Are Winds Aloft used for TOD calculation?

[marcvc]

First of all: this is a truly amazing plane – huge respect for what the developers have put together here!

 

This morning I made a flight from LFPB to LFSB, with strong winds (as Eunice was getting close).

At FL330, the tailwind component was well above 80kts for most of the time.

The flightplan was created with Simbrief and winds were downloaded via de VHF link.

The descent was done through VNAV, from TOD down to 4000ft – and all worked fine.

However, I was surprised that I needed to (fully) deploy the flight spoilers pretty much throughout the entire descent in order to stay on the vpath and maintain 250kts (at idle, commanded by ATS). As all of that was programmed in the fms and the winds aloft were available (and verified on LEG WINDS screen), I would have expected that the TOD point calculation would have factored that in – and hence TOD would come earlier to allow for a descent without spoilers.

Don’t know if that’s the way it is supposed to work in IRL, or whether I’m still missing some setup somewhere?

 

Thanks,

 

Marc

[marts747]

As far as I know, the VNAV descent defaults to 3.0 degrees, so it makes no difference what the winds are. I would imagine the downloaded winds are used for fuel planning purposes only.

If you had a steady 80Kt tailwind on descent then it would make sense that speedbrakes would be required as the wind would be 'pushing' you off the computed 3.0 degree descent angle.

I'm no real life pilot but I would hazard a guess that this is correct.

[Rodeo]

Indeed, in the CL650 you get a geometric path VNAV descent, rather than performance-based VNAV descent. Thus neither winds nor target speeds make any difference. You can manually change the vertical path angle via one of the VNAV setup pages to be steeper or shallower depending on the expected wind component during descent as well as personal preference.

[richjb]

On 2/21/2022 at 11:53 PM, Rodeo said:

Indeed, in the CL650 you get a geometric path VNAV descent, rather than performance-based VNAV descent. Thus neither winds nor target speeds make any difference. You can manually change the vertical path angle via one of the VNAV setup pages to be steeper or shallower depending on the expected wind component during descent as well as personal preference.

You can also change the VNAV path angle from the LEGS page.  For example, on the LVZ4 STAR into KTEB, it is common to get STENT at FL350.  The LEGS page will look something like this:

JHW

HOXIE

DMACK

                             3.0 

STENT                 /FL350

In the scratch pad, you can type in "1.5" and then line select it over the "/FL350" for STENT.  That will change the descent angle to 1.5 to STENT at FL350:

                            1.5

STENT                /FL350

A 1.5 descent angle results in a 1000 - 1200 FPM descent rate, depending on headwind or tailwind. This keeps you within the RVSM descent rate requirements, i.e., not to exceed 1500 FPM.  It doesn't affect your default settings either.  It's a one time change. 

You can also use this method to restore the default vertical angle.    The next waypoint after STENT is LVZ, which is an "expect FL180" constraint.  Often times, we'll program that constraint ahead of time.  When you enter the AT FL350 for STENT, the VNAV draws a geometric path between STENT at FL350 and LVZ at FL180, which usually equates to a 1.8 degree or so path, which might be too shallow.  You can type "3.0" in the scratch pad, and line select it up on the LEGS page next to the FL180 at LVZ and that will change the angle back to 3.0.  It also makes for a smoother path transition to join the VNAV path from LVZ to MUGZY at 6000. 

 

Rich 

Edited March 4, 2022 by richjb

[marcvc]

On 3/4/2022 at 9:52 PM, richjb said:

You can also change the VNAV path angle from the LEGS page.  For example, on the LVZ4 STAR into KTEB, it is common to get STENT at FL350.  The LEGS page will look something like this:

JHW

HOXIE

DMACK

                             3.0 

STENT                 /FL350

In the scratch pad, you can type in "1.5" and then line select it over the "/FL350" for STENT.  That will change the descent angle to 1.5 to STENT at FL350:

                            1.5

STENT                /FL350

A 1.5 descent angle results in a 1000 - 1200 FPM descent rate, depending on headwind or tailwind. This keeps you within the RVSM descent rate requirements, i.e., not to exceed 1500 FPM.  It doesn't affect your default settings either.  It's a one time change. 

You can also use this method to restore the default vertical angle.    The next waypoint after STENT is LVZ, which is an "expect FL180" constraint.  Often times, we'll program that constraint ahead of time.  When you enter the AT FL350 for STENT, the VNAV draws a geometric path between STENT at FL350 and LVZ at FL180, which usually equates to a 1.8 degree or so path, which might be too shallow.  You can type "3.0" in the scratch pad, and line select it up on the LEGS page next to the FL180 at LVZ and that will change the angle back to 3.0.  It also makes for a smoother path transition to join the VNAV path from LVZ to MUGZY at 6000. 

 

Rich 

Tried this and it works great.

Thanks for sharing!

 

Marc

[richjb]

On 3/7/2022 at 12:21 PM, marcvc said:

Tried this and it works great.

Thanks for sharing!

 

Marc

I am amazed at how close HS got to the real Collins FMS!