What is the FAA regulation on how low a plane can fly?

FAR 91.119 (14 CFR 91.119) establishes some basic minimums, as follows (paraphrased):

1. The airplane must always be high enough to land safely without harming people or property.

2. Over inhabited areas like cities and towns (the FAA interprets this very broadly), at least 1000 feet above the highest obstacle on the ground within a 2000-foot radius of the airplane. So if there is a 1100-foot skyscraper within 1500 feet of the aircraft, it must fly at 2100 feet above the ground or more.

3. Out in the middle of nowhere, at least 500 feet above the ground and 500 feet from any person or structure. If it's truly in a deserted area like a salt flat, or if it's over open water, there is no minimum.

4. Helicopters, hang gliders, ultralights, and some other craft are allowed exceptions.

There are also many other regulations that control minimum altitudes. The restrictions of 91.119 are the bare minimums; in other regulations, the minimums may be far higher. For example, aircraft flying under instrument flight rules (such as airliners) generally must fly considerably higher (typically thousands of feet at the very least).

It's not just an article or anything like that. It's called airspace. Different types of airspaces have different types of rules regarding how low you can fly depending on what is under you! Check out this Wikipedia article. It does a pretty good job at explaining the different types of airspaces in the US and rules for flying in them including minimum and maximum altitudes.

http://en.wikipedia.org/wiki/Airspace_class_(Unite...

Title 14, section 91.119:

http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ec...

I suspect that there are other sections that have some applicability as well.

they taught me in the academy to fly 100ft higher than the tallest building.... but my sargent was a crazy nut... don't take my advise :D