Date: 201301
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: IMC
Weather elements / visibility: 0.25
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Airspace: Class B ZZZ
ATC / advisory: Tower ZZZ
Aircraft component: Autothrottle/Speed Control
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Second reporter function: Pilot Flying; Captain
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Loss Of Aircraft Control
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Executed Go Around / Missed Approach; Flight Crew Regained Aircraft Control; General Maintenance Action
Contributing factors: Aircraft; Human Factors
Primary problem: Aircraft

Narrative: On go-around; the Pilot Flying selected the go-around button once with the autothrottles engaged.  The autothrottles commanded full thrust and the Pilot Flying wasn't ready for the uncommanded thrust setting.  This caused a big pitch increase and subsequent stick shaker.  We landed on the next approach and wrote up the autothrottle system.  The Mechanic found a fault in the system and put the autothrottle on MEL.


Second narrative: Upon commencing the ILS approach; visibility was reported to be 1800/1800/1800 RVR.  The approach was normal until the minimums call.  With no part of the runway environment or approach lights in sight; I initiated the missed approach by pressing the Takeoff/Go-Around button on the left throttle one time and calling for Flaps 15 and landing gear up.  I flew the pitch up to the flight director cue and; about the time the pitch was correct for the procedure; the autothrottles abruptly went to full MAX power causing an abrupt pitch up beyond 20 degrees.  I remember that during the initial procedure; I was thinking that we were very close to our pre-briefed fuel state for a diversion versus a second approach decision.  I diverted my attention momentarily to the fuel gauges.  I was following the thrust levers lightly with my hands rather than having them firmly in my grasp.  The missed approach was very controlled and smooth up to that point and I felt quite relaxed with how it was proceeding.  As the thrust levers moved rapidly to MAX; I lost physical contact with them while the nose pitched abruptly up.My initial reaction (after one of those 'What the heck was that?!' moments) was to reach for and grasp the thrust levers and begin pulling them back while pushing on the yoke. The thrust levers came back more quickly than the nose came down; as I had underestimated the amount of force required; and an already decreasing airspeed decayed more rapidly.  At this point; I had just disconnected the autothrottles and; simultaneous with the AIRSPEED LOW annunciation and stick shaker activation; quickly moved the thrust levers forward; further adding to difficulty in holding the nose level.  I was hesitant to lower the nose further to break the impending stall because of the low altitude involved.  Holding essentially level (deviations both up and down due to the forces involved); the aircraft powered out and airspeed increased sufficiently to resume the climb and missed approach.Remarkably; with the airspeed having decayed as low as 97 KIAS; the aircraft never stalled.  We were just under our max landing gross weight; having an aircraft at full capacity and sufficient fuel to divert to XXX.  At heavier approach weights; it is quite apparent to me that; even with max power; airspeed will decay quite rapidly if the aircraft is allowed to pitch up as the power moves up.  I could probably have handled the uncommanded power application better had I been totally focused on the task at hand and not trying to rethink ahead to something we had already planned and briefed.  It took no time at all from a seconds inattention to a highly undesired aircraft state.I subsequently debriefed this incident thoroughly with my First Officer and wrote up the autothrottle system.  Maintenance confirmed the erroneous max power application and deferred the item via MEL.  That was a relief in a way as I had been reviewing the event in my mind and not seeing where I might have keyed Takeoff/Go-Around a second time to cause the event.1. Stress total focus on the task at hand during critical phases of flight. 2. During some PCs; practice an approach to missed approach at higher gross weights.3. Always be ready to immediately deactivate elements of automation during critical phases of flight.


Synopsis: B737-700 flight crew experiences an autothrust anomaly during go-around that causes an inappropriate maximum thrust setting with resulting pitch up and low airspeed with stick shaker.