Engaging FLCH on the ground

[Hardy Heinlin]

Hi,

when FLCH is engaged on the ground and the A/T is armed, will this also engage the THR mode (respectively the HOLD mode if the MCP altitude is below us).

On the ground!


Regards,

|-|ardy

[John H Watson]

I've always assumed you couldn't engage anything on the ground other than T/O. You can get the A/P to go into FLCH and V/S modes however (on the ground). With respect to V/S mode, don't the books say you can't engage speed mode below 400'? (or does it just mean the SPD button won't react to a push below 400')?

(respectively the HOLD mode if the MCP altitude is below us).

If it did, would current airspeed also be a factor. As mentioned previously, when FLCH is selected on the ground at zero knots (engines not running) with the MCP altitude above or below, the pitch bar heads south to try to build up to the target speed. In the air with the target altitude below,  does FLCH normally compensate for low airspeed by providing thrust as well as elevator input (or does it try one first, then the other if the airspeed doesn't go up fast enough)? May it not even engage at all if the airspeed is below even the minimum selectable MCP speed?

Re A/T arming prior to takeoff. It seems that some airlines do disarm the A/T prior to takeoff. My old airline didn't.

[Britjet]

Big sim test today, on ground with Autothrottle switch armed. Press FLCH...
TOGA TOGA changes to TOGA FLCH SPD.
Command pitch bar moves to horizon.
No autothrottle response.

Peter

[Hardy Heinlin]

Thanks, Peter! :-)


Cheers,

|-|ardy

[Hardy Heinlin]

Peter, do you remember whether the flaps were up?


|-|ardy

[Britjet]

Flaps were down, Hardy.
Peter

[John H Watson]

Is TOGA (for takeoff) inhibited in a specific flap range, or do the flaps just have to be out of up?

Big sim test today, on ground with Autothrottle switch armed. Press FLCH...
TOGA TOGA changes to TOGA FLCH SPD.
Command pitch bar moves to horizon.
No autothrottle response.

Were you at any particular airspeed, Peter?

(EDIT) I ask this because I have seen the FD bar go well below the horizon with the engines not running (airspeed 0). See video below:

http://www.iinet.net.au/~b744er@ozemail.com.au/PSX/TOGAVS&FLCH.AVI

(video may take some time to load)

[Britjet]

That video is weird, John - I've never seen a pitch bar go all the way down there!
The test I did was just before take-off, with Flap 20 set, airspeed zero.

Peter

[Hardy Heinlin]

I think when the flaps are up, the maximum of the pitch down command is -15°.

The video certainly refers to flaps up.

When you select a V/S of more than -4500 on the ground at 0 KIAS, it goes to -15° as well.


|-|ardy

[John H Watson]

That video is weird, John - I've never seen a pitch bar go all the way down there!

I've reproduced it many times and I'm sure your aircraft will do the same. It may be because flaps are up or maybe because the system knows there is no chance of thrust being used?

[Britjet]

Ok - I'll have a play next week and see if I can reproduce it.
Peter

[emerydc8]

Interesting video, John. Well worth the download and possibly could win me a beer someday.

[John H Watson]

Here's another one which might win you a beer... How low can the airspeed bug go on the PFD?

http://www.iinet.net.au/~b744er@ozemail.com.au/PSX/SpdwoFD.gif

[emerydc8]

I thought it was 100 knots, but obviously it is much lower through engineering legerdemain.

[John H Watson]

There is a trick to it. At the gate, if you switch off the FD and push the speed intervene knob on the MCP (I can't remember if the other FD has to be off/on). You can do this in PSX, too 

I don't know the logic behind it, however.

[torrence]

A couple of comments re current PSX behavior re: 'win a beer items':

FLCH on ground:  As I inferred from the discussion, PSX on the ground (with or without engines running) - FLCH deflects FD pitch to below the horizon.  Also if engines running, it engages the AT and starts advancing throttles.  I gather the big sim doesn't.

Speed bug at 0 kts:  PSX produces the PFD display in John's pix - speed bug way low, 0 at top of speed tape.  I think the trick needs both FD's off, then SPD sel.  FD's back on, the 0 speed bug remains, with 000 shown in the open SPD window.  At this point I found that I could take off more or less normally with 0 as the speed bug, with LNAV armed and with TOGA for vertical guidance (or even FLCH SPD with the caveats noted above).  AP on after 500 ft or so - will have various odd behaviors trying to respond to the 0 kt speed in the box.  VNAV selection restores more or less normal ops.  Strange situation - it's a good thing Watson's trick would be hard to do by accident  .

Cheers,
Torrence

[Hardy Heinlin]

FLCH engagement on the ground will no longer engage A/T in PSX 10.0.9-beta2:

http://aerowinx.com/board/index.php?topic=3685.0

[Britjet]

I just tested in the big sim. All ready for take-off, TOGA TOGA on FMA with a sensible ALTSEL.
Nothing happened. No light either.
Peter

[Hardy Heinlin]

FLCH didn't engage?

[Britjet]

No.