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uhntissbaby111

AP commanding minimum speed on climb

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Ive had a problems a few times now while flying the 777. It doesnt happen all the time, just sometimes and i cannot seem to replicate it. Sometimes while im climbing out from an airport, while on LNAV/VNAV the AP will command the lowest possible speed, just above the red marks. When i go to the legs page, there will be no manually entered speed restriction, but it will say that the speed that the aircraft wants to be at is, 170 kts for example. Or, it will be a normal speed, such as m .828 as i had just a few minutes ago, but the AP was commanding 188 knots. I had to enter FLCH mode to get my speed up. Im assuming im doing something wrong, just cant figure out why it happens on some flights and not others.

 

Adam

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That's what I do and it works fairly straight forward:

 

Set V2 on the MCP during preflight.

After rotation set initially attitude to achieve V2+20 to V2+30 knots (which is around FLAPS5).

At acceleration altitude wait for the engines to stabilize at CLB

Set speed for FLAPS5 on the MCP

Center the FDs and engage AP

 

Read through FCTM Chapter 3...

You will see that my procedure is not precisely by the book, but you might also come up with something similar that will work for you.

Hope this will help you to sort is out.


 

Regards,

Martin Martinov / VATSIM 1207931

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Thank you for the reply. That is a good workaround, but I'm stil curious as to why I need a workaround. Am I doing something wrong, or is it a bug of some sort?

 

Adam

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Am I doing something wrong, or is it a bug of some sort?

 

My first idea is almost always user error, to be honest.  Can you reproduce this, and if so, what are the steps to reproduce it?


Kyle Rodgers

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Ive had a problems a few times now while flying the 777. It doesnt happen all the time, just sometimes and i cannot seem to replicate it. Sometimes while im climbing out from an airport, while on LNAV/VNAV the AP will command the lowest possible speed, just above the red marks. When i go to the legs page, there will be no manually entered speed restriction, but it will say that the speed that the aircraft wants to be at is, 170 kts for example. Or, it will be a normal speed, such as m .828 as i had just a few minutes ago, but the AP was commanding 188 knots. I had to enter FLCH mode to get my speed up. Im assuming im doing something wrong, just cant figure out why it happens on some flights and not others.

 

Adam

 

Yesterday I was on descent and speed entered was 300 yet it kept falling below 250IAS until I clicked FLCH then back to VNAV.

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I was thinking the same Kyle, as if it was aircraft error I'm sure someone else would have posted it. I'll have to wait till I get home to try to reproduce. But so far, it's been random. And yesterday, I was on the active at LTBA, route programmed, lnav/vnav armed, AT switches on, and the only constraint was 11000B on the 3rd waypoint of the SID. I engaged TOGA, rotated, climbed out at v2+10, reached accel point, commanded speed went to 250, retracted flaps, then a few minutes later, commanded speed dropped to 170 all of a sudden. And when I pulled up the legs page, it was showing 170 for speed for the next 2 waypoints, then M .82. Even when I reached the .82 waypoint, speed was still slow. A bit confusing I must say

 

Adam

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Some screen shots might help. The only time I've ever seen 170 kts on the LEGS page is at the destination runway. Do you have any discontinuities in your path?


Dan Downs KCRP

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I'll be following this thread because I experienced a similar scenario as the OP. I was on a short flight (CYVR-KSFO), very light, and in clean config in a nominal climb to FL350 with LNAV/VNAV active. FMS was set for derated T/O and CLB-1. I walked away for about 10 minutes, and when I next looked at the sim I was at ~FL300 with the nose way up and IAS around 210. The MCP was properly configured, A/T and A/P active, and there was a 17 KT crosswind. I use AS2012 and unfortunately did not check OAT. I tried SPD INT to increase IAS but no response. So I simply let it struggle up to cruise at which point it correctly achieved M.84 as set in the FMS.

 

I wish I had recorded more info on weights, N1 and such, so I have nothing worthwhile to add to the discussion at this point, and chances are it's the result of something I'm doing wrong. If it occurs again I'll be sure to come back and contribute.


- Jev McKee, AVSIM member since 2006.
Specs: i7-2600K oc to 4.7GHz, 8GB, GTX580-1.5GB, 512GB SSD, Saitek Pro Flight Yoke System, FSX-Acceleration 

 

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The MCP was properly configured, A/T and A/P active, and there was a 17 KT crosswind.

 

AT and AP being active is only about 25% of the battle.  The important notes are the flight modes annunciated on the PFD.


Kyle Rodgers

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I'm going to hop on this one for the ride as well.  This has happened to me three times in the past couple days and I've used FLCH to recover each time with no follow-on problems.  With the complexity of this aircraft I would definitely not be surprised that I was causing this, but being a creature of habit and trying to be very precise each and every flight, I just don't see what I'm doing wrong.  Like Jev stated above, I will also attempt another flight soon, document each step along the way, and if it happens again, post the results here for public consumption.

 

And thanks for the comment, Kyle.  I'll pay close attention to those modes.



Doug Miannay

PC: i9-13900K (OC 6.1) | ASUS Maximus Z790 Hero | ASUS Strix RTX4080 (OC) | ASUS ROG Strix LC II 360 AIO | 32GB G.Skill DDR5 TridentZ RGB 6400Hz | Samsung 990 Pro 1TB M.2 (OS/Apps) | Samsung 990 Pro 2TB M.2 (Sim) | Samsung 990 Pro 2TB M.2 (Games) | Fractal Design Define R7 Blackout Case | Win11 Pro x64

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AT and AP being active is only about 25% of the battle.  The important notes are the flight modes annunciated on the PFD.

OK, Kyle. I'll be sure to note those if it happens again. Thanks.


- Jev McKee, AVSIM member since 2006.
Specs: i7-2600K oc to 4.7GHz, 8GB, GTX580-1.5GB, 512GB SSD, Saitek Pro Flight Yoke System, FSX-Acceleration 

 

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And thanks for the comment, Kyle.  I'll pay close attention to those modes.

 

 

 


OK, Kyle. I'll be sure to note those if it happens again. Thanks.

 

Welcome.  Yeah, there's some discussion of it in either the intro or the tutorial.  I don't have either in front of me or I'd refer you to the specific page, but it's a more in-depth answer as to why the FMA is much more important than simply the lights on the MCP.


Kyle Rodgers

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Yeah, there's some discussion of it in either the intro or the tutorial.  I don't have either in front of me or I'd refer you to the specific page, but it's a more in-depth answer as to why the FMA is much more important than simply the lights on the MCP.

 

I keep all the docs on a USB stick in my briefcase :)  Incredibly thorough documentation for sure, and nice to have with me for those moments when I have some free time.

 

There is ample discussion of FMA modes in the tutorial starting on page 51 and then numerous times thereafter as each specific flight regime is discussed.  I'll go over that in much more detail as I do my next flight.



Doug Miannay

PC: i9-13900K (OC 6.1) | ASUS Maximus Z790 Hero | ASUS Strix RTX4080 (OC) | ASUS ROG Strix LC II 360 AIO | 32GB G.Skill DDR5 TridentZ RGB 6400Hz | Samsung 990 Pro 1TB M.2 (OS/Apps) | Samsung 990 Pro 2TB M.2 (Sim) | Samsung 990 Pro 2TB M.2 (Games) | Fractal Design Define R7 Blackout Case | Win11 Pro x64

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Ok so I managed to replicate it. Same setup, 30% fuel, 98% payload. LTBA runway 36L departure VADE1E SID. First waypoint had a 220/2200A, then /4000B, then /13000B. After that the next waypoint was at my cruise alt. Now the strange thing is, even on the ground, the FMC had 170 as the speed it was shooting for after the 220/2200A waypoint. Then at the first waypoint at my cruise altitude, the speed was m .82. So I took off, and sure enough, I reached the first waypoint at 220 kts per the restriction, then afterward it reduced back to 170. I had to manually enter 250 as the speed restriction to get the plane to speed up. The FMA was showing SPD/LNAV/VNAV PTH. And when I entered the manual 250 restriction, the AT didn't advance, it stayed at around 77% and used pitch to pickup speed. So why is this happening? Maybe it's specific to the SID? But I've had it happen at another airport as well, so I'm at a loss

 

 

Adam

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