We know that a transponder must be calibrated every 2 years and signed off by a qualified avionics technician. I assume that if this is not done, the aircraft is unairworthy (maybe only in class B & C airspace where a transponder is required). I personally know of several pilots/owners who rarely seem to bother with this requirement. How serious is it? I think for IFR flying it is more serious, but it is also a requirement for VFR flying. Has anyone ever been ramp checked or been dinged for this? I don't believe you are required to carry the calibration documentation on the aircraft. No big deal here; I'm just curious.