The AV-20 fits into a standard 2.25-inch clock hole. This instrument is available in two versions, the AV-20 at $499 and the AV-20-S at $895. The difference? The AV-20-S has pitot/static input so it can display real airspeed, not GPS-derived groundspeed and it also functions as a capable attitude gyro. The entry-level AV-20 lacks the plumbing input and has no gyro, but it has multilevel clock and timer functions. Both are approved for installation under the FAA's NORSEE rule-non-required safety enhancing equipment. Technically, that means it can't replace a panel clock if the airplane requires one, nor can it provide a legal attitude source backup if one of those is needed.