Narrative:

After receiving updated altimeter setting, aircraft altitude was 100 ft low. Used manual pitch wheel of autoplt control to correct, but initial movement was incorrect (commanded descent vice climb). Noticing the incorrect input, the correct input (climb) was selected and an approximately 1000 FPM climb initiated. Upon reaching 4000 ft MSL, the altitude capture mode of the flight director was pressed. The aircraft began a series of 'hunting porpoises' to capture the altitude. The oscillations became dramatically divergent, finally resulting in a 20 degree nose up altitude, 800 ft high, and airspeed diminishing from 220 KIAS to 140 KIAS. The autoplt was deselected and the aircraft was brought manually under control and back to straight and level at 4000 ft MSL. Entire evolution lasted approximately 20 seconds. It is being determined whether there was a material failure of either the autoplt, flight director, or trim system, or if there was pilot error involved with engaging the altitude capture mode in a high rate of climb. The extreme oscillations and altitude deviation could have been prevented by the pilot taking action sooner and not allowing the aircraft to diverge so much, however the evolution happened very fast and only went through 5 oscillations in 20 seconds. If it is determined that the pilot was in error for engaging in the relatively high rate of climb, then better training and publication of such limits would have prevented the incident.

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Original NASA ASRS Text

Title: UC12B FLT CREW EXPERIENCES AUTOPLT INDUCED ALT EXCURSIONS.

Narrative: AFTER RECEIVING UPDATED ALTIMETER SETTING, ACFT ALT WAS 100 FT LOW. USED MANUAL PITCH WHEEL OF AUTOPLT CTL TO CORRECT, BUT INITIAL MOVEMENT WAS INCORRECT (COMMANDED DSCNT VICE CLB). NOTICING THE INCORRECT INPUT, THE CORRECT INPUT (CLB) WAS SELECTED AND AN APPROX 1000 FPM CLB INITIATED. UPON REACHING 4000 FT MSL, THE ALT CAPTURE MODE OF THE FLT DIRECTOR WAS PRESSED. THE ACFT BEGAN A SERIES OF 'HUNTING PORPOISES' TO CAPTURE THE ALT. THE OSCILLATIONS BECAME DRAMATICALLY DIVERGENT, FINALLY RESULTING IN A 20 DEG NOSE UP ALT, 800 FT HIGH, AND AIRSPD DIMINISHING FROM 220 KIAS TO 140 KIAS. THE AUTOPLT WAS DESELECTED AND THE ACFT WAS BROUGHT MANUALLY UNDER CTL AND BACK TO STRAIGHT AND LEVEL AT 4000 FT MSL. ENTIRE EVOLUTION LASTED APPROX 20 SECONDS. IT IS BEING DETERMINED WHETHER THERE WAS A MATERIAL FAILURE OF EITHER THE AUTOPLT, FLT DIRECTOR, OR TRIM SYS, OR IF THERE WAS PLT ERROR INVOLVED WITH ENGAGING THE ALT CAPTURE MODE IN A HIGH RATE OF CLB. THE EXTREME OSCILLATIONS AND ALTDEV COULD HAVE BEEN PREVENTED BY THE PLT TAKING ACTION SOONER AND NOT ALLOWING THE ACFT TO DIVERGE SO MUCH, HOWEVER THE EVOLUTION HAPPENED VERY FAST AND ONLY WENT THROUGH 5 OSCILLATIONS IN 20 SECONDS. IF IT IS DETERMINED THAT THE PLT WAS IN ERROR FOR ENGAGING IN THE RELATIVELY HIGH RATE OF CLB, THEN BETTER TRAINING AND PUB OF SUCH LIMITS WOULD HAVE PREVENTED THE INCIDENT.

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.