A B767-300 ON APPROACH AT 4000 FT ON AUTOPILOT ACFT PITCH OVER AT 6000 FT FPM. CREW UNABLE TO MOVE ELEVATORS AND GAINED CONTROL WITH STABILIZER TRIM.

Narrative

Z DURING APPROACH WHILE CONFIGURING FOR LANDING I NOTICED THAT THE ACFT WAS NOT RESPONDING TO AFDC PANEL IAS COMMANDS. THE SPEED WAS INCREASING INSTEAD OF DECREASING AS COMMANDED. IN AN ATTEMPT TO SLOW THE ACFT DEPLOYED THE SPEED BRAKES. AFTER A SHORT PERIOD THE VERTICAL INSTANTANEOUS CLIMB INDICATOR WENT TO 6000 DN. AT THAT TIME I DISCONNECTED THE AP AND PULLED ON THE YOKE. THE YOKE WOULD NOT MOVE FORWARD OR AFT. IT WAS JAMMED. THEN I APPLIED ANU STAB TRIM TO ARREST DESCENT. DESCENT STOPPED AT 3500 MSL. RETURNED TO 4000 USING STAB TRIM FOR PITCH CONTROL. ANNOUNCED TO THE CAPT THAT THE ACFT WAS BEING FLOWN WITH A JAMMED ELEVATOR-STAB TRIM ONLY FOR PITCH CONTROL. HE THEN TOOK CONTROL OF THE ACFT VERIFIED CONDITION OF ELEVATOR CONTROL AND MADE THE LANDING USING APPROXIMATELY 13 ANU UNITS OF STAB TRIM TO MAINTAIN PITCH ALTITUDE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED IN CONFIGURING THE ACFT FOR LANDING WITH THE AUTOPILOT ENGAGED THE IAS WAS LOWERED ON THE AUTOPILOT CONTROL PANEL; BUT THE ACFT SPEED UP. THE RPTR SAID THE AUTOPILOT WAS DISENGAGED AND THE ACFT PITCHED OVER 6000 FEET PER MINUTE. THE RPTR STATED BOTH PILOTS PULLED ON THE YOKE ACFT NOSE UP; BUT COULD NOT RECOVER ELEVATOR CONTROL. THE RPTR SAID THE ACFT CONTROL WAS GAINED WITH THE STABILIZER TRIM. THE RPTR SAID ON THE GROUND THE RIGHT HAND ELEVATOR THREE POWER CONTROL UNIT COMPARTMENTS WERE FULL OF ICE PREVENTING INPUTS TO THE ELEVATOR. THE RPTR STATED THE SUSPECTED CAUSE WAS AN AUXILIARY POWER UNIT INLET DOOR SEAL THAT ALLOWED WATER TO RUN DOWN INTO THE ELEVATOR POWER UNIT COMPARTMENTS. CALLBACK FROM ACN 506193. THE RPTR STATED THE ACFT WAS PARKED FOR SEVERAL HOURS PRIOR TO THE FLIGHT IN A TORRENTIAL RAINSTORM. THE RPTR SAID THE ACFT WAS DISPATCHED FOR A TEN-HOUR FLIGHT WITH THE RIGHT ELEVATOR COMPARTMENTS FULL OF WATER; WHICH FROZE. THE RPTR SAID WHEN THE AIRPLANE WAS BEING CONFIGURED FOR LANDING THE IAS WAS SELECTED FOR DESCENT; BUT THE ACFT SPEED INCREASED. THE RPTR STATED THE AUTOPILOT WAS DISCONNECTED AND THE ACFT PITCHED OVER AT A 6000 FT PER MINUTE RATE OF DESCENT. THE RPTR SAID BOTH PILOTS PULLED BACK ON THE YOKE WITH FULL FORCE RECORDED ON THE DIGITAL FLIGHT RECORDER AS 20 DEGREES OF ELEVATOR DEFLECTION; BUT THE ELEVATOR ONLY MOVED 1 DEGREE. THE RPTR SAID CONTROL WAS REGAINED WITH STABILIZER TRIM AT 3550 FT AND ACFT WAS RETURNED TO 3800 FT AND THEN LANDED USING 13 UNITS OF NOSE UP STABILIZER TRIM. THE RPTR STATED THAT FORTUNATELY ONE OF THE CREW WAS TRAINED IN THE TECHNIQUE OF LANDING WITH ONLY STABILIZER TRIM IMPUTTING SMALL GRADUAL CHANGES. THE RPTR SAID INVESTIGATION FOUND WATER IN THE ELEVATOR POWER UNIT COMPARTMENTS AND BELIEVES A DETERIORATED AUXILIARY POWER UNIT INLET DOOR SEAL WAS THE SOURCE OF THE WATER.