Quick little tutorial for those in trouble with VNAV descents

[gheeD 62]

If the STAR and/or APP has any altitude restrictions (either from the navdata or any that you have applied yourself) the VNAV will continue to descend following the approach. You dont need to re-plan it; as long as the waypoints have more altitude restrictions...

 

What I do is set VNAV to descend to first altitude restricted point on my STAR and that's it. It will follow the path down to when you can capture localizer and glideslope for ILS in my experience.

[groentesoep 22]

@@Kyle and Kim.

Thanks for the explanations.

[n4gix 4,947]

Promoted to a "Tutorial" for posterity...

[dmbusmc 46]

Promoted to a "Tutorial" for posterity...

 

^_^  Thanks Bill!

[jose88 50]

If the STAR and/or APP has any altitude restrictions (either from the navdata or any that you have applied yourself) the VNAV will continue to descend following the approach. You dont need to re-plan it; as long as the waypoints have more altitude restrictions...

 

What I do is set VNAV to descend to first altitude restricted point on my STAR and that's it. It will follow the path down to when you can capture localizer and glideslope for ILS in my experience.

 

So if the STAR has restrictions the plane will fly it correctly?

[William J.S.S 173]

Still a little confused. Lets say you have 4 alt constraints. I set up the vnav for the last one and it will still hit the 3 before it? Or is is a kinda, pick one and go with it kinda thing.

 

Reason is, I like flying into KLAX and we all know about the multiple constraints one after the other there.

 

 

Sent from my Apple communications device.

[Boeing or not going 578]

I believe setting up vnav is more for guidance to show you where you are in relation to the approach, it doesn't seem like vnav descents would be used that often with such a poor fms design.

[William J.S.S 173]

Oh well. Hand flying Her is nice either way and I grew tired of letting the NGX fly me around and started to hand fly SIDS/STARS so I am no stranger.

 

Also, flying online makes it better too as you have usually ATC giving out altitudes rather than looking at the box

 

 

Sent from my Apple communications device.

[jose88 50]

Anyone can confirm the restrictions on VNAV mode?

[groentesoep 22]

Well, I tested the vnav again today on the DATBO1 arrival into CYLW. And to me it seemed that it works as long as you enter the first restricted waypoint in the vnav. The restrictions for subsequent waypoints will than also be met.

 

I setup VNAV for the first restricted waypoint of the STAR: VIDRI with an altitude of 12000 feet. I realize that this is not the way the STAR would normaly be flown, as 12000 ft is the minimum altitude for this waypoint. After overflying the waypoint the AP stayed in VNAV PATH mode and the rate of descent decreased a lot, but the plane was still descending. The next waypoints altitude restriction was 9000 ft or above. And indeed the airplane overflew the XOTUV waypoint at 9000 ft.

 

So my conclusion is that for VNAV to work on multiple waypoints, with multiple altitude restrictions, you should enter the first restricted waypoint in the VNAV page of the CDU. I do think that the VNAV will descent to the altitude thats entered at the specific waypoint and will not take into account any MIN or MAX values. So for optimum descent on, for example, the DATBO1 arrival, you should enter the first exact altitude restriction (by that I mean altitude restrictions that are not a MIN altitude).

[RudiJG1 172]

Well stated, Bastiaan, and that's my experience as well.

[William J.S.S 173]

@@groentesoep

 

Thanks for the HU!

I will try Her out tonight. KLAX, here I come.

[n4gix 4,947]

I believe setting up vnav is more for guidance to show you where you are in relation to the approach, it doesn't seem like vnav descents would be used that often with such a poor fms design.

One should actually study the real world manual before making such a sweeping generalization as that...

 

See Section V - Operations, especially pages 5-59, et seq. (page 237/442 in the PDF)

 

VNAV Setup and Activation

VNAV operations are controlled by the vertical speed input by the

pilot for the first waypoint and the vertical speed computed, based on

the altitudes, for second and subsequent vertical waypoints. The

following information describes how vertical speed is used in the

VNAV function.

...

VNAV Flight Path Definition Procedure

NOTE: A Flight Plan must already be active, as flight plan

waypoints are used to define the VNAV waypoints.

 

1. Press the VNAV function key. The VNAV Cruise page is

displayed, and all entry fields are prefilled with the current fight

plan waypoints.

 

2. Press the TO line select key to define the TO VNAV waypoint.

The Vertical Waypoint Select page is displayed. Use the PREV

and NEXT keys to view other flight plan waypoints (ahead of the

FMS present position).

 

3. Enter the desired flight plan waypoint using the numeric

selection process. This waypoint will be used as a reference to

define the VNAV waypoint location.

 

4. The cursor expands on the VNAV Cruise page to cover the

offset distance entry field. Use the ± key to change the offset

sign ("-" for offset prior to the flight plan waypoint, "+" for offset

beyond the flight plan waypoint). Enter the offset distance.

 

5. The cursor expands to cover the target altitude entry field. If an

altitude field entry is three digits and less than 640, then it is

assumed to be a flight level entry. Otherwise it is assumed to be

a regular altitude entry. For example, if "370" is entered, the

altitude field will automatically convert to "FL370" for display. If,

however, the pilot wants to enter an actual altitude of 370 feet,

he must input "0370" in the altitude field. If an altitude is entered

which results in a flight path angle greater than the configured

maximum FPA, then it will flash and not be accepted.

 

6. Using line select keys [3L, 4L and 5L] to access the NX entry

fields, repeat the above process to define succeeding VNAV

waypoints.

•  Above each NX line is the vertical speed required to follow

the defined vertical flight path to that VNAV waypoint.

•  All VNAV waypoints must lie ahead of the FMS present

position and must be sequenced in the same order as they

occur on the lateral flight plan.

 

7. If vertical flight guidance is desired to the "TO" VNAV waypoint,

press line select key [1R] to access the TGT V/S field and enter

the desired vertical speed. Invalid entries will cause the V/S

entry to flash, and an error message will be displayed (see

following section).

 

8. When the TO VNAV waypoint and the TGT V/S entries have

been made, the distance to the TOD will be displayed as well as

Flight Path Angle. Vertical Deviation indication will be

referenced to the flight path defined by the first VNAV waypoint

and VSR. TOD is the point at which the pilot should begin the

capture maneuver for the FPA.

 

9. When the aircraft reaches the last programmed VNAV waypoint,

subsequent VNAV waypoints will automatically fill with current

VNAV altitude, causing guidance to be to zero VSR and

maintaining level flight.

 

 

 

 

 

 

 

[Boeing or not going 578]

One should actually study the real world manual before making such a sweeping generalization as that...

 

See Section V - Operations, especially pages 5-59, et seq. (page 237/442 in the PDF)

 

I'm not making a generalization, I'm commenting on the fact that pilots hate the fms and speed hold or vertical speed is the preferred method for descent.

[Overload 91]

I'm not making a generalization, I'm commenting on the fact that pilots hate the fms and speed hold or vertical speed is the preferred method for descent.

I agree, in today's complex ATC environment, pilots can expect many level offs in descent, I really couldn't see the rather limited VNAV implementation being used much. Even with aircraft with more more sophisticated VNAV modelling, it's not always trusted or used.

 

I enjoy playing around with it from an academic point of view, but really don't think it's a very practical.

As ever, I'm always ready to listen and learn, any RW 400 pilots care to comment?

Eugene