Narrative:

During cruise flight at 16000 ft and approximately 310 KIAS; aircraft started shaking and fluttering without warning. The autoplt was engaged at the time. A few seconds later the master warning light and chime sounded; with the message 'pitch trim main inoperative' and 'pitch trim backup inoperative.' at that time I disconnected the autoplt using the quick disconnect button. The aircraft was very hard to control about the pitch axis; requiring excessive force inputs in order to maintain controled and level flight. Declared emergency and requested direct ZZZ. ATC approved and gave a descent clearance to 11000 ft. Airspeed was reduced; and while fighting with the vertical control of the aircraft; we began losing altitude in a shallow descent. Very strong back pressure had to be kept on the controls to keep the descent shallow. Pitch trim indications were showing '1' on the EICAS. The first officer then ran the appropriate QRH -- 'pitch trim main inoperative.' during the descent to 11000 ft; I don't remember the exact airspeed; the EICAS messages cleared; trim command was re-established; and pitch control returned to seemingly normal. We did not go through the circuit breakers as suggested by the QRH since EICAS had cleared. At this point; having already declared the emergency; begun our descent and questioning our controllability; I determined that landing at ZZZ would be the best course of action. We continued the flight to ZZZ normally; with a normal descent; visual approach; and landing on runway 35L. The flight attendant was called and told of the diversion; and that we had some EICAS messages show up; and to prepare the cabin for landing. The autoplt was not re-engaged during the remainder of the flight; nor were the speed brakes used. The emergency crews at ZZZ were present during our landing and followed us to our gate. A maintenance write-up was entered in the logbook. I spoke with dispatch and maintenance control once on the ground. We were assigned a new aircraft and continued the flight.supplemental information from acn 712019: we were at cruise flight at 16000 ft about 20 NM southwest of ZZZ. It was a calm and mostly clear night. Suddenly the airplane shook violently; followed by a master warning light and chime. Immediately after that we got two EICAS messages of 'pitch trim main inoperative' and 'pitch trim backup inoperative.' the captain was the PF and took control of the airplane by disconnecting the autoplt. As I reached for the QRH he said the airplane is very hard to control and that we need to get it on the ground now and to declare an emergency. I called center and declared an emergency. Since we had just flown over ZZZ we decided to turn around and land there. ATC cleared us direct to ZZZ and gave us a descent clearance to 11000 ft. I continued to go through the QRH checklist. As I was going through the QRH; the 2 EICAS messages had cleared. I asked the captain if the controls felt any better and he stated they seemed normal again. Since the EICAS messages went away; we did not attempt to reset the pitch trim circuit breakers. Having already declared an emergency and not fully sure of the status of our flight controls we decided to continue to ZZZ and land the airplane. I told ATC the information that they requested (number of persons on board and fuel remaining). We notified the flight attendant of the situation and told her to prepare the cabin for landing in ZZZ. I went through the appropriate checklists (descent; approach and landing). We got cleared for the runway 35L visual approach by approach. A normal landing had been accomplished. Emergency vehicles met us on the ground and followed us to the gate. The passenger were deplaned safely by jetway. I did a post flight inspection and did not notice anything wrong with the airplane. All pitch trim systems operated properly. We notified maintenance control and dispatch of the situation. Later on that evening we changed airplanes and continued flight.

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Original NASA ASRS Text

Title: AN EMB145 FLT CREW EXPERIENCED AIRFRAME VIBRATION IN CRUISE FLT; AND PITCH TRIM WARNING LIGHTS ILLUMINATED. THEY DECLARED AN EMER AND DIVERTED TO NEAREST SUITABLE ARPT.

Narrative: DURING CRUISE FLT AT 16000 FT AND APPROX 310 KIAS; ACFT STARTED SHAKING AND FLUTTERING WITHOUT WARNING. THE AUTOPLT WAS ENGAGED AT THE TIME. A FEW SECONDS LATER THE MASTER WARNING LIGHT AND CHIME SOUNDED; WITH THE MESSAGE 'PITCH TRIM MAIN INOP' AND 'PITCH TRIM BACKUP INOP.' AT THAT TIME I DISCONNECTED THE AUTOPLT USING THE QUICK DISCONNECT BUTTON. THE ACFT WAS VERY HARD TO CTL ABOUT THE PITCH AXIS; REQUIRING EXCESSIVE FORCE INPUTS IN ORDER TO MAINTAIN CTLED AND LEVEL FLT. DECLARED EMER AND REQUESTED DIRECT ZZZ. ATC APPROVED AND GAVE A DSCNT CLRNC TO 11000 FT. AIRSPD WAS REDUCED; AND WHILE FIGHTING WITH THE VERT CTL OF THE ACFT; WE BEGAN LOSING ALT IN A SHALLOW DSCNT. VERY STRONG BACK PRESSURE HAD TO BE KEPT ON THE CTLS TO KEEP THE DSCNT SHALLOW. PITCH TRIM INDICATIONS WERE SHOWING '1' ON THE EICAS. THE FO THEN RAN THE APPROPRIATE QRH -- 'PITCH TRIM MAIN INOP.' DURING THE DSCNT TO 11000 FT; I DON'T REMEMBER THE EXACT AIRSPD; THE EICAS MESSAGES CLRED; TRIM COMMAND WAS RE-ESTABLISHED; AND PITCH CTL RETURNED TO SEEMINGLY NORMAL. WE DID NOT GO THROUGH THE CIRCUIT BREAKERS AS SUGGESTED BY THE QRH SINCE EICAS HAD CLRED. AT THIS POINT; HAVING ALREADY DECLARED THE EMER; BEGUN OUR DSCNT AND QUESTIONING OUR CONTROLLABILITY; I DETERMINED THAT LNDG AT ZZZ WOULD BE THE BEST COURSE OF ACTION. WE CONTINUED THE FLT TO ZZZ NORMALLY; WITH A NORMAL DSCNT; VISUAL APCH; AND LNDG ON RWY 35L. THE FLT ATTENDANT WAS CALLED AND TOLD OF THE DIVERSION; AND THAT WE HAD SOME EICAS MESSAGES SHOW UP; AND TO PREPARE THE CABIN FOR LNDG. THE AUTOPLT WAS NOT RE-ENGAGED DURING THE REMAINDER OF THE FLT; NOR WERE THE SPD BRAKES USED. THE EMER CREWS AT ZZZ WERE PRESENT DURING OUR LNDG AND FOLLOWED US TO OUR GATE. A MAINT WRITE-UP WAS ENTERED IN THE LOGBOOK. I SPOKE WITH DISPATCH AND MAINT CTL ONCE ON THE GND. WE WERE ASSIGNED A NEW ACFT AND CONTINUED THE FLT.SUPPLEMENTAL INFO FROM ACN 712019: WE WERE AT CRUISE FLT AT 16000 FT ABOUT 20 NM SW OF ZZZ. IT WAS A CALM AND MOSTLY CLEAR NIGHT. SUDDENLY THE AIRPLANE SHOOK VIOLENTLY; FOLLOWED BY A MASTER WARNING LIGHT AND CHIME. IMMEDIATELY AFTER THAT WE GOT TWO EICAS MESSAGES OF 'PITCH TRIM MAIN INOP' AND 'PITCH TRIM BACKUP INOP.' THE CAPT WAS THE PF AND TOOK CTL OF THE AIRPLANE BY DISCONNECTING THE AUTOPLT. AS I REACHED FOR THE QRH HE SAID THE AIRPLANE IS VERY HARD TO CONTROL AND THAT WE NEED TO GET IT ON THE GND NOW AND TO DECLARE AN EMER. I CALLED CTR AND DECLARED AN EMER. SINCE WE HAD JUST FLOWN OVER ZZZ WE DECIDED TO TURN AROUND AND LAND THERE. ATC CLRED US DIRECT TO ZZZ AND GAVE US A DSCNT CLRNC TO 11000 FT. I CONTINUED TO GO THROUGH THE QRH CHKLIST. AS I WAS GOING THROUGH THE QRH; THE 2 EICAS MESSAGES HAD CLEARED. I ASKED THE CAPT IF THE CTLS FELT ANY BETTER AND HE STATED THEY SEEMED NORMAL AGAIN. SINCE THE EICAS MESSAGES WENT AWAY; WE DID NOT ATTEMPT TO RESET THE PITCH TRIM CIRCUIT BREAKERS. HAVING ALREADY DECLARED AN EMER AND NOT FULLY SURE OF THE STATUS OF OUR FLT CTLS WE DECIDED TO CONTINUE TO ZZZ AND LAND THE AIRPLANE. I TOLD ATC THE INFO THAT THEY REQUESTED (NUMBER OF PERSONS ON BOARD AND FUEL REMAINING). WE NOTIFIED THE FLT ATTENDANT OF THE SITUATION AND TOLD HER TO PREPARE THE CABIN FOR LNDG IN ZZZ. I WENT THROUGH THE APPROPRIATE CHKLISTS (DSCNT; APCH AND LNDG). WE GOT CLRED FOR THE RWY 35L VISUAL APCH BY APCH. A NORMAL LNDG HAD BEEN ACCOMPLISHED. EMER VEHICLES MET US ON THE GND AND FOLLOWED US TO THE GATE. THE PAX WERE DEPLANED SAFELY BY JETWAY. I DID A POST FLT INSPECTION AND DID NOT NOTICE ANYTHING WRONG WITH THE AIRPLANE. ALL PITCH TRIM SYSTEMS OPERATED PROPERLY. WE NOTIFIED MAINT CTL AND DISPATCH OF THE SITUATION. LATER ON THAT EVENING WE CHANGED AIRPLANES AND CONTINUED FLT.

Data retrieved from NASA's ASRS site as of January 2009 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.