I’ve been flying, and around aviation for many years, and don’t know of any such rules of thumb for visually estimating altitude. With practice, you can develope some skill, but I know of several instances where my altitude was badly estimated by people who should have aquired such skill.
However, knowing the FARs (federal aviation regulations) can be useful. Over a populated area, or gathering of people, fixed wing aircraft are required to maintain at least 1000’ altiude unless taking off or landing. Over sparsely populated areas they are to remain at least 500’ from any person, structure, or vehicle. Most pilots take that to mean that 500’ is minimum altiude over farm land and such. That altitude gives you few options in the event of engine failure, though, so most will fly higher unless giving cousin Bob an arial view of the family farm or such.
There is no minimum altitude requirement over bodies of water.
It is common at General Aviation (small) airports for aircraft to enter the downwind leg of their landing pattern ( parlell to runway where they will land, but going opposite direction) at 1000-1200’ AGL. Aircraft flying over the airport, but not intending to land (they make good landmarks, and it is generally safer to plan a flight where there are opportunities for unforseen landings) will cross over the airport above 2000’ AGL. Specific airports may have exceptions to the above due to geographic or leagle issues.
Over generally flat areas, small aircraft and helicopters tend to fly at under 4-5000’ AGL in my experience. Their engines lose performance with altitude, and climbing cost fuel and airspeed. If they are on a path that will require them to cross mountainous terrain, then they will not wait to get there to climb, so they will then be flying much higher.
Exeptions to these guidlines abound, and some pilots ignore them.
I know some Ag pilots (crop dusters) who don’t beleave in wasting fuel that will fly just above power line level, and one that normally flys between there and fence level.
Flying above 18,000MSL normally requires the aircraft to be on an instrument flight plan, and many small airplanes can’t fly that high anyway. Any operation above 14,000, or more than 1/2 hour above 12,500’MSL requires oxygen. Operation above 10,000 MSL requires a mode-C transponder. So over Iowa, it is a pretty safe bet that any small airplane is flying at under 10,000 MSL. In the mountain west, it is probably below 12,500’, and almost certainly below 14,000. Gliders use altitude as “fuel in the tank” and carry oxygen, so they will climb as high as thermals and regulation permit. In the mountain west, sailplanes (gliders) routinely climb to 18,000 MSL in summer.
Finally, small aircraft flying at level altitude have some rules of the road: Generally eastbound traffic is to fly at odd 1000’+500 altitudes, (9,500, 11,500 etc.) while generally westbound traffic is to fly at even 1000’+500 altitude. (8,500’, 10,500’ etc.) Note that these are MSL (mean sea level) figures not AGL (above ground level). This means that based on the direction of travel, if you can estimate the altitude to within 1000’ you can be reasonably sure that the airplane is flying at the altitude specified above. Instrument flight is at similar altitudes without the 500’ addition. So an expensive turboprop, or corporate jet is probably in one of those slots.
