AN AVRO RJ-85; AFTER TKOF THE ACFT REMAINED IN THE GND MODE DUE TO STRUT EXTENSION FAILURE AND OVERRIDE USED TO RETRACT LANDING GEAR. WHEN STARTING A CRUISE DESCENT AT FL240 WITH THE AUTOPLT ENGAGED; THE STABILIZER TRIM RAN AWAY NOSE DOWN. AUTOPLT SWITCHED OFF AND THEN LOST NAIADS ON AUTOPLT MODE CONTROL PANEL. WITH AUTOPLT OFF YAW DAMPER COMMANDED A HARD LEFT. SOLUTION WAS YAW DAMPER OFF AND AUTOPLT ON TO EFFECT LANDING.

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AFTER TAKEOFF; GEAR LEVER OVERRIDE SWITCH HAD TO BE USED TO RETRACT THE LANDING GEAR (NEWLY INSTALLED GEAR SEALS ON THE STRUT PREVENTS THE GEAR STRUT FROM EXTENDING; THUS SQUAT PITCHED DON'T DISCONNECT; IE ACFT STILL THINKS IT'S ON THE GROUND). THIS CAUSED LOSS OF FLIGHT DIRECTORS AND AUTO THROTTLES. WE WERE ABLE TO GET THE GEAR BACK UP IN CRUISE. WHEN STARTING A CRUISE DESCENT INTO RDU; WITH AUTOPLT ON; OVERSPEED (VNE) OCCURRED; THE LVL CHG MODE CAME ON AUTOMATICALLY; BUT INSTEAD OF REDUCING AIRSPEED THE AUTOPLT SWITCHED THE NOSE FURTHER DOWN; INCREASING AIRSPEED EVEN MORE. PLT FLYING DISCONNECTED THE AUTOPLT; BEGAN HAND FLYING. THE TRIM WAS ACTUATED NOSE DOWN MOMENTARILY; BUT THE SERVO AND NOSE DOWN TRIM CONTINUED UNCONTROLLABLY; TRIM RAN AWAY NOSE DOWN. PF CALLED 'RUNAWAY TRIM;' PNF SWITCHED AUTOPLT MASTER SWITCH OFF. THIS CAUSED THE ENTIRE MASTER CONTROL PANEL (GLARE SHIELD) INCLUDING NAVAID FREQ BOXES TO GO BLANK. SINCE WE COULDN'T GET DOWN VISUALLY (WX 8 OVC) WE TRIED RESETTING THE AUTOPLT MASTER SWITCH TO ON. TRIM RAN AGAIN AND THE NOSE YAWED HARD LEFT (YAW DAMPER RAN AWAY?); SO THE AUTOPLT MASTER WAS SHUT OFF AGAIN. OUR QUICK REFERENCE HANDBOOK WAS CONSULTED BEFORE THE AUTOPLT MASTER WAS TURNED BACK ON; BUT WE HAVE NO PROCEDURE FOR THIS TYPE OF EVENT; THE MANUFACTURER DENIES THAT THIS CAN HAPPEN IN THEIR ACFT (BAE 146-AVRO); SO WE WERE MAKING UP THIS PROCEDURE AS WE WENT. FINALLY THE APPARENT SOLUTION WAS TO SHUT THE YAW DAMPER MASTER SWITCH OFF; THEN WE WERE ABLE TO TURN THE AUTOPLT MASTER BACK ON; WITHOUT THE YAW DAMPER AND ELECTRIC TRIM GOING CRAZY ON US; AND WERE ABLE TO GET DOWN (WE NEEDED THE AUTOPLT MASTER ON TO GET OUR NAIADS BACK(. UNEVENTFUL LANDING FOLLOWED. THE DFGS (DIGITAL FLT GUIDANCE COMPUTER) WAS DOWNLOADED AND ANALYZED; BUT NO EXPLANATION TO THE RUNAWAY TRIM EVENT WAS FOUND. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT AFTER TKOF THE AIRPLANE REMAINED IN THE GROUND MODE DUE TO FAILURE OF THE SHOCK STRUT TO EXTEND CAUSED BY STRUT SEAL REPLACEMENT. THE RPTR SAID THE GEAR WAS RAISED USING THE OVERRIDE CONTROL. THE RPTR STATED ON START OF CRUISE DESCENT THE AUTOPLT STARTED TO TRIM NOSE DOWN WITH NO CONTROL AND THEY ALMOST LOST THE AIRPLANE. THE RPTR SAID THE AUTOPLT WAS SWITCHED OFF. THE RPTR SAID WITH THE AUTOPLT OFF; THE YAW DAMPER COMMANDED A HARD LEFT AND THE AUTOPLT MODE CONTROL PANEL WENT BLANK. THE RPTR SAID THE NAVAIDS WERE LOST WHEN THE AUTOPLT MODE CONTROL PANEL WENT BLANK. THE RPTR STATED THE SOLUTION WAS TO SWITCH OFF THE YAW DAMPER AND TURN ON THE AUTOPLT; WHICH BROUGHT BACK THE NAVAIDS AND TRIM CONTROL. THE RPTR STATED WHEN ON THE GROUNDS MAINT SELF TESTED THE AUTOPLT AND FOUND NOTHING; BUT REPLACED THE DIGITAL FLT CONTROL COMPUTER. THE RPTER SAID MAINT SENT THE COMPUTER TO THE MANUFACTURER AS THIS WAS THE SECOND INCIDENT OF UNWANTED TRIM; ONLY IN A FULL NOSE UP. THE RPTR SAID AN EMER WAS NOT DECLARED AS THE TRIM COULD BE CONTROLLED MANUALLY; BUT IS NOT CERTAIN IF THE PICKLE SWITCH MAIN ELECTRIC ACTUATOR IS ALSO THE AUTOPLT ACTUATOR.