Always overshooting if levelling off after takeoff/go around.

[scandinavian13 5,192]

 

 

With all due respect, and a lot is due trust me I highly respect your knowledge and your input on this forum, but sometimes one should not always be so quick to dismiss things.

 

While I understand the sentiment, imagine how difficult our jobs would be if we chased down everyone's gut reaction of some video they watched somewhere with some guy doing some thing. Facts, evidence, and references make all the difference.

 

Heck, even a legitimate issue like the FBW, as you mentioned, would have been a lot quicker to be looked into if people had brought more facts to the discussion. To be honest, "this doesn't feel right" isn't generally going to get someone to re-visit something. Just look at all of the false "issues" that pop up here in the forum because people don't read the intro manual.

 

Guys, I understand that certain issues can depend on many factors and there may be a lot of discussion about them, but this issue is clear and there is little to say, other than facts. Forget about takeoff or go around, think of the following configuration: A/P on, A/T on in SPEED mode (say 180 to 200 kts), V/S with a shallow climb, the airplane overshoots the assigned altitude by 600 to 1000 ft.  It's cristal clear...

 

As to the go around procedure, you advice not to use V/S. Ok, try and follow the Boeing procedure in order to level off at 4000 ft: "1. At flap retraction altitude, set speed to maneuvering speed for the desired flap. 2. After flap retraction, push VNAV (or FLCH), VNAV activates with A/T in THR REF (or FLCH activates with A/T in THR mode)". The airplane will reach 4800-5000 ft or more, then will realize and descend... So, no way...

 

Then submit a ticket. If you've submitted one already, and it's been acknowledged, then continued discussion here is superfluous. Regardless, it might be better to speak in less certain terms if you don't have experience in the aircraft, and you're not bringing references.

[vr-pilot 11]

Facts, evidence, and references make all the difference.

Kyle, I am not a T7-Pro and my office is not a cockpit. Yesterday I "noticed" that the T7F I use to fly has THR RED ALT 3000FT and ACC ALT 1500FT set in the options.

I was a bit surprised because out of my poor sim experience I thought that vice versa values actually would be correct. THR RED ALT 1500FT (LH Cargo uses even 1000FT on their T7F) and ACC ALT 3000FT.

Does this circumstance perhaps lead to problematic F/D and A/P commands in conjunction with A/T?

[kevinh 409]

There is some confusion creeping in here between climbout after takeoff (a different subject) and go-around (the thread subject). For go-around there is no derate available. GA is GA.

 

Regarding VS mode after selecting go-around, in effect the first push of the TOGA button is a VS mode. It gives you enough thrust to maintain the target VS (which is determined automatically) at the airspeed selected. If you press TOGA twice you get full GA thrust and that might well cause an altitude overshoot. According to the FCOM in go-around mode the AP will intercept the MCP ALT without any firther intervention. So no need to select VNAV, FLCH, etc which will increase thrust and climb rate.

 

So there is no need to select another vertical mode, including VS. As long as the missed approach altitude is selected and you only press TOGA once, the aircraft should climb at a comfortable VS and intercept it without overshoot. If it doesn't do this then it may well be a ticket needs to be opened about it.

[downscc 1,327]

FCTM 5.64:

The TO/GA pitch mode initially commands a go-around attitude and then transitions to speed [pitch mode] as rate of climb increases

 

Contininuing on pg 5.65:

When flaps are retracted to desired position and airspeed approaches maneuver speed, select FLCH or VNAV and ensure CLB thrust is set. Verify airplane levels off at selected altitude and proper speed is maintained.

 

The above are Boeing recommended procedures in the FCTM, which should be referred as an assist in understanding how to operate the aircraft. Note: No V/S pitch mode is suggested, the key factors are speed and pitch. Any overshoot is a result of the pilot not following procedures.

[kevinh 409]

FCTM 5.64:

The TO/GA pitch mode initially commands a go-around attitude and then transitions to speed [pitch mode] as rate of climb increases

 

Contininuing on pg 5.65:

When flaps are retracted to desired position and airspeed approaches maneuver speed, select FLCH or VNAV and ensure CLB thrust is set. Verify airplane levels off at selected altitude and proper speed is maintained.

 

The above are Boeing recommended procedures in the FCTM, which should be referred as an assist in understanding how to operate the aircraft. Note: No V/S pitch mode is suggested, the key factors are speed and pitch. Any overshoot is a result of the pilot not following procedures.

Yes, but check the FCOM to see how TOGA actually operates (it sets a vertical speed of 2,000 fpm initially).

 

Also later on FCTM page 5.65 it says this:

 

"To prevent an altitude and/or airspeed overshoot. the crew should consider doing one or more of the following things:

• use the autothrottle
• press TO/GA switch once to command thrust sufficient for a 2,000 fpm climb rate
• if full go-around thrust is used, reduce to climb thrust earlier than normal
• disconnect the AFDS and complete the level off manually if there is a possibility of an overshoot
• if autothrottle is not available, be prepared to use manual thrust control as needed to manage speed and prevent flap overspeed"

The second item is what I would consider the most important if someone is experiencing an altitude overshoot using AP and A/T. If full thrust has to be used then the third item becomes useful. The pitch mode you select subsequently will not necessarily help you prevent an overshoot if you are already at THR REF.

[downscc 1,327]

We're sorta saying the same thing... two different Englishes maybe (Haha).

 

Toga sets thrust for 2000 fpm and pitch for speed control... that's my point. Pitch should be controlling speed not rate of climb.

[kevinh 409]

Yes, I agree, pitch for speed is the preferred control law. However pitch for speed with GA thrust will result in a high VS. So the thrust setting is the likely cause of any overshoot.

[tcas_climb 45]

I believe this is what the OP is experiencing. Sorry for the jerkiness, this is the first time i am recording fsx and the sim was smooth until i started the recording.

 

 

 

It seems that as ALT capture occurs the thrust accelerates to full G/A thrust, because the mcp speed is set automatically to 220, and this increase in thrust is what causes the overshoot. it is worse if you have poor framerates.

[jgoggi 65]

Exactly, Bryan. As you can see, the airplane reaches 3900 ft before realizing that has overshot and then descends back to 3000 ft. This without touching anything, it's simply a standard go around. In order to avoid this, I intervene with V/S at 1000 ft, but even in that case I get an overshoot. I did submit a ticket some months ago.

Anyway the same thing happens during takeoff, if for some reason I need to level off say at 4000 ft after takeoff, the airplane overshoots and reaches almost 5000 ft.

[tcas_climb 45]

Exactly, Bryan. As you can see, the airplane reaches 3900 ft before realizing that has overshot and then descends back to 3000 ft. This without touching anything, it's simply a standard go around. In order to avoid this, I intervene with V/S at 1000 ft, but even in that case I get an overshoot. I did submit a ticket some months ago.

Anyway the same thing happens during takeoff, if for some reason I need to level off say at 4000 ft after takeoff, the airplane overshoots and reaches almost 5000 ft.

I've never experienced an overshoot after takeoff though even when the altitude is set to 3000. I have a few work around that may or may not work to prevent this but I won't be able to try them for a few days, but maybe you could to see if it helps.

 

1. Try using vnav or flch during takeoff or go around after you've passed acceleration altitude.

 

Or

 

2. If you really really really want to use v/s during the go around, after passing acceleration altitude try setting your mcp speed higher so you beat the autopilot to it and don't get that huge burst of thrust at the last moment that the autopilot can't keep up with.

 

Only the real T7 pilots or pmdg themselves can look at the video and tell if something wonky is going on, but until then, stay ahead of the aircraft and takeover manually if things don't work as they should.

[kevinh 409]

As Dan said selecting CLB thrust rating is part of the FCTM go-around procedure. If you do that thrust can't increase to the GA limit.

 

Whether the 777 should go to full rated thrust during alt capture is another issue which someone with real 777 experience may be able to answer. It doesn't sound right but you never know.

[kevinh 409]

I just tried a go-around at EGKK to 2000 ft and didn't get a thrust increase to GA as it captured the altitude. It stayed at the same thrust I got after selecting TOGA. No significant altitude overshoot.

[tcas_climb 45]

What did the speed bug do? Did it move? Did you leave it at Vapp throughout the procedure? Or did you set it to something else?

 

What is the acceleration altitude set to?

[kevinh 409]

I increased speed to match flap configuration. Acceleration altitude as it was for takeoff, the default value. I didn't change vertical mode.

 

I think the important thing is if you set a new vertical mode such as VNAV or FLCH you need to select CLB thrust as well.

[tcas_climb 45]

I increased speed to match flap configuration.

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.