ALTDEV ALT UNDERSHOT IN MISSED APCH PROC.

Narrative

WX AT THE DEST HAD CHANGED RAPIDLY IN THE LAST HR. RPTED WX WAS BELOW MINIMUMS FOR THE APCH AS RPTED BY ATIS. I DECIDED TO SHOOT THE APCH ANYWAY FOR PRACTICE AND TO CONFIRM THE WX WAS AS RPTED (-X 1/16 F). RWY ENVIRONMENT WAS NOT IN SIGHT AT DECISION HT AND THE MISSED APCH WAS INITIATED. AFTER THE CLB WAS STARTED AND TURN TO THE W INITIATED; I GLANCED DOWN AT MY KNEE BOARD TO CONFIRM THE CLRED ALT AS 2000 FT. I ALSO STARTED TO THINK ABOUT HOW I WAS GOING TO CONTACT THE PEOPLE THAT EXPECTED TO MEET ME AT TACOMA NARROWS ARPT. WHEN I LOOKED UP I READ THE ALTIMETER AS PASSING THROUGH 2000 AND STOPPED MY CLB AND DSNDED TO WHAT I THOUGHT WAS MY CLRED ALT. I HAD ALSO SWITCHED FROM TWR FREQ DURING THIS TIME AND CHKED IN WITH DEP AS LEVEL AT 2000 FT. THEY QUICKLY REPLIED WITH 'CONFIRM LEVEL 2000 FT' AND THEN RPTED AN ALT ALERT AND A COMMAND TO 'CLB IMMEDIATELY TO 2000 FT.' AT THIS TIME I AGAIN LOOKED AT THE ALTIMETER AND REALIZED THAT I WAS ACTUALLY LEVEL AT 1000 FT. CLB TO 2000 FT WAS UNEVENTFUL AND THE FLT CONTINUED TO THE ALTERNATE AIRFIELD (BFI). HOW DID IT HAPPEN: PLT DISTRACTED FROM FLYING AND SCANNING INSTS TO CONFIRM MISSED APCH ALT AND BY CONCERN ABOUT CONTACTING PEOPLE WAITING FOR THE PLT AT THE DEST. ALTIMETER IS CLASSIC CLOCK STYLE WITHOUT A NUMERICAL DRUM READOUT. I UNDERSTAND NOW WHY MANY PEOPLE HAVE MISREAD THIS TYPE OF ALTIMETER IN THE PAST. CLOUD TOPS AT THE OM WERE ABOUT 2000 FT. TOPS OVER THE ARPT WERE JUST BELOW 1000 FT. WHEN I BROKE OUT ON TOP AFTER THE MISSED APCH; I ASSUMED I MUST BE ABOUT 2000 FT. IN FACT I THOUGHT TO MYSELF; BETTER CHK YOUR ALT. MY MIND EXPECTED TO SEE 2000 FT AND WHEN I SAW THE BIG HAND PASSING THROUGH 12 O'CLOCK I THOUGHT I WAS PASSING MY ASSIGNED ALT.