Date: 201406
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Weather elements / visibility: Clear
Light: Daylight
Aircraft: PC-12
Operator: Air Taxi
Operating under: Part 135
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class E ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Horizontal Stabilizer Trim
Component problem: Malfunctioning
Reporter organization: Air Taxi
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Flight Instructor
Human factors: Workload; Situational Awareness; Confusion; Communication Breakdown
Second reporter function: Captain; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Loss Of Aircraft Control
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Provided Assistance; Aircraft Automation Overrode Flight Crew; Flight Crew Took Evasive Action; Flight Crew FLC Overrode Automation; General Declared Emergency; General Maintenance Action
Contributing factors: Aircraft; Chart Or Publication; Manuals
Primary problem: Aircraft

Narrative: Flight departed in VMC late afternoon with two passengers aboard. I was the pilot flying; and the Captain was the pilot monitoring. Climbing through approximately 9;000 FT; a tone that sounded like the gear warning horn began to sound continuously. The airplane was properly configured with the gear and flaps up; climb power set. There were no annunciators illuminated. The sound made it difficult to hear ATC transmissions and to communicate between us. As the Captain was looking into a possible cause; I noticed upon pulling the 'trigger' on the yolk to trim the aircraft; that this silenced the horn. We agreed that I should hold the trigger in a depressed position while hand flying to facilitate communications. Hand flying was necessary as the trigger serves to disengage the autopilot as well as to activate the trim. The Captain continued to look into a cause and corrective action for the sound; and we continued our climb toward our assigned altitude of 16;000 FT.Approximately 30 miles southwest of the airport climbing through approximately 15;000 FT; I felt the nose of the aircraft begin to pitch down. I resisted the nose down force and depressed the autopilot disengage switch; but the nose down forces continued to increase rapidly. I told the Captain that the aircraft was pitching down uncommanded; and noticed a 2000 FPM descent on my VSI. I continued to resist the nose down forces with both hands; and told the Captain I suspected a trim runaway. The Captain said 'my controls;' and took over flying the airplane. After relinquishing the controls; I pressed the 'trim interrupt' switch and referred to the Quick Reference Handbook for the proper checklist. The Captain retarded the throttle to reduce control forces; and continued to hold strong aft yolk pressure to try to maintain altitude.At about this time; Center addressed us saying our altitude was 13;000 FT; and I responded telling them we had a trim runaway and were addressing the problem. They assigned us a block altitude. The Captain and I exchanged flight controls again to enable him to pull the stabilizer trim circuit breaker per the checklist; located on the left side of the cockpit. When the Captain pulled the circuit breaker; the horn sound stopped. Continuing with the checklist; the Captain returned the Trim Interrupt Switch to the 'Normal' position; and I re-trimmed the aircraft using the alternate stabilizer trim. About this time; Center contacted us for a frequency change and informed us that they had declared an emergency on our behalf. We continued on our assigned course with the aircraft hand-trimmed for level flight at 13;000 FT. As I was the pilot flying; the Captain attempted several times to contact the company; but was unsuccessful. With the aircraft returned to a properly trimmed state; we elected to continue to our filed destination; and briefed our approach; agreeing that if either of us noticed any further abnormalities or difficulty controlling the aircraft; we would divert to a nearby larger airport. I set up for and flew an extended final leg. This allowed me to configure the aircraft early with gear down; flaps thirty degrees; and trimmed for near approach speed; assuring that the airplane handled normally and I could be assured of a safe landing. No further problems were encountered during our landing; taxi; or shutdown.I feel that the Captain and I handled the situation well. I suggested to the Captain in flight that we divert to a larger airport nearby; because they have a longer runway and more advanced accident response equipment than our destination. Since we had regained normal aircraft controllability; he stated that he preferred to go to our filed destination. I consented; feeling confident that I could fly the airplane safely. Ensuring I was in a stabilized descent on extended final as described above would serve as a final check that all factors were indicating a safe landing would be made. Despite this; diverting would have given us a greater margin for error. My decision to consent to go to our filed destination was affected by my perception that the Captain has final authority; and by my tendency to yield to authority figures; even when I may have doubts. I yielded to his judgment because I didn't want to cause a dispute over something that he may have seen as unimportant if I didn't feel sure that it would directly jeopardize the safety of flight. If I were Captain; however; I would have made a different decision because I feel that when safety is involved; a bigger margin for error is better than a smaller one. This serves as a reminder to me not to be afraid to stand up for myself when I am making a judgment on safety.

Callback: The reporter stated that they had never heard the gear warning horn sound because the conditions to alert have not been met while flying. This statement came about as a result of the Captain's comment. She is fairly certain the gear warning and trim alert horn sounds are not identical but she cannot say that for certain.

Second narrative: We determined that the primary stabilizer trim was the runaway item. Per the QRH; its circuit breaker was pulled; and we continued flying with the alternate stabilizer trim. Since the aircraft operated normally again; we elected to continue to our destination with an uneventful and safe landing. A bad relay caused the primary stabilizer trim to runaway.Misunderstanding of the warning horn. What I refer to as the gear warning horn; was in fact; the warning horns for a runaway stabilizer trim. In actuality; the 2 warning horns do sound very similar. It should be noted that there is only one sentence in the Pilatus PC-12 POH that referred to the warning horn for a runaway stabilizer trim; and nothing is noted in the QRH.To prevent future occurrences; commit to memory the runaway stabilizer trim procedure; and know the different warning horns; including the one for runaway stabilizer trim.


Synopsis: During climb through about 9;000 FT a PC-12 warning horn similar to the gear warning sounded; but with horizontal trim activation it ceased. At about 15;000 FT the aircraft began an uncommanded pitch over indicating a runaway stabilizer trim; so the QRH was completed; the warning silenced and the flight continued to the filed destination with alternate trim. A failed relay was found.