747-400 Go Around FMAs

[StopStart]

I'm slightly confused about the sequence of FMAs during a TO/GA missed approach and was wondering if someone could edify me.....?

I'm quite happy with the mechanics etc of the go around - as far as selecting VNAV at 1000ft agl. Does the AT go into THR REF or stay in THR at this point? If it's THR REF, what limit is it referencing? CLB?

Initially TOGA pitch is aiming to achieve 2000fpm (at the MCP or engagement speed) and THR applies/adjusts power to achieve that. Once VNAV is engaged and pitch reverts to VNAV SPD are the "roles" not effectively reversed, with the AT supplying a fixed thrust (eg. CLB) and the pitch axis now adjusting to achieve that speed?

Would really appreciate any wisdom and insight that could be offered!

I'm new to the type and the FCOM is a little vague on the subject....

[SMOC]

Pitch modes TOGA/FLCH SPD/VNAV SPD are all speed controlled by elevator,

One push of the TOGA switches gives you THR as you said which aims initially for 2000fpm roc it doesn't chase it there's a transitional phase where things are blended, it depends on the pitch rate and acceleration for when the algorithms/computer think it's added sufficient thrust to achieve the 2000fpm. If the rotation is very slow you'll end up with over 2000fpm. Way beyond the scope of flying. Eventually you'll be in TOGA with speed on elevator and the resultant roc a reflection of the performance during the rotation.

Selecting VNAV at 1000'aal will give you THR REF (CLB thrust) & VNAV SPD it's basically exactly the same, however it also closes the speed window thereby allowing the A/C to accelerate, so the A/C will pitch down or command a pitch down as it's now at the wrong commanded speed.

Opening the speed window in TOGA and winding the speed up would do the same thing except the thrust would stay fixed in THR at the original thrust it locked onto while attempting to achieve 2000fpm roc.

[StopStart]

SMOC - that's excellent, thanks very much, all much clearer now.

To take it a little further (if I may) just to further my understanding of the whole thing...!

Is full CLB thrust a significant increase on the TOGA/2000fpm thrust?

Am I correct in thinking that selecting FLCH at 1000ft (window remains open) will leave the AT in THR? Selecting the mysterious THR button post acceleration will then engage CLB and a change to THR REF?

Out of interest would using FLCH instead of VNAV be a more "controllable" way of managing a go around into a busy environment with, say, a relatively low missed approach altitude (and low MSA)? My thinking being you avoid the thrust increase to CLB and the requirement to reopen the window and wind the speed back from that commanded by VNAV SPD (VRef+100?)

Thanks! :8

[SMOC]

Is full CLB thrust a significant increase on the TOGA/2000fpm thrust?

It could be an increase it could be a decrease all depends on weight and how accurate the transition was towards 2000fpm I guess.

THR button I think you're correct it's been a while since I've thought about it.

For a very low level off or near level off, I'm spring loaded to press TOGA and immediately press VNAV as it's the window closing and jumping to over 200kts that I find most benificial in keeping the nose down. However if below 1000aal and unable to accelerate I choose V/S once the A/C has pitched up sufficiently, I just hit it while it pitches through 800-1000fpm, it slows everything down nicely especially bringing the thrust under control. Best briefed and thought about often as we all know GAs aren't always when we expect them, I also try and pause for a sec if I have time available to think about my actions after hitting TOGA. Did a GA at 900aal years ago but had time to think so within the next 3sec hit TOGA then LNAV then VNAV was the smoothest one I've ever done levelled off at 2000' as instructed by ATC.

[JammedStab]

Selecting the mysterious THR button post acceleration will then engage CLB and a change to THR REF?

Selecting the THR button will adjust thrust as appropriate for the situation in terms of engines operating. For all engines operating, after takeoff whether in VNAV or FLCH the thrust will be changed to the armed climb limit. The climb limit was armed on the ground as CLB, CLB1 or CLB2. Which one is selected? it depends on the situation. Heavy departure and a high initial altitude or close-in minimum crossing height and CLB(full climb thrust) is a likely choice. Light weight with a low altitude level off or you don't want fuel pump lights on due to a high pitch attitude, then CLB2 is better. Either way, selecting it selects the armed climb thrust. But why do that when it will happen automatically anyways from the FMC entry on the takeoff ref page. In other words, it doesn't get pushed. I have never touched it once in the aircraft. In FLCH, it is a different story as you are operating at a lower automation level and so if you were to a FLCH takeoff(perhaps for QFE ops where VNAV is not allowed), pressing THR selects climb thrust. After a go-around, pushing THR selects CLB thrust.

If you have one engine inop on takeoff, The FMC is smart and knows this and knows that you don't want climb thrust. Once again, while in VNAV, there is an automatic thrust change, but this time to max continuous thrust at the appropriate speed when the flaps are up which is Vref+98 . But if in FLCH, you manually select MCT with the THR button at around Vref+100. This I have done in the sim due to a screw up in setting up the FMC during repeat exercises where VNAV was leading to a descent so FLCH is selected. I think it was due to not setting a cruise altitude in the FMC. After a go-around, pushing THR selects MCT.

In other pitch modes such as ALT or V/S, pushing it can give you THR REF for the appropriate reference thrust limit.

In other words, the button never really gets pushed.

At least I think this is how it works.