Vac - Mismatch between AFM and FMS?

[StrikerTed]

Hi,

I am FO on a CL605 fleet, with a long background in aeronautical engineering. I developed an interest in desktop flight simulation at a time when Flight Simulator still was a subLogic product   - let me say that this CL650 simulation is absolutely amazing! 

Now, let's go to the point: regarding Vac, I noticed a mismatch between the AFM and the FMS calculation. It looks like the latter doesn't take into account the saturation which can be seen in the attached AFM table - basically, at low pressure altitudes and landing weight not too close to the maximum, Vac is always 129 KIAS; in all other cases, it looks like Vac = Vref or very close to.

I did a couple of tests both on the real FMS and with the Performance Software we use, and I (as expected) got exactly the AFM numbers, whereas the Hotstart FMS seems to always give Vac close to or equal to Vref.

Note: the AFM numbers for Approach Climb Speed look the same between 605 and 650 (and 604 as well)

Do I miss anything?

 

 

[Pils]

On 1/30/2025 at 5:42 AM, StrikerTed said:

I did a couple of tests both on the real FMS and with the Performance Software we use, and I (as expected) got exactly the AFM numbers, whereas the Hotstart FMS seems to always give Vac close to or equal to Vref.

Can you provide screenshots of some examples, please?

[StrikerTed]

Pressure altitude around SL in this case (EGSS, RWY 04, Q1026). LW = 32000 pounds. HotStart FMS gives Vac = 121, whereas AFM gives Vac = 129, as you can see from the table above (consistent with the real FMS and the performance sw I use). On the other hand, Vref = 121 is ok among HotStart FMS, AFM, real FMS and Perfo SW.

PS unfortunately I do not have a picture of the real FMS at hand, and cannot share screenshots/details of my Perfo SW. Anyway, AFM is the reference to be considered here

[Pils]

55 minutes ago, StrikerTed said:

Confirm this is an extract of the AFM and the lines are baked in, not something you've added?

[Pils]

57 minutes ago, StrikerTed said:

129

It looks like 128.5 kts technically, but I guess it's rounded up.

[StrikerTed]

On 2/9/2025 at 11:41 PM, Pils said:

Confirm this is an extract of the AFM and the lines are baked in, not something you've added?

Yes I confirm it's from the AFM - btw it's somehow curious that the example in the Vac chart is given for a weight of 18 tons, well above the MLW (38000 pounds)

On 2/9/2025 at 11:43 PM, Pils said:

It looks like 128.5 kts technically, but I guess it's rounded up.

Yes of course it's rounded up  there can be many ways to put those AFM tables into an FMS 

 

Do you think that it will be possible to have a correct Vac computation in a future release of HotStart 650?

[Pils]

5 hours ago, StrikerTed said:

Do you think that it will be possible to have a correct Vac computation in a future release of HotStart 650?

Yes I’ll try to being it up with the developer.