B737-700
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 149
Workload; Training / Qualification; Situational Awareness; Communication Breakdown
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Inflight Event / Encounter Weather / Turbulence
Flight Crew Requested ATC Assistance / Clarification; Flight Crew Took Evasive Action
Procedure; Weather
This was an early morning flight. We were passing over a line of weather below us in level flight at FL400. Ride conditions were light chop with occasional light turbulence. FMC optimum altitude was FL403 and the performance computer enroute page computed 1.48 G available for the flight conditions. Winds had just increased to 95 KTS. We received a RT SIDE Window Overheat and Master Caution light; which we reset in accordance with the QRH. Finishing the QRH procedure; the First Officer (pilot flying) noticed the airspeed decreasing and applied power. I (pilot not flying) noted 35-knot drop/shift in head wind component (220/95 to 220/60). The First Officer applied emergency thrust and disconnected autopilot in accordance with stall recovery procedure. The airspeed continued to decrease to 210 IAS. I attempted to request lower altitude but ATC frequency was congested. We started a descent down to FL380 and airspeed decreased slightly more but stabilized in descent and then slowly increased (momentarily touched yellow arc but no buffet or clacker). Passing through FL390; I was able to get word in to ATC that we were descending and we were cleared to FL380. I reported the large windshear event to them as well. Before leveling at FL380; we had regained normal airspeed and re-engaged autopilot and autothrottles. No aircraft limitations were exceeded and flight continued without further incident. We had no prior alerts or warnings of ride and wind conditions at FL400.  The First Officer (pilot flying) did a good job of recognizing the airspeed decay and applying the high altitude stall procedures including trading altitude for the airspeed when on the backside of the power curve.
Good CRM and crew coordination were key in handling a situation that could have gone bad. Simulator training in high altitude stall awareness/recovery from a prior PC was also very helpful in expedient recognition and recovery.
A B737-700 at FL400 encountered a 35 KT headwind increase which resulted in an airspeed decaying to 210 KTS so the First Officer applied emergency power during a descent; finally getting ATC clearance to FL380.