SMA HAS ALTDEV DURING IFR FLT.

Narrative

DEPARTED BARTLESVILLE; OK (BVO) AFTER BEING CLRED TO LEWIS UNIVERSITY (LOT) AS FILED (DIRECT BVO VOR RNAV DIRECT JOT DIRECT). WX WAS CLR WITH UNLIMITED VISIBILITY AT TKOF AND DURING ENTIRE TRIP; EXCEPT FOR LAST 15 MINS OF 2.5 HR TRIP WHEN THERE WAS ABOUT A 200 FT THICK BROKEN DECK AT ABOUT 3000 FT MSL. THE NEBOUND FLT PARALLELED A COLD FRONT WHICH LAY ABOUT 100 NM E ALONG THE ENTIRE RTE AND THE ALTIMETER SETTING CHANGED CONTINUOUSLY FROM ABOUT 30.38 AT BVO TO 30.08 AREA SETTING AT LOT. THE AUTOPLT ALT HOLD AND RNAV TRACKING WERE USED AT CRUISE (11000 FT) THE ENTIRE RTE. TO CHANGE ALT AND CONTINUE RTE TRACKING; THE ALT HOLD MUST BE DISENGAGED AND THE AUTOPLT ALT TRIM ADJUSTED FOR AN ALT CHANGE. THEN THE ALT HOLD MUST BE MANUALLY RE-ENGAGED WHEN THE DESIRED ALT IS REACHED. AN ATC HDOF ABOUT 1 HR 45 MINS AFTER TKOF (NEAR QUINCEY; IL) RESULTED IN AN ALTIMETER SETTING FROM 30.26 TO 30.22. THE 30.22 WAS SET IN THE KOHLSMAN WINDOW (RESULTING IN 10900 FT INDICATED INSTEAD OF THE 11000 FT ASSIGNED) AND A SLIGHT NOSE UP TRIM WAS ADJUSTED TO CLB TO THE ASSIGNED 11000 FT. COCKPIT DISTRS (FUEL MGMNT; NAV; MAP READING; ETC) RESULTED IN A CLB THROUGH 11000 FT. ABOUT 10 MINS LATER ATC CURTLY REITERATED THE 30.22 ALTIMETER SETTING AND SAID I WAS 300 FT HIGH. AN IMMEDIATE CHK OF THE ALTIMETER SHOWED ME 100 FT HIGH AND I SO INFORMED ATC AND SAID I WAS RETURNING TO 11000 FT (WHICH I DID WITHIN 15- 30 SECONDS). HAVING THE ABILITY TO MONITOR THE XPONDER ALT SENT TO ATC; I CONFIRMED THE XPONDER ALT IS ABOUT 100 FT HIGHER THAN THE ALTIMETER. BOTH WERE CHKED WITHIN THE LAST 60 DAYS AS PART OF THE IFR BIENNIAL CALIBRATION CHK AND ARE APPARENTLY OK. IN THE FUTURE I WILL REQUEST CALIBRATION SO THAT THE BLIND ENCODER AND THE ALTIMETER AGREE TO LESS THAN 100 FT AND I WILL MAKE ALT CHANGES IN SUCH A MANNER THAT COCKPIT DISTRS ARE LESS LIKELY TO CAUSE FORGETTING TO RE-ENGAGE ALT HOLD.