H25B crew had altitude overshoot when the autoplt malfunctioned.

Attributes
ACN	457217
Time
Date	199912
Day	Wed
Local Time Of Day	1801 To 2400
Place
State Reference	NV
Altitude	msl bound lower : 34700 msl bound upper : 35000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zla.artcc
Operator	general aviation : corporate
Make Model Name	HS 125 Series 700
Operating Under FAR Part	Part 91
Navigation In Use	other
Flight Phase	descent : vacating altitude
Flight Plan	IFR
Person 1
Affiliation	company : corporate
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 28 flight time total : 12540 flight time type : 17
ASRS Report	457217
Person 2
Affiliation	company : corporate
Function	flight crew : first officer
Qualification	pilot : multi engine pilot : commercial pilot : instrument
Events
Anomaly	aircraft equipment problem : critical altitude deviation : overshoot non adherence : published procedure non adherence : far non adherence : clearance
Independent Detector	other flight crewa other flight crewb
Resolutory Action	flight crew : returned to assigned altitude none taken : detected after the fact
Consequence	other
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

On descent, the autoplt captured the preselected altitude but then suddenly pitched the aircraft down. I was PNF and both of us grabbed the yoke to stop the descent and get back on altitude. We barely stopped the descent 300 ft below assigned altitude. The autoplt had failed to capture correctly on occasion in the past, but never went off altitude that quickly before. We will not use the altitude capture until this intermittent problem is resolved.

Original NASA ASRS Text

Title: H25B CREW HAD ALT OVERSHOOT WHEN THE AUTOPLT MALFUNCTIONED.

Narrative: ON DSCNT, THE AUTOPLT CAPTURED THE PRESELECTED ALT BUT THEN SUDDENLY PITCHED THE ACFT DOWN. I WAS PNF AND BOTH OF US GRABBED THE YOKE TO STOP THE DSCNT AND GET BACK ON ALT. WE BARELY STOPPED THE DSCNT 300 FT BELOW ASSIGNED ALT. THE AUTOPLT HAD FAILED TO CAPTURE CORRECTLY ON OCCASION IN THE PAST, BUT NEVER WENT OFF ALT THAT QUICKLY BEFORE. WE WILL NOT USE THE ALT CAPTURE UNTIL THIS INTERMITTENT PROB IS RESOLVED.

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.