from: suninmyeyes
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In the scenario described with V/S of 1500 fpm the autothrottle would have maintained the speed at Vref plus 5 knots once the speed had come back to that point. So at some later stage the mode must have been changed to one where the autothrottle stayed in "hold" and maintained idle thrust. FLCH or VNAV SPD would do this. Or as has been mentioned earlier if the handling pilot is resting his hands heavily on the thrust levers this death grip could prevent the thrust levers from opening up.
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Exactly. Since they disabled the autopilot at 1600 ft according to Hersman/.NTSB, it would seem VNAV SPD was not active. Perhaps it was still in VNAV SPD and the pilots were confused? From the B777 Systems Summary [Automatic Flight] manual by Boeing:
With the autothrottle armed, the autothrottle automatically activates if:
-no autopilot or flight director active, or
-an autopilot or flight director is in VNAV XXX, FPA, ALT, V/S, or G/S
AND
-speed less than FMC calculated value for one second
-thrust below reference thrust
-airplane altitude above 100 feet RA on approach or airplane barometric altitude 400 ft above airport on takeoff
Note: During a descent in VNAV SPD, the autothrottle may activate in HOLD mode and will not support stall protection
In particular the last two items are of importance, if they were in VNAV SPD mode (autopilot on), then A/T would be put in HOLD mode.
Or if the CDU thinks it is in takeoff, then the 400 ft elevation may apply and autothrottle is not active . It seems that it was when below 500 ft when the speed dropped more and more below their programmed SPD (137). Also from the Boeing manual:
Autothrottle (A/T) Switch
Push - above 400 feet, with the autothrottle armed, activates the appropriate autothrottle mode for the selected AFDS pitch mode, or if no pitch mode, in the speed (SPD) mode.