B744 Autothrottle on Manual Landing
 

I was going through an old study guide and read something that I was not aware of with regard to the autothrottle. It may not even be true, but I was hoping to get some feedback here on the forum:

Any thoughts on what they mean by "off"? Is this idle or does it just go into a type of HOLD submode where it stays where it was? This is theoretical, but what if you did a manual landing with the autothrottles engaged but never used reverse? Would the A/T attempt to maintain the MCP speed on rollout while you were braking?

I have never left the A/T engaged all the way to touchdown on a manual landing, so I would have never discovered this. I always thought that it was the reversers that disengaged the A/T if left on during a manual landing, but perhaps the autothrottle behavior on an autoland is different than on a manual landing.

It would make sense that the autothrottles would be affected because TOGA is disabled any time you are less than 5' for more than 2 seconds. Maybe that's why TOGA is disabled -- because the autothrottles are off?

Thanks.

This is from a current 744 FCOM:

Flare

The flare maneuver brings the airplane to a smooth automatic landing touchdown. The flare mode is not intended for single autopilot or flight director only operation.

Flare arms when LAND 3 or LAND 2 annunciates. At approximately 50 feet radio altitude, the autopilots start the flare maneuver. FLARE replaces the G/S pitch flight mode annunciation.

During flare:

at 25 feet radio altitude, the autothrottle retards thrust levers to idle
IDLE replaces the SPD autothrottle flight mode annunciation
at touchdown, the FLARE annunciation no longer displays, and the nose lowers to the runway.

Rollout

Rollout provides localizer centerline rollout guidance. Rollout arms when LAND 3 or LAND 2 annunciates.

At approximately five feet radio altitude, rollout activates. ROLLOUT replaces the LOC roll flight mode annunciation.

The autopilot controls rudder and nose wheel steering to track the localizer centerline.
During rollout, autothrottle IDLE mode remains active until the autothrottle disconnects with thrust levers in reverse.

Rollout guidance continues until the autopilots are disengaged.

Note: Ten seconds after reverse thrust levers are down, autothrottle is armed until flaps are UP. Pushing a TO/GA switch while the autothrottle is armed activates the autothrottle in THR REF mode.

Thanks for the reply, Eckhard, but you are describing the autoland. You won't get those FMA indications like FLARE and ROLLOUT on a manual landing.

Been a while, but I suspect they are alluding to the fact that in that scenario (more than 2secs below 5' ??) you will have to push the thrust levers up manually. A subsequent push of the a TOGA button will re-connect the autothrottle.

But only if you are above 5'.

Correct! Hopefully the GA is going well enough at that point for that to be the case . . .

So we both agree that below 5' for more than 2 seconds disables (for lack of a more descriptive word) the A/T for purposes of TOGA.

The question I have is on a manual landing, what is the effect of this disabled state? Below 5' for 2 seconds would likely be right after you touch down. If you didn't use reverse, would the A/T feed in power as you braked the a/c on rollout? Would it go to idle? Or would it just stay at whatever setting it was when the A/T became disabled? What would be displayed on the A/T FMA? Would it stay in SPD?

Once disabled it will stay where it is, but you should be retarding thrust to idle in the flare and selecting idle reverse thrust as a minimum for a full stop landing. Suggest you look out the front for a manual landing rather than looking at the FMA annunciations.

Like you, I have made all my manual landings looking out the front. That's why neither of us knows exactly what is going on with the A/T at that point (e.g., Does it go to idle or hold? Does the FMA blank? I don't know). As I said, this question was theoretical and I am at least willing to admit that I don't know; thus, my question for someone who does.

Thanks anyway.

I've always referred to this as the "TOGA Dead Zone."

You can click the button but it ain't gonna work.

So build the habit of advancing the levers *every* time on go around and *then* click TOGA. You'll never go wrong.

Interesting to know. There is certainly nothing in maintenance manuals about this.

I doubt the thrust levers would go into idle, however. Without cues from the FMC or A/P, the A/T must surely go into a hold mode for safety. Having said that, the 737NG has a flare thrust lever retard mode at 27' RA if the A/P is not engaged (with the aircraft on the ILS and flaps extended). The NG A/T disconnects 2 seconds after touchdown.

I assume in the absence of a valid Rad Alt signal, the A/T would remain in SPD mode.

I am aware of the dead zone, but I never questioned the reason for it. Could it be that TOGA doesn't work below 5' because the throttles are disabled for manual landings? What other reason would Boeing disable TOGA in this area?
I think the 777 also disengages at around 25' for manual landings.

With the autothrottle armed (MCP) and no autopilot or autothrottle engaged, with 3 or 4 engines, normal EEC mode, the autothrottle remains ARMED (available for a 'controlled' 2000 fpm climb) until 2 secs after 5'R (when the TOGA switches are inhibited) until 3 secs after 5'R in the climb. If there is no valid radalt signal presumably the inhibit doesn't occur?
So, as I understand it, the AT remains armed throughout but the TOGA switches themselves are effectively unpowered.

Not sure if that answers your original question. It's been a while......

Basically, below 50' radar altitude the automation is configuring itself for the flare and roll out.

Flare begins at 50'. FMA changes from G/S to FLARE.
Between 30-15' Thrust goes to idle. FMA changes from SPD to IDLE

At 5' TOGA is disabled and FMA LOC changes to ROLLOUT

At touchdown FMA FLARE goes blank and nose is lowered to runway, spoilers should deploy and once the thrust reversers are deployed you're essentially committed to the landing.

The above is mostly a summary from a company training document.

I think the design essentially puts the burden on the pilot to MANUALLY begin the go around procedure at such a LATE POINT in the landing maneuver. The automation is busy preparing for the roll out portion and while it will not block the manual advancement of the power levers, neither will it be done automatically--remember below 5' it's transitioning to ROLLOUT with IDLE thrust.

Hope that clarifies it a little.

Percy: Then you are saying that the A/T will attempt to maintain the MCP speed upon touchdown on a manual landing if no reverse is used?

Zero: You are describbing an autoland and that is completely different than a manual landing. There is no FLARE, IDLE or ROLLOUT on a manual landing.

I'm pretty sure Percy is saying that the A/T will not do anything (as your books say)

There are a couple of not-so-cheap desktop flight simulators on the market which model an active SPD mode during non-A/P coupled rollouts, but I would be more inclined to believe your FCOM.

No. The autothrottle is not engaged, so the thrust & levers will stay where you put them.
The A/T is ARMED so it's available for TOGA except when the TOGA switches are inhibited.
Which is from 2secs after 5' on descent until 3secs after 5' in the climb!

Well, if you are now speaking for Percy, then maybe you can explain how the A/T can "remain[] armed throughout" but the "A/T will not do anything."

My books don't address this issue, as I'm sure yours don't either or you would have an answer.

To simplify the question: If you do a manual landing with the A/T engaged and you do not use reverse, will the A/T attempt to maintain the MCP speed on rollout? According to Percy, if the A/T is armed, then when the speed starts to bleed off on the rollout the A/T will advance power to maintain the MCP speed.

My mistake. I didn't register the contents of his second sentence. All I saw was

"the autothrottle remains ARMED (available for a 'controlled' 2000 fpm climb) until 2 secs after 5'R"

Your books say something similar, ergo my comments

Nonresponsive? My first post said that your comment was interesting to know and told you that my books lacked that information. I thought I was being supportive, not nonresponsive.

None of the Boeing or company manuals I have addresses my question. That's why I'm throwing it out here in hopes that someone might know. Granted that landing without using at least idle reverse is not something I've ever done, nor would I recommend it.

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.

If the A/T does not attempt to maintain the MCP speed on rollout, then what mode is it in, and when did it get there? At 5', as one study guide suggests? I don't know.

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:

---

"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.