Sign in to follow this  
pdavies

777 P3D Shooting Through Descent VNAV Profile

Recommended Posts

Hey Guys - 

 

I'm finally back at it with P3D instead of FSX.  I completed my first round the world without a single hitch for the first time since...well...I can't even remember how long!!  Anyway, I've noticed the plane fails to maintain the VNAV profile coming down.  So far it only happens while getting close to the approach, say 5,000'.  It'll totally ignore the VNAV profile.  Any ideas?

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

 

 


Anyway, I've noticed the plane fails to maintain the VNAV profile coming down.  So far it only happens while getting close to the approach, say 5,000'.  It'll totally ignore the VNAV profile.  Any ideas?

 

Can you be a little more specific?

 

What's the route/STAR/approach? Is it "ignoring" the VNAV profile on the high or low side (not descending fast enough, or descending too fast)? What mode is the A/T in (watch the FMA at the top of the PFD on this one)?

 

To be honest, the "finally back at it" is leading me to believe there's something going wrong in the procedure of how you're doing things rather than the 777 choosing at random to ignore something, but we'll certainly get to the bottom of that here.

Share this post


Link to post
Share on other sites

Also, remember you're flying about 250 tons and she may not respond like a roller coaster on a track.  For example, the RIIVR2 arrival into KLAX there's a segment where she slows the descent following the vertical path around 14000 (this is from unreliable memory) then the path increases it's descent rate. The B777 doesn't immediate turn nose down and follow it but give her time and she will nose down slowly and in the end you will catch up with the path where the arrival levels off at 10000 for a segment. She is a big lady and she moves like one.

Share this post


Link to post
Share on other sites

So far it has done the same thing for each approach that I've shot, so I've just disengaged and hand flown the landing.  It will descend too fast, but I haven't paid enough attention to see what mode the A/T is in.  I'm sure it's something I'm doing wrong.  Nearly every flight I tried to finish in FSX ended in a graphic failure, despite every tweak I tried, so I rarely got to the point of landing!!  So needless to say I'm great at setting the FMC up, not so much with the landing phase...


Also, remember you're flying about 250 tons and she may not respond like a roller coaster on a track.  For example, the RIIVR2 arrival into KLAX there's a segment where she slows the descent following the vertical path around 14000 (this is from unreliable memory) then the path increases it's descent rate. The B777 doesn't immediate turn nose down and follow it but give her time and she will nose down slowly and in the end you will catch up with the path where the arrival levels off at 10000 for a segment. She is a big lady and she moves like one.

 

Hey Dan - 

 

She's a hog and can respond like that, but no, this will be at 5,000' and dropping at 2,000 FPM.  I haven't been brave enough to see if she'll level off and where, but she doesn't give much of an indication that she's going to.  I'll have to play around with it tonight when I'm not trying to jeopardize a trans-pacific flight!

Share this post


Link to post
Share on other sites

 

 


So far it has done the same thing for each approach that I've shot, so I've just disengaged and hand flown the landing.

 

Have you flown the tutorial yet? What is a graphic failure?

 

 

 


at 5,000' and dropping at 2,000 FPM.

 

What was she doing before 5000? Regardless, I recommend we focus on your first comment before getting into arrivals.  Please explain what you mean by graphic failure and lets try to get you to the point where you can arrive at the STAR. How did you Tutorial flight work out for you?

Share this post


Link to post
Share on other sites

 

 


Have you flown the tutorial yet?

 

I flew it a long time ago when I first got it - and I've flown STAR's hundreds of times in real life and the Sim. It won't obey the altitude restrictions at waypoints or anything.  It's bizarre, but again I need to go pay more attention to it tonight and see if I can recreate it.

 

 

 


What is a graphic failure?

 

It would completely glitch out, either flickering, or it would drag part of the screen and blur it if you moved screens, or nothing would show up at all.  But I didn't change anything and installed P3D and have had zero issues.  And the performance is infinitely faster than what I was doing in FSX - sliders nearly maxed out.  I only need to take it easy on traffic. I am thrilled with the performance.  It was getting super frustrating having a pretty decent setup and not seeing better performance.

Share this post


Link to post
Share on other sites

Please report back on the VNAV issues.. sounds like you have P3D up a running okay.

 

The B777 should do pretty good on the vertical path especially observing constraints so any problem you're having should be interesting to figure out.

Share this post


Link to post
Share on other sites

So it's my use of the FLCH button to slow down during the descent.  The annunciator on the screen says "FLCH SPD" as I'm descending down.  It does go to the altitude that I have entered in the MCP, but it doesn't follow the VNAV profile.  That's because I told it not to!  I'm going to need to hit the manuals and read up on how to use the FLCH button in conjunction with VNAV.  As is normally the case, this is a perfect example of pilot error!!

Share this post


Link to post
Share on other sites

Hey Guys - 

 

I'm finally back at it with P3D instead of FSX.  I completed my first round the world without a single hitch for the first time since...well...I can't even remember how long!!  Anyway, I've noticed the plane fails to maintain the VNAV profile coming down.  So far it only happens while getting close to the approach, say 5,000'.  It'll totally ignore the VNAV profile.  Any ideas?

 

Descent WINDS data updated as well? if not could be the issue, since lets say tail wind will cause odd vnav descend if not properly entered.

Share this post


Link to post
Share on other sites

So it's my use of the FLCH button to slow down during the descent.  The annunciator on the screen says "FLCH SPD" as I'm descending down.  It does go to the altitude that I have entered in the MCP, but it doesn't follow the VNAV profile.  That's because I told it not to!  I'm going to need to hit the manuals and read up on how to use the FLCH button in conjunction with VNAV.  As is normally the case, this is a perfect example of pilot error!!

 

Interestingly enough, this exactly thing also caused a crash in the last few years. At least you've had the presence of mind to kill the AP/AT and take control.

 

 

Descent WINDS data updated as well? if not could be the issue, since lets say tail wind will cause odd vnav descend if not properly entered.

 

Not that low, no.

Share this post


Link to post
Share on other sites

So it's my use of the FLCH button to slow down during the descent.  The annunciator on the screen says "FLCH SPD" as I'm descending down.  It does go to the altitude that I have entered in the MCP, but it doesn't follow the VNAV profile.  That's because I told it not to!  I'm going to need to hit the manuals and read up on how to use the FLCH button in conjunction with VNAV.  As is normally the case, this is a perfect example of pilot error!!

I don't ever remember having to use FLCH in a VNAV arrival.  The trick I use for speed is well before TOD I set up my descent speed in the FMS for often it will be 0.8xx/314 and since 314 is a not exceed speed the need for speed brake is almost constant; I change my descent to something like .84/310 and problem is solved.  There are also many STARS that instruct mach until 280 then hold 280 until whatever and again I set this up before TOD.  Speed control is then built into the forecast vertical path.

 

FLCH is used when you are following ATC instead of the magenta line, then it is very useful.

Share this post


Link to post
Share on other sites

 

 


Interestingly enough, this exactly thing also caused a crash in the last few years. At least you've had the presence of mind to kill the AP/AT and take control.

 

I can see how this could cause a crash.  It's easy to forget you're off VNAV.

 

 

 


I don't ever remember having to use FLCH in a VNAV arrival.  The trick I use for speed is well before TOD I set up my descent speed in the FMS for often it will be 0.8xx/314 and since 314 is a not exceed speed the need for speed brake is almost constant; I change my descent to something like .84/310 and problem is solved.  There are also many STARS that instruct mach until 280 then hold 280 until whatever and again I set this up before TOD.  Speed control is then built into the forecast vertical path

 

I think I need to do a better job at setting up my speeds in the FMC.  Is there a way to get the plane slowed down to Vref + 5 automatically?  Or is that something that you have to enter in on the legs page?  That's the only reason I'm using FLCH is to get it slowed to approach speed while still following LNAV.

Share this post


Link to post
Share on other sites

 

 


Is there a way to get the plane slowed down to Vref + 5 automatically?

 

You could try but I don't recommend it. The B737 is real good at managing approach speeds but the B777 is not designed to do that.  I usually start using MCP SPD when slowing into the flap speeds. The only trick is to keep an eye on the FMA and if pitch mode changes from VNAV PTH to VNAV SPD it might be time for some speedbrakes.

Share this post


Link to post
Share on other sites

 

 


I usually start using MCP SPD when slowing into the flap speeds.

 

So does this override VNAV?  Do you have to control altitude with the ALT Hold and VS?  So many questions!  I'm telling you, I am really good at setting the flight up but I would NEVER get to the landing phase!!  Haha!

Share this post


Link to post
Share on other sites

 

 


So does this override VNAV? Do you have to control altitude with the ALT Hold and VS?

 

No not at all, separate the three FMA modes in your conceptual picture.  You have thrust, roll and pitch and you have options within each.  You can select MCP SPD and in VNAV the pitch mode will usually be VNAV PTH (this means the speed you selected is within the range of speeds that can sustain the VNAV path) or VNAV SPD, which means that the speed setting is overriding the VNAV path. 

 

For example, on an arrival in VNAV and you are at 240 KIAS and want to slow to 210.  You can enter the 210 constraint in the LEGS page before you start down the path or you can switch to MCP SPD setting 210 and the pitch mode will change to VNAV SPD where it keeps the nose up to slow to 210 regardless of vertical path.  This might be okay or you might want to add speedbrake depending on your deviation from path and looking at the segments ahead.  A little pilotage going on here. I am always trying to manage my energy such that I minimize the use of speedbrake but still end up using it on at least half my arrivals, which seems to be realistic based on posts from T7 pilots I've seen.

 

Come on down from Moses Lake to KLAX and instead of arriving on the SADDE, come in from the NE and use the RIIVR arrival. It's perfect for playing with the various ways to manage energy in the descent.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this