Date: 201907
Aircraft: B767 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight phase: Other All
Aircraft component: Horizontal Stabilizer Trim
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Maintenance
Detector: Person Flight Crew
When detected: Taxi
Result: General Work Refused
Contributing factors: Aircraft; Procedure
Primary problem: Ambiguous

Narrative: UNSCHED STAB TRIM: On the first occurrence; I was assigned to operate a delayed departure from the day prior. Got to the aircraft and all preflight items and procedures were completed. ZZZ was experiencing multiple delays due to thunderstorms and the ramp was closed for some time. Once we were able to depart; we pushed back; started both engines and started to taxi. Upon moving barely 10 feet on the taxi out we heard a Master Caution Tone and glanced the EICAS message UNSCHED STAB TRIM. The message quickly disappeared and initially everyone in the cockpit thought maybe we hit a bump on the taxi out that triggered the nuisance message as there was no change in the pitch trim. After moving another couple of feet; the same message UNSCHED STAB TRIM appeared at this point in time we all decided it is best we get in contact with Maintenance Control for further troubleshooting. I coordinated with the Ground/Ramp Controllers to find a spot to we could hold while contacting Maintenance Control at this point we also submitted and logbook entry. We contacted Maintenance Control and explained the situation and we told them we did not notice any STAB Trim movement to they then gave us instructions to perform a system reset. After complying with Maintenance Control's instructions they signed the A/C off and sent us a new release. Maintenance Control also mentioned that there was no history of this issue on the A/C. The relief pilot suggested we review the actual QRH procedure in the event that this did happen in flight and so we did take a look at it together. We then taxied out for the next 30 minutes and everything was normal and we had a very uneventful flight. I was later scheduled to operate a second flight with no revenue passengers. All preflight items were completed and we pushed back; started both engines; and started taxing. After about 3 Minutes of taxiing we got a Master Caution Light; Aural Tone and the EICAS message UNSCHED STAB TRIM. The Captain then instructed me to coordinate with Ground to hold somewhere for troubleshooting and to contact Maintenance Control. I did this and we were instructed to hold in a de-ice bay. We contacted Maintenance Control and we were instructed to run the same reset procedure as we did on the first flight. The Captain looked at both FO's [First Officers] and asked if we were comfortable; we said all said no; we would rather go back to the blocks and have MX [Maintenance] take a closer look so the Captain and Maintenance Control agreed and we did a gate return. Upon getting to the gate Local Maintenance ran some diagnostics and mentioned that they are seeing some sort of fault code and they advised us we are getting a nuisance message. They released the aircraft sent us a new release and we pushed back again. We taxied out and departed; everything was normal. On the climb out passing around 12;000 feet we got the Master Caution Light; Aural Tone and the EICAS message UNSCHED STAB TRIM. The Captain disconnected the AP and manually flew the aircraft. He then passed the airplane to me while he and the Relief Pilot looked at the QRH. The QRH stated under the Condition Line 'Stabilizer Movement occurs without signal to trim.' This condition did not exist as the STAB trim did not move so we turned on the autopilot and monitored the system. At this point we decided we will not cross the North Atlantic with a situation like this as it is distracting and there is clearly something wrong with the aircraft. I flew the airplane and coordinated a level off at FL240 and the Captain contacted Dispatch and Maintenance Control via SATCOM call. At this point I was focused on ATC radio communications and flying the aircraft so I was not monitoring the call with Maintenance Control/Dispatch. After the call the Captain and Dispatch and his First Officers decided we need to divert and we ultimately agreed on a divert airport. The Captain took the airplane over and I started to get set up for ZZZ1 and coordinated withATC. During this flight the Master Caution came on multiple times startling us as a crew which was very distracting. On approach into the divert airport while the gear was transitioning to down and locked; we got the Master Caution Light; Aural Tone and the EICAS message UNSCHED STAB TRIM I glanced at the trim indication and said the trim is moving the Captain disconnected the autopilot and flew it manually and said the controls feels normal. We then believed the trim movement was due to the reconfiguration of the airplane in that phase of flight. We did a normal underweight landing and taxied to the stand and accomplished all necessary post flight procedures. On the third occurrence we showed up and saw the mechanics working on the aircraft and completing their operational checks. They signed the aircraft off and sent us a release. We accomplished all preflight procedures and upon clearing the EICAS on the Before Push Checklist we noticed the UNSCHED STAB TRIM Message; there was no master caution or aural tone. We submitted a logbook entry and contacted Dispatch/Maintenance Control. The flight ended up cancelling and we went to the hotel.




Synopsis: B-767 first officer reported experiencing recurring EICAS warnings for UNSCHED STAB TRIM.