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 altitude 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.

Related reports:

Google
 

Original NASA ASRS Text

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

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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.