Narrative:

The aircraft, an mdt was climbing to FL190. We had the aircraft on autoplt during the climb. We had last talked to memphis center on 133.85 climbing through 16500', 45 mi west of nashville, tn. At 50' below FL190 we started to level off but the autoplt would neither respond to the controls nor disengage. We tried to contact memphis center to let them them know we were having problems with the aircraft but we could not contact them on any radio or frequency. We then pulled the circuit breaker but the autoplt would still not disengage. We tried to overpwr the autoplt but were afraid to overstress the control system. We finally got the autoplt to disengage by flipping the master switch repeatedly while hitting the control box with a flashlight. By this time we were at FL220 and now descending to the assigned altitude of FL190. At this time we were 118 mi past nashville and tried successfully to pick up indy center on 125.25. We told them of or problem and that we were under control. They said that they were concerned but that we had caused no problems and our radio xmissions were clear. We landed in charleston, wv, with no other incident and gave the problem to our mechanics. I believe the problem to be a simple mechanical failure of the autoplt and dirty connectors in the radio with the problem not to be repeated. Callback conversation with reporter revealed the following information. This is the only incident of this nature that the reporter has ever heard of and that west/O any maintenance activity the aircraft functioned normally after this flight. The recovery to assigned altitude was done by over pwring the autoplt and the reason they overshot so far was the surprise factor of the situation. All autoplts have been replaced with a newer type.

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Original NASA ASRS Text

Title: COMMUTER FRT ALT DEVIATION OVERSHOT WHEN AUTOPLT FAILED TO CAPTURE AND THE FLT CREW COULD NOT DISENGAGE BY NORMAL MEANS.

Narrative: THE ACFT, AN MDT WAS CLBING TO FL190. WE HAD THE ACFT ON AUTOPLT DURING THE CLB. WE HAD LAST TALKED TO MEMPHIS CTR ON 133.85 CLBING THROUGH 16500', 45 MI W OF NASHVILLE, TN. AT 50' BELOW FL190 WE STARTED TO LEVEL OFF BUT THE AUTOPLT WOULD NEITHER RESPOND TO THE CTLS NOR DISENGAGE. WE TRIED TO CONTACT MEMPHIS CTR TO LET THEM THEM KNOW WE WERE HAVING PROBS WITH THE ACFT BUT WE COULD NOT CONTACT THEM ON ANY RADIO OR FREQ. WE THEN PULLED THE CB BUT THE AUTOPLT WOULD STILL NOT DISENGAGE. WE TRIED TO OVERPWR THE AUTOPLT BUT WERE AFRAID TO OVERSTRESS THE CTL SYS. WE FINALLY GOT THE AUTOPLT TO DISENGAGE BY FLIPPING THE MASTER SWITCH REPEATEDLY WHILE HITTING THE CTL BOX WITH A FLASHLIGHT. BY THIS TIME WE WERE AT FL220 AND NOW DSNDING TO THE ASSIGNED ALT OF FL190. AT THIS TIME WE WERE 118 MI PAST NASHVILLE AND TRIED SUCCESSFULLY TO PICK UP INDY CTR ON 125.25. WE TOLD THEM OF OR PROB AND THAT WE WERE UNDER CTL. THEY SAID THAT THEY WERE CONCERNED BUT THAT WE HAD CAUSED NO PROBS AND OUR RADIO XMISSIONS WERE CLR. WE LANDED IN CHARLESTON, WV, WITH NO OTHER INCIDENT AND GAVE THE PROB TO OUR MECHS. I BELIEVE THE PROB TO BE A SIMPLE MECHANICAL FAILURE OF THE AUTOPLT AND DIRTY CONNECTORS IN THE RADIO WITH THE PROB NOT TO BE REPEATED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO. THIS IS THE ONLY INCIDENT OF THIS NATURE THAT THE RPTR HAS EVER HEARD OF AND THAT W/O ANY MAINT ACTIVITY THE ACFT FUNCTIONED NORMALLY AFTER THIS FLT. THE RECOVERY TO ASSIGNED ALT WAS DONE BY OVER PWRING THE AUTOPLT AND THE REASON THEY OVERSHOT SO FAR WAS THE SURPRISE FACTOR OF THE SITUATION. ALL AUTOPLTS HAVE BEEN REPLACED WITH A NEWER TYPE.

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.