Sparsely Populated Precedent

The Federal Aviation Administration regulates two industries:  aviation and space transportation.  Congress has directed the agencies to regulate these industries under two different laws, which means that the FAA has two separate sets of regulations for air and space transportation.  The FAA does not necessarily view the one as precedent for the other.  Nonetheless, they are awfully interesting. For example, both the air and space regulations address overflight of populated areas. They use different terminology, but they circle around the same principles.

The rules for experimental suborbital rockets contain a number of safety requirements, including the requirement that the vehicle operator contain its reusable suborbital rocket’s instantaneous impact point within an operating area, a three-dimensional region where permitted flights may take place, and outside any exclusion area.

How does the operator figure out what an operating area is?  14 C.F.R. § 437.57(b)(2) states that an operating area must contain enough unpopulated or sparsely populated area to perform key flight-safety events.  A key flight-safety event means a permitted flight activity that has an increased likelihood of causing a launch accident compared with other portions of flight.    Additionally, an operating area may not contain or be adjacent to a densely populated area or large concentrations of members of the public.  If there are too many people within an operating area, the operator may establish an exclusion area, which is an area, within an operating area, that a reusable suborbital rocket’s instantaneous impact point may not traverse.

Although the FAA acknowledged that it had considered defining “unpopulated” and “densely” and “sparsely” populated the FAA’s space office deliberately avoided defining them until it gained experience with their application.  The notice where the FAA proposed its requirements provides considerations and guidance for an operator trying to figure out whether its operating area would be acceptable.

The FAA did consider, but does not propose to adopt, the following definitions:  Unpopulated means devoid of people.  Sparsely populated means a population density of less than 10 people per square statute mile in an area of at least one square statute mile. Densely populated areameans a census designated place, as defined by the United States Census Bureau, with a population in excess of 100,000 people, or any area with a population density in excess of 1,000 people per square statute mile and an area of at least one square statute mile. Although proposing precise definitions may be premature, the FAA offers the following observations as preliminary guidance. The term ‘‘unpopulated’’ would mean no people, period. The term ‘‘sparsely populated’’ suggests an area with a few scattered people where the risk to those few persons from the overflight of a suborbital rocket, even one being tested, would likely be negligible. The term ‘‘densely populated area’’ would have two characteristics. One would be strictly related to numbers of people, without regard to population density. Any area with 100,000 people is not a good area to test rockets. The second characteristic would be density—an area would have to be large enough to allow an applicant to find a workable operating area in certain parts of the country, but small enough to keep the risk to the people within the area negligible, given the flight constraints discussed below.

One interesting aspect of this discussion is the focus on risk.  In the FAA’s regulations governing expendable launch vehicles, the FAA sets a quantified cap on risk.  For an experimental vehicle, where some of the variables, such as a vehicle’s probability of failure, for quantifying risk are unknown, the FAA did not impose a quantifiable risk threshold.  Nonetheless, the FAA still addresses risk in the experimental context, and appears to rely upon an uncodified, qualitative standard for risk, namely, that it be negligible.  Does this mean that there is a probability of failure that the FAA assigns to experimental reusable suborbital rockets?

When it published its final rule governing experimental permits, the FAA affirmed its decision not to define the terms.  Commenters had agreed.

The Federation and XCOR agree that the FAA should not define these terms. The Federation commented that operating areas are site dependent. The Federation’s statement is true because similarly sized operating areas with identical total populations may have a different distribution of the population, leading to different risks. Likewise, how the calculations are performed may change the apparent population density.For example, there may be an area of 100 square miles, with all the population clustered in the southeast corner in a town. The density would appear to be low if the population were distributed over the whole 100 square miles. On the other hand, if the operating area were assessed in blocks of one square mile at a time, certain areas would show high density. Because the FAA wants to gain experience in assessing these questions, the FAA will define these terms on a case-by-case basis for now.

On the aviation side of the fence, under 14 C.F.R. § 91.119, the FAA addresses minimum safe altitudes over a “congested area.”  That provision states that no person may operate an aircraft “[o]ver any congested area of a city, town, or settlement, or over any open air assembly of persons, an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the aircraft.”

The first thing we notice, of course, is that the aviation and space regulations use different terminology, with the former referring to congested areas, and the latter addressing densely, unpopulated, or sparsely populated areas.  The use of different terminology indicates that the FAA means different things.  Accordingly, it is not likely that applications of one set of rules would provide precedent for the other.  More significantly, the FAA said as much when it issued a legal interpretation in 2010 addressing the meaning of a “congested area.”  Specifically, when the person requesting the interpretation asked if he could use the definitions discussed in the preamble to the proposed space regulations, the FAA replied that those definitions were discussed in relationt to the commercial space rules, arose out of  a different statute, and did not apply to the general operating and flight rules of part 91.

The converse must be true as well.  Nonetheless, the interpretation’s review of a number of situations where the FAA applied the term “congested area” might bear a look-see when setting up an operating area.  Presumably, certificated aircraft should have a higher reliability than an untested experimental, reusable suborbital rocket, suggesting that if something is too congested for the more reliable vehicle an operating area might not be sparsely populated enough for a newer vehicle.  Other variables might be in play, as well, and I’d love to hear any thoughts on the matter.