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VNAV help asked


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Yesterday I was flying my flight from KSEA to KLAX following the IRMN2 STAR as indicated below. I figured I would try to use the VNAV mode as there were so many steps and I didn't really feel like constantly adjusting the ALT knob. Assuming that the VNAV constraints would be respected by the flight computer I set an Altitude of FL100 at the start of BURGL and enabled VNAV. However as soon as it hit the TOD it started descending quite rapidly, in fact it went below the low constraints. As such I ended up needing to adjust the ALT so that it would fall within the limits of the constraints, but this is not the behaviour I know from i.e. the CJ4 which respects constraints. Did I assume wrong functionality or should I have done something else here? Thanks!

 

Second question:

- I would have expected a calculated and averaged out descent, instead I found the AP to have very rapid descent trying to make the constraints. In the beginning it went as fast -4 VS for a little bit before I noticed that it was trying to go under the lower constraint. Does the CRJ AP know how to fly a smooth VNAV descent or what should I expect from it?

 

Thanks!

 

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Well, first of all I would look at what you were doing with the throttle during the descent.  If all you did was cut it to idle you are asking for issues.  The CRJ does not have auto-throttle so no it will not fly a smooth descent.  It will not calculate a descent at idle.  You, as the pilot still have to monitor what is happening and adjust the throttle accordingly.  Even if you select the VNAV option in the EFB, you still need to treat it as slightly smarter advisory vnav.  Apparently in the CJ4 they modeled, or are trying to model, the Thrust Sense A/T.  So, that is why the CJ4 can fly a "smooth" vertical profile.  Trying to compare the CRJ to the CJ4 only works with the C and the J.  I am reminded of this post a while ago....

 

 

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I held the throttle to keep around the posted speed of about 280kts. But besides I don't think it would matter at what percentage you have the throttle as the CRJ doesn't have a smart throttle anyway. If you flying at cruise and your speed is too low you will have the well documented problem that your plane will stall itself out without giving a warning until you are indeed at the stall speed. So why would changing the power setting matter now for a descent? As said my speed was around the posted 280 (in the beginning), 250 and later 210 as the speed limits change so it was a speed within limits of the plane and it would be able to execute changes.

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8 hours ago, Ircghost said:

I held the throttle to keep around the posted speed of about 280kts. But besides I don't think it would matter at what percentage you have the throttle as the CRJ doesn't have a smart throttle anyway. If you flying at cruise and your speed is too low you will have the well documented problem that your plane will stall itself out without giving a warning until you are indeed at the stall speed. So why would changing the power setting matter now for a descent? As said my speed was around the posted 280 (in the beginning), 250 and later 210 as the speed limits change so it was a speed within limits of the plane and it would be able to execute changes.

It is not a documented problem that a plane stalls at cruise or otherwise, if you speed is to slow, its a fact of flying an aircraft and one of the first signs of pilot failure.   If you don't know why working throttles on descent means then, well....

It does not matter what the speed constraints at the different points tell you, it matters what you do between those points.  You have to work the throttle to maintain an airspeed AND follow the snowflake.  for descent.  I will repeat this.  YOU CANNOT HAVE TRUE AUTOMATIC VNAV WITHOUT AUTO THROTTLE.  

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1 hour ago, Crabby said:

It does not matter what the speed constraints at the different points tell you, it matters what you do between those points.  You have to work the throttle to maintain an airspeed AND follow the snowflake.  for descent.  I will repeat this.  YOU CANNOT HAVE TRUE AUTOMATIC VNAV WITHOUT AUTO THROTTLE.  


Coupled VNAV mode is responsible for following the snowflake. The autopilot should fly the required vertical speed, and “working the throttle” has nothing to do with the rate of descent in this mode (it’s thrust on a jet by the way). All you have to do is keep the speed high enough to not stall and coupled VNAV should keep you on the snowflake. 


If @Ircghost is maintaining the speeds programmed in the VNAV pages, there is no reason the coupled VNAV should dive below the restrictions. Normally VNAV will take whatever descent angle is set in the VNAV pages and calculate a path from the last altitude restriction in your flight plan, back along your track to your cruise level. In this case that would be 6000ft at DAHJR. Working back from there it seems most restrictions are on a bit less than 3.0 degree descent angle.
 

If the path does not fit within other above/below restrictions along the way, it will adjust the actual flown track to meet those, unless it requires a descent angle that is less than 1 degree or steeper than allowed for the aircraft. It should not go below any restrictions at all. 

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34 minutes ago, CRJay said:


Coupled VNAV mode is responsible for following the snowflake. The autopilot should fly the required vertical speed, and “working the throttle” has nothing to do with the rate of descent in this mode (it’s thrust on a jet by the way). All you have to do is keep the speed high enough to not stall and coupled VNAV should keep you on the snowflake. 


If @Ircghost is maintaining the speeds programmed in the VNAV pages, there is no reason the coupled VNAV should dive below the restrictions. Normally VNAV will take whatever descent angle is set in the VNAV pages and calculate a path from the last altitude restriction in your flight plan, back along your track to your cruise level. In this case that would be 6000ft at DAHJR. Working back from there it seems most restrictions are on a bit less than 3.0 degree descent angle.
 

If the path does not fit within other above/below restrictions along the way, it will adjust the actual flown track to meet those, unless it requires a descent angle that is less than 1 degree or steeper than allowed for the aircraft. It should not go below any restrictions at all. 

No one said working the throttle or thrust, has any thing to do with the snowflake.  It has everything to do with maintaining speed.  I do not think that the CRJ will blindly follow the snow flake and stall the aircraft.  Maybe it will.  If so, more to my point.  You cannot have fully automated VNAV without an auto-throttle.  You are also saying that the VNAV calculates from TOD to LAST RESTRICTION at ~3.0 degree angle.  Yes, however, you have other restrictions in between that VNAV does not really account for in the CRJ because it has no way of let's say leveling off and pushing the throttles forward.  Why?  No auto throttle.  If you are a real world pilot, you know the CRJ is not a B737, A320 or even in this case a CJ4.  Those other aircraft have auto-throttles.  The funny thing is even in a more automated aircraft, there are MANY times when intervention is required by the pilot to fly a VNAV descent.  You see MORE DRAG NEEDED or DRAG NEEDED messages because from POINT A to B the descent will cause the aircraft to fly faster at idle than it should.  Then you have to add spoilers.  So to sum it up, a pilot is required to work the throttles (those are the handles the pilot touches, he cannot touch thrust) to maintain the airspeed that is set on the VNAV descent page so that the calculations done by system can be executed.

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3 hours ago, Crabby said:

It is not a documented problem that a plane stalls at cruise or otherwise, if you speed is to slow, its a fact of flying an aircraft and one of the first signs of pilot failure.   If you don't know why working throttles on descent means then, well....

It does not matter what the speed constraints at the different points tell you, it matters what you do between those points.  You have to work the throttle to maintain an airspeed AND follow the snowflake.  for descent.  I will repeat this.  YOU CANNOT HAVE TRUE AUTOMATIC VNAV WITHOUT AUTO THROTTLE.  

If you can't respond in a normal way without always attacking / being rude towards the user who is asking a question, please don't bother to respond to me anymore.

 

What I meant with well documented problem is not that it is a bug, it's indeed intended plane behaviour, I have just never in all the planes I have flown found such a plane that really wants to stall on the pilot that badly due to big speed changes i.e. due to weather changes and what not. But that is not what I was asking about anyway.

 

@CRJay Thanks for your response. I will try the flight again soon just to verify it wasn't a problem between monitor and chair ;), but i'm 100% positive I saw the Alt constraints in the FMC correctly and I hit VNAV so i'm curious if I did something else wrong or that I missed something. Do I read it correctly that you are suggesting to keep the speed slower? Or do you have other tips I might follow-up on?

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14 minutes ago, Crabby said:

No one said working the throttle or thrust, has any thing to do with the snowflake.  It has everything to do with maintaining speed.  I do not think that the CRJ will blindly follow the snow flake and stall the aircraft.  Maybe it will.  If so, more to my point.  You cannot have fully automated VNAV without an auto-throttle.  You are also saying that the VNAV calculates from TOD to LAST RESTRICTION at ~3.0 degree angle.  Yes, however, you have other restrictions in between that VNAV does not really account for in the CRJ because it has no way of let's say leveling off and pushing the throttles forward.  Why?  No auto throttle.  If you are a real world pilot, you know the CRJ is not a B737, A320 or even in this case a CJ4.  Those other aircraft have auto-throttles.  The funny thing is even in a more automated aircraft, there are MANY times when intervention is required by the pilot to fly a VNAV descent.  You see MORE DRAG NEEDED or DRAG NEEDED messages because from POINT A to B the descent will cause the aircraft to fly faster at idle than it should.  Then you have to add spoilers.  So to sum it up, a pilot is required to work the throttles (those are the handles the pilot touches, he cannot touch thrust) to maintain the airspeed that is set on the VNAV descent page so that the calculations done by system can be executed.

 

Most of your response is correct with a couple of corrections. After attempting to allow the CRJ to follow the "snowflake" on many a VNAV attempt while keeping the AS at about 290 kts, it is apparent that the VNAV implementation by AEROSOFT is POOR at best. I usually see a decent rate of 4K-5k ft per minute right after flying over a restraint waypoint and then it will pull up to something more reasonable. During this short time, it is impossible to keep the AS anywhere near 290 kts even with the speed brake fully deployed at idle. Even as an advisory VNAV, it does not work well. The Working Title CJ4 also does not have an auto-throttle but the VNAV is nearly spot-on in that implementation so an auto-throttle is not absolutely necessary. You will be happier to manually manage the decent rather than to try to let the CRJ do it.

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@Crabby You might want to find yourself an FMS guide to learn how the VNAV works for the CRJ before you say more silly things. You know, read the manual and all that. The VNAV will apply vertical path smoothing and respect restrictions in between. If the required angle of descent is less than one degree it will even plan a level segment.

 

 

@Ircghost Honestly, I think it is more likely that the VNAV in the CRJ does not work as it should. Maintaining the speeds you mentioned should not have caused any issues with VNAV. 

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1 hour ago, CRJay said:

@Crabby You might want to find yourself an FMS guide to learn how the VNAV works for the CRJ before you say more silly things. You know, read the manual and all that. The VNAV will apply vertical path smoothing and respect restrictions in between. If the required angle of descent is less than one degree it will even plan a level segment.

 

 

I understand the smoothing, now when it smooths, what do you do with the throttles or thrust? 

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1 hour ago, rwilsonlj said:

 

Most of your response is correct with a couple of corrections. After attempting to allow the CRJ to follow the "snowflake" on many a VNAV attempt while keeping the AS at about 290 kts, it is apparent that the VNAV implementation by AEROSOFT is POOR at best. I usually see a decent rate of 4K-5k ft per minute right after flying over a restraint waypoint and then it will pull up to something more reasonable. During this short time, it is impossible to keep the AS anywhere near 290 kts even with the speed brake fully deployed at idle. Even as an advisory VNAV, it does not work well. The Working Title CJ4 also does not have an auto-throttle but the VNAV is nearly spot-on in that implementation so an auto-throttle is not absolutely necessary. You will be happier to manually manage the decent rather than to try to let the CRJ do it.

I have never flown the CJ4 working title or otherwise, but when I searched it was said it has auto throttles.  Either way, I do happily manage my descent manually, don't have the VNAV enabled in the EFB and so I don't encounter problems, unless I cause them. 

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2 hours ago, rwilsonlj said:

 

Most of your response is correct with a couple of corrections. After attempting to allow the CRJ to follow the "snowflake" on many a VNAV attempt while keeping the AS at about 290 kts, it is apparent that the VNAV implementation by AEROSOFT is POOR at best. I usually see a decent rate of 4K-5k ft per minute right after flying over a restraint waypoint and then it will pull up to something more reasonable. During this short time, it is impossible to keep the AS anywhere near 290 kts even with the speed brake fully deployed at idle. Even as an advisory VNAV, it does not work well. The Working Title CJ4 also does not have an auto-throttle but the VNAV is nearly spot-on in that implementation so an auto-throttle is not absolutely necessary. You will be happier to manually manage the decent rather than to try to let the CRJ do it.

The part in bold is indeed exactly what I saw as well, glad it's not just me.

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20 minutes ago, Crabby said:

I understand the smoothing,

 

Do you really understand it though? You just posted it does not do any of the things it does. And why are you so hung up on auto thrust and full automatic VNAV? The only one bringing that up is you. We've already established the correct speeds were maintained yet you keep going on about thrust. All coupled VNAV does is basically a managed VS mode. You can fly it with thrust at idle, or 60% N1 and the only change will be your air and ground speed and thus the VS the AP will command.

 

So, try to be helpful for once. Explain to us why the CRJ went below the altitude restrictions while maintaining normal speeds when it should be fully capable of flying a smooth VNAV descent using those speeds?

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VNAV doesn’t really work properly without an auto throttle, it’s really just advisory information. All VNAV can do (in this case) is calculate descent profiles based on the descent restrictions (you’ve added to the procedure) at idle power. 
 

Normally VNAV will look at all the descent restrictions and calculate fuel efficient descent profiles for each segment. The least efficient are “AT” restrictions as they’re hard heights. Ideally you’d want “above” or “below” as then the AFDS can calculate more efficient profiles, using an auto throttle. 
 

Obviously without an auto throttle all the aircraft can do is use idle power profiles (including descent winds etc) which are pretty agricultural. 


Then when you reach the hard height you have to increase the throttle or advance the thrust levers to maintain the speed. 
 

I think if you don’t the aircraft will stall. More advance aircraft would enter a slow descent though but the CRJ doesn’t have this protection. 

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1 hour ago, Gstove said:

VNAV doesn’t really work properly without an auto throttle, it’s really just advisory information. All VNAV can do (in this case) is calculate descent profiles based on the descent restrictions (you’ve added to the procedure) at idle power. 
 

Normally VNAV will look at all the descent restrictions and calculate fuel efficient descent profiles for each segment. The least efficient are “AT” restrictions as they’re hard heights. Ideally you’d want “above” or “below” as then the AFDS can calculate more efficient profiles, using an auto throttle. 
 

Obviously without an auto throttle all the aircraft can do is use idle power profiles (including descent winds etc) which are pretty agricultural. 


Then when you reach the hard height you have to increase the throttle or advance the thrust levers to maintain the speed. 
 

I think if you don’t the aircraft will stall. More advance aircraft would enter a slow descent though but the CRJ doesn’t have this protection. 


The CRJ does not calculate based on idle descent. I have written it before in this thread so you might want to read a few posts back. It is based on speed profiles and a descent angle. It will adjust/optimize the requested descent angle and path based on restrictions in between present position and the last altitude restriction in your flight plan or last restriction before a discontinuity.

 

All you have to do is fly the speed you have set in the FMS and you should fly a pretty nice, smooth VNAV descent. No auto thrust required.

 

The AS CRJ uses a default 3.0 degree VPA. In the real plane something like 3.5-3.8 would work better for full idle descents if you are so inclined. 

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I read it and assumed you were wrong tbh, what kind of aircraft does that? Seems a very agricultural way to calculate a descent profile. Is this just a sim thing or is that actually how the real one works? 
 

I’ve never operated an aircraft with VNAV that works like that. 
 


 

 

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It’s how the real plane/FMS does it. That’s what you get with the CRJ… It’s one step into what was then still somewhat Next Gen  for regional aircraft, with the glass cockpit and lots of automated systems. And one foot also still firmly planted in the last gen from back then with things that could have been clearly more automated but were not.  
 

It is likely one of the more efficient ways to handle VNAV when you don’t have auto thrust since it is more flexible and customizable than only calculating for idle descent. Usually the fuel flow difference between the thrust required to keep your VNAV speed and idle descent is relatively minimal, so it is not all that bad. And if you are comfortable with the plane and you do a bit of mental arithmetic to set a custom VPA, you can end up not touching the thrust levers down to 1000ft or even 500ft AGL. 

 

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10 hours ago, Crabby said:

So you are butt hurt that I could not read your mind?  Wow.  Caution tape and teddy bears for you.

I'm just saying that you are full of assumptions in each and every topic and always assume the worst of everyone, and that just makes you come across as a total ass. So I couldn't care less about your comments, I'm just saying that you don't need to bother show up.

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7 hours ago, Ircghost said:

I'm just saying that you are full of assumptions in each and every topic and always assume the worst of everyone, and that just makes you come across as a total ass. So I couldn't care less about your comments, I'm just saying that you don't need to bother show up.

Then why do you keep reading my comments.  That choice is yours.  I will not impede it.  Who are you to impede mine?  It is funny, the only people that are running around acting like an ass, down voting everything, calling people asses, using vulgarity etc., well let's just say that group does not include me.  LOL.

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  • 1 year later...

Apart from the demonstration of toxicity in the flight sim community, did we come to any conclusions as to why it breaks the limit altitudes? Does the real one level off at the limits even if the Alt knob is set below?

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