AN ACR B757-200 FLC HAS A NOSE DOWN TRIM INPUT FROM AN UNDETERMINED SOURCE CAUSING THE DSCNT RATE TO INCREASE TO APPROX 10000 FPM AND THE AIRSPD TO INCREASE FROM 280 KIAS TO 350 KIAS. THE FLC MANUALLY RECOVERED.

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DSNDING THROUGH 13000 FT; 280 KTS; WE WERE RECLRED TO 2600 FT. ANTICIPATING A STRAIGHT-IN ILS TO RWY 17L AND ONLY 40 MI OUT; I EXTENDED THE SPOILERS AND GAVE 2 QUICK CLICKS OF NOSE DOWN TRIP. THE NOSE PITCHED DOWN; BUT CONTINUED PITCHING DOWN STEEPLY. I FELT NOSE DOWN FORCES INCREASING AS I BEGAN PULLING BACK. THINKING A SPOILER MALFUNCTION; I GRADUALLY RETRACTED THE SPOILERS. NOSE DOWN FORCE BEGAN DECREASING AND I INITIATED RECOVERY. FLT RECORDER DATA SHOWED 10000 FPM DSCNT MAX; 350 KTS AND 15 DEGS DOWN BODY ANGLE. RECOVERY COMPLETED BY 8500 FT. EVENT LASTED 32 SECONDS. CONTINUED TO NORMAL APCH AND LNDG AT PIE. EMER NOT DECLARED. FLT RECORDER DATA; PRESENTED NEXT DAY IN SDF; SHOWED CONTINUOUS 6 SECONDS OF NOSE DOWN TRIM INPUTTED AFTER MY 2 CLICKS AT SPOILER EXTENSION. MOVEMENT WAS TOO SLOW TO BE AUTOPLT. DATA DOES NOT SHOW IF NOSE DOWN SIGNAL FROM CTL COLUMN TRIM SWITCHES; PEDESTAL ALT ELECTRICAL TRIM LEVERS; OR SOMEWHERE ELSE (EG; MALFUNCTION). I DID NOT MOVE COLUMN SWITCHES 6 SECONDS DOWN. COMPANY THEORIZING I ACCIDENTALLY MOVED ALT LEVERS OR PEDESTAL AS I EXTENDED SPOILERS. I'VE GONE TO THE PLANE -- MY ARM IS ALMOST 4 INCHES AFT OF TRIM LEVERS WITH SPOILER HANDLE AFT. COMPANY HAD INCIDENT 3-4 YRS AGO OF SIMILAR INCIDENT WHERE CAPT MOVED ALT TRIM SWITCHES WHEN EXTENDING SPOILERS (EARLIER TAIL NUMBER PLANE). THEIR SCENARIO OF ME MOVING STIFF LEVERS IS ERGONOMICALLY IMPOSSIBLE. AIRPLANE WAS TO BE TEST FLOWN YESTERDAY (FEB/SAT/97). AS OF SUNDAY; MAINT HAD FOUND NOTHING WRONG. I'M AWAITING FURTHER INFO/ACTION FROM COMPANY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE CAPT SAID THAT THE INITIAL FOCUS WAS ON WHETHER HE HAD ACCIDENTALLY ENGAGED THE MANUAL TRIM PADDLES AS HE DEPLOYED THE SPD BRAKES. THIS LATER MODEL B757-200 DID NOT HAVE THE PADDLES; ONLY ALTERNATE TRIM SWITCHES; SO THIS TYPE OF ACCIDENTAL ACTUATION WAS PHYSICALLY IMPOSSIBLE; THE RPTR SAID. THE FLT TEST CREW WAS ABLE TO REPLICATE THE FLT PROFILE BY DRIVING THE TRIM DOWN FOR 6 SECONDS; BUT THE SOURCE FOR THE TRIM ACTUATION IN THIS EVENT WAS NOT DETERMINED. THIS PARTICULAR ACFT DOES NOT HAVE THE FULL MONITORING RECORDER THAT THE NEWEST B757'S HAVE SO ALTHOUGH IT SHOWED THAT THE TRIM WAS DRIVING THE CAUSE WAS NOT RECORDED.