I have read the NTSB report on Asiana flight 214 that crashed at KSFO, which includes a good technical description of the B777 auto-thrust system.
(My bold throughout)
The PF disconnected the A/P and moved the thrust levers to idle, which caused the autothrottle (A/T) to change to the HOLD mode, a mode in which the A/T does not control airspeed. The PF then pitched the airplane down and increased the descent rate. Neither the PF, the pilot monitoring (PM), nor the observer noted the change in A/T mode to HOLD.
The safety issues discussed in the report relate to the need for the following:
Adherence of Asiana pilots to standard operating procedures (SOP) regarding callouts. The flight crew did not consistently adhere to Asiana’s SOPs involving selections and callouts pertaining to the autoflight system’s mode control panel. This lack of adherence is likely the reason that the PF did not call out “flight level change” when he selected FLCH SPD. As a result, and because the PM’s attention was likely on changing the flap setting at that time, the PM did not notice that FLCH SPD was engaged.
Reduced design complexity and enhanced training on the airplane’s autoflight system. The PF had an inaccurate understanding of how the Boeing 777 A/P and A/T systems interact to control airspeed in FLCH SPD mode, what happens when the A/T is overridden and the throttles transition to HOLD in a FLCH SPD descent, and how the A/T automatic engagement feature operates. The PF’s faulty mental model of the airplane’s automation logic led to his inadvertent deactivation of automatic airspeed control. Both reduced design complexity and improved systems training can help reduce the type of error that the PF made.
The autothrottle can support stall protection when armed and not activated. If speed decreases to near stick shaker activation, the autothrottle automatically activates in the appropriate mode (SPD or THR REF) and advances thrust to maintain minimum maneuvering speed (approximately the top of the amber band) or the speed set in the mode control speed window, whichever is greater. The EICAS message AIRSPEED LOW displays.
Note: When the pitch mode is FLCH or TOGA, or the airplane is below 400 feet above the airport on takeoff, or below 100 feet radio altitude on approach, the autothrottle will not automatically activate
Note: During a descent in VNAV SPD, the autothrottle may activate in HOLD mode and will not support stall protection.
Obviously I am not an aircraft designer or aircraft systems designer, but the terminology still seems slightly ambiguous to me, because it will annunciate 'HOLD' even at idle thrust. Since there are buttons on the MCP to select HOLD of the Heading and HOLD of the Altitude, the word 'HOLD' would seem to suggest the automatics are actively modulating some parameter in order to hold it, i.e. maintain it. However, as I understand it; 'HOLD' as applied to auto-thrust is simply where the back-driving motors are just switched off and the thrust levers stay wherever they last were - including flight idle.
I personally think that 'IDLE' or 'THR IDLE' - ideally in pulsating amber - on the PFD mode annunciators would be a much more informative caption for better crew awareness whenever the thrust levers were at the flight idle stop, no matter how they got there.
Interesting.
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