Aircraft autoplt has trim runaway and results in sharp nose up gaining 3000 ft of altitude.

Attributes
ACN	327477
Time
Date	199602
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : cmh
State Reference	OH
Altitude	msl bound lower : 5000 msl bound upper : 8000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	tracon : cmh
Operator	common carrier : air taxi
Make Model Name	Small Transport, Low Wing, 2 Turboprop Eng
Flight Phase	climbout : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : air taxi
Function	flight crew : first officer
Qualification	pilot : atp pilot : cfi
Experience	flight time last 90 days : 100 flight time total : 3000 flight time type : 700
ASRS Report	327477
Person 2
Affiliation	company : air taxi
Function	flight crew : captain oversight : pic
Qualification	pilot : commercial pilot : instrument pilot : atp
Experience	flight time last 90 days : 150 flight time total : 2900 flight time type : 800
ASRS Report	327579
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot non adherence : clearance non adherence : far other anomaly other
Independent Detector	other flight crewa
Resolutory Action	flight crew : overcame equipment problem flight crew : returned to intended course or assigned course flight crew : regained aircraft control
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

On the leveloff at 5000 ft MSL (assigned), we had an autoplt trim runaway. The aircraft violently pitched up and climbed approximately 3000 ft before we regained control of the aircraft. Disabling the autoplt circuit breaker solved the problem. We advised ATC of the problem and returned to cmh without incident. Supplemental information from acn 327579: at the initial leveloff, my copilot was having a problem maintaining 5000 ft MSL. He was attempting to engage the autoplt (altitude hold). I assumed that he was only trying to trim the aircraft. The autoplt fail light was flickering and I noticed the trim was rolling nose up, taking us further off our altitude. He said, 'we have a problem.' I saw he was fighting with the yoke. The trim continued to roll the aircraft nose up. I pushed the trim disconnect on my yoke and the trim continued to roll. I turned the trim off on the pedestal and the trim was still rolling. I told him to give me the aircraft controls so that he could pull the elevator trim circuit breaker. He gave me the controls, and I realized we had a serious problem. I couldn't hold enough force on the controls to hold altitude. He pulled the breaker and the aircraft was still climbing with both of us using amazing forward pressure on the controls. I alerted ATC that we needed to divert to cmh and that we were having a trim runaway. We turned the autoplt off and on with the autoplt controller and it finally stopped moving. Being unable to move the manual trim wheel before, we were now able to trim the aircraft manually back to level flight. We gained 3000 ft in the occurrence. We found out on the ground that the autoplt trim had failed and that control forces couldn't override the autoplt system. We were aware of the fact that our next step would have been to shut down the avionics or pull the avionics master circuit breaker. The time frame from start to finish was probably 1-2 mins.

Original NASA ASRS Text

Title: ACFT AUTOPLT HAS TRIM RUNAWAY AND RESULTS IN SHARP NOSE UP GAINING 3000 FT OF ALT.

Narrative: ON THE LEVELOFF AT 5000 FT MSL (ASSIGNED), WE HAD AN AUTOPLT TRIM RUNAWAY. THE ACFT VIOLENTLY PITCHED UP AND CLBED APPROX 3000 FT BEFORE WE REGAINED CTL OF THE ACFT. DISABLING THE AUTOPLT CIRCUIT BREAKER SOLVED THE PROB. WE ADVISED ATC OF THE PROB AND RETURNED TO CMH WITHOUT INCIDENT. SUPPLEMENTAL INFO FROM ACN 327579: AT THE INITIAL LEVELOFF, MY COPLT WAS HAVING A PROB MAINTAINING 5000 FT MSL. HE WAS ATTEMPTING TO ENGAGE THE AUTOPLT (ALT HOLD). I ASSUMED THAT HE WAS ONLY TRYING TO TRIM THE ACFT. THE AUTOPLT FAIL LIGHT WAS FLICKERING AND I NOTICED THE TRIM WAS ROLLING NOSE UP, TAKING US FURTHER OFF OUR ALT. HE SAID, 'WE HAVE A PROB.' I SAW HE WAS FIGHTING WITH THE YOKE. THE TRIM CONTINUED TO ROLL THE ACFT NOSE UP. I PUSHED THE TRIM DISCONNECT ON MY YOKE AND THE TRIM CONTINUED TO ROLL. I TURNED THE TRIM OFF ON THE PEDESTAL AND THE TRIM WAS STILL ROLLING. I TOLD HIM TO GIVE ME THE ACFT CTLS SO THAT HE COULD PULL THE ELEVATOR TRIM CIRCUIT BREAKER. HE GAVE ME THE CTLS, AND I REALIZED WE HAD A SERIOUS PROB. I COULDN'T HOLD ENOUGH FORCE ON THE CTLS TO HOLD ALT. HE PULLED THE BREAKER AND THE ACFT WAS STILL CLBING WITH BOTH OF US USING AMAZING FORWARD PRESSURE ON THE CTLS. I ALERTED ATC THAT WE NEEDED TO DIVERT TO CMH AND THAT WE WERE HAVING A TRIM RUNAWAY. WE TURNED THE AUTOPLT OFF AND ON WITH THE AUTOPLT CTLR AND IT FINALLY STOPPED MOVING. BEING UNABLE TO MOVE THE MANUAL TRIM WHEEL BEFORE, WE WERE NOW ABLE TO TRIM THE ACFT MANUALLY BACK TO LEVEL FLT. WE GAINED 3000 FT IN THE OCCURRENCE. WE FOUND OUT ON THE GND THAT THE AUTOPLT TRIM HAD FAILED AND THAT CTL FORCES COULDN'T OVERRIDE THE AUTOPLT SYS. WE WERE AWARE OF THE FACT THAT OUR NEXT STEP WOULD HAVE BEEN TO SHUT DOWN THE AVIONICS OR PULL THE AVIONICS MASTER CIRCUIT BREAKER. THE TIME FRAME FROM START TO FINISH WAS PROBABLY 1-2 MINS.

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.