TheBusFlyer and
Vessbot
In FLCH SPD mode, the rate at which the thrust levers move depends on the difference between the current altitude and the target altitude .The A/T is limited by the thrust limit at the forward range of thrust lever travel and by idle at the back range of travel. During a FLCH descent, HOLD mode will engage when the thrust levers reach the aft stop or if the pilot manually overrides the A/T. During a FLCH climb, HOLD mode will engage only if the pilot manually overrides the A/T. When the HOLD mode engages, the annunciation for the A/T mode will change from “THR” to “HOLD,” and the annunciation will be surrounded by a green box for 10 seconds. The A/T will remain in HOLD mode until one of the following conditions is met:
(1) The airplane reaches the MCP target altitude.
(2) The pilot engages a new AFDS pitch mode or new A/T mode.
(3) The A/T arm switches are turned off
(4) The thrust is manually commanded to increase past the thrust limit.
(5) The A/P is disconnected, and both F/D switches are turned off.
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.
An event analysis provided by Honeywell indicated that the accident flight did not meet the criteria to trigger any EGPWS alerts.
The above are quotes from the NTSB report on the analysis of the Asiana 214 Accident in SFO some years ago. I suggest to you that this supports, though not explicitly, my contention that FLCH controls speed and does not compute and call for a particular ROC. Rather it applies thrust and then regulates IAS on elevator to achieve selected altitude and maintains selected IAS. This contention is supported by years of watching how it actually happens.
Vessbot.
Let's say the VS went below target. If the THR behavior as described above makes a correction for it, and bumps the thrust up a little, the airspeed wont be "locked" to the airspeed target but will go up a little bit, until the pitch channel in FLCH responds to that and pitches up, bringing the speed back down to target. AKA, the phugoid cycle.
While it's pitching up, what's happening to the VS? It's also going up, to which THR will respond by pulling the thrust back. That will also pull the speed back, causing FLCH to pitch down, reversing the original correction. As a result of this, the VS is below target, bringing us back to step one.
In this case the THR will not respond because it is on the limit set by FLCH and it is thinking about getting to altitude, not how rapidly it will get there. As it pitches up THR will increase to the limit, where it will stay until ALT CAP or manual intervention. In VS, different story. But VS is not in discussion.
Vessbott.
Obviously it is a solved problem as the 777 is not known for anything like this happening. But this is what the issue is, in the first place.
It is my contention that issue/possibility has been resolved by Boeing and Honeywell by making FLCH responsible only for thrust and IAS.
Uplinker.
What Boeing actually meant was that the pilot must hold the thrust levers with their hand !!
Is that referring to the comment in the earlier
FairyTale? If so, perhaps it would be useful for you to delve into the meaning of the expression FAIRY TALE.
Maui