A/T autodisconnection during manual flare

[emerydc8]

Thanks, Will. I guess it's up to Hardy how much of this he believes applies to the 744. Aside from the 777 A/T wakeup, it is beginning to appear that the similarities between the A/T systems on the two aircraft are notable.

[Britjet]

Big sim test today..
All set up for auto land with autothrottle engaged at Vref+5.
Disconnect A/P and make manual landing without disconnecting A/T....

Autothrottle disengages at 5R with blank FMA mode, EICAS AUTOTHROTTLE DISCONNECT caution.
Throttles remain at last position they were in to maintain the bugged speed, (ie they don't retard).

Peter

[Will]

Thanks, Peter.

[emerydc8]

Thanks, Peter. So, either the Atlas material I have is right (said at 5'RA it goes to "off"), or their sims are like BA's.

Did you happen to check what happens if you manually override the throttles during a FLCH climb?

Jon D.

[EDIT]: I just saw it goes to HOLD in your other post. Thanks again.

[John H Watson]

Could there be a small period (2 seconds) when the A/T and A/P GA logic is different? i.e the A/T is inhibited at 5', but the A/P is inhibited at 5' plus 2 seconds?

[Britjet]

They might be the same, I wasn't timing it...5ft radio is also a guess, really, it was just about touchdown...
Peter

[Hardy Heinlin]

I will use the same logic the TOGA inhibit uses (5R + 2 sec, landing config etc.).

Otherwise there would be unwanted side effects during non-landing operations.


|-|ardy

[Magoo]

Big sim test today..
All set up for auto land with autothrottle engaged at Vref+5.
Disconnect A/P and make manual landing without disconnecting A/T....

Autothrottle disengages at 5R with blank FMA mode, EICAS AUTOTHROTTLE DISCONNECT caution.
Throttles remain at last position they were in to maintain the bugged speed, (ie they don't retard).

Peter

If you are on an ILS, @1500' you'll get LAND 3, ROLL OUT and FLARE mode armed, wether you are manually flying or not after that makes no difference, at 50' RA you'll get the FLARE mode engaging and at 25' IDLE will annunciate and the A/T will retard the throttles to IDLE and the FMA will go blank in all modes except for ROLL OUT replacing LOC... The thrust is at idle during the ROLL OUT, if you were to disengage the A/P before landing, leaving the A/T in, the exact same thing happens except the retard speed of the A/T is a bit slow for most. But on the 744 we tend to disconnect the A/T if the A/P is disconnected, unlike the light twin cousin, the 777.

[Hardy Heinlin]

Are you saying that you get ROLLOUT and FLARE even when only 1 A/P is engaged and NO AUTOLAND is annunciated? So will the AFDS still provide autoland roll and pitch commands even when just 1 or zero A/Ps are engaged?

[Magoo]

wow, now you've got me thinking...
We tend not to fly the 400 with A/T on when the A/P is off. The 400 A/T is a bit slow and tends to be out of sync with your flying and doesn't help, especially during level offs.as we also fly the -8,the company has introduced the "let's keep the A/T engage" during manual flight on the -8 and I've done a few landings with the A/T left on with the -8 and the result isn't all that great as you end up overriding the A/T during the flare as they move too slowly...
Now, there never was, as far as I can remember, a big no no to do the same on the 400, ie I think it will behave the same way, meaning the A/T will retard to IDLE at 25' RA.
Now as to what happens to the lateral and vertical modes after the A/P is dis-engaged, you still have LOC and G/S engaged but I can think for the life of me, wether ROLL OUT and FLARE stay armed or not. I'll get back to you on that on.

[Magoo]

Ok, I think when you dis-engage the A/P after 1500' the FLARE and ROLL OUT mode disappear and you are left with LOC and G/S, but I think at 25', if the A/T were to be left engaged, they would retard to IDLE just like during an autoland.

[Hardy Heinlin]

Perhaps it's an illusion when you think the A/T retards to idle automatically; you just wrote the A/T is very slow and therefore you override it to accelerate the idle run. So maybe you think the A/T servo assists you under your hand, while in fact it doesn't move at all?

On the other hand, it's strange that Peter got an A/T disconnect caution. Perhaps the caution is inhibited when the throttles are at the idle stop.

[Britjet]

Well, I did the test...the A/T stays in SPD, so the thrust levers stay wherever they were for the bugged speed.
I had to close the thrust levers manually on my landing (long after touchdown, because I hadn't noticed that they hadn't retarded).

Peter

[Britjet]

Just found this in the FCTM out of interest...

'Autothrottle use is recommended during takeoff and climb in either automatic or manual flight. During all other phases of flight, autothrottle use is recommended only when the autopilot is engaged."

Peter.

[Hardy Heinlin]

So, what are the latest facts if we refer to the BA sim?

1. When the AFDS status is "NO AUTOLAND" or "FD", the ROLLOUT and FLARE modes are not available.

2. During a "no autoland" flare, when TOGA is armed for landing (G/S captured or flaps out etc.), the A/T mode will never change from SPD to IDLE. Instead, when the aircraft is below 5 ft (+2 sec), the A/T will automatically disconnect with a caution alert and the throttles will stay at their last positions.

(If the A/T were disconnected by the reverser levers, the caution alert would be inhibited.)


|-|ardy

[Britjet]

Sounds about right to me. Always happy to do another test - maybe with video next time.
Peter

[tango4]

Sorry to chime in here,
I just have a small comment regarding what Magoo said about the fact that the 747-400 A/T is not very fast.
Actually, I remember talking to an Air France B777 pilot who was a former 747-400 driver and told me exactly the same thing: that A/T was much more efficient on the B777 than on the B747.
Now I have always had the feeling that A/T system in PSX was pretty darn efficient. Of course, as you know I have no real life comparison. The only reference I have would be PMDG B777, on which the A/T FEELS (I insist on this word, as I did not do any "scientific" measurement on that point) much slower, while it should be the opposite.
I'm curious to get real pilots opinion here ? Is the real A/T system that efficient, even for a "slow" one ?

Hardy, if you wish, you can create a "donate " button on your site: I'd be glad to pay for a few aspirins... I think you might need some.

Charles

[Will]

"Autothrottle use is recommended during takeoff and climb in either automatic or manual flight. During all other phases of flight, autothrottle use is recommended only when the autopilot is engaged"

This makes some kind of sense. It seems that Boeing is recommending the combination of autothrottle + manual flight only in situations in which the thrust is fixed at max, and the pilot is using the pitch to maintain airspeed. In cruise, when the throttles would maintain speed, autothrottle + manual flight is not recommended, presumably because of the less than perfect coupling.

Interesting thread. Thanks for your testing, Peter. I'd love to see a video from your sim session.

[Britjet]

"presumably because of the less than perfect coupling".

It's a disaster with the pitch-power couple. Of course the 777 doesn't have this problem as thrust-induced pitch changes are cancelled by the fly-by-wire.

Peter

[Hardy Heinlin]

Hi Charles,

a donate button is not necessary, but a new thread :-) Could you start a new one re "744 autothrottle rates" or something like that?

The subject mix in the threads is confusing these days ... :-)


Cheers,

|-|ardy