B737 Undifferentiated or Other Model
Aircraft Equipment Problem Critical
Flight Crew Landed in Emergency Condition; General Declared Emergency
Aircraft
During cruise flight at FL360; the STAB OUT OF TRIM light illuminated. I transferred the controls to the First Officer (FO) and ran the QRH for the light. The stabilizer was not trimming and neither pilots yoke could control the trim electrically except for one very brief movement from the FO control wheel switch. It never responded again from either side. The checklist directed us to the Electric Trim Stabilizer Inoperative checklist. We also checked the circuit breakers to see if any were extended. All circuit breakers that weren't collared; were closed. We continued with the Electric Trim Stabilizer Inoperative checklist. Step 2 states: 'Use sufficient force to disengage the disengage clutch. Approximately 1/2 turn of the trim wheel may be needed.' It took both of us using a significant amount of force and approximately 1-1/2 to 2 turns of the trim wheel to get the clutch to disengage; significantly more than the QRH noted. After the clutch felt as if it disengaged; manual movement felt normal; although trimming the aircraft nose down felt as though it required slightly more effort. My FO and I spoke about this and attributed it to aerodynamic loads at altitude. We decided to check it throughout the flight regime to see if it changed. I informed ATC of our problem with the STAB trim and that we were not Reduced Vertical Separation Minimums (RVSM) compliant. ATC subsequently cleared us to FL280. During the descent I contacted ARINC to get a patch through to our dispatcher. Once on the line with dispatch he conferenced [Maintenance Control] in to discuss the problem. We discussed the issue and I asked our dispatcher to help me locate an airport that was VMC; since we would be hand flying the aircraft and executing a flaps 15 landing. I also started to pull weather for surrounding airports. The solution was to head to ZZZ. I advised them that if we had any further change in the manual trimming of aircraft; that I would be declaring an emergency. I then notified the flight attendants of what was going on (T.E.S.T) and that we would be landing in ZZZ. I made an announcement to the passengers; explaining why we were going to ZZZ and assuring them that everything was ok. After the conference call; we discussed the descent and approach into ZZZ; reviewed all of the deferred item checklists and checked the Non-Normal Configuration Landing Distance table. We then checked to see if the resistance to aircraft nose down trim (A.N.D.) had subsided; it had not and in fact felt as if it were binding. Explanation: we would manually trim A.N.D. 1/4 to 1/2 of a full turn and the wheel would return to its starting point as pressure was slowly released from the handle. Note: if we were to let go of the handle the wheel would have surely 'snapped' back. This explanation is to illustrate the amount of resistance to movement to A.N.D. We checked aircraft nose up (A.N.U.) and it was markedly normal and very smooth. Trimming the aircraft A.N.D. was possible but required more force to effectuate movement of the stabilizer; also we would have to move the trim past the point where we wanted it to stop. We discussed this further and decided that there was potentially another issue. During this time; the aircraft was stable and completely controllable albeit requiring a bit more effort. I decided to have my FO look at the QRH and ensure that I wasn't missing anything; we transferred the controls and he started from the STAB OUT OF TRIM light and concurred with what we had accomplished up to that point. This gave me a chance to hand fly the aircraft and get a feel of the aircraft. The aircraft flew fine but when it needed A.N.D. trim; the added force required to trim while hand flying increased the workload and scan. We discussed the issues with trying to comply with approach clearances going into ZZZ; I decided to declare an emergency as this was a flight control issue and would allow us a longer approach assisting us in attaining a stabilized approach.We would also be able to slowly adjust speed and trim accordingly. I contacted ARINC and requested a phone patch to dispatch. I explained what had transpired with the manual trim and that I would be declaring an emergency. Dispatch agreed. I declared an emergency with ATC and explained the situation. After ending the call; I received an ACARS message to contact Dispatch again. I reestablished contact with dispatch and spoke to the FODM. It was explained to me that after my latest report on the manual trim; maintenance wanted the aircraft on the ground immediately as there may be a bigger problem. Dispatch said that ZZZ would be the closest and gave us the weather. I agreed and proceeded to inform ATC. We needed to lose more altitude and requested two 360 degree turns. During this time I informed the flight attendants as to what we were doing and followed up with a short announcement to the passengers. I decided that my FO should fly the approach as he had the most time flying the aircraft since the start of the event. He had been doing an excellent job and his stick and rudder skills were readily exceeding what was required. I asked him if he was ok with flying the approach and landing. He affirmed that he was completely comfortable with the approach and landing. Before we started the approach; I told him that if he felt any change in the controls and or pitch to immediately tell me; also that we didn't want many more changes to the trim once on the glide slope if possible. My FO landed to aircraft and we taxied to the gate uneventfully. All checklists were completed and write ups in the logbook were finished. I did not move the trim after landing to check its function as I didn't want to cause any other issues with the system. I felt as though our training in the simulator to include TEM and CRM helped in the successful outcome of this event. I tried to elicit ideas; thoughts and concerns from my FO throughout the event and he was a great asset offering the aforementioned. Last comment: We kept losing our patch with ARINC and or Dispatch; requiring us to reacquire communication. This was a large distraction and hindered our ability to convey information to Dispatch and for them to convey information to us. ACARS only helps so much. Also; while the iPads are an enormously helpful tool and I am thankful that we have them; they are only as good as the applications and processor speed. My iPad was fully charged when we left ZZZ. During this event; my Content Locker shut down twice and the JeppFD-Pro was very sluggish.
B737 Captain reported loss of stabilizer trim in cruise flight that affected both Captain and First Officer positions.