B737 Undifferentiated or Other Model
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Distraction; Situational Awareness; Human-Machine Interface
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Inflight Event / Encounter Weather / Turbulence
Flight Crew Regained Aircraft Control
Weather
As we were deviating right to avoid a magenta area associated with a minor buildup we encountered an area of choppy air the quickly turned to turbulence. Airspeed was fluctuating but then continued to decrease. Corrective action was taken that led to a 3;000 foot loss. Center was advised and gave instructions to climb to 38;000 feet. Before the event I noticed the winds aloft to be a left quartering head wind; and after the event it was a left quartering tailwind. It's highly suspect that a high altitude windshear was the cause.
At 40;000 feet light chop; seatbelt sign on; purple area on radar (no green; yellow nor red) started deviation to the right. Chop increased to turbulence; visibility good in wispy tops; airspeed then decelerated with altitude drop fairly rapidly (believe wind at the time was 240/50). Corrective action was taken. Wings level; max power; nose down for airspeed gain as approached amber hook; altitude loss 3;000 [feet] to 3;7000 feet. As recovery began ATC said to level at 38;000 feet then airspeed rapidly accelerated. Corrected nose up and some power reduction; climbed to 38;500 feet then back to 38;000 [feet]. Event lasted approx 3 to 4 minutes; no injuries High altitude windshear; moderate turbulence; wind shift quartering tail to quartering headwind VNAV engaged and maintaining .78. No stall; but airspeed decelerated and no power available to maintain so lost altitude. At the time in right turn for deviation of magenta area on radar.
B737 flight crew reported unexpected turbulence and windshear causing a 3;000 feet altitude loss.