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VNAV v Coupled VNAV explanation


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Total noob here, so please don't shoot me.

 

Can anyone explain (or point me at an explanation) of how VNAV for the CRJ works and why/how this is different to Coupled VNAV?

 

I understand that VNAV is advisory - so I think that means the descent is advised and you have to fly the plane to follow it - it won't do that automtically. And I think that is VNAV. However, in the EFB there is an option that says "Coupled VNAV available". I've searched the documentation and can't find anything to explain what this is or does and can't see enything in these forums, but I may well have missed it.

 

If anyone is able to shed any light on this it would be much appreciated.

 

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I guess that with coupled VNAV, the autopilot will actually control the VNAV Path.

For climbs and descends it will manage the various altitude restrictions.

For the descends I asume that VNAV is calculated for a engines idle descend, meaning that when VNAV starts the descend, the throttle levers should be set to idle.

But since there isn´t  any autothrottle, You must closely supervise Your speed and be prepared to set engine thrust when it levels out at the various descend restrictions (both altitude and speed).

 

 

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9 minutes ago, Finn said:

I guess that with coupled VNAV, the autopilot will actually control the VNAV Path

Thanks for the reply. Can you explain more what the bit I've quoted means. As there is no autothrottle, is the AP controlling pitch to manage the descent assuming the throttles are set to allow that?

 

Your words on VNAV only make sense. I've seen a lot of comments on the MSFS forum about VNAV rapidly diving to get the to the next altitude step - but not sure what that really means.

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Let´s say You cruise at FL150 40nm from You destination airfield. 

The flightplan incooperates a STAR which ends in an ILS approach.

30nm from the airport along Your STAR You have an altitude restriction at FL070 marked by a waypoint.

20nm from the airport there is an altitude restiction at FL050 marked by a waypoint.

Your platform altitude (The altitude where You would intercept the ILS glideslope - if doing an ILS approach) is at 2500 ft.

 

Before reaching TOD (Top of descend where the aircraft goes from cruise to descend phase) You set You MCP (Autopilot) altitude at 2500 ft (Platform altitude).

The VNAV part of the autoflightsystem will calculate the path to fly with the throttles at idle, taking grossweight, wind (if entered) and a few other parameters into account.

Reaching TOD it will then start the descend and You will have to manually set the throttles at idle.

It will then automatically ensure that the altitude restrictions at these points are kept - remember taht altitude restrictions can be marked as "AT", "Below" or Above".

So if the restriction are "AT" then You pass the restrictions "AT" FL070 and FL050.

If the restriction are "Above" then You pass the restrictions "AT" FL070 and FL050, but if adequite for the rest of the path also will be above, then the aircraft is allowed to be above FL070 and/ FL050, but not below.

If the restriction are "Below" then You pass the restrictions "AT" FL070 and FL050, but if adequite for the rest of the path also will be above, then the aircraft is allowed to be belowFL070 and/ FL050, but not above.

 

The aircraft might level off if it reaches these altitudes before the waypoint and will then continue descending to the next restriction and then again, if reaching the next altitude restriction before that waypoint level out.

 

It will then end at the platform altitude of 2500 ft - kind of guarded by You MCP set altitude of 2500 ft, where You would change to Approach mode and then follow the ILS down for landing.

 

Since the CRJ does not have autothrottle, You must keep an eye on the airspeed and set the thrust manually to manage airspeed.

 

VNAV is mostly good for STAR's that doesn´t consiste vectoring by ATC. It is a "heads down" mode meaning that You spend more time looking at You flightinstruments rather than looking outside - that's why many Airlines does not accept the use of VNAV or restrict's it´s use to above 10.000 ft.

 

Often it is better to manage the descend using V/S or FLC (IAS / Speed) mode.

 

The CRJ is a small aircraft used between smaller airports or in conjested airspace where ATC vectoring is common.

Therefore is VNAV not a mode that is well suited for descend management.

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Speaking of non coupled VNAV, You have to set all restrictions manually for each waypoint.

VNAV advisory will simply indicate if You are on the correct path and Your deviation from it, and guide You via the star symbol on the VNAV "glideslope" indicator.

 

Actually both VNAV and AUTOLAND modes are highly overrated in the flightsimming community and are alot less used in real life than in the sim world.

Personally though I really like to use both functions cause it´s cool to see all the automation do it´s job.

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  • 4 weeks later...

Are you sure that with coupled vnav the throttle has to be set to idle? Surely that would make the speed drop below the vnav speeds set in the fms for the desent phase. I assumed that with coupled vnav you need to constantly adjust the throttle so that you match the fms desent speeds. Can someone clarify?

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2 minutes ago, RF pilot said:

Are you sure that with coupled vnav the throttle has to be set to idle? Surely that would make the speed drop below the vnav speeds set in the fms for the desent phase. I assumed that with coupled vnav you need to constantly adjust the throttle so that you match the fms desent speeds. Can someone clarify?

 

You're more or less correct. The CRJ VNAV doesn't really plan for descents at idle thrust. Ideally it should since typically flight plans are built using idle thrust descents. The CRJ VNAV is set by default to descend on a 3° slope. So it takes the first hard altitude restriction and builds a 3° slope back from that and that's where it places your TOD point. Three degrees isn't really steep enough for an idle descent. If you like, there is an option to change the VNAV descent slope if you like on the VNAV page in the FMS. On the right hand side you can change the 3.0 to something else. 3.4 should get you close. I've tried this a few times and it does the calculations well but the automation in the sim doesn't seem to like it so I just leave it at 3.0. In the real plane I used to set this a 3.8 and it worked pretty well.

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46 minutes ago, Chaxterium said:

 

You're more or less correct. The CRJ VNAV doesn't really plan for descents at idle thrust. Ideally it should since typically flight plans are built using idle thrust descents. The CRJ VNAV is set by default to descend on a 3° slope. So it takes the first hard altitude restriction and builds a 3° slope back from that and that's where it places your TOD point. Three degrees isn't really steep enough for an idle descent. If you like, there is an option to change the VNAV descent slope if you like on the VNAV page in the FMS. On the right hand side you can change the 3.0 to something else. 3.4 should get you close. I've tried this a few times and it does the calculations well but the automation in the sim doesn't seem to like it so I just leave it at 3.0. In the real plane I used to set this a 3.8 and it worked pretty well.

Thanks!  That explains it very well.

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