747-400 Autothrottle Disconnect

[NSEU]

Gents, I'm just wondering what will reactivate the A/T automatically on a 747-400, if anything, after the A/T has been disconnected with a double push of the disco button/s on the thrust levers and, say, with the A/P off (double push also).

I know the TOGA switches will manually reactivate the A/T, but is there any alpha protection (nothing is mentioned in the engineering manuals)?

If the F/D is still on in VNAV, FLCH, G/S, etc, will anything re-engage the A/T automatically? i.e. including F/D mode automatic mode changes.

Thanks!

[JammedStab]

Manual activation only which is to activate or re-activate....

AUTOTHROTTLE ARM switch........ARM

For pitch mode TO/GA:

Push either TO/GA switch


If pitch mode is in VNAV PTH, VNAV ALT, VNAV SPD, or FLCH SPD:

Select the AUTOTHROTTLE ARM switch to OFF, then ARM
Verify THR REF, THR, SPD, IDLE, or HOLD displays on flight mode
annunciation.


If pitch mode ALT, V/S, G/S, or no pitch mode:

Push the SPEED switch


Disconnecting the autothrottle is one activation of either disconnect switch. The second activation to resets the master caution lights and EICAS message.

Alpha protection.....The autothrottle and AFDS independently provide speed protection for all operations except during V/S pitch mode or engine failure above maximum engine-out altitude. Autothrottle speed protection is limited by the reference thrust limit (CLB, CRZ, CON, etc.) and idle.

[NSEU]

Much appreciated.. thanks.. I think that answers most of my questions :ok:

How you guys remember the myriad of variations in flight and under pressure is beyond me :}

[JammedStab]

I just try to remember that if there are two words in the FMA pitch mode, you can reset with two actions on the A/T switch being off then on. If one word then one push of the speed button.

You mentioned two pushes to disengage the autopilot(s) but it is one push of the disconnect switch on either control wheel(or the bar on the MCP). The second push resets:the master warning lights, EICAS warning message and the aural warning. Pilots just do it quickly so you don't even get the warnings but the A/T switch seems to have to be done less quickly and in a more deliberate manner to avoid the warnings.

[NSEU]

Brilliant aide memoire, thanks.

Sure, I understand the two-button push, although our a/p disconnect seems to sound at least once aural cycle no matter how fast you double push.

Re VNAV approaches, with final approach under manual control (no ILS), I'm wondering what happens on landing. Does the VNAV pushbutton extinguish on landing (and the FMA blank)? Otherwise, you might re-engage VNAV mode (throttles moving) by cycling the A/T switch. I understand that the A/T is disengaged with the use of (EDIT) reversers, but it re-arms after a certain time period of stowing the reversers (flaps not in UP)

I understand that VNAV is not normally available until above 400', but if the mode is still annunciated in green, does this mean it is available.

I guess this is something to try in the sim.

[Pontius]

I'm wondering what happens on landing.

From The Book, regarding VNAV approaches:


The FMC transitions out of "on approach" under the following conditions:
• selecting TOGA
• the airplane lands
• the airplane flies beyond the last waypoint in the approach (missed
approach waypoint or runway) and the VNAV page title changes from "ACT xxxxxx DES" to "ACT END OF DES"

It doesn't get any more specific than that, so I assume it must be a weight-on-wheels switch or tilt switch that gets rid of VNAV.

[SMOC]

I know the TOGA switches will manually reactivate the A/T, but is there any alpha protection (nothing is mentioned in the engineering manuals)?

If the F/D is still on in VNAV, FLCH, G/S, etc, will anything re-engage the A/T automatically? i.e. including F/D mode automatic mode changes.

Managed to do this in the sim FL370 with normal modes such as SPD . LNAV . VNAV PTH auto throttle disconnected (SPD mode blanks) thrust reduced to idle this is what happened.

A/C slows towards MERS (amber band) once below MERS maybe 10kts the airspeed box is highlighted in amber and "airspeed low" EICAS (if fitted).

VNAV PTH changes to VNAV and the A/C goes into pitch hold, shortly thereafter VNAV has and amber line struck through it and the pitch F/D command disappears.

The A/C would have descended in pitch hold around 10knots above the red bricks. (Because the amber hockey stick varies in size so the mode changes vary its not exact).

Now if you're nasty and hit ALT HOLD (pitch F/D command reappears) the speed reduces to the buffet and stick shaker below the red bricks.

Some figures, A/C was above optimum below max alt forgotten what weight.

Maintaining FL370 pitch was 7 degrees up airspeed 222 / M.696 with major buffet no stick shaker just below the red bricks.

Recovery was pitch down to the horizon or a little more the A/C descended at over 5000 fpm then Max thrust set which took at least 15-20sec to achieve. Once above MERS level off and accelerate towards bug speed. Recovered at just over FL340.

So to answer you question no the A/T doesn't wake up at anytime but the A/C will attempt to do the right thing and give you plenty of indications that its time to do the right thing.

[NSEU]

Thanks! Very interesting.

[JammedStab]

Re VNAV approaches, with final approach under manual control (no ILS), I'm wondering what happens on landing. Does the VNAV pushbutton extinguish on landing (and the FMA blank)?

To quote....
"When an arrival or approach procedure is selected on the ARRIVALS page and incorporated into the flight plan, the FMC creates an E/D. The E/D is located 50 feet above the runway threshold (RW waypoint) for all approaches except VOR approaches. The E/D for VOR approaches is the missed approach point"

I would think that the VNAV Path ends at the E/D point but that is to be confirmed.


Otherwise, you might re-engage VNAV mode (throttles moving) by cycling the A/T switch. I understand that the A/T is disengaged with the use of (EDIT) reversers, but it re-arms after a certain time period of stowing the reversers (flaps not in UP)

I understand that VNAV is not normally available until above 400', but if the mode is still annunciated in green, does this mean it is available.



VNAV is selected or armed by pressing the VNAV button, not by cycling the autothrottle switch. VNAV in green means that VNAV is active. It is white when only armed. It is available below 400 feet on approach.

[NSEU]

VNAV is selected or armed by pressing the VNAV button, not by cycling the autothrottle switch. VNAV in green means that VNAV is active. It is white when only armed. It is available below 400 feet on approach.

Understood. However, the A/T in this situation has been manually disconnected leaving VNAV still annunciated in green and the aircraft has been flown onto the ground. E/D speed is not usually zero, so _if_ VNAV is still shown in green after landing, will cycling the A/T switch reeactivate the A/T (as it does in the air) and have the engines spooling up to achieve E/D speed.

Possibly, results may vary according to flap position and reverser use.

(Edited for typos)

[JammedStab]

To quote....
"When an arrival or approach procedure is selected on the ARRIVALS page and incorporated into the flight plan, the FMC creates an E/D. The E/D is located 50 feet above the runway threshold (RW waypoint) for all approaches except VOR approaches. The E/D for VOR approaches is the missed approach point"

I would think that the VNAV Path ends at the E/D point but that is to be confirmed.



Did an RNAV approach in the sim today. VNAV PTH in the FMA stays annunciated all the way to touchdown.

Was told that this is the case for a Loc or RNAV approach because there is a runway waypoint. But, this is not the case for a VOR approach because it does not have a runway waypoint.

[NSEU]

Did an RNAV approach in the sim today. VNAV PTH in the FMA stays annunciated all the way to touchdown.

Interesting. Do you recall if VNAV PTH remained annunciated ON the ground? If so, cycling the A/T switch may reactivate the A/T (big maybe). Reversers would have to be stowed, among other prerequisites.

Thanks

[JammedStab]

VNAV PTH disappeared on touchdown, but I don't think it would be anything to do with an air/ground sensor. Maybe just being on the runway location in the FMC.

[NSEU]

Just checking... Mode change after landing wasn't co-incident with reverser deployment was it?

[JammedStab]

I believe was touchdown but will try to find out.

[NSEU]

No urgency, just curiosity, thanks )

Looking through some notes, I see the 744 A/P doesn't seem to use the Air/Ground System as a trigger for mode changes, at least not directly. The Rad Alt provides an in-air/on-ground signal to the FCCs at 5'. The Air/Ground System only enables testing. However, the FMC may be controlling mode changes at this point.