Vessbot
Is it my use of “locked” that has caused you technical offence? That term is derived from days of old. 727’s and DC9’s had a rotary selector that among other things had an IAS lock. MD80 series had an IAS button which “locked” IAS. From memory, on an MD 11 audit fight I think I might have seen the same. And from memory 747 200 and 300 had a rotary variety of speed control selection. The purpose of these controls was to effectively “lock” in, the existing IAS at the time of selection. For convenience and commonality I have used the same nomenclature and inference. If that caused confusion I apologise.
Vessbot # 31
You made a point before about vertical speed being a consequence, and not controlled target, of pitch controlling airspeed. While that is true, it is a physical event in the chain of events all the same. And, if (your interpretation of) the FCOM quote above is true, then 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.
Vessbot #47
You make this point a few times and yes it's of course true... for the pitch channel. But we're talking about the throttle channel.
I was under the impression that we were talking about the functioning and output of the entire box of tricks, as a unit. I would posit that the “throttle channel” has control of exactly that, throttle/thrust. It doesn’t command anything but thrust.
NTSB say this;
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.
FCOM Section 6. says
The performance database supplies all the necessary performance data to the flight crew. It supplies the FMC with the necessary data to calculate pitch and thrust commands. All necessary data can be shown on the CDU. The database includes: (FCOM 6.11.69)
And
Autothrottle Thrust Lever Operation The autothrottle system moves either or both thrust levers to provide speed or thrust control, depending on the mode engaged.
Vessbot # 31
You made a point before about vertical speed being a consequence, and not controlled target, of pitch controlling airspeed. While that is true, it is a physical event in the chain of events all the same. And, if the FCOM quote above is true, then 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.
As previously stated. It doesn’t because the thrust is already on a limit.
Functionally, the amount of thrust required is computed by the FMC utilising the pilot inputs, the existing environmental factors, and the FMC performance database,
and then transmitted to the TLC.
The upshot of all of this is that we have 4 possible conditions to consider. Long descent, Long Climb, Short climb, and short descent.
On the long cases I think you agree, that thrust levers will be at upper or lower limits.
On long climb the thrust levers will be at full noise and the FMA will show THR, until ALT capture mode or pilot intervention whereupon it will transition to HOLD. Whilst in THR any negative speed excursion will be rectified by pitch down, no thrust change, cos its already on the stops. Any speed excursion up will be countered by a pitch up to regain “locked speed”. If the pilot has intervened on thrust it will have gone to HOLD, and thrust will remain as set, until further intervention, such as ALT cap or selection of another mode i.e. VS.
Long Descent is handled the same way. THR will reduce the noise until the stops. FMA will show THR initially then HOLD after the noise is minimum. The thrust will remain at min until something intervenes be that ALT cap or an alternate mode selection, or pilot intervention. Any speed variation will be responded to by pitch alone, as the thrust levers are at the back stop and the FMA is showing HOLD.
In both these scenarios, IAS is controlled by pitch. Thrust is at limit. ROC and ROD is a
consequence of pitch changes to maintain at “locked” IAS.
So then we come to the
short climb or descent. According to the NTSB quote above, the FMC using the current environmental conditions and the pilot commanded action, will determine if this to be a long climb/descent (actions as above) or a short climb/descent (less than or equal to approximately 2 minutes. For the short It will then calculate an
appropriate thrust setting, which it will command from TLC. From observation it will only make one command and the live with it for the next minute or so (2minutes minus, initial power adjustment time plus flare time). It will not hunt, it will sit, unless a major upset invokes stall protection. Highly unlikely.
In each of the 4 scenarios the controlling factors are
THR and IAS. ROC/ROD would only complicate a simple equation. If you get the hots for a ROC or ROD use VS mode. At all times in the above scenarios, there is no
TARGET ROC or ROD. What you see is what you get THR and IAS. When you select FLCH the speed window will open if it is not already. The VS/FPA window will remain closed. If you want to nominate a VS you will have to change modes.
Vessbot I entered this conversation because whilst I see this as a great forum for those desiring to expand knowledge, It is incumbent upon us all to ensure that the knowledge imparted is valid AND useful. I firmly believe that those following us should be afforded useful information unburdened by minutae of limited use. Make a simple concept complicated and you are sure to end up with confusion.
I would be surprised if anything discussed in this thread will result in any reader modifying their mode of operation.
Maui