Go around thrust logic?

[tcas_climb 45]

Rob, very strange, I did some more tests and, if the aircraft has touched the ground before performing the go around (therefore not starting the flight from the air), I manage to get THR REF during go around when pressing TOGA twice. The same happened with the other problem of SPD not turning into THR at go around. Did you test any further?

What do you mean? TOGA is supposed to be inhibited if the plane descends below 2ft RA and some seconds after climbing above 5ft RA. are you saying that you can engage TOGA after the plane touches down?

[jgoggi 65]

are you saying that you can engage TOGA after the plane touches down?

 

Sorry, maybe I was not clear. If I save the flight with the airplane on the ILS, then load that flight and perform the go around, I don't get THR REF. If instead I load that flight and, before performing the approach and go around, I slew the plane until it touches the ground, then reposition it on the ILS, exit the slew mode and go ahead with the approach and go around, I get THR REF. At least that's what happened two times today, I used the slew because I did not have time to perform a whole flight, takeoff, circle and land. As soon as I can I will try. I realized something like that also when I was testing why I did not get THR when pressing TOGA once, and the same thing happened, it worked if the airplane had been on the ground before. If this is true and not casual, the flight should always be initiated on the ground, saving while airborne and then restarting from there may cause anomalies. I will do more tests.

[777simmer 117]

Rob, very strange, I did some more tests and, if the aircraft has touched the ground before performing the go around (therefore not starting the flight from the air), I manage to get THR REF during go around when pressing TOGA twice. The same happened with the other problem of SPD not turning into THR at go around. Did you test any further?

Yes I did, today.

 

Same here.

I am glad I did not submit my report yet because there seems to be more to it.

 

The tests I did before were from a saved flight. On the Gnd ready for take off, then come back to land and then 3 approaches with go arounds.

I never saw THR REF in the FMA during those tests.(thust did go to go around thrust though....as reported earlier).

 

But today I started a new flight from scratch.

FSX boots into the default PMDG situation with both engines already running at the gate of my choice.

Then I selected the SHORT turn around panel state (an original PMDG panel state).

Then I did my preflight setup, taxied to the rwy, took off and returned for landing at my departure aerodrome.

 

1st thing I noticed:

I monitored go around performance and must say that things seem a bit too slow.

With AP engaged it really takes a long time before I got a positive rate (as in 4-5 seconds or so).

I think the LevelD sim pitches up more deliberately.

 

2nd

During the first go around I did I pressed TOGA a second time at around 1000ft AGL and got....yep...THR REF indicated and set ....all perfect :-)

 

3rd

During the second approach and go around I could not engage THR REF with a second click.

As before, thrust did advance to Go Around thrust, but the FMA stayed in THR. So not perfect.

 

4th

And the last approach and go around things were even weirder.

I pressed TOGA a second time but this time right after getting a positive rate.

And nothing happened at all.

THR REF was not displayed, but this time thr thrust level did not go to the Go Around thrust limit either.

I clicked again and again and only after passing 400ft AGL did I get Go Around thrust.

But still no THR REF indication.

 

5th

level off in 5000ft from all three go arounds was not good.

Altitude busted by 200-300ft (AP engaged).

The pitch mode goes from TOGA to ALT but it seems that the thrust mode wanted  to keep a high climb rate (due to being in that "invisible" THR REF mode). The result was N1 fluctuation at a too high N1 setting  as if it was not sure what to do.

At the same time the ALT mode want to level off but it cant decide if it should pitch to keep speed at Vref+xx or if it should start the level off.

 

 

So....things are very inconsistent.

It could be the SHORT panel state.....it could be that PMDG programmed TOGA thrust wrong so that a second click will not do anything until 400ft AGL is passed.

The level off ALT mode needs to be looked at for sure though.

 

I will be doing more tests like this from different panel states.

[Budbud 227]

Yep I agree with Rob's observations except only in my case, I did go around above 400ft (ILS 16R at KPAE MDA is 770ft) and did not get THR REF either. I didn't think of changing the panel state though, as I use a slightly personalised short turn panel but never got any other issue with despite hundreds of hours of flight.

[jgoggi 65]

Yes, after doing some more tests I detected a strange behaviour, too. There seems to be no logic concerning THR or THR REF engagement, and very often the aircraft is very slow in pitching up. I think it's something developers should seriously look into... 

[jgoggi 65]

I have just watched the AOA training video where the 777 performs a go around in fog due to no runway in sight at decision height. It seems to work correctly, there, the pilot presses TOGA once and gets THR, then, since thrust is fluctuating, presses TOGA again and THR REF appears. I suppose that video was made when SP1 was not out yet (even because the fluctuating thrust at go around was a problem corrected by SP1), therefore I think the lack of THR REF indication is a bug that has been introduced with SP1 (or SP1b or c).

[777simmer 117]

I have not had the time to test more, sorry.

 

Too many Xmass addons to try out ;-)

 

Not sure I will either.......I think I will just report that TOGA mode could be improved (more deliberate) and THR REF modes sometimes works and sometimes does not.

 

To pin point where these problems come from will require many flights from many panel states and situations, while talking to a software developer from the PMDG team.

[jgoggi 65]

 

 

To pin point where these problems come from will require many flights from many panel states and situations

 

Anyway I don't think it has anything to do with the panel states, I tried 1000 states, configurations and situations but in no case I get THR REF as soon as I press TOGA the second time...

[777simmer 117]

Anyway I don't think it has anything to do with the panel states, I tried 1000 states, configurations and situations but in no case I get THR REF as soon as I press TOGA the second time...

ok, if you already tried that, then it is just a general bug.

[jgoggi 65]

I am really getting crazy! I did some more tests after cleaning up everything about the 777 and installing the whole package with the new SP1d (FSX, not SE). I CAN'T get the go around thrust to work! Everytime I press the TOGA with the thrust mode in SPD, while I get TOGA/TOGA for heading and pitch, SPD doesn't change into THR and the airplane doesn't climb! I can't use the 777 because it can't do go arounds! The weirdest thing is that only ONCE, after reinstalling as administrator, I fired up FSX, chose the airport, the default PMDG livery 777 LR, I went to about 3000 ft, started descending with V/S, A/P and A/T, flaps 30, and, after pressing TOGA the first time, I got THR, the second time THR REF and it worked! BUT only ONCE and I did about 20 tries (each time quitting FS and restarting it). The other 19 times SPD never changed into THR (and THR REF). Everything else is working perfectly with the 777 and the computer, I don't have problems with the BIOS clock, with the system clock, nothing! I tried not using FSUIPC and also reinstalling SP1 (the one released in July, not 1b, or 1c or 1d) to see if the problem was not there, but nothing! What is that controls the go around thrust mode? What may interfere with it? 

I haven't yet understood to what extent it is a problem in my system and to what extent a bug, therefore I would be grateful to all those who can test some go arounds and report, in order to see:

1. if SPD changes to THR during the first TOGA push

2. if THR changes to THR REF during the second TOGA push.

Thank you!

[Michael Moe 494]

I just did three automatic approaches followed by a go around.

 

The system works just fine but you are correct that the THRUST LIMIT indication does not change to THR REF as it should.

 

Just so we are all talking about the same thing first:

 

- The THRUST LIMIT indication is what the autothrottle will use maximum.

The green line at the end of the N1 dial will show you at what N1 this maximum/limit will be reached.

 

(Do not confuse this with the N1 redline limit by the way.

The red line is a real limit beyond which structural damage can occur.

The green thrust limits are just calculated limits that extend engine life just like reduced TO thrust does. You can exceed the green limits (safely) by disengaged the AT and pushing the AT full forward if you need that much thrust.)

 

- So, if the thrust limit says CLB, then the AT will not use more than the calculated CLIMB thrust limit and the green line is set at an N1 speed that defines this climb thrust limit (under the current atmospheric conditions).

- If it says GA then the green line will be a bit higher up the dial at the N1 setting that defines Go Around thrust (under the current atmospheric conditions).

 

But that does not mean the AT will use all that thrust available. The AT can use less than the limit (green line), but not more.

 

So the Green line in the N1 dial and the corresponding CLB/CRZ/TO/GA indication just show a safe limit.....this has nothing to do with what the AT is actually doing.

 

What the AT is doing, is displayed on the FMA (on top of the PFD over on the left). In SPD mode for example, the AT will advance and retard to keep the target speed. The AT is smart though and it will add/retard as much as is required to keep the speed. It will not just go back and forth between idle N1 and full CLB or CRZ or GA thrust limit (green line).

That would be really uncomfortable!

 

 

The same is true for a go around.

The go around thrust limit (GA) is shown on top of the N1 dial and the green line sits high up the dial showing you that IF you wanted maximum GA thrust....that green line is where you would have to set your N1

.

The whole final approach you will see that GA is your thrust limit, but neither you nor the AT if engaged will require that much power under normal circumstances.

During approach the AT is in SPD mode and varies thrust to keep the target speed.

 

And during a go around (single TOGA click) the AT will switch from SPD to THR mode.

The PMDG777 does this correctly.

With the AT in THR mode, +2000ft/min is the go around target for the AFDS (Auto pilot Flight Director Sytem).

 

Note that the GA thrust limit does not change.....you still have GA as your limit and the same green line as during the approach.

The AT could use up to that GA limit green line, but usually is will require much less than that to reach a vertical speed of +2000ft/min.

 

The AP and FD will pitch up to maintain speed at Vref+5 to25kt (pitch for speed) and the AT in THR mode will add thrust as required to keep those +2000ft/min.

During my go arounds N1 advance to around 81%.....on a hot day or at heavier weights more N1 will be required to keep the same +2000ft/min.

THR is telling you that go around thrust is set variably.

Wind shifts can also require the AT to advance more....and it will do so up to the GA thrust limit if required.

 

A second click of the TOGA switches should engage the AT in THR REF mode.

It does so correctly in the PMDG777.....I could clearly see on each go around how N1 went up from those initial 81% to the GA thrust limit green line!

But on the PMDG777 the AT mode indication still says THR.....it should be saying THR REF.

So the system does work....but the AT mode annunciation (FMA) is not changing correctly with what the AT is actually doing.

Not a big deal.....but a small bug that should be reported yes.

 

The videos you have posted where the PMDG777 simply stays in SPD mode after pressing TOGA is something else completely.

That is just all wrong but I dont know what happened there.

If you still have that problem as well then I would suspect a corrupt install since you have already tried deleting the FSUIPC.ini and you are no longer using a saved panel state or saved flight.

 

 

 

Great stuff and very well explained.

 

Thank you

 

Michael

[777simmer 117]

:-) thank you.

[Michael Moe 494]

:-) thank you.

You are most welcome.

 

Just one thing more. What if the AT gets disconnected just 100 feet more or less before DH /DA. What should the logic procedure then be?

 

Thanks Michael

[777simmer 117]

It depends a little on company procedures (SOP) as to what you can do and can not do I guess.

 

Below Alert Height (200 ft) the 777 inhibits all Master Warnings and Master Cautions except:

AUTOPILOT DISC

AUTOPILOT

AUTOTHROTTLE DISC

NO AUTOLAND

SPEEDBRAKE EXTENDED

 

If any of these messages come and you planned to do a CAT IIIB autoland,then you must fly a go around.

In the case of AUTOTHROTTLE DISC dont forget to manually add thrust!

 

Then you can do another approach with minima set for a CAT IIIA with manual thrust (our SOP allow this).

Of course if the weather does not allow a CAT IIIA approach then you have to divert.

[jgoggi 65]

Ok, after some more tests (testing for almost a month, now), I can state that, aside from the THR or THR REF bugged activation, there is no logic in the PMDG 777 go around overall mode activation and this needs fixing. The go around mode does not arm everytime the flaps are out of up or the airplane is on the glideslope. Maybe the first time it works but, if you do a second go around, it often does not activate again and at times you don't even get TOGA/TOGA modes. Then it may work again or work no more, I couldn't find a logic in this. It should instead fully activate **everytime** you push TOGA and the flaps are out of up or you are on the glideslope, period.