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Recently a final report for accident of an Emirates B777-300 at Dubai on 3rd August 2016 was published. An important factor there was that when the pilot tried to recover from long landing by going around, the TO/GA buttons were inhibited because the wheels were on the ground and the pilots did not realize this.

Now at first sight, this seems like an unnecessary complication of the auto-thrust system that just adds a point the pilots need to learn. But I suppose the engineers did have some reason to add this bit of logic.

So what dangerous condition is inhibiting the TO/GA button when weight on wheels or radio altitude below 2 feet for 3 seconds supposed to prevent?

I would understand after reversers deployed, but before that going around is considered appropriate solution for long landings and bounces, both of which might trigger the inhibition.

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  • $\begingroup$ I feel like a helpful clarification is whether the 777 has an auto-takeoff, or if it is assumed that only a human will control the plane when it's on the ground (auto-land excluded). $\endgroup$ – zymhan Feb 6 at 20:54
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    $\begingroup$ @zymhan, I believe it uses the TO/GA button on take-off, but it does different action as take-off thrust is determined differently than go-around thrust. $\endgroup$ – Jan Hudec Feb 7 at 21:00
  • $\begingroup$ The TOGA button is not inhibited, but the autothrottle disconnects exactly for the reasons mentioned above. if you push TOGA, the FD will still direct you for a go-around position of the aircraft nose, if you have the correct velocity of course.. What happened there was that they pushed toga, and because they still had enough speed- the FD directed them to pull, leading to a bleed of speed which led eventually to a stall and belly landing. $\endgroup$ – Stan Feb 8 at 20:09
  • $\begingroup$ @StavNemirovsky, but doesn't the TOGA button engage the A/T if armed in other cases—on take-off and if it was turned off on approach for some reason? Since it does not do this after touch-down, it is still inhibited in this sense. $\endgroup$ – Jan Hudec Feb 8 at 23:31
  • $\begingroup$ @janhudec the TOGA engages the take off/ go atound mode. This includes FD instructions and the Autothrottle. After landing the button still works, but it does not re-engage the AT. From here on- ir's only semantics. Yo can ssy that AT engagement by the TOGA button after touchdown is inhibited, that would be more correct. Refer to the 777 FCOM for more. $\endgroup$ – Stan Feb 9 at 6:30
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Boeing's officially stated reason in the report is:

... to prevent pilots from inadvertently activating TO/GA mode at or after touchdown. Inadvertent activation of TO/GA could result, among other things, in the aircraft departing[1] the runway. ...

[1] As in runway overrun/excursion.

Which is in accordance with AC120-29A (also from the report), where:

... If an automatic go-around mode can be engaged at or after touchdown, it should be shown to be safe. The ability to initiate an automatic or flight director go-around at or after touchdown is not required or appropriate. ...

It's either Boeing did not want to, or could not, demonstrate it would be safe, hence the inhibition (nor is it a required system). Boeing does not elaborate on the "among other things".


Some remarks on the 777's TO/GA switch:

Applying full power on a go-around on a 777 after consuming most of the trip fuel is tricky because of how powerful the engines are. It would produce a big nose up.

For this reason, pressing TO/GA – unlike most planes – does not command full power, rather enough power for a 2,000 ft/min climb. Pressing TO/GA again, would command the full power (source: FCOM 6.4 Page 51).

So there's logic involved.

The closest Boeing patent I found was the Total energy based flight control system published in 1984, which does exactly that, among other things. A system for commanding elevator and/or thrust based on the plane's current and commanded energies for a target flight path. (Download the PDF for the logic diagram.)

For that to work, the plane needs to know its current flight path. If the wheels touch, and with the drag of the ground friction, which is unaccounted for in the logic, the system can't function as designed, or additional logic will need to be added and certified.

Note: The same inhibition, climb target logic, and a double-push for THR REF is also on the 747-400 (source: FCOM 4.10.17). The patent's date makes it plausible that the 747-400 was of the first to receive this system, and it was carried over to the 777.

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  • $\begingroup$ It would seem that 777 has control authority issues, as it is fbw and thus reacting to full throttle when light should not be a problem if control surfaces were sufficient? $\endgroup$ – Jpe61 Feb 7 at 8:43
  • $\begingroup$ @Jpe61: I don't think so. One can counter with a fwd control column input, but it isn't what you need to focus on when going around. $\endgroup$ – ymb1 Feb 7 at 8:50
  • $\begingroup$ So the logic of 777 fbw is fundamentally different from Airbus. $\endgroup$ – Jpe61 Feb 7 at 8:56
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    $\begingroup$ @Jpe61: Airbus doesn't use a TOGA button. In normal operation the levers act as thrust limit selectors. For an A380 go-around for example: If TOGA thrust is not required, set the thrust levers to the TOGA detent then, retard the thrust levers as required. If you did the same in a Boeing, the A/T will take them back to THR REF. After manual positioning, the autothrottle system repositions the thrust levers to comply with the engaged mode. $\endgroup$ – ymb1 Feb 7 at 9:04
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    $\begingroup$ @Jpe61, the “move to the TOGA detent then retart as required” is because moving to the TOGA detent does the secondary functions of a TOGA button: ensuring the spoilers are stowed and in types that can't go-around with full flaps, partially retracting those (I am not sure which Airbus models need to). $\endgroup$ – Jan Hudec Feb 7 at 20:58

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