TOGA Between 50' & 400'

[emerydc8]

Hi Hardy,

I was studying the FCOM and happened to notice that if TOGA is pressed between 50' and 400' after takeoff, the TOGA roll mode will display. Have you or Peter done any experimenting with this in the sim? PSX will correctly take out the derates and activate the THR REF mode when TOGA is pressed between 50' and 400', but it stays in LNAV. It does display TOGA roll and pitch modes if TOGA is pressed after 400'. I'm not sure if TOGA roll should replace LNAV after liftoff, but perhaps displaying TOGA roll below 400' duplicates the logic that occurs when pressing TOGA on a missed approach (below 400') with the A/P and F/D off -- THR|TOGA|TOGA appears in this case. Any thoughts on this? Thanks.

Jon

[Hardy Heinlin]

Hi Jon,

as far as I recall, the mode transition logic in PSX is based on a transition table from a maintenance manual.

In the TO-GA logic, TO conditions are not always identical to GA conditions (in TO you are initially climbing, in GA you are initially descending).

For example, in TO logic LNAV cannot engage below 50 ft, in GA logic not below 400 ft.


Cheers,

|-|ardy

[emerydc8]

Thanks, Hardy. It's definitely unusual to press TOGA prior to 400' on takeoff. I guess a windshear encounter would be the exception, so you can get max TO thrust when the derates are taken out, but this would only be a backup to manually firewalling the throttles. I'm not really sure what Boeing's logic would be to replace LNAV with TOGA once LNAV is already active. Maybe Peter knows.

Jon

[Hardy Heinlin]

In TO mode, maybe LNAV guidance is safer than maintaining the current track* because the FMC route usually provides lateral obstacle clearance. In TO you are past the runway threshold, flying away from the airport; in GA you are in front of the runway threshold, approaching the airport.

* Think of the Kai Tak departure from runway 31. What if you get a windshear and you push TOGA to get max thrust? You would not only get max thrust, which is good, you would also get a command to fly the current track into the mountains.

I'll recheck the logic anyway. Perhaps it's wrong in PSX, or a special FCC software version.

[emerydc8]

Good points. That's why I don't understand Boeing's rationale here. The way the FCOM is worded, it does seem to be referring to takeoff (after liftoff) -- not go-around. On a go-around, we all know it will go THR|TOGA|TOGA if pushed below 400' because we see it every time. I don't think I've ever pressed TOGA between liftoff and 400' in the sim or the airplane.

[Britjet]

It am pretty sure it should not stay in LNAV. It should be TOGA TOGA.
The windshear scenario is one example where this is used, and an LNAV turn would definitely not be useful in such a scenario, where wings level is a performance optimum, but also it provides a way to achieve max thrust of course for an engine failure after take-off at heavy weight, not that this should usually be necessary.
I used to suggest this method in the sim as an alternative to selecting in the FMC as it seemed that "heads down" in the FMC was not a good idea at low altitude. It did require a reselection of LNAV.  In fact BA at a later stage amended the FCOM to use the FMC method, but the TOGA method would still work.
I hadn't noticed this. I can probably test it in the sim this weekend just to confirm.
Peter.

[Hardy Heinlin]

Thanks, Peter. What is the "FMC method"?


|-|ardy

[Hardy Heinlin]

I just rechecked the roll mode transition table and it says that none of the roll modes can be changed to the T/O mode (but they can be changed to the G/A mode). However, I probably misinterpreted the definition of the T/O mode: I thought if you press TOGA during take-off below 400 ft, "TOGA" still refers to T/O mode, not to G/A mode. This is a confusing mix of modes and logics because if I apply the G/A logic here, the actual go-around logic of "first THR, then THR REF" will be applied as well. This means, I need to create a second special G/A submode, the first one being for take-off (THR REF), the second one for the true go-around (THR, THR REF).

Design-wise, calling this take-off phase a go-around mode is pure nonsense, in my opinion. I would call it a T/O mode and rewrite the transition table instead. Anyway, it looks like the actual T/O mode in the "TOGA" world is the mode you get when you push TOGA on the ground before 50 KIAS; any TOGA push thereafter will engage the G/A mode.


|-|ardy

[Jeroen Hoppenbrouwers]

... or cancel GA mode ... when you go around, I understand most pilots press the TO/GA button four times  :-D


Hoppie

[Britjet]

Thanks, Peter. What is the "FMC method"?


|-|ardy

Just going into the FMC to select GA in the THR LIM Page...
Peter.

[Hardy Heinlin]

Ah, this! :-)

Windshear, windshear! ... head down ... turbulence ... hand on CDU ... turbulence ... push INIT REF ... push 6R ... push 2L ... bingo: GA :-)


|-|

[emerydc8]

That would be the EK 777 technique.

[Magoo]

When encountering wind shear during the takeoff roll, firewall the thrust levers, once airborne press TOGA to remove the derates and prevent VNAV to reduce back the thrust at 400'.

[Britjet]

Ah, this! :-)

Windshear, windshear! ... head down ... turbulence ... hand on CDU ... turbulence ... push INIT REF ... push 6R ... push 2L ... bingo: GA :-)


|-|

Indeed. They wouldn't do that for a windshear warning of course, just to increase thrust for some other reason.
Peter.

[Hardy Heinlin]

Makes sense. I forgot the fact that the <GA key is not on the THRUST LIM page until CLB or CON is activated, which typically happens when VNAV engages or when the MCP THR switch is pushed -- all that can't happen below 400 ft. (Also, in that phase, the INIT REF key will promptly open the THRUST LIM page.)


|-|ardy

[emerydc8]

Anyway, it looks like the actual T/O mode in the "TOGA" world is the mode you get when you push TOGA on the ground before 50 KIAS; any TOGA push thereafter will engage the G/A mode.

|-|ardy

Are you referring to any push of TOGA while airborne? I'm not sure, but I think if you push TOGA on the ground a second time while in HOLD, nothing will happen. Is that your understanding too?

Jon

[Hardy Heinlin]

TOGA engagement is inhibited above 50 KIAS on the ground. (A strong headwind may inhibit it even before you push the button the first time).


|-|ardy

[emerydc8]

You're right. I was thinking more along the lines of what pushing TOGA a second time (once it's already in HOLD) would do to an armed LNAV, but now that I think about it, the roll mode should be locked out above 50 knots too -- until 50' AGL.

[Britjet]

I just tried a TOGA push in the big sim between 50ft and 400ft.
LNAV (green) and VNAV (armed) bith disappear and you just get THR TOGA TOGA.
HTH.

Peter.

[Hardy Heinlin]

Thanks, Peter.

Just to be sure it's not a typo: You got "THR", not "THR REF"?


|-|ardy