Narrative:

After takeoff on runway from teb we were on the teb 5 departure which calls for runway heading until 1500 ft then a turn to 280 degrees. I was flying and chose to engage the FMS for the departure (honeywell fmz ver. 5.2) but not the VNAV portion; and also flew the aircraft manually. After what I thought was 1500 ft the FMS did not call for a turn to 280 degrees. The controller reminded us of the turn and also said we were only just barely beyond where we should have turned. At that point I noted that my altimeter actually said about 1490 ft whereas the copilot's said 1500 ft. Apparently the FMS must actually see exactly 1500 ft on the side selected in the fgp for it to recognize we're at 1500 ft and initiate the turn. The copilot; looking at his side; noted we were at 1500 ft and was about to remind me to turn when the controller spoke. I think I was expecting the FMS to start the turn momentarily. I began the turn nevertheless. However; the v-bars remained uncoordinated and we had to switch to from LNAV to heading mode to get them to move properly. (A slight tug on the yoke; we realized later; might have raised us that 10 ft and caused the FMS to turn in sync with us as well.) I have seen this kind of FMS issue before and should have briefed it and been more prepared for it. The FMS is a wonderful device but sometimes it's very literal. Ten ft may be within pts standards but not FMS standards. It's unwise to let the FMS do all the thinking and one should be prepared to downgrade to a lower level of instrument display at times; if necessary. FMS turns when reaching a predetermined altitude are one of these times.

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Original NASA ASRS Text

Title: GIV ON TEB SID OFF RWY 24 FAILS TO MAKE TURN TO 280 DEGS IN A TIMELY FASHION.

Narrative: AFTER TAKEOFF ON RWY FROM TEB WE WERE ON THE TEB 5 DEP WHICH CALLS FOR RWY HDG UNTIL 1500 FT THEN A TURN TO 280 DEGS. I WAS FLYING AND CHOSE TO ENGAGE THE FMS FOR THE DEP (HONEYWELL FMZ VER. 5.2) BUT NOT THE VNAV PORTION; AND ALSO FLEW THE ACFT MANUALLY. AFTER WHAT I THOUGHT WAS 1500 FT THE FMS DID NOT CALL FOR A TURN TO 280 DEGS. THE CTLR REMINDED US OF THE TURN AND ALSO SAID WE WERE ONLY JUST BARELY BEYOND WHERE WE SHOULD HAVE TURNED. AT THAT POINT I NOTED THAT MY ALTIMETER ACTUALLY SAID ABOUT 1490 FT WHEREAS THE COPILOT'S SAID 1500 FT. APPARENTLY THE FMS MUST ACTUALLY SEE EXACTLY 1500 FT ON THE SIDE SELECTED IN THE FGP FOR IT TO RECOGNIZE WE'RE AT 1500 FT AND INITIATE THE TURN. THE COPILOT; LOOKING AT HIS SIDE; NOTED WE WERE AT 1500 FT AND WAS ABOUT TO REMIND ME TO TURN WHEN THE CTLR SPOKE. I THINK I WAS EXPECTING THE FMS TO START THE TURN MOMENTARILY. I BEGAN THE TURN NEVERTHELESS. HOWEVER; THE V-BARS REMAINED UNCOORDINATED AND WE HAD TO SWITCH TO FROM LNAV TO HDG MODE TO GET THEM TO MOVE PROPERLY. (A SLIGHT TUG ON THE YOKE; WE REALIZED LATER; MIGHT HAVE RAISED US THAT 10 FT AND CAUSED THE FMS TO TURN IN SYNC WITH US AS WELL.) I HAVE SEEN THIS KIND OF FMS ISSUE BEFORE AND SHOULD HAVE BRIEFED IT AND BEEN MORE PREPARED FOR IT. THE FMS IS A WONDERFUL DEVICE BUT SOMETIMES IT'S VERY LITERAL. TEN FT MAY BE WITHIN PTS STANDARDS BUT NOT FMS STANDARDS. IT'S UNWISE TO LET THE FMS DO ALL THE THINKING AND ONE SHOULD BE PREPARED TO DOWNGRADE TO A LOWER LEVEL OF INSTRUMENT DISPLAY AT TIMES; IF NECESSARY. FMS TURNS WHEN REACHING A PREDETERMINED ALTITUDE ARE ONE OF THESE TIMES.

Data retrieved from NASA's ASRS site as of May 2009 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.