Narrative:

Conducting ILS runway 13R approach to bfi in seattle. At the LOM, checked in with bfi tower. Were told we were #2 behind another aircraft, we would receive landing clearance further in. At about 500-700 ft, received landing clearance and also that a cessna would be departing prior to our arrival. Due to some fog we were unable to see the cessna until about 200 ft AGL. The cessna had not begun its takeoff roll. Bfi tower was yelling at the cessna to takeoff and that we were still cleared to land. We had an approach reference speed of 145 KIAS and closing in on cessna so elected to go around. On the go around we were cleared to 6000 ft. Then climbing through 4000 ft we were cleared to 5000 ft. The first officer and autoplt was acting strange, trying to pitch aircraft excessively high. At 5000 ft we leveled off and I then began to load approach back into flight computer. At this point the captain disconnected autoplt and the excessive nose-up trim caused us to climb to 5300 ft. I called out the altitude deviation and the captain corrected back to 5000 ft. As the PNF, once you start loading the computer, you're out of the loop. The captain should have announced that he was disconnecting the autoplt. This would have given me notice to help him to scan the flight instruments. The rest of flight and next approach were normal.

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Original NASA ASRS Text

Title: B767-300 ACFT ON A GAR AND LEVELOFF, PF, CAPT, DISCONNECTED AUTOPLT AND ACFT PITCHED UP GAINING 300 FT BEFORE FLC COULD RETURN TO ASSIGNED ALT.

Narrative: CONDUCTING ILS RWY 13R APCH TO BFI IN SEATTLE. AT THE LOM, CHKED IN WITH BFI TWR. WERE TOLD WE WERE #2 BEHIND ANOTHER ACFT, WE WOULD RECEIVE LNDG CLRNC FURTHER IN. AT ABOUT 500-700 FT, RECEIVED LNDG CLRNC AND ALSO THAT A CESSNA WOULD BE DEPARTING PRIOR TO OUR ARR. DUE TO SOME FOG WE WERE UNABLE TO SEE THE CESSNA UNTIL ABOUT 200 FT AGL. THE CESSNA HAD NOT BEGUN ITS TKOF ROLL. BFI TWR WAS YELLING AT THE CESSNA TO TKOF AND THAT WE WERE STILL CLRED TO LAND. WE HAD AN APCH REF SPD OF 145 KIAS AND CLOSING IN ON CESSNA SO ELECTED TO GAR. ON THE GAR WE WERE CLRED TO 6000 FT. THEN CLBING THROUGH 4000 FT WE WERE CLRED TO 5000 FT. THE FO AND AUTOPLT WAS ACTING STRANGE, TRYING TO PITCH ACFT EXCESSIVELY HIGH. AT 5000 FT WE LEVELED OFF AND I THEN BEGAN TO LOAD APCH BACK INTO FLT COMPUTER. AT THIS POINT THE CAPT DISCONNECTED AUTOPLT AND THE EXCESSIVE NOSE-UP TRIM CAUSED US TO CLB TO 5300 FT. I CALLED OUT THE ALTDEV AND THE CAPT CORRECTED BACK TO 5000 FT. AS THE PNF, ONCE YOU START LOADING THE COMPUTER, YOU'RE OUT OF THE LOOP. THE CAPT SHOULD HAVE ANNOUNCED THAT HE WAS DISCONNECTING THE AUTOPLT. THIS WOULD HAVE GIVEN ME NOTICE TO HELP HIM TO SCAN THE FLT INSTS. THE REST OF FLT AND NEXT APCH WERE NORMAL.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.