cf6-802b5f


"With the autopilot off, the autothrottle goes off at 5’."


A very interesting scenario indeed. I'm thinking misprint, and it should read... the autothrottle disengages (but remains armed) at 5' RA. Perhaps a B744 Avionics engineer could throw some light on this. Any out there?


McHale. :)

Your hypothetical scenario also assumes that you haven't manually brought the thrust levers to idle during the flare. If you have, then we get into the area of dormant servomotors due to manual intervention.

From the BAMM
"(h) The *control A/T servo is responsible for throttle rate servo command, manual override detection and line voltage compensation.
1) Manual override occurs when the throttle rate does not track the commanded servo
rate. This may be caused by the pilot manually opposing the A/T lever motion or the
levers reaching their physical position (?**). Whatever the cause, the control A/T servo will
detect it and cause the throttles to become dormant."

(EDIT) This statement, too, is somewhat ambiguous. Does the A/T servo only become dormant while you are opposing the command, or does it put the A/T servo permanently "on hold".

*The "control A/T servo" is actually a control function within the FMC and not the A/T servomotor itself
** Did the author mean "physical limit" rather than physical position?

"maybe you can explain how the A/T can "remain[] armed throughout" but the "A/T will not do anything."


Flaps in landing configuration, aircraft in ground mode (WOW...weight on wheels) perhaps. Just my two cents worth.


McHale. :)

Correct, NSEU -- No need to throw that into the mix too. For the hypothetical, we just won't select reverse at all.
Not to change the subject, but this sounds like what would occur when in FLCH (either climb or descent) and you manually override the throttles. They will go to HOLD, just as Asiana in SFO discovered when they manually overrode the throttles and pulled them back to idle after pressing FLCH at 1600'.

I'm curious, does your BAMM state whether this throttle rate is a specific TLA or a specific period of time that it diverges from the commanded servo rate, or both? I have read the figure 1.3 seconds to set it into HOLD during a FLCH climb or descent, but I have also read that movement of more than one throttle knob-width will do it. Maybe I should make a separate post for this.

Sorry, there is no mention of the rates or distances in the maintenance manual for manual override.

A separate post for this might be a good idea considering the ambiguities found in the manuals for this subject: e.g. looking at an old AOM for one particular airline, I see the following statement ...

"Thrust levers can be manually positioned without disconnecting the autothrottle.
After manual positioning and release, the autothrottle repositions thrust levers to
comply with the active mode. The autothrottle system does not reposition thrust
levers while in HOLD mode."

If moving the levers produces HOLD mode, then how can the levers reposition?

FLCH is a special situation. When the throttles are manually overridden in FLCH, or they automatically reach the idle stop on their own (for an idle descent), the FMA will indicate HOLD and the throttles will not re-activate if the speed gets slow (unless or until you reach the MCP altitude you are heading for -- then they will go to SPD||ALT).

[This is assuming the A/P is disengaged].

Quote:

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"With the autopilot off, the autothrottle goes off at 5’."

---

I don't know if it's a misprint. I think that since TOGA is disabled below 5' for 2 seconds, there is also something that affects the A/T at this point. Or, maybe TOGA is disabled precisely because the A/T is disabled.

Which means the quote in the old AOM I have is quite wrong (Who writes these things?)

I don't think it's wrong -- it just doesn't explain it very well. If you are descending in SPD mode on the A/T (say SPD||V/S) and you manually override the throttles by pulling the power back, the throttles will just go back to maintaining the select speed when you let go of them.

But if you do that while in THR||FLCH SPD, the A/T goes to HOLD||FLCH SPD and the power will not move back when you let go of the throttles.

CF6: I think I've caught your drift now. What you are asking is:
On a manual approach (or if you're daft enough, using V/S and one autopilot! But anyway, without arming FLARE mode) WITH the A/T engaged, when you close the thrust levers to idle against the demand of the servo, will the T/Ls always move forward again to maintain your selected Vref? Or perhaps after 5'R plus 2 secs (or WOW squat switch?) will they remain closed?
Simple answer: Don't know.

Exactly! That makes at least two of us who don't know. Thanks.

Very good!. Thanks! This largely confirms what I am hearing from someone who just tried it in a sim. I would have bet that the A/T would disconnect on manual landing at least by touchdown if not by 5', but not that you would see IDLE at 25'. This is interesting. Is this based on observations in the sim, airplane or both? Thanks again.

If your 744 manuals even mention a distance (15' or 25') they are miles ahead of anything I can find. I sure would like to see something on this in writing if it's not too much trouble. So, you are pretty sure the -8 A/T works the same as the 744 in this regard? I know the -8 has A/T wakeup when disengaged and the 744 doesn't, but I don't know that it would necessarily affect the scenario above.

Thanks. Just to be sure we are both on the same sheet of music -- and I think we are -- this is for a manual landing (without the autopilot). I'm pretty sure that's what you mean. If so, your manuals must be spectacular. I haven't seen that figure anywhere.

When I transitioned from the DC-8 to the DC-10 back in 2000 the A/T was a novel feature -- It actually worked on every airplane. Imagine that.

Yes, it is.

Why would you have the A/T in use on a manual landing. Surely a manual landing is just that no A/P no A/T. On the B744 in my Company use of A/T was not recommended on a hand flown approach.

I am not advocating use of the A/T on a manual landing. As I said in my first post, this is theoretical. I eventually got my answer.

Incidentally, the Asiana crew in SFO never planned on pressing FLCH inside the outer marker, nor did their manual authorize it; but they found themselves in that situation one summer day in 2013. If they had better understood how the airplane would behave, perhaps they could have averted the outcome. I thought that's what this tech forum was for.

It is interesting that you ask this type of question. Almost five years before the Asiana crash a poster asked a theoretical question about the 777 A/T trap that caused the Asiana crash. You can read the post here, http://www.pprune.org/tech-log/34257...peed-prot.html but I'll save you the time and share the replies to his honest question below. Cheers.

I'm just curious whether the A/T would attempt to maintain MCP speed on the rollout during a manual landing if no reverse is used. If it does, the A/T FMA would probably stay in SPD mode.

Irrespective, do you think Boeing would have not thought of this in the system design?
Do you think that this might happen?
Are you really flying 747's ?

It's hard to tell whether you are being sarcastic or just daft. Do you think Boeing would have not thought to provide better warnings about the FLCH trap?

Irrespective, if the original question was so intuitive to everyone, it would not have taken five days to find someone with the answer. I don't recall your making any substantive contribution during that time, but if you have something constructive to say on this question, I invite you to share it.