Even if you just fly VFR, having a tested and calibrated Altimeter and encoder is important.  Example: We were flying east bound near Palmdale, CA (just east of the LA basin) at 15,500' with flight following.  Several air carriers were desending west bound about a mile south, close enough for us to appreciate the rate of closure.  Center called "Hawker 12:00, 2 miles, 16,000".  It went from a dot, to what seemed like 100' or less directly over us, in about 3 seconds.  I'm sure it was 500', or was it??  There was no room for error, and little time to react.

Steve Colwell  Legacy