Always overshooting if levelling off after takeoff/go around.

[downscc 1,327]

 

 

when alt capture engages that's when the speed bug jumps to 220

 

Just guessing here, but that sounds like the FMS is changing state from go around to missed approach and it is dialing in V_A , at some point you need to stop letting the computer fly the airplane and take over as pilot.

[kevinh 409]

In my video I did not change the speed from Vapp and it's when alt capture engages that's when the speed bug jumps to 220 causing the engines to accelerate and causing the overshoot. Next time I get the chance I'll try changing the speed as you did and see if that makes a difference.

I just tried doing exactly what you did, with MCP ALT set to 3000 and leaving speed alone. My 777 didn't command a new speed and captured the altitude with only a small overshoot. I can only imagine there might be something in the missed approach procedure for the runway you used which changed the speed bug to 220. But I don't think it should change speed by itself with the MCP speed in view.

 

I've found that if I select FLCH or VNAV after go-around my thrust limit changes to CLB automatically so full GA thrust is never going to be set whatever speed command change occurs.

[tcas_climb 45]

You know what, come to think of it I think the FO might be messing with me, the speed window remains open so the fms shouldn't have control of that. Fs2crew might be treating it as a manual go around and sets 220 as an approximate min clean speed without the aircraft being totally clean.

[Mike777 163]

I just tried doing exactly what you did, with MCP ALT set to 3000 and leaving speed alone. My 777 didn't command a new speed and captured the altitude with only a small overshoot. I can only imagine there might be something in the missed approach procedure for the runway you used which changed the speed bug to 220. But I don't think it should change speed by itself with the MCP speed in view.

 

I've found that if I select FLCH or VNAV after go-around my thrust limit changes to CLB automatically so full GA thrust is never going to be set whatever speed command change occurs.

 

 

I've just done a couple of go-arounds by pressing TOGA once.  If anything, I don't get enough thrust.  A/T goes into THR (not THR REF).  My final approach speed set in MCP is 144, but this is too low to retract flaps to 20 (as in a standard go-around), so I increase MCP speed to 180.  But the aircraft is very sluggish at getting there.

 

Despite the low speed and fairly modest climb rate, it does overshoot the MCP set altitude of 3000 by @ 500 ft and then go back down.  I don't think this overshoot has much to do with throttle engagement, as when I near the target altitude, A/T goes into SPD and vertical mode goes from TOGA to ALT.   It does seem to do better if I press VNAV shortly after the go-around starts, but I need to test further.

 

VNAV update: just ran another go-around.  VNAV works better, because it gives THR REF (in CLB mode) and appropriate, increasing speed, which it controls with pitch.  But I am still getting about a 300ft overshoot, even though A/T goes into SPD and vertical mode goes into ALT before I get to 3000.

 

 

 

You know what, come to think of it I think the FO might be messing with me, the speed window remains open so the fms shouldn't have control of that. Fs2crew might be treating it as a manual go around and sets 220 as an approximate min clean speed without the aircraft being totally clean.

 

I haven't seen any uncommanded jump in MCP speed, and was just about to ask if an addon might be doing it.

 

Mike

[jgoggi 65]

The MCP speed change to 220 knots is correct, check out this video at about min. 1:22.

 

[skelsey 765]

My final approach speed set in MCP is 144, but this is too low to retract flaps to 20

Hi Mike,

 

What makes you say that?

 

If you are flying at Vref+whatever, there should be no problem selecting flap 20. Vref is, as a rule, approximately equal to V2: your margin with flap 20 and ref+5 should be no less than during a normal takeoff.

 

Flap 30 is very draggy and will compromise your climb performance.

[Mike777 163]

 

 

If you are flying at Vref+whatever, there should be no problem selecting flap 20. Vref is, as a rule, approximately equal to V2: your margin with flap 20 and ref+5 should be no less than during a normal takeoff.

Flap 30 is very draggy and will compromise your climb performance.

 

Hi, Simon,

 

Thanks for your comment.  I hadn't thought of the issue that way -- certainly in a normal takeoff you're not going for speed, but altitude during the initial climbout until the acceleration altitude.  I understand about flaps 30 being draggy, but I am just uncomfortable "low and slow."  But what would be the problem with using VNAV, which gives THR REF (TOGA OR CLB thrust depending upon how high I am) and therefore greater acceleration?  Or to put it another way, why not have more thrust in the THR mode during a go-around (pressing TOGA once only) to produce greater acceleration?   Thinking aloud a little here: would the answer be that greater acceleration = a more shallow climb profile that might bring the aircraft closer to obstacles located further from the airport?:

 

Mike

[skelsey 765]

No problem Mike.

 

Most jet aircraft, and especially the very overpowered 777, will do a very passable imitation of a Saturn V rocket when full thrust is applied at light weights. This means everything happens very, very quickly, especially if the missed approach altitude is low (here's a good example of what can happen, albeit there were a lot of other errors made:

https://assets.digital.cabinet-office.gov.uk/media/547f0781e5274a42900001e9/Boeing_757-236_G-TCBC_10-14.pdf ).

 

To get around that prob​lem, the first click of the TOGA switches will just give you enough thrust for a 2000fpm climb, which in most cases (i.e. unless you are OEI and/or the missed approach requires a particularly high climb gradient) is more than sufficient and makes everything a bit calmer. If for any reason you need full GA thrust, this is available at any time via a second click of the TOGA switches.

 

There's no need to accelerate the aircraft in the early stage of a go-around from low altitude. Just like a takeoff, your initial objective is to get away from the ground quickly: so you effectively return the aircraft to the takeoff configuration (Flap 20, gear up with positive rate and climb at V2 - V2+20).

 

Again, just like a takeoff, once you reach your acceleration altitude (say 1000ft aal), you can then select VNAV/FLCH and clean the aircraft up.

 

Obviously you have to select TOGA in the first place to put the AFDS in to the appropriate modes (and get out of GS | LOC if this was what you were in). Selecting VNAV before Aa, as you reason, will likely result in the aircraft accelerating which will reduce the climb gradient and may compromise your obstacle clearance: FLCH would work but you would lose the ability to get full GA thrust (I hvave a feeling you might get CLB automatically with FLCH selection) and the manoeuvre may become rather more sporty than you would have liked (see first paragraph).

 

You can, of course, select LNAV (or HDG SEL) above the appropriate minimum engagement heights for these modes without changing the pitch mode (400ft on the 747, might be different on the 777).

 

Hope that's helpful!

[kevinh 409]

The MCP speed change to 220 knots is correct, check out this video at about min. 1:22.

The speed change to 220 is fine, but the sim doing it by itself wouldn't be. If it was an FS2Crew input the problem comes if it's not coordinated with what the pilot is doing. In the video they are retracting flap as speed increases so everything stays under control.

 

Edit: just had a chance to watch the video again full screen, and I see what you mean now. Watching it on my phone it wasn't clear the change to 220 was automatic. It hasn't done that with my installation. Maybe it's an option? The pilot kept on top of things by retracting flaps to match though.

[kevinh 409]

Further to the above, the FCOM, Vol2 page 4.20.16, states that during a TO/GA level off at the set altitude, if altitude is captured or if V/S or FPA is active the MCP speed is automatically reset to flap placard speed minus 5 knots or a speed value entered into the MCP after TO/GA is pushed. For the 777-200LR, 777F and 777-300ER the flap placard speed for Flap 20 is 225, which means it would set 220. So it seems there are circumstances where the MCP speed will change automatically.

 

Possibly because I usually enter a new speed after TO/GA the 220 speed input didn't occur. Regardless, my overall feeling is that the FCTM wouldn't give advice on avoiding altitude overshoots during GA if it wasn't a possiblity in the aircraft. The autopilot isn't magic and doesn't always follow commands exactly and without errors.

[tcas_climb 45]

 

 

Possibly because I usually enter a new speed after TO/GA the 220 speed input didn't occur.

I suppose then to prevent a massive over shoot in the sim aside from doing things manually would be to select a speed after selecting toga or hitting vnav after passing acceleration altitude.

[kevinh 409]

I suppose then to prevent a massive over shoot in the sim aside from doing things manually would be to select a speed after selecting toga or hitting vnav after passing acceleration altitude.

It's worth trying. The main thing is to check that CLB thrust rating is selected, so if the engines go to full thrust it's not such a big increase. Also, as Simon said, retract flaps to 20 after initiating go-around.