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Quick little tutorial for those in trouble with VNAV descents

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Hi guys,

 

Reading through here and other forums I see a few are having issues getting VNAV to work. Here is just a little tutorial I whipped up on my way to LOWW.

 

Step 1. During cruise.

 

Ok, so you're at your cruise altitude, and in my case this is 21000'. I brought up my 2D FMS.

 

- Select FPL- what you are looking for are your altitude constraints and also the altitude you want to be at for your approach. For this example I'm going to ignore some of the constraints and only worry about getting myself at the correct altitude for the approach, which is the ILS 29 into LOWW. Use the NEXT and PREV keys to cycle through your flightplan to find a reference waypoint.

 

- In this screen you will see WW951 at 3000 and I29 at 3000. BRK is technically my FAF, but, I want to ensure I'm at altitude a little sooner, so I'll use WW951 as my reference point.

 

1achkwpt.jpg

 

 

Step 2. Click VNAV on the FMS.

 

- On the right you will see your future waypoints laid out. Use the NEXT and PREV keys to cycle through them, and find WW951 (or your reference waypoint.

 

- Dial in the number associated with the waypoint, in my case "10", and press ENTER.

 

1bvnavslctwpt.jpg

 

 

 

Step 3. Now you will be at the VNAV PATH page.

 

- You will see a white box and an altitude next to the TO waypoint, which is WW951 (or your reference waypoint).

 

1cvnaventer.jpg

 

 

- Press enter. Now you will see the altitude box is highlighted. Verify it is the correct altitude that you want to be at that waypoint by. It is set at 3000' already.

 

2013423132623483.jpg

 

 

- Press enter. Now you will see the target vertical speed (TGT V/S) is highlighted. Input your desired vertical speed for the descent.

 

2013423132626101.jpg

 

 

- Press enter. Now, you've got yourself a TOD!

 

1dtod.jpg

 

 

Step 3. The descent.

 

Okay, so now you've got your TOD, and you will see it on your ND.

 

- When you are 2 minutes out from TOD, you will see your VNAV indicator pop up on the PFD.

 

2013423133130194.jpg

 

 

- Now, dial in your altitude into the autopilot on the AFCS, in this case 3000.

 

- Click VNAV on your AFCS to arm VNAV.

 

2013423133137737.jpg

 

 

And that's it! At TOD, the autopilot will start the descent, so be ready to back out the power a bit!

 

There may be other tutorials out there already, but I've seen this come up here and elsewhere a few times today anyways. This was a little hasty in putting together, but hopefully it will help a little bit! Just apply what I said here to your flight, and you should be good to go!

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Regards,

Kyle

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Thanks very much, this is extremely helpful!  I'd figured out some of it already, but you've really helped confirm what I was doing right versus what I was doing wrong!


Jim Stewart

Platforms: X-Plane 11 | P3Dv4 | DCS
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Thank you for clear explanations. I hope we'll have less questions about VNAV on our support forum :)


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Thank you for clear explanations. I hope we'll have less questions about VNAV on our support forum :)

 

No problem, let me know if you need any help with anything.


Regards,

Kyle

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I have a problem that the fms still is using altitude setting constraints from the sid. At least I think that's what is happening. I see in the ND that other waypoints after the sid show the same altitude setting.

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Great tutorial!  Thanks much!


Dylan Charles

"The aircraft G-limits are only there in case there is another flight by that particular airplane. If subsequent flights do not appear likely, there are no G-limits."

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Nice explanation.

 

Couple questions Im trying to figure out. Does inputing the target of WW951/3000 in this case mean that all previous constraints in the STAR are ignored? 

 

Seems like it just skips them all and just focuses on getting you to 3000. But if ATC or the STAR has restrictions, putting in a waypoint somewhere midway as the VNAV point,I assume the rest of the way will be manual flying or similar to "LVL change". 


Dave Seminchuk  CYVR LSZH FDX Virtual, Fly Virtual.
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Good question David.

 

Does the FMC adhere to hard restraints?


- Dean

P3Dv4 & XP11

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Nice explanation.

 

Couple questions Im trying to figure out. Does inputing the target of WW951/3000 in this case mean that all previous constraints in the STAR are ignored? 

 

Seems like it just skips them all and just focuses on getting you to 3000. But if ATC or the STAR has restrictions, putting in a waypoint somewhere midway as the VNAV point,I assume the rest of the way will be manual flying or similar to "LVL change". 

 

I was wondering the same thing.

 

If I want to fly a star, with multiple altitude restrictions, should I usethe first or the last waypoint (IAF) for the vnav. And if I use the first waypoint, should I update the vnav page for every following waypoint or should I use vspeed or IAS?


Bastiaan

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Thanks for the tutorial it's very useful, but same question...


José Fco. Ibáñez /// i7 6700k (Delid) @ 4,6 Ghz /// Asrock Z170 OC Formula /// 16GB RAM G.Skill Ripjaws V 3200 /// GTX 1070 Founders Edition 8GB /// LG 27UD58 4K 27' // OCZ Vertex 4 SSD (X-Plane 10) & SAMSUNG 850 EVO SSD (P3D V3) /// Windows 10 Pro x64

 

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Nice explanation.

 

Couple questions Im trying to figure out. Does inputing the target of WW951/3000 in this case mean that all previous constraints in the STAR are ignored? 

 

Seems like it just skips them all and just focuses on getting you to 3000. But if ATC or the STAR has restrictions, putting in a waypoint somewhere midway as the VNAV point,I assume the rest of the way will be manual flying or similar to "LVL change". 

 

Taken from the System Description manual 16.10.7:

 

"When the altitude information is entered, the VNAV function will calculate the vertical path to that waypoint, based on the vertical speed entered by the pilot. Any subsequent flight plan waypoints will then be imported into the vertical flight plan, as vertical waypoints. The altitude restriction for those waypoints will also be taken from the flight plan if they exist. If not, they will be automatically pre-filled from the previous waypoints which had such restriction."

 

This passage hints that there might be a way, if there is, I personally have not found it. My understanding and experimentation is shown perfectly by this photo from 16.10.7:

 

vnav.jpg

 

 

In other words VNAV will go to one specific waypoint, and afterwards vertical guidance is free. That means you will have to control the descent using either Pitch Hold, IAS hold, or V/S.

So, if you have a complicated STAR, you will have to enter the first constraint, then use other AP modes afterwards.

 

This is just my observations so far, I'll experiment a bit more and see if I can come up with anything else. Otherwise it would be helpful to hear from someone with Dash 8 time, a reliable linked source, or one of the developers, just to clarify further.

 

Best thing to do though....hand fly that STAR :wink2:


Regards,

Kyle

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