I didn't know the 744 did this either (as a former Avionics Maintenance Engineer). My maintenance training notes tell me that the command to idle the thrust levers during Autoland comes from the AFDS system. Would you so readily assume that the command also happens with the AFDS disengaged if you had no previous Boeing experience?

I made an earlier comment on the 737NG about its thrust levers going to idle at 27' with the A/P not engaged (but the aircraft on the ILS) because I thought it different from the 744. Apparently it's not. The training notes for the 737NG and the Maintenance Manual are much clearer in this respect. The signal path and logic is shown in diagrams. The text tells you what happens.

There are lots of things on the 744 and other Boeing aircraft which go against intuition.

Hypothetical or unrealistic?
 

CF6

I think it's great that people can post questions and discuss the answers. I'm always ready to learn which is why I read this thread so nothing wrong with that.
However, your question was, in my humble opinion, not so much hypothetical but unrealistic. You may disagree but I'll explain my reasoning.

On a manually flown approach my company always recommends disengaging the autothrust. I don't think it's prohibited as such but the pitch power couple makes it clumsy so no autopilot then we use manual thrust. In fact with the CF6 and metric weights it's a easy to remember the power setting as it's 6 plus the middle digit of the weight eg 240 tonnes use 64%. Add about 8% so 72% if one engine inop. Useful info every 6 months or so!

I'd never thought about it but yes I suppose if, contrary to the recommended technique, one left the autothrust (A/T) engaged then perhaps it will still retard the thrust in "idle" mode at about 25 ft radalt. Of course that's one of the cues one looks for in an autoland along with "flare".

I'd never heard of FLCH trap until the Asiana crash and as someone else mentioned it might be an issue on the 777 and 748 but not the 744 although I'm happy to be corrected but it doesn't ring any bells and I've read the FCOMs a few times.

If there is a trap of sorts then it's the fact that TOGA as a mode is not available after a prescribed time and height - 5 radalt and 2 seconds? While I remembered that in the context of an autoland it surprised a colleague who rejected an approach very late, the classic "floater", then realised hitting TOGA wouldn't work. No problem once he realised of course back to basics. There but for the grace of God!

Hope that helps.

BBK

My airline is two merged. One of the two would always turn off the AT on a hand flown approach, the other would always leave the AT on during hand flown. I now do both of the above, plus sometimes leave both on, until very late, or auto land. If I am using AT on during hand flown, I turn it off at about 50', just before I retard the throttles.

I like using the AT during gusty conditions...it keeps the plane from getting too fast. It is slow in advancing thrust, so if the plane starts to get slow I manually push the throttles up a bit. I find this much less of a workload in gusty conditions than manual throttles.

Also, at my airline, we were all taught that the AT would maintain min safe speed in all conditions. We didn't know about this AT slow speed trap that caught Asiana until their crash. It was not in our publications. Our check airman tried the Asiana situation in the sim and were surprised that the airspeed was not maintained.

In gusty winds I MUCH prefer to use manual thrust. Autothrust does not respond well to gusts in all airplanes, and I find it easier to simply allow the airspeed to vary a bit around the bug speed, resulting in a more stable approach. After all, that IS why you increase the bug speed for gusts, isn't it?

With flaps out of up, but not in landing position, activation of VNAV in VNAV PTH
changes the reference thrust limit to CRZ. However, pressing TO/GA changes the
reference thrust limit to GA and GA thrust is available.
Pushing either TO/GA switch activates a go-around. The mode remains active even
if the airplane touches down while executing the go-around.
When the flight director switches are off, pushing either TO/GA switch displays the
Flight Director Bars.
An automatic go-around cannot be initiated after touchdown.
Note:
An automatic go-around cannot be initiated after touchdown or if the aircraft
is below 5 ft radio altitude for more than 2 seconds.

Now I would have thought that the implication of that was that the auto throttles are closed and will not re-open.

It also would seem obvious that the automatic go-around functions if the autopilot is in or out, and/or the auto throttle is engaged/disengaged.

Why would you imagine that the auto throttle would try to maintain the selected speed during the ground roll?

cf6,


Firstly I must thank you for starting a very good thread. It has produced many a theory on the AT system. Some on the money, some close and some nowhere near the ballpark so to speak.


Anyhow after much digging around and harassing some of our engineers, I may have had a "Eureka" moment. So here's my two cents worth.....


On approach at 25ft RA the AFDS sends a flare retard signal to the AT and causes the AT servo to drive the throttles to the aft stops (interestingly this happens during a MANUAL or AUTO landing). SPD will be replaced by IDLE on the FMA.


At <5ft RA TOGA is inhibited (but AT is still armed) until >5ft RA or reverse thrust selected (and AT disarmed). If you do not select reverse thrust and use only braking, the AT mode will remain at IDLE and will not resume the SPD mode.


For the SPD mode to engage several conditions must be met. These are....
- AT armed
- AIRCRAFT IN THE AIR>400ft
- Thrust limit mode not takeoff
- Go-around mode not active
- FLCH mode not active
- VNAV mode not active
- G/S mode not active
- Select SPD mode on MCP or
- Default SPD selection, for example, exit of FLCH or VNAV.


Hope this helps.


McHale.

Thanks for the input, Captain.

It is interesting. None of my books says anything about how the A/T will behave during a manual landing. During a manual landing, as you know, you are not going to see the FLARE at 50' or the ROLLOUT at 5' on the FMA. If there's no FLARE and no ROLLOUT then I think it would be a stretch to imply that it would "seem obvious" that the AFDS would be sending a flare retard signal to the A/T servo to drive the A/T to IDLE. Nevertheless, this must be taking place behind the scene, even in the absence of the FLARE and ROLLOUT commands.

If you have any material on this and it's not too much of a burden, please PM me with it. Thanks again!

cf6


Have sent PM (hopefully)


McHale.

Ditto


Thinking about the finer details... Do we know if IDLE would be annunciated on the FMA for A/T during rollout in this hypothetical scenario? Assuming the levers reach the aft stops before the aircraft reaches 5' plus 2 seconds, would the A/T go into HOLD mode (annunciated/not annunciated)?

Is the A/T system both inhibiting the TO/GA switches and putting the system into a hold mode where manual manipulation of the thrust levers is not opposed?

I can only go by what the A/T does when in autoland. If manual is the same as autoland, there is no HOLD.

What is not clear to me is when the A/T actually goes from IDLE to blank.

Is it:

1. At touchdown;
2. When less than 5'RA;
3. When less than 5'RA for >2 seconds; or
4. When reverse is applied?

I'm still looking for something in writing on this one.

Thanks. I'm looking at my FCOM and this is what it says under the autoland section:

Could there be a software revision or specific airline setting that would account for the disparity?

Having just done two manual landings in a 744ERF with a NGFMC I can tell you it stayed in SPD until just AFTER touchdown (as in not immediately on touchdown) the FMA went blank and an EICAS >AUTOTHROTTLE caution resulted.

I had to manually override the servo motors during the flare until it self disconnected, I then cancelled the caution with the thumb switch (didn't need any surprises) and selected reverse.

Well this totally throws a monkey wrench into the spokes! Thanks for the feedback CCA. Looking forward to hearing Silberfuchs' experience.

According to one source, this behaviour is backed up in the simulator. i.e. no idle during flare with the A/P not engaged. Manual intervention required.

Manual is manual; auto is auto. Do NOT mix the two.
If you truly feel the airplane speed control can do a better job than you can, you are already behind the eight ball. The speed control on the 744 sucks! The systems are logically good, but the automation is geared for the lowest common denominator.

Still hoping to hear from Silberfuchs.

Thanks! Did this -400 have the NG software? If so, it's looking like the NG software is the reason for the IDLE at 25' versus just going from SPD to blank when below 5' for 2 seconds.

Well, that blows my theory. I was hoping you would say it had the NG software. Thanks again for trying this, Silberfuchs.

Thanks for the thread.

I can't see that it would have been any other way. IDLE after touchdown makes sense to keep the thrust levers from adding power by holding them aft. There is no way the aircraft would have been certified if it went to SPD on touchdown. We know the autothrottle disengages at reverse selection which would of course, blank the associated FMA indication. Going to IDLE at 25 feet is exactly what it does during an autoland but thinking about it, I doubt the autothrottle has any idea how whether the autopilot is engaged or not so it just does its thing as usual.

I think that the only thing that happens below 2'(or whatever the very low altitude is after a certain number of seconds is that the TOGA switches are deactivated meaning...manual advancement of thrust levers for the go-around.

Good to hear from you, JammedStab.
It doesn't go to SPD on touchdown; what was seen in the sim was that somewhere below 5' the SPD FMA just went blank and on touchdown the throttles remained wherever they were when it blanked. I have a study guide that states the A/T will go off at 5'.

I'm just trying to figure out if some of the 744s (maybe older versions) actually don't go to IDLE at 25' as Silberfuchs experienced. I have a friend who is going to try it on a 744F in a few hours, conditions permitting, and hopefully he will report back.

As I said earlier, the idle retard command actually comes from the AFDS (according to the engineering manuals). One would hope that the AFDS knew if it was engaged or not :)

Cheers

Thanks,

Obviously your manuals have more details than our manuals. But the real question would be, is there any difference in autothrottle operation based on the use of the autopilot and if so, why.

That pretty much restates my original question in a more concise way. So far, we've got peole saying it behaves the same as the autoland and people saying it stays in SPD until anywhere from 25' RA to a few seconds after touchdown, then the autothrottle FMA goes blank -- no IDLE. CCA said he even got the AUTOTHROTTLE caution when it went from SPD to blank after touchdown. It is appalling that Boeing doesn't explain this better.

Re the Boeing manuals it's just a thought but maybe they're written to give pilots the necessary info not answer hypothetical questions?:E

It's a "hypothetical question" until it causes an accident, like the Asian 777 in SFO. If you're happy being ignorant about this, you can always skip this string. Thanks again for your valuable input.

BTW, regarding your prior post:

This demonstrates how poorly Boeing writes its manuals. You've read them a few times and you still don't know whether the FLCH trap applies to the 744? I can tell you without hesitation that it does.

Two questions for you Cf6, why do want to know so badly what happens in this situation which, in my company, shouldn't occur anyway.

In your considered opinion is FLCH an appropriate mode for final approach.

rgds

BBK

PS Oh and I word searched "FLCH trap" in the FCTM and Vol 1/2 of the FCOM and no results.

PPS I believe the Asiana crash may have something to do with the aircraft's speed decaying to something like Vref minus 17!

BBK:

I'm not sure why you feel compelled to police the forum for questions that you believe are inappropriate simply because "in [your] company, [it] shouldn't occur anyway." Really, you should probably refrain from clicking on this thread if it bothers you that much. So far, your posts have demonstrated that you have nothing of value to add to it.

Of course, FLCH isn't appropriate on final approach, but it was pressed and it resulted in a crash. Most crashes are due to something that shouldn't occur.

The "FLCH trap" was coined by the investigators and attorneys after the Asiana crash. If you did the same thing in the 744, the logic would react the same way.

Calm down dear! (sorry English joke)
 

CF6 just to refresh your mind this is what I wrote originally. I thought it was polite enough.

"I think it's great that people can post questions and discuss the answers. I'm always ready to learn which is why I read this thread so nothing wrong with that.
However, your question was, in my humble opinion, not so much hypothetical but unrealistic. You may disagree but I'll explain my reasoning......

.....Hope that helps. "

I certainly don't intend to "police" this forum or any other and I'm at a loss why you think so as I only asked why your keen interest. Just idle curiosity. I assume, from what you have written, you are not flying the 744 so thought your comment about the "appalling" Boeing manuals a little odd. I reiterate that using FLCH on the final approach is not a good idea nor is losing control due to stalling on short finals.

Anyway, I consulted said manuals and found the following:

"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

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."

I still think manual flight and manual thrust works just fine as did my trainers some of whom have flown the 400 since the 90s.

BBK

Ps exception I've just remembered might be a Cat3A with a dual FMC failure.

There have been reports on this thread that are contrary to the above, unless you take the first sentence as meaning that none of the description that follows the first sentence applies to a manual landing. Check out CCA's post.

I do fly the 744, but I have been out on a medical leave. As I dug into the FMC logic with time to kill, I realized how little I really knew about how it would behave under certain circumstances. When I dug further, it became apparent that my peers were equally clueless. The answers I have received on this forum confirm that it is not just limited to my peers at my airline. We have first-hand testimony from one pilot that it will go from SPD to IDLE and another guy saying that IDLE never happens -- it just goes from SPD to a blank FMA. How could there be such a discrepancy from 744 to 744 with nothing written in the manuals to explain it? Anyway, thanks for the input.

CF6

Just goes to show one shouldn't jump to conclusions and happy to stand corrected. On reflection I wrote my penultimate post after a long night flight so maybe it wasn't as well phrased as it could have been.

Anyway, as you will know very well there are lots of things going behind the scenes with the AFDS that are not annunciated eg runway alignment. I can't offer an explanation for the discrepancies other than maybe they're are different modification states of the FMC perhaps. When you get into how waypoints are coded in the FMC and things like "on approach logic" 'tis a dark art indeed. Anyway, anything I can add I'm sure you would know as a fellow operator so I'll leave it at that.

BBK

Thanks, BBK. You may very well be right about the different modification states. Whether it was a delivery option from the factory or a subsequent modification, it would be interesting to know the background on this. But like you said, it's a dark art, and at this point most of the engineers who initially worked on the 744 are probably long retired.