2025-11 · NASA ASRS report 2302774
Air carrier flight crew reported experiencing the auto throttles retard while on departure; resulting in the flight crew disengaging the auto throttles to regain control of the aircraft's speed. The flight crew returned to the departure airport where they landed safely.
Right after takeoff between 800' and 1000' the throttles began to retard as airspeed bug had dropped down to the top of the amber band (Minimum Maneuvering Speed). The auto throttle was disconnected and the MCP (Mode Control Panel) speed window was opened to set a higher speed. The FO and Relief Pilot confirmed that we did NOT have an unreliable airspeed indication. After completing the After Takeoff Checklist; we used FLCH; V/S and ALT HOLD to complete the climb to cruise altitude.During the climb; we tried multiple times to select VNAV; but it would always result in the airspeed bug dropping to the top of the amber band and had to resort to other MCP modes. Attempts to change cruise speed or CI (Cost Index) values had no effect on the the airspeed bug.We contacted Dispatch who brought in Maintenance Control and Fleet Support to attempt to troubleshoot the problem with no success.With poor weather and low ceilings forecast at destination and alternates; we elected to return to ZZZ. Fuel dumping was NOT required to land below Max Landing Weight. Landing at ZZZ was uneventful; but VNAV seemed to struggle with calculations.
ZZZ - ZZZ1During T/O roll; all FMA'S (Flight Mode Annunciator) seemed to be normal. Once airborne climbing through approximately 800' to 1000' MSL; I saw the Relief Pilot's hand reach up to the throttles as I was making a radio call. I looked down and saw the throttles retarded half way and the PF(Capt) hand was not on them. We were leveling off with airspeed indicated around V2. I pushed up the throttles at the same time noticing the VNAV commanded speed bug dropped to the top of the speed tape Amber Bar (Min Maneuver Speed). Capt regain control of throttles and disengaged auto throttles. With no further trend to an undesired aircraft state; I opened the MCP (Mode Control Panel) speed window and set a higher speed approx 210 kts to climb and help accelerate to retract flaps followed with setting CMS (Climb Speed) during steady acceleration and climb trends. During the maneuver; I asked the Relief Pilot to verify we didn't have an unreliable airspeed issue or such and it was determined that was not the case. After Take Off Checklist complete; we used MCP modes; FLCH; V/S; and Alt Hold; for rest of climb with normal Auto Throttle response. We tried at higher intermediate altitudes to re-select VNAV with little success. Airspeed bug would drop to top of amber band and had to revert to other MCP modes. Once level at FL360 the we were able to have the FMS go from CLMB to CRUISE and VNAV speed would revert to the current speed indicated. While manipulating the FMC VNAV Cruise speed or CI (Cost Index) values had zero effect on the Flight display speed bug or a response by the Auto Throttle.Dispatch; Maintenance Control and Fleet support were brought in and helped with further trouble shooting. With Fleet Support; we repeated many of initial attempts and also deleted/changed arrival/APP; Changed to RTE2 (Route); and many other FMC manipulation with no success. With TS (Thunderstorm) forecast and lower weather at destinations and ETOPS ALTN (Alternate); It was determined by crew and all parties to return to ZZZ. Landing would be below max landing weight and fuel dumping was not required. During descent VNAV seemed to struggle with calculations and crew used other MCP MODES. Return to ZZZ was uneventful. Maintenance performed procedures and aircraft was released with new crew to continue to ZZZ1.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.
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