Narrative:

Loss of aircraft separation occurred due to convective turbulence causing momentary altitude deviation of 150-200 ft from the assigned altitude of 6000 ft MSL. Other aircraft was at an assigned altitude of 7000 ft MSL. The PF was hand flying the aircraft; the autoplt was not engaged. The altitude capture function of the altitude preselect feature was not functioning correctly. It was noticed on the previous leg; and was intermittently inoperative. Crew corrected to 6000 ft immediately; and the rest of the flight was uneventful. Crew was notified on the loss of separation after landing at frg airport. Supplemental information from acn 703156: we did observe a slight deviation above 6000 ft as our altitude select did not capture at exactly 6000 ft. I do remember making the observation to my first officer that the altitude hold did not engage at 6000 ft. This happened at the end of a 13 hour duty day in cumulus clouds in an area of convective activity.

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Original NASA ASRS Text

Title: FALCON 50EX FLT CREW HAS AN ALTDEV RESULTING IN LOSS OF SEPARATION DURING DEP FROM IPT.

Narrative: LOSS OF ACFT SEPARATION OCCURRED DUE TO CONVECTIVE TURB CAUSING MOMENTARY ALTDEV OF 150-200 FT FROM THE ASSIGNED ALT OF 6000 FT MSL. OTHER ACFT WAS AT AN ASSIGNED ALT OF 7000 FT MSL. THE PF WAS HAND FLYING THE ACFT; THE AUTOPLT WAS NOT ENGAGED. THE ALT CAPTURE FUNCTION OF THE ALT PRESELECT FEATURE WAS NOT FUNCTIONING CORRECTLY. IT WAS NOTICED ON THE PREVIOUS LEG; AND WAS INTERMITTENTLY INOP. CREW CORRECTED TO 6000 FT IMMEDIATELY; AND THE REST OF THE FLT WAS UNEVENTFUL. CREW WAS NOTIFIED ON THE LOSS OF SEPARATION AFTER LNDG AT FRG ARPT. SUPPLEMENTAL INFO FROM ACN 703156: WE DID OBSERVE A SLIGHT DEV ABOVE 6000 FT AS OUR ALT SELECT DID NOT CAPTURE AT EXACTLY 6000 FT. I DO REMEMBER MAKING THE OBSERVATION TO MY FO THAT THE ALT HOLD DID NOT ENGAGE AT 6000 FT. THIS HAPPENED AT THE END OF A 13 HR DUTY DAY IN CUMULUS CLOUDS IN AN AREA OF CONVECTIVE ACTIVITY.

Data retrieved from NASA's ASRS site as of January 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.