Go-Around procedure

[Leif9619 0]

Hello guys,

 

firstly, I want to congratulate PMDG for this wonderful airplane.

 

Secondly, I have a little problem. I am not sure if this is a bug or just my stupidity.

 

Since I had to go around today, I noticed, that after pressing TO/GA the thrust levers increased and my A/T activated in THR mode for a 2000 FPM climb. Roll and pitch modes changed to TO/GA. That's how the FCOM 4.20.15 (PDF page 673) describes it. But despite changng the target airspeed to the actual airspeed the targed airspeed remained the same as for the approach. Then, after having engaged CMD 1 and after LNAV was activated I selected a higher speed to retract the flaps. After that, I selected FLCH at about 1300 ft AGL. Thereafter, pitch decreased slightly and climb thrust was set. The engines' thrust also decreased slightly and the aircraft had a hard time to accelerate while climbing to the missed approach altittude with about 600 FPM. I retracted the flaps according the flap retraction schedule and selected flap 1 maneuvering speed, but then the engines spooled down further and the aircraft nearly maintained level flight. I pushed the THRUST switch at about 2100 ft AGL, but nothing happend. When I selected V/S and A/T speed mode was activated the aircraft began to climb again. FCOM NP.21.53 (PDF page 147) points towards selecting FLCH at acceleration height and pushing the THRUST switch after flap retraction to the planned flap setting. Also, the FCOM says that climb thrust is set after the THRUST switch is pushed and not after FLCH is pushed.

 

What do you think? Did I do something wrong or is this normal behavior? I also recorded the scene and uploaded it on youtube: 

 

Thank you in advance!

 

Leif Gehre

[scandinavian13 5,192]

 

 

What do you think?

 

Right before the modes changed, I noticed that thrust wasn't at target. What are your hardware AT settings in the CDU menus? Additionally, I heard a few clicks before it switched into SPD + V/S. What were you hitting?

[Leif9619 0]

Thank you for the quick response, 

 

I have A/T override set to NEVER. I think I had pushed the TO/GA button again before I pushed V/S.

[Leif9619 0]

Hello,

 

isn't there anybody who can help me?

Shouldn't the thrust increase to the selected climb limit when pushing THR while FLCH SPD is the active pitch mode?

 

Regards,

 

Leif Gehre

[skelsey 765]

Hi Leif,

 

The THR switch is sometimes referred to as "the button that makes the wings fall off". In other words, it's a slightly confusing multi-function switch which does things you can accomplish in a more user-friendly way elsewhere, and so many adopt the view that they don't really understand it so they don't touch it .

 

If the THR switch stays blank when you press it, the thrust LIMIT will change to the next one down (GA to CLB or CON, CLB to CRZ etc). The actual thrust may or may not change.

 

If it lights up, the thrust levers will advance to THR REF.

[Leif9619 0]

Thanks Simon,

 

this is interesting. Sice as you might have seen in the video nothing happened after I had pressed the THR switch. I found it very confusing that after I had pressed FLCH and the thrust limit changed from GA to CLB the airplane decreased pitch slightly to about 10 degrees, but the engines weren't turning at climb limit. Instead, N1 even decreased to about 83%. So the aircraft couldn't accelerate.

I did some go-arounds yesterday as well. When I pushed the THR switch the engines should have turned at climb limit, but actually nothing happened. In the video you can also see that when I selected the speed to 210 KIAS while in FLCH SPD the aircraft was still at nearly 10 degrees of pitch and the engines reduced N1 further to about 75%. The aircraft wasn't accelerating and couldn't climb as well.

I have also watched this CBT (https://youtu.be/kbPFCCwfahI?t=6m19s) and it is said that pushing THR in FLCH changes thrust to the selected clb thrust limit while in my case the speed mode should stay in THR.

 

I am not sure, but I think that there's something wrong. I can't imagine that this is normal behavior.

[skelsey 765]

it is said that pushing THR in FLCH changes thrust to the selected clb thrust limit while in my case the speed mode should stay in THR.

 

Yes. In FLCH the thrust limit should change (from GA to CLB) but the actual thrust mode will not change.

 

So if you are in THR REF (let's say you pressed the TOGA switches twice), pressing the THR switch will change the thrust limit to CLB. Since this is below the GA thrust limit, this will result in the thrust coming back to CLB.

 

Pressing the THR switch whilst in FLCH will change the limit, but (probably, unless the current thrust is above the new limit) not the actual thrust output. For what it's worth, if I was going to touch the THR switch at all I'd do it after levelling off at the missed approach altitude - there's not really any need to change the thrust limit whilst you are in THR.

 

Either way, I agree with you that the behaviour you are experiencing is incorrect -- the thrust limit should stay at GA when you select FLCH.

 

Incidentally, strictly speaking the sequence should be to press FLCH then SPD (the press of the SPD button will not normally have any effect -- it is just in case the aircraft has captured the missed approach altitude whilst still in TOGA which will result in the speed running away) but this should not have any impact on what you are seeing.

[Leif9619 0]

Ahhhh, ok. Thank you for clarifying. I seem to have misunderstood the part with the climb limit. 

 

 

 

Incidentally, strictly speaking the sequence should be to press FLCH then SPD (the press of the SPD button will not normally have any effect -- it is just in case the aircraft has captured the missed approach altitude whilst still in TOGA which will result in the speed running away) but this should not have any impact on what you are seeing.

 

Shouldn't the SPD mode engage automatically when the ALT mode engages?

 

 

But what about the strange behavior after I had selected a higher speed? Can you tell me why the airplane didn't acellerate to that speed? The engines spooled down immediately after I had selected speed to 210kts and the plane nearly maintained level flight. Should I have pressed the SPD switch before selecting a higher speed?

[skelsey 765]

 

 

Shouldn't the SPD mode engage automatically when the ALT mode engages?

 

It doesn't if you capture the altitude in TOGA mode -- you get THR (REF) | LNAV/HDG/TOGA | TOGA and the speed starts getting very fast, very quickly .

 

 

 

But what about the strange behavior after I had selected a higher speed? Can you tell me why the airplane didn't acellerate to that speed?

 

Nope, that behaviour looks wrong to me. It should accelerate just fine in FLCH -- though I'd be careful about selecting too high a speed as I seem to recall that FLCH, VNAV etc may allow the aircraft to sink during acceleration (whereas TOGA pitch will not). Pressing SPD, as I say, should have no effect in that regard -- as I say, the only reason for being in the habit of automatically pressing it after FLCH is to take care of the case where you've got a low missed approach altitude, you're perhaps a bit late hitting FLCH and the aircraft captures the altitude before FLCH can engage).

[Leif9619 0]

That's interesting. I will keep that in mind.

Hopefully PMDG will correct this in a future update.

However, thank you very much for your help.

 

Regards,

Leif

[emvaos 13]

Leif,

I've seen the video.

First : FLCH is an FMC driven mode aiming to reach next altitude in 125 seconds or more. It will accordingly set appropriate thrust (not necessarily max as in VNAV SPD). This why you see THR and not THR REF in the FMA window when you climb in FLCH.
In this example at the altitude engaged it is shooting at around 1000 fpm or slightly less at target climb speed in MCP window and current flaps! Not a lot of thrust required for this.

Second: you retract flaps 20 knots below the green marker. You go from 20->10 at the 10- marker and 10->5 at the 5- marker. Not 10->5 at the 10 marker as in the video. Otherwise you have the aircraft hanging out at a very high angle of attack and hit the wrong side of the drag curve. Take also into account that you order the plane to accelerate too and you got very low climb and acceleration rate.

 

PS. FLCH and VNAV SPD do NOT let the aircraft (any Boeing plane in fact) sink in a climb or climb in a descend!

 

Best,

[Leif9619 0]

EMV,

 

I didn't know that FLCH aims to reach the next altitude in a certain time. If so, it seems much more logically to me.

Regarding the flap retraction: Yes, having seen that in the recording I was shocked by what I did myself.

So if I had retracted the flaps on schedule and give the aircraft time to reach the selected speed, the aircraft would have reached the selected 4000ft in >125 seconds with an appropriate thrust setting, right?

 

Thank you for the detailed description.

 

Regards,

Leif 

[emvaos 13]

I believe so.

 

This nice feature is there since the 757 for all Big Boeings ie. the 57/67/47 and 77 chronologically.

 

I believe Boeing introduced this given the vast weight-to-thrust differences these airplanes can have in different flight conditions and as an incentive for the pilot to use FLCH rather than VS. FLCH always protects the plane from going outside the speed envelope while VS does not.

 

Makes sense also for small altitude changes and pax comfort. On a light 744 going from 5000 to 7000 at full thrust it might become a 6000 fpm maneuver. On the other hand, it can be a PITA if you intend to climb only 1000 ft and you have to do it at 500 fpm (use VS instead).

 

Our 777 if you own it has the exact same model.

 

Best,

 

Vangelis

[Captain_Al 66]

There are a couple of problems here that I see.

At the acceleration height (1000 feet in this case) you want to select a pitch mode of VNAV or FLCH, which you did. VNAV will increase your speed automatically up to the current flap limit speed minus 5 knots, then you retract flaps on schedule. If you use FLCH, then you need to "Bug Up" to your desired maneuvering speed. Could be 250 knots and you are going to clean up to Flaps Up, or maybe 200 knots and keep the flaps at 5, your choice if you are going to hold. But, procedurally you should increase the speed at 1000 feet more than you did up to whatever maneuvering speed you want to hold.

Second problem, you retracted to Flaps 5 almost 20 knots below the minimum maneuvering speed and you did not bug the speed up. This combination is not a good one on a go-around. You are going to suffer climb wise by doing that and is not safe.

The other problem though seems to be how the model responded to that problem. In FLCH SPD the elevator pitches to maintain your speed and the A/T controls your rate of climb. Clearly, the thrust reduced and should have increased up to G/A thrust. It did not. In fact, it came out of G/A to CLB.

So, in my eyes this is a combination of an improper Go Around procedure, but also the model did not respond as it should have and increase the power up to G/A. Your climb rate would have really been impaired, but you would have kept climbing toward 4000 feet as your speed limped up slowly accelerating.

Edited February 8, 2017 by Captain_Al

[emvaos 13]

The other problem though seems to be how the model responded to that problem. In FLCH SPD the elevator pitches to maintain your speed and the A/T controls your rate of climb. Clearly, the thrust reduced and should have increased up to G/A thrust. It did not. In fact, it came out of G/A to CLB.

Erm.. No !

 

On FLCH the thrust limit will switch to either CLB (all engines operating) or CON (engine out).

 

FLCH does not necessarily drive the throttles to max available thrust. See why in post(s) above