We’ve all read and sort of absorbed the minimum safe altitude numbers in 14 CFR 91.119 and generally know that we’re supposed to stay 500 feet above the ground and 1,000 feet above congested areas. However, the regulation is a little more complex than those basic numbers.
The problem is “congested area.” Rather than publish a definition so pilots can know how to shape their aeronautical behavior, the FAA purposefully doesn’t—it comes up with its definition on a case-by-case basis. The FAA says it does that so it can balance the pilot’s interests with the need to protect persons and property. In enforcement actions, the FAA has successfully declared that a congested area includes a group of people on an airport ramp, sunbathers on a beach, a small subdivision covering less than a quarter mile, and traffic on an Interstate highway.
If there is a group of people on the ramp watching airplanes, the FAA has considered that group to make the area “congested.” That triggers the clearance distances of 14 CFR 91.119(b). If the pilot is within 2,000 feet horizontally, he or she must be flying 1,000 feet above the highest obstruction unless it’s necessary for taking off or landing. The FAA has long held that a low pass is not necessary for taking off or landing (and if the gear and flaps were up on final, the FAA has taken the position that the pilot wasn’t approaching to land and then executed a go-around). So, if a group of people is hanging around the ramp at a fly-in and you want to make a low pass down the runway, make sure the runway is more than 2,000 feet away—although you may still be subject to the 500-foot minimum altitude regulation. If no one is around and the runway area is considered sparsely populated, the only minimum altitude is to be high enough to land safely if the engine quits; however, you still have to be more than 500 feet horizontally from vehicles and structures—including an airplane parked near the runway.
Today, everyone has a cellphone camera, and those images will be used against you if you are charged with illegal low flying. In addition, the FAA will subpoena your GPS data and use it.