Here are some interesting tidbits I was able to find:
General Aviation Operations Inspector's Handbook, Order 8700.1 wrote:The congested nature of an area is defined by what exists on the surface, not the size of the area. While the presence of the nonparticipating public is the most important determination of congested, the area may also be congested with structures or objects. An area considered congested for airplane operations could be equally congested for helicopters. If an airplane flying over a congested area at less than 1,000 feet above ground level (AGL) is in violation of 14 CFR § 91.119(b), the area may also be a congested area for a helicopter conducting external load operations. However, the most important word in this concept is 'over.' Helicopters can operate over relatively small uncongested areas because of their maneuvering abilities.
(b) Densely Populated Area. Title 14 CFR §§ 91.313 and 133.45(d) use the term "densely populated" area. Those areas of a city, town, or settlement that contain a large number of occupied homes, factories, stores, schools, and other structures are considered densely populated. Additionally, a densely populated area may not contain any buildings but could consist of a large gathering of persons on a beach, at an airshow, at a ball game, or at a fairground. NOTE: While the presence of the nonparticipating public is the most important determination of congested, this definition also applies to structures, buildings and personal property. The congested nature of an area is defined by what exists on the surface, not the size of the area.
European Regulations (JAR-OPS 3) wrote:
Congested areas,as defined in JAR-OPS 3, are essentially any densely populated town or city where no open spaces exist to permit a safe emergency landing in the event of an engine failure.
FAA wrote:There is no regulatory definition of 'congested area'. Administrative case law has determined what is congested on a case-by-case basis. [Case references are available on request]).
The public should be aware that an area does not have to be completely free of persons or properties to be considered noncongested. Additionally, it is possible that small, noncongested areas as small as an acre or two, may allow aerobatics to be performed without violating 91.303's stipulations." DMH note: 91.303 prohibits aerobatics over any congested area of a city, town, or settlement or over an open air assembly of persons.
Another concept created by JAR-OPS 3 was that of "exposure times." Performance Class 1, Category A helicopters can fly over hostile and congested areas, but JAR-OPS 3 allows Performance 2, Category A helicopters to fly over hostile, non-congested areas with an exposure time, the length of which depends on a target engine failure probability of 5 x 10-8. This translates, in practical terms, to exposures ranging from a few seconds to several minutes (for a helicopter).
Fly-ul.com wrote:What is Uncontrolled Airspace? Uncontrolled airspace is the area in which air traffic control separation services are not provided.
This area is usually below 1,200 feet above ground level (AGL). When nearing airports with established instrument approaches, the ceiling of uncontrolled airspace usually lowers to 700 feet AGL, and, if a control zone exists, uncontrolled airspace remains outside of the control zone horizontal limits, thus putting the airport within controlled airspace. In some geographic areas, primarily west of the Mississippi River, uncontrolled airspace ceilings are above 1,200 feet AGL. This is an exception, rather than the rule. The ceiling of uncontrolled airspace may be determined by reference to Sectional Aeronautical Charts used for aviation (see item h., Airspace and the Chart).
http://www.risingup.com/fars/info/part91-119-FAR.shtml]RisingUp.com[/url] wrote:Except when necessary for takeoff or landing, no person may operate an aircraft below the following altitudes:
(a) Anywhere: An altitude allowing, if a power unit fails, an emergency landing without undue hazard to persons or property on the surface.
(b) Over congested areas: Over 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.
(c) Over other than congested areas: An altitude of 500 feet above the surface, except over open water or sparsely populated areas. In those cases, the aircraft may not be operated closer than 500 feet to any person, vessel, vehicle, or structure.
(d) Helicopters. Helicopters may be operated at less than the minimums prescribed in paragraph (b) or (c) of this section if the operation is conducted without hazard to persons or property on the surface. In addition, each person operating a helicopter shall comply with any routes or altitudes specifically prescribed for helicopters by the Administrator.
So, PPGs are not bound by any minimum altitude restrictions except when flying over certain areas like national parks and wildlife reserves. Additionally, there is no definition of "congested area", but the most prevalent factor is having enough time to make a safe forced landing (easy as pie with a PPG). Also,
"The congested nature of an area is defined by what exists on the surface, not the size of the area" and
"...the most important word in this concept is 'over.' Helicopters can operate over relatively small non congested areas because of their maneuvering abilities." Basically, the congestion of an area is not determined on a wide or general scale, but is based on the area
directly beneath you (not off to the side, not in front, not behind, etc). PPGs are tiny, and like helicopters, are very easily maneuvered. With that being said, you should be able to fly just about anywhere you want so long as there aren't many people or structures in a 500ft bubble around you. 1 building here, a couple cars there, NBD.
Back to the issue of flying under a radar, since the vast majority of aircraft are not allowed to fly under 500 ft, I wouldn't think many radars are set up to detect objects at that level. Some
are however, so as long as you stay close to the ground (like 50-200 feet) except for when necessary (when multiple structures/vehicles/people will be in your 500ft bubble) then you'd probably be ok, legally and otherwise.