Date: 201908
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Light: Daylight
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Situational Awareness
Second reporter function: First Officer; Pilot Flying
Anomaly: Deviation - Altitude Undershoot; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Clearance
Contributing factors: Human Factors; Weather
Primary problem: Weather

Narrative: En route at FL360 constant light chop began increasing to moderate chop; so we requested climb to FL380 which was reported mostly smooth. We checked the FMC Cruise page which displayed a max altitude of 39;700 FT and optimum of 37;500 FT. When cleared to FL380 the Controller requested a climb rate of 1;000 FPM; which we did using Vertical Speed mode at 1;200 FPM. ATC assigned a turn to [VOR] which we initiated via LNAV. Climb performance was good with Mach steady at .76. Approaching FL370; turbulence rapidly increased to light to moderate turbulence and airspeed decreased to the top of the minimum for clean configuration (yellow band). An uncommanded increase in vertical speed due to turbulence caused the aircraft to enter ALT Acquire mode shortly followed by a pitch down and FMC Speed mode and altitude loss from 37;800 FT to 37;500 FT. We disconnected the autopilot; rolled wings level from the right turn toward [VOR]; and leveled altitude to stabilize airspeed. At that same time; I advised ATC of the altitude loss; the controller queried whether we were turning toward [VOR]. Within moments the turbulence stopped; airspeed and climb rate restored and we completed the turn to [VOR] and leveled at FL380. Although aircraft performance was good initially; it decreased rapidly due to unexpected deterioration in ride quality. The turn assigned toward [VOR] after accepting the climb clearance also reduced climb performance. The turn toward [VOR] was the controller's effort to increase traffic separation but it also reduced our climb performance. Also he advised after the fact that standard climb rate would have been ok due to the turn toward [VOR]. It all happened pretty quickly so a shallower climb rate may or may not have helped anyway. Probably doing the turn in heading select mode at 15 degrees may have increased performance as well once the turbulence increased.


Second narrative: Although aircraft performance capability was initially good; the unexpected increase in turbulence coupled with the turn to [VOR]; resulted in a situation where we could not maintain the ATC required climb rate. To mitigate these conditions and to prevent a similar event from occurring again; I would not accept a climb with a required climb rate at such a high altitude. In hindsight; there was no rush to climb and if the controller could not allow a 'standard' climb; then we could have waited. Furthermore; given the controller required climb rate; the turn to [VOR] could have been accomplished at a shallower bank to ensure the clearance was met.


Synopsis: B737-800 flight crew reported encountering light to moderate turbulence in their climb to FL380 that resulted in an uncommanded descent.