B737-700
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 211
Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown
Flight Crew Overcame Equipment Problem; Flight Crew Returned To Clearance; Flight Crew Became Reoriented; Flight Crew FLC Overrode Automation
Aircraft; Human Factors
Climbing thru FL380 to FL390; we noticed the aircraft getting slow. PF disconnected autopilot and pitched down three degrees to accelerate preventing stick shaker/stall. We lost at least 1;000 feet then; after accelerating; climbed back to FL390. ATC never inquired of our altitude loss nor did we see any conflicting traffic in our vicinity. Somehow we were in VS mode instead of VNAV. Neither PM or PF caught it.I believe that automation in the cockpit is taking away our scan and awareness of what the airplane is doing; I trust it too much! I also know we were tired as this was a long day; and; the last leg of the night.
We noted the aircraft was climbing in Vertical Speed instead of VNAV. As the PF; I reached up and hit the VNAV and the plane pitched down to accelerate. At that point; I turned off the autopilot to try to mitigate the decent; since we were supposed to be climbing. Because we were at high altitude; I wanted to try to make less abrupt inputs. Minimize distractions (keep conversations to a minimum) while climbing out so that we're not distracted from flying/monitoring the climbout.
When their B737-700 was climbing out of FL380 for FL390 the flight crew discovered they were in Vertical Speed mode instead of NVAV and the airspeed was rapidly declining into the yellow arc. The PF took over manual control to re-accelerate which resulted in the loss of approximately 1;000 feet. They then continued climb to their cleared altitude.