2011-10 · NASA ASRS report 975507
A B737-800 crossed a convective frontal boundary and after the tailwind increased to 105 kts the airspeed decreased to MACH .74; then oversped before returning to .78. But the airspeed then bled off until the crew intervened to increase N1.
Copilot's leg. Takeoff and departure uneventful. At FL390; line of weather extending from our departure airport northeast with lots of convective activity. Our flight was on top of all weather. After crossing the frontal boundary and without notice; winds increased to 105 knots out of the west (strong tailwind) and airspeed surged to about 5-10 knots into the red band (overspeed) and clacker sounded. Throttles manually retarded to 70% N1 and airspeed reduced back toward .78M but continued a slow decrease without any automatic addition of thrust from either engine toward selected Mach. Thrust then manually added to selected cruise 'CLB' setting (green carrot on N1 gauge) and engines did not respond. In addition; airspeed continued to slowly decrease. After about 10 seconds; Captain selected engine Anti-ice ON and after approximately 5 seconds the thrust from both engines responded as selected. Both events; airframe overspeed and lack of response with autothrottles engaged; were recorded in aircraft maintenance logbook and a thorough discussion with Maintenance upon arrival.
Flight departed late due to weather passing through the area. We received a reroute to the our south eastern destination for weather avoidance. Once airborne and at cruise altitude FL390; (all climb and cruise checklists completed) we determined that we were well above the tops of the front and we requested a turn more south easterly to save gas and time. Shortly after the turn; we noticed about a 105 knot direct tailwind. Flight conditions were flight level 390; on top of weather; continuous light to moderate chop; FMC optimum altitude 39.5 / FMC maximum altitude 41.0; LNAV/VNAV with FMC window opened flying .78 Mach. Auto Throttles ON. SAT -53/ISA 4. We noticed the airspeed get slower; to approximately .74 then return to .78 few minutes later. Then speed goes from .78 to an overspeed condition; approximately 8 seconds of the overspeed clacker sounding. I reduced throttles to approximately 70%. The Captain applied 'medium to light' speedbrakes; with the airspeed returning to .78. We simultaneously stowed speedbrakes and increased the throttles back to normal cruise N1. That's when we noticed that the engines (both) did not respond to throttle movement and the airspeed again started to 'bleed off' for approx 5-10 seconds. The engine N1s (both) stayed at approximately 70%. The Captain applied engine anti-ice (although we had been in the clear above the weather the whole time; SAT 53) and the engines both came back to normal climb N1 speed as well as the airplanes airspeed stabilized at .78 with no more increases or decreases in airspeed. Although I believe that both the Captain and myself did nothing wrong; every phase of flight was by the book I have 12 yrs and approximately 5700 hrs in the 737 and have never witnessed the aircraft perform in this manner. In the future I may not cross over the top of a frontal boundary of weather with the winds as high as they were.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.
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