Vessbot
Thank you for the link to Continental’s FCOM. It has been a trip down memory lane to pick through that tome. A useful reference in this discussion.
With regard to your last post:
My Questions 1 & 2 and your reply 1 & 2.
They were rhetorical questions directed to TheBusFlyer. Upon reflection maybe I should have been less obtuse, so here is a more direct reframing of the questions.
1) If FLCH is capable of direct control of V/s in the manner you are proposing, what could an independent VS input contribute that the FLCH cannot?
2) Is it possible to select FLCH and VS mode simultaneously. (note V/s is a parameter and VS is a command)
To end the pain I will supply the answers. 1) Nothing 2) You can’t
To separate the various worms in this can, Continental FCOM Section 6.4 is useful.
FLCH SPD (engaged) - Pushing the MCP FLCH switch opens the IAS / MACH window (if blanked). Pitch commands maintain IAS / MACH window airspeed or MACH.
Nothing there about V/s.
And
• The thrust (THR) and pitch (FLCH SPD) modes command a climb or descent to the altitude set in the MCP altitude window at a thrust setting and vertical speed that will acquire the MCP altitude in 125 seconds.
The FCOM is silent on what happens if the 125 second time limit can’t be achieved.
What happens in the real world is that the thrust levers will be advanced or retarded to their limit be it idle for go down or full loud ( as determined by Thrust Limit Computer (TLC)) for go up. At about the point of max travel FMA will announce HOLD on the way down or THR or CLB (??) on the way up.
In your previous quotation of the FCOM, you omitted the part about the 125 seconds. Doing that eliminates one conditional element, and in my view, invalidates your assertion about control over V/s.
FLCH cannot command a V/s that is the role of mode VS. It (FLCH) can only command auto throttle. You are correct in your contention that VS and FLCH are conflicting modes, thus in the following statement, the only mention of vertical speed is in the context of achieving the 125 second, soft, target.
• The thrust (THR) and pitch (FLCH SPD) modes command a climb or descent to the altitude set in the MCP altitude window at a thrust setting and vertical speed that will acquire the MCP altitude in 125 seconds. • When changing from TO/GA to FLCH: • If the current speed is greater than the IAS / MACH window speed, the IAS / MACH window speed changes to the current speed. • If the current speed is less than the IAS / MACH window speed, the IAS / MACH window speed does not change. • The autothrottle automatically engages: • For climb - Engages in THR mode; the thrust limit is CLB thrust • For descent - Engages in THR mode, followed by HOLD if the thrust levers reach idle.
Then, your
“And, if the FCOM quote above is true, when the pitch and thrust channels in FLCH are both resulting in vertical speed changes, which creates the potential for interference as THR responds to it.”
There is no potential interference there because the THR is out of it until some other input occurs viz Alt cap or physical intervention such as mode change. This is so ‘cos thrust is either at HOLD or full noise until an intervention. And it will be telling you that with HOLD or THR annunciations on the FMA.
With the other comment, which I directed to Capt Bloggs, don’t sweat it. It refers to a long ago intercourse I had with him in which he was similarly highly critical of the design of the 777 AFCS. A system that if understood and operated as designed, is enormously competent and versatile. Note that there is a significant difference between the A/T being asleep as opposed to being switched off.
Maui