In real flying, and more particularly in low-altitude GA aircraft, how much is a pilot allowed to deviate from cleared cruising altitude in controlled airspace before getting an earful from ATC? For instance, if a pilot is cleared for a cruising altitude of 4,000ft, are they allowed to stray within 3,900 and 4,100ft (i.e. +- 100ft)? What about 3,800 and 4,200ft?

My experience is that ATC usually will say something if you're off an assigned IFR altitude by more than 200', but instrument pilots are trained to be able to maintain +/-100' at least.A center controller's radar updates every 12 seconds or so, while approach radar udates every few seconds. Center controllers tend to manage large areas, so they might not notice immediately if you're off your altitude. Approach controllers manage smaller areas and will probably notice more quickly if you're off course or altitude.If you are VFR, you are expected to fly an appropriate VFR altitude for your magnetic course, within whatever restrictions ATC has given you (if any). If ATC hasn't given you a restriction, they simply expect you to maintain VFR - fly an appropriate altitude for your magnetic course.In Class B, ATC requires more accuracy from pilots because they are providing separation for all aircraft. If your lack of heading or altitude control creates a separation issue, you can be sure ATC will be on your case.If you ecounter strong updrafts and downdrafts that make it difficult to maintain a specific IFR altitude, you can request a block altitude. ATC will give a range of altitudes and this lets you roll with the punches in a 1000' or 2000' range.Hope that helps.John

Thanks for the detailed explanation John, much appreciated.

With a VFR clearance in controlled airspace You are usually given a "not above altitude" so "you are cleared to transit VFR not above 2000 feet on xyz pressure setting.Interesting point is that if you are given an altitude to fly what are the tolerances expected from a pure VFR non instrument rated pilot?Peter

VFR checkride is 150 feet as far as I remember off the top of my head. In IFR, I personally try my best to keep it within 10 feet. For further reading on VFR altitudes, check www.faa.gov for FAR