B737-800
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Distraction; Workload
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Weight And Balance
Flight Crew Overcame Equipment Problem; Flight Crew Returned To Departure Airport
Aircraft; Company Policy; Human Factors
I; the CA (Captain); was PM (Pilot Monitoring); the FO (First Officer) was PF (Pilot Flying). During the climb-out phase in level flight in mildly hazy VMC at 17;000 MSL at 290 KIAS with Autopilot B engaged the Master Caution; the 'FLT CONT' item on the CA-side SAP ('6-pack'); and the right half only of the 'STAB OUT OF TRIM' light illuminated. Press-to-test of the 'Stab out of trim' light showed both bulbs operable. After establishing that FO was actively remaining the PF; I performed 'Stabilizer Out of Trim' QRH procedure with the result that the autopilot electric trim and both pilot yokes trim switches were inoperative in both directions. I notified ATC that we had a mechanical problem that would eventually result in [requesting priority] and landing at a nearby airport; probably ZZZ; but for now we were content to remain at 17;000 slowly outbound from ZZZ while we worked the problem. Knowing we were also over landing weight; nicely under control; and still had a lot of checklists to run; I had no desire to rush into diving towards some final approach fix before we were ready to land. I performed the 'Stabilizer Trim Inoperative' QRH procedure with the result that manual hand-cranked trimming was possible in either direction; albeit against unusually high resistance and with a 'cogging' feel as if pushing against a powered but stalled electric motor. This was true even when the aircraft was in-trim. One pilot could move the trim using one arm; but two pilots using one arm each worked much better for anything more than a couple turns of the trim wheel. We found that one pilot could both hand-fly and crank the trim; but tended to make unwanted aileron inputs while doing so. So it was doable in a pinch but not smooth. The forces required to turn the trim wheel did not appreciably reduce as we later slowed & configured. In the course of reading & discussing or doing the 'deferred items' section of the 'Stabilizer Trim Inoperative' QRH I started to activate the GPWS Flap inhibit switch.  The FO stopped me saying we were above the 250 KIAS speed limit at that time.  That sounded familiar to me; but no such limitation is stated at that point in the QRH.  Nevertheless; we chose to delay activating the GPWS flap inhibit until we had slowed to first configuration speed around 220KIAS. Discussed with [the] FO whether ZZZ was the best divert option and which runway to use. ZZZ & longer Runway XXL was the unanimous choice. By that time we were a few miles south of [the] VOR on a south-southwesterly course for ZZZZZ; roughly 100nm from ZZZ.I then performed the 'Non-routine Landing' QRH procedure; alerting ATC for a return to ZZZ XXL with ARFF (Airport Rescue Firefighting) standing by. Coordinated with ATC for return via direct arrival for ILS XXL. Asked Company explicitly to alert ZZZ Approach & Tower of the intended non-standard XXL landing. Notified Dispatch via ACARS report. Briefed FAs via intercom; and passengers via PA on the situation; my expectations; and their roles.  We selected the non-standard landing Runway XXL based on that runway being much longer than the more typical landing Runway XXR.  That was a minor miscue; because I/we had not considered the effect of the large displaced threshold. Which I now consider insignificant.  For landings eastbound the difference is 2500 feet and would be significant. Lesson learned: the big runway length number on the 10-9 page can be significantly misleading; take the time to dig into the 10-9A details page. The fact that was our takeoff runway and within Jepp FDPro I'd used the green highlighter to extend the highlighting all the way to the takeoff threshold didn't help.  What had been useful to help direct my taxi to the correct threshold for takeoff became mildly misleading when evaluating which runway to land on. Which was not a side effect I noticed at the time or had considered previously. Still working as PM I performed normal divert preparations and top-of-descentprocedures: Reconfigured FMS & ACARS for ZZZ arrival; obtained ATIS; set up radios and performed approach brief; landing assessment; etc. Notified ZZZ Ops of our return by direct radio call to obtain a gate assignment.We discussed that we'd be a little above the normal max landing weight; but the already requested priority covers for that administrative requirement; and the runway available was nearly double what the QRH procedure's performance chart said was required. Performed descent checklist and before landing checklists and flows as normal.  Throughout the evolution the FO kept us neatly in control; tight on the FD despite sometimes having to hand-crank the trim himself; caught a few radio calls I missed; and demonstrated overall high situational awareness and excellent systems knowledge. This is especially impressive for someone fairly new to the airplane who is just back from 6 months of furlough. During descent below 10;000 ft approaching downwind we changed control to CA as PF w FO as PM. We sighted the field while on base. During the turn-on to final I was using visual lookout primary; backed up by the LNAV purple line and the ILS. There was a moment of confusion as the ILS and LNAV came together before I was aligned with what I had thought was the landing runway.  Having landed on XXR dozens (hundreds?) of times over the decades; that habit was unhelpful for a few seconds before we got fully established pointing at the correct runway. We then realized that Runway XXR is concrete and very easy to see in mildly hazy conditions while Runway XXL is asphalt and much harder to see. In fact XXL was invisible until we got closer in following the LNAV & LOC. My general habit on any landing involving parallel runways is to verbalize seeing all the relevant runways by name and explicitly saying which one I'm aligning / aligned with. E.g. 'I see XXL and XXR. I'm aligning with XXL.'  For whatever reason that verbalization didn't occur; with sorta predictable results.Intercepted final outside intermediate fix in level flight at 3;000 ft. Configured to flaps 15 early and slowed gradually to Vapp of approximately 160KIAS before glideslope intercept at final approach fix. FO kept the airplane nicely in trim on my requests.Made a flaps 15 slightly overweight (approx. 148K#) approach and landing to a smooth touchdown on ZZZ Runway XXL just shy of the 2000ft marker. Upon stopping made a PA to reassure the passengers & remind them the safety vehicles would be seen around the airplane. ATC gave us an alternate frequency to contact ARFF on. Which surprised both of us a bit. Once we were communicating with ARFF I requested they inspect us for hot brakes or damaged tires.  ARFF found normal brake temps and no tire damage. At ARFF's request I released them at that point and we taxied normally to [the] gate.Following shutdown the FO attempted to manually crank the trim to the usual post-shutdown 5 units from the 8-9 units we'd used at touchdown.  At zero airspeed the trim was still 'cogging' and almost as difficult to move as it had been in flight. So we left the trim at the landing setting for Maintenance's edification. Which point we discussed with the Chief Pilot who met the flight and again later with the maintenance folks who also met the flight. I made appropriate aircraft maintenance log book entries for the stab trim failure and for the overweight landing. Spoke with Dispatch by phone before leaving aircraft per FOM. The next morning I filed the required report. In all I think this event worked out pretty close to textbook; minus the small glitches identified above.The main electric trim failing was the causal factor. Everything else is just details.1.	 Subsequent research on the ground shows that activating the GPWS Flap Inhibit switch is called for in 11 different QRH procedures; and in the Flaps 15 landing procedure.  Only the 'One Engine Inoperative' QRH procedure and the 'Flaps 15 Landing' procedures mention that limit; the other 10 QRH procedures do not.I do not know whether those differences are deliberate or an oversight. Suggest someone on the 737 fleet team look into this. If the difference is deliberate there probably ought to some verbiage available in QRH or [procedures] to explain the rationale so others don't mistakenly apply a remembered restriction where it's not intended. 2.	 We were both surprised to see that only half of the 'Stab out of trim' annunciator illuminated.  Which left us wondering at first if we had a warning system malfunction or a trim malfunction.  I surmise now that the right bulb is connected to FCC (Flight Control Computer)  B and the left is connected to FCC A.  And that had the Captain been PF; we'd have seen the left half illuminate instead.However it works; the details of left & right are not documented in [manual] where the annunciator is described. At a minimum there should be a statement like 'Only half of the annunciator may illuminate. That is still a valid indication of an out-of-trim condition.'3.	Overall; from the 'Stab out of trim' annunciator illuminating to touchdown was about 30 minutes.  From reviewing the ACARS logs I know it was at least 28 minutes and less than 35.  As the CA / PM / QRH operator I was working quickly and diligently almost the entire time to accomplish all the non-routine and normal procedures in full without hurrying.  Had we turned back even a couple minutes earlier the airplane would have been approaching the airport before we were fully caught up and mentally transitioned from handling an [urgent inflight situation] to performing a normal low-stress; albeit flaps 15; landing.Given the reality that following all the procedures fully takes nearly 30 minutes; I believe we do a disservice to pilots in the sim; rushing around the traffic pattern with an engine out. That encourages hurrying; doing QRH steps without really thinking them and their implications through; etc. We fight how we train; and we're training to hurry. I question the wisdom of that.Further I question the wisdom of planned arrival fuels that only allow for approximately 30 minutes to flameout at traffic pattern altitudes.  The so-called FAR '45 minute reserve' is 45 minutes at cruise; not 45 minutes at traffic pattern altitude with e.g. flaps stuck partly extended. I now conclude that performing a major QRH evolution discovered in the arrival traffic pattern while down near Company planned arrival fuels would necessitate skipping a lot of steps or giving them at best a lick and a promise.I believe it is unwise to plan arrival fuels that tightly. Yes; we commonly have additional hold and / or Dispatch fuels.  But each of those may legitimately be burned in delays encountered before the [urgent situation] appears.  This saves Company a lot of money on the many flights that don't turn out worst-case. But it really sucks to be the pilot who draws that short straw; and is faced with either doing a rushed job of handling the [situation] or running out of fuel.That's planning; in extremis; to fail. An industry-leading FAR 121 carrier should not be planning to fail. In extremis or otherwise.
At a level off of 17;000 ft we noticed stab out of trim light. I was the PF so it was my aircraft. The Captain ran the stab out of trim QRH checklist which lead to the QRH stab trim inoperative checklist. We did not regain electric trim. Manual trim was what we were left with. Then the non-routine QRH checklist was performed. The manual trim wheel was difficult to work for both us even when the plane was near an in trim condition. Speed was slowed to about 270kts from about 290kts then. [ATC was advised] and we planned and set up for an arrival back to ZZZ.  I flew the aircraft and worked the manual trim while the CA performed checklists; set up for the arrival and approach; and the briefings. Manual trim did not become much easier until on approach when aircraft was slowed to approach speed; however even then it was difficult. The CA took the aircraft and I became the PM. The landing was nice and smooth. Once on the ground using the manual trim wheel to reset the stab trim to 5 units was still very difficult.
Air carrier flight crew reported experiencing a Stabilizer Out of Trim problem during climb out. They completed the appropriate QRH checklist and performed an air turn back.