Autoplt failed to release resulting in altitude overshoot of 600'.

Attributes
ACN	103978
Time
Date	198901
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : scy
State Reference	TX
Altitude	msl bound lower : 4400 msl bound upper : 5000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : dfw
Operator	general aviation : corporate
Make Model Name	Small Transport, Low Wing, 2 Turboprop Eng
Flight Phase	descent other
Route In Use	arrival star : star
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : commercial pilot : cfi pilot : instrument
Experience	flight time last 90 days : 110 flight time total : 8200 flight time type : 150
ASRS Report	103978
Person 2
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	flight crew : overcame equipment problem
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Altitude hold light and aural warning came on at 4960 (not unusual for this autoplt), but at 4900 there is still no pitch change. I waited too long. At 4800' I engaged the trim up, the autoplt disengaged, warning came on, but the forward pressure on the yoke continued. I was pulling against 'something' solid, with both hands. I tried the red autoplt disengage: no change. I released the wheel with my right, reached across my left arm to pull the electrical trim breaker, but before I was able to do so, the forward pressure released, gently and smoothly. I was almost to 4400' before the descent was stopped. It was about here before I heard the controller, and I could hear the alarm in his voice. He had probably called earlier, but I didn't hear him. Anyhow, I greatly appreciated the concern that sounded so genuine. Although I waited too long to disengaged the autoplt, I should have been able to recover the altitude within 250' of 5000'. The failure of the trim to let go of the controls still confuses me. I have not been able to duplicate it. When I started flying this aircraft several months ago, I was told by the pilot that this problem existed. I checked the altitude preselect several times and never had more than a 200' overshoot--and that only when I failed to add power for the preselected cruise speed in level flight (which I did in this case, 825 pounds/ft per side to maintain 180 KIAS, as instructed by ATC). If you have any other reports on the autoplt similar to this, I would appreciate seeing them or receiving some information. Callback conversation with reporter revealed the following: had not had any previous problems with the autoplt disconnecting when manual trim used. Three means of disconnect are provided for the autoplt, control wheel button, manual trim activation, automatic pilot control on the instrument panel and actually the fourth, use of the circuit breaker. Reporter admitted he became excited when the elevator control did not release and failed to use the autoplt control on the instrument panel. Advised the air load being carried on the elevator at the time of manual trim probably prevented the movement of the elevator. In reaching for the circuit breaker he relaxed the pressure and the aircraft recovered from the descent.

Original NASA ASRS Text

Title: AUTOPLT FAILED TO RELEASE RESULTING IN ALT OVERSHOOT OF 600'.

Narrative: ALT HOLD LIGHT AND AURAL WARNING CAME ON AT 4960 (NOT UNUSUAL FOR THIS AUTOPLT), BUT AT 4900 THERE IS STILL NO PITCH CHANGE. I WAITED TOO LONG. AT 4800' I ENGAGED THE TRIM UP, THE AUTOPLT DISENGAGED, WARNING CAME ON, BUT THE FORWARD PRESSURE ON THE YOKE CONTINUED. I WAS PULLING AGAINST 'SOMETHING' SOLID, WITH BOTH HANDS. I TRIED THE RED AUTOPLT DISENGAGE: NO CHANGE. I RELEASED THE WHEEL WITH MY RIGHT, REACHED ACROSS MY LEFT ARM TO PULL THE ELECTRICAL TRIM BREAKER, BUT BEFORE I WAS ABLE TO DO SO, THE FORWARD PRESSURE RELEASED, GENTLY AND SMOOTHLY. I WAS ALMOST TO 4400' BEFORE THE DSCNT WAS STOPPED. IT WAS ABOUT HERE BEFORE I HEARD THE CTLR, AND I COULD HEAR THE ALARM IN HIS VOICE. HE HAD PROBABLY CALLED EARLIER, BUT I DIDN'T HEAR HIM. ANYHOW, I GREATLY APPRECIATED THE CONCERN THAT SOUNDED SO GENUINE. ALTHOUGH I WAITED TOO LONG TO DISENGAGED THE AUTOPLT, I SHOULD HAVE BEEN ABLE TO RECOVER THE ALT WITHIN 250' OF 5000'. THE FAILURE OF THE TRIM TO LET GO OF THE CONTROLS STILL CONFUSES ME. I HAVE NOT BEEN ABLE TO DUPLICATE IT. WHEN I STARTED FLYING THIS ACFT SEVERAL MONTHS AGO, I WAS TOLD BY THE PLT THAT THIS PROB EXISTED. I CHKED THE ALT PRESELECT SEVERAL TIMES AND NEVER HAD MORE THAN A 200' OVERSHOOT--AND THAT ONLY WHEN I FAILED TO ADD PWR FOR THE PRESELECTED CRUISE SPD IN LEVEL FLT (WHICH I DID IN THIS CASE, 825 LBS/FT PER SIDE TO MAINTAIN 180 KIAS, AS INSTRUCTED BY ATC). IF YOU HAVE ANY OTHER RPTS ON THE AUTOPLT SIMILAR TO THIS, I WOULD APPRECIATE SEEING THEM OR RECEIVING SOME INFO. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: HAD NOT HAD ANY PREVIOUS PROBS WITH THE AUTOPLT DISCONNECTING WHEN MANUAL TRIM USED. THREE MEANS OF DISCONNECT ARE PROVIDED FOR THE AUTOPLT, CONTROL WHEEL BUTTON, MANUAL TRIM ACTIVATION, AUTO PLT CONTROL ON THE INSTRUMENT PANEL AND ACTUALLY THE FOURTH, USE OF THE CB. RPTR ADMITTED HE BECAME EXCITED WHEN THE ELEVATOR CONTROL DID NOT RELEASE AND FAILED TO USE THE AUTOPLT CONTROL ON THE INSTRUMENT PANEL. ADVISED THE AIR LOAD BEING CARRIED ON THE ELEVATOR AT THE TIME OF MANUAL TRIM PROBABLY PREVENTED THE MOVEMENT OF THE ELEVATOR. IN REACHING FOR THE CB HE RELAXED THE PRESSURE AND THE ACFT RECOVERED FROM THE DSCNT.

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