One important thing also to know: Alt encoders have heaters in them. When the heater acts up or is intermittent, it gives erroneous alt reporting. This is especially true with GA a/c. If you go to Flightaware.com and pull up a known tail number and check out the flight profile, you may see the alt vary for a specific flt. That is the anomolly I am referring to. I am sure that most commercial and corporate a/c, especially those certified for RVSM probably don't suffer from this problem, so it was prolly a GA a/c.