate and the Constitution grants states the powers not vested in the federal government.  This argument has been advanced in a case called Huerta v. Haughwout, currently pending in federal district court in Connecticut, but has not so far been resolved.  The effect of granting states jurisdiction over air space below the navigable air space is that states could pass laws regulating what can use that space, what parts of that space, and when that space can be used.

But this argument reads far too much into Causby, a case that raised more questions than answers about the limits of what is “navigable” airspace—and what might constitute a taking of private property.  In Causby, the Supreme Court observed that “The airspace, apart from the immediate reaches above the land, is part of the public domain. We need not determine at this time what those precise limits are. Flights over private land are not a taking, unless they are so low and so frequent as to be a direct and immediate interference with the enjoyment and use of the land.”  The Court noted that if the “navigable” airspace boundary had been set low enough, it might raise issues about whether that line validly defined the “public domain.”

Perhaps just as importantly, Causby recognized even where airspace was not “navigable,” aircraft had to be able to transit that airspace in order to get to and from what is “navigable.”  This right of transit might or might not constitute a compensable taking, depending on whether it was temporary or permanent, and just how intensive the use was.  The disposition of the case suggests, though, that the use has to be extreme in order to cross that line.  In Causby, even “four motored heavy bombers [and] fighter planes . . . in considerable numbers and rather close together,” flying at tree-top level, were not enough to constitute a “direct and immediate interference with the land” per se, despite the fact that these flights caused “the destruction of the property as a commercial chicken farm.  Instead, the Court referred the matter back to the Court of Claims for further proceedings on the frequency, permissible altitude, and type of aircraft at issue in the case.

So what can we draw from Causby?  It is certainly not that there is a hard and fast line below which airspace ceases to be navigable and state sovereignty kicks in.  Rather, Causby stands for the proposition that the airspace is a public good.  The Court recognized that traditional principles of property law necessarily gave way to the technological evolution represented by flight, If they had not, the air could not be used as a “public highway.”  To allow traditional trespass suits would “clog these highways, seriously interfere with their control and development in the public interest, and transfer into private ownership that to which only the public has a just claim.”

Causby did not define the bottom edge of this public good, but there is no reason to think that Causby’s recognition of the evolution in technology should be frozen in amber as of 1946 (Causby) or 1958 (Federal Aviation Act).  To the contrary, the lesson of Causby is that as new classes of aircraft are developed that can operate safely at lower altitudes, the “navigable airspace” should expand to embrace them, just as the traditional notion that the property owner had title to the airspace all the way to the stars had to yield to manned flight.  The proliferation of UAS flights is a sign that, in practical terms, such an expansion is occurring.  In legal terms, the FAA has already recognized this expansion by adopting Part 107, which dispenses with the 500-foot floor for small UAS, and instead imposes a 400 foot ceiling.

The Court in Causby deferred to the federal government’s judgment about the lower boundary of the navigable airspace.  Although it noted that the government’s discretion was not unbounded (saying that “[i]f [the] agency prescribed 83 feet as the minimum safe altitude, then we would have presented the question of the validity of the regulation”), the factors that go into determining a “minimum safe altitude” for four-engined bombers and fighters are clearly different than for small UAS, as the FAA has concluded in adopting Part 107.  There is no reason to think that courts interpreting Causby in the context of small UAS should feel bound by that 83 foot number, which arose in a very different context.

Moreover, Causby tells us that even if there is a lower limit to what is “navigable,” the public (through the federal government) still maintains an interest in regulating flight in these areas.  That interest is twofold.

First, there is the obvious point that in order to access “navigable” airspace, an aircraft must traverse non-navigable airspace (at the time of Causby, this glide path was not included in the definition of “navigable air space,” though Congress subsequently added it). Take away that access, and the public good above is useless. Of course, Causby acknowledged that this may lead to a taking in extreme circumstances, but even there the doctrine of takings has evolved substantially since the Causby decision.

This idea is not unique to aviation.  With respect to navigable waters, for example, the Supreme Court has stated (in a case called Braniff Airways) that “the title to the beds and the banks are in the states and the riparian owners, subject to the federal power over navigation.”  As long as the waterways have some sort of nexus with interstate commerce, the federal government has a jurisdictional hook over these waters.  Thus, federal government could still regulate a stream that mainly runs through a landowner’s backyard if that stream flows into an interstate waterway.

Second, there is the other simple fact that aircraft (and operators) who may fly in any “non-navigable” space (to the extent such a concept makes sense) are likely to also fly, at some point, in navigable space.   Thus, it is well-settled that an aircraft that flies below 500 feet is still subject to federal aircraft safety rules.  Likewise, the Federal Aviation Administration sets design standards for aircraft – no matter where they end up flying.  How else could the FAA preserve air safety?

At bottom, claims of state and local sovereignty over airspace cannot be justified simply by invoking Causby.  The scope of what constitutes “navigable” airspace has evolved, and it is the FAA and the federal government, by statute, that establish where this boundary lies.  And even in any liminal space between the ground and airspace that can be safely navigated, the federal government retains a clear interest in ensuring access to the “public good” of the navigable airspace and the enforcement of uniform safety standards.

States and localities should consider these issues carefully before adopting their own regulations.  Otherwise, they may ground an entire industry before it ever has a chance to spread its wings.


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