An AC690B failed to level off at assigned altitude of 12000 ft.

Attributes
ACN	428645
Time
Date	199902
Day	Tue
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : zob.artcc
State Reference	OH
Altitude	msl bound lower : 12000 msl bound upper : 12400
Environment
Flight Conditions	IMC
Weather Elements	Ice
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zob.artcc
Operator	general aviation : corporate
Make Model Name	Turbo Commander 690C
Operating Under FAR Part	Part 91
Flight Phase	climbout other
Flight Plan	IFR
Person 1
Affiliation	company : corporate
Function	flight crew : single pilot oversight : pic
Qualification	pilot : multi engine pilot : instrument pilot : atp pilot : cfi pilot : commercial
Experience	flight time last 90 days : 70 flight time total : 9600 flight time type : 400
ASRS Report	428645
Person 2
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Events
Anomaly	altitude deviation : overshoot non adherence : clearance
Independent Detector	aircraft equipment : altitude alert
Resolutory Action	flight crew : returned to original clearance flight crew : overrode automation
Supplementary
Problem Areas	Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

I had just leveled off at 12000 ft as I encountered icing (light) conditions. The climb was being conducted with autoplt engaged. At 11500 ft, I decreased rate of climb to 500 FPM and engaged altitude hold at 12000 ft. I then placed my attention on the overhead switch panel to select propeller deice and windshield heat and to verify that engine ignition was on. (Engine anti-ice is annunciated on a lower annunciator panel and was observed to be activated.) at that moment, the altitude alerter gave an aural alert that target altitude was exceeded and I looked at flight instruments to find the aircraft in an uncommanded nose high, wings level climb at 1100 FPM. I immediately took manual control of the yoke, pitching down and simultaneously disengaging the autoplt. I recovered at 12400 ft as ATC alerted me to traffic in my vicinity. I trimmed the aircraft and cautiously re-engaged the autoplt which functioned correctly for the duration of the flight. I believe this occurred because of the multi tasking involved with simultaneous leveling off and encounter with icing conditions. The altitude alerter served its purpose and this was a never before observed autoplt malfunction in this particular aircraft.

Original NASA ASRS Text

Title: AN AC690B FAILED TO LEVEL OFF AT ASSIGNED ALT OF 12000 FT.

Narrative: I HAD JUST LEVELED OFF AT 12000 FT AS I ENCOUNTERED ICING (LIGHT) CONDITIONS. THE CLB WAS BEING CONDUCTED WITH AUTOPLT ENGAGED. AT 11500 FT, I DECREASED RATE OF CLB TO 500 FPM AND ENGAGED ALT HOLD AT 12000 FT. I THEN PLACED MY ATTN ON THE OVERHEAD SWITCH PANEL TO SELECT PROP DEICE AND WINDSHIELD HEAT AND TO VERIFY THAT ENG IGNITION WAS ON. (ENG ANTI-ICE IS ANNUNCIATED ON A LOWER ANNUNCIATOR PANEL AND WAS OBSERVED TO BE ACTIVATED.) AT THAT MOMENT, THE ALT ALERTER GAVE AN AURAL ALERT THAT TARGET ALT WAS EXCEEDED AND I LOOKED AT FLT INSTS TO FIND THE ACFT IN AN UNCOMMANDED NOSE HIGH, WINGS LEVEL CLB AT 1100 FPM. I IMMEDIATELY TOOK MANUAL CTL OF THE YOKE, PITCHING DOWN AND SIMULTANEOUSLY DISENGAGING THE AUTOPLT. I RECOVERED AT 12400 FT AS ATC ALERTED ME TO TFC IN MY VICINITY. I TRIMMED THE ACFT AND CAUTIOUSLY RE-ENGAGED THE AUTOPLT WHICH FUNCTIONED CORRECTLY FOR THE DURATION OF THE FLT. I BELIEVE THIS OCCURRED BECAUSE OF THE MULTI TASKING INVOLVED WITH SIMULTANEOUS LEVELING OFF AND ENCOUNTER WITH ICING CONDITIONS. THE ALT ALERTER SERVED ITS PURPOSE AND THIS WAS A NEVER BEFORE OBSERVED AUTOPLT MALFUNCTION IN THIS PARTICULAR ACFT.

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.