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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.

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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?

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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?

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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.

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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.

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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.

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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.

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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.

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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.

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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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I did the exact same flight with the NGX to see if it somehow pertains to the SID, but the NGX flew it just fine. So for some reason the 777 is commanding the low speed

 

 

Adam

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I saw this same thing this morning, figured operator error (not uncommon for me).. On climbout, LAX, PRCH departure, it went into THR REF/LNAV/VNAV SPD and the active mode in the CDU VNAV page was REDUCED SPEED CLIMB (or something to that effect). No thrust reduction at accel height, no speed-up at 10K. Ended up doing the climb in speed intervene, didn't get to play around and figure out why it wasn't in just ECON CLB.

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Looking at my VNAV page while on the runway ready for departure from LTBA, the vnav page is ACT 174KT CLB. And I don't see anything about a reduced climb. Still can't figure out why the FMC is calling for such a slow speed

 

 

Adam

Also, the progress page shows 174 as the speed selected. I didn't input that myself, it was there automatically

 

Adam

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Was reading in the FCOM just now and read that if the progess page shoes SEL SPD, it means that it was entered manually in the CDU, which i never did. So for some reason, the speed is getting input by itself. I did a test, i ran through all departures from LTBA, and they all call for the low speed. I then went to EDDM, and no problems. Could anyone else see if they experience the same from LTBA?

 

Adam

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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

 

Adam,

 

Can you please post the exact details of this flight (exact route etc)? I just tried it going LTBA-EDDF using that SID and I did not see this issue at all.

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Of course Ryan,

 

Ok, so i spent the past hour trying different setups to see exactly what causes this and at what point. And i have narrowed it down. The flight is LTBA-LSZH. Take off from 36R (the PMDG navdata has it as 35R) VADE1 VADEN L610 UTEKA BABIT UT23 GRZ NEGRA (and i found that the STAR doesnt effect the slow climb speed). 30% fuel, 98% payload. The steps that cause the issue are as follows: Start up FSX, select the 722LR, LTBA, start on RWY 36R. I have the plane load in the defauly config, engines running. First thing, go to the FMC and set the fuel and payload. Then i go to the ident page, then init, then to route. Now here is the important part, i enter LTBA for departure, and LSZH for arrival. THEN, i go straight to the DEP/ARR page. Pick RWY 35R, and the VADE1 dep, activate and execute. Then i go back to the RTE, (and VADE1 to VADEN is already entered from selecting them from the DEP/ARR page), and i build the rest of the route from there, l610 UTEKA ect ect. When i check the LEGS page, as expected all speed/altitude spaces have dashes, except for the constraints. I then go to the PERF page and enter my ZFW, 5000 for reserves, FL350 and 50 for the CI. Now, after i execute that the LEGS page is populated with speed/altitude information. And thats where the problem is, after the first waypoint which has a constraint of 220/2200A, the commanded speed for waypoints BA041 and RIMBO is 170 kts.

 

Now, after trial and error, i found that this problem only happens if i go to the DEP/ARR page and pick my RWY and SID BEFORE i enter the route and execute. If, when i get to the RTE page for the first time, i enter LTBA and LSZH, then go to page 2, and enter VADEN L610 UTEKA ect ect, execute, got to the PERF page and enter the same info and execute,THEN i go to the DEP/ARR page, and select VADE1 and 35R and execute. when i go to the LEGS page, the speed commanded for BA041 and RIMBO is no longer 170, its 250 and 320 respectively just like it should!!

 

So, if i select the SID and RWY before entering the route and filling out the PERF page, the FMC commands to slow of a speed. If i enter the route, then fill out the PERF page, THEN select my RWY and SID, the speeds are normal. I hope this makes sense and you are able to replicate it and its not just my computer. Because already did a full FSX reinstall and the problem occurs both before and after the install. If you have any further questions, or need clarification, please ask!!

 

Adam


And i realized that the PROG page behavior i noticed earlier is actually correct. After you select your V Speeds the PROG page shows a SPD SEL of V2+15, so thats not part of the problem

 

Adam

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i had this happen a few times, but it didn't seem related to the fmc order (i always do the route first)

 

in my case it was because i didn't de-rate my thrust and had a light takeoff weight. i shot up so fast that i was at 250 knots before i knew it while still hand flying and basically had to clean up my flaps just a few seconds after takeoff.

 

somehow this skipped the part where it resets the vnav speed bug from v2 to flaps 5 speed (i think, i was at 250 and cleaned up before i hit accel height??), and also the subsequent reset to full speed after 10000 didn't take either..

 

climbing with flch seemed to work and once i hit cruise everything worked like normal.

 

i do not have a comprehensive understanding of those modes yet so i figured it was user error. i find that if i de-rate a bit and carefully fly the FD (and watch closely for the speed bug to jump up) the issue never happens, it seemed like something about shooting past the accel height too quickly...

 

cheers

-andy crosby

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Ok, I am not alone with that. ^_^

Yesterday I started out of KMIA and I had this behaviour with the 170 kts (rising with higher altitude).

That was the first time. FLCH is needed in that case.

Next time I try to make some sreenshots.

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