I'm not understanding vnav in this plane

[eslader 1,783]

Hoping someone has some insight. I'll set the vnav page up, and it will tell me X time until I should start descending. The first odd thing is that the countdown moves at 3x real time. So, every one second, about 3 seconds pass on the countdown.

Then I'll get to where I'm supposed to start the descent, and I'll descend at the programmed rate, but the required rate display keeps increasing. For instance, if I set vnav up for an 800fpm descent, and then descend at 800 when it tells me to, the required descent will soar well over 2,000 by the time I get to the waypoint, and I'll be high. Even if I descend at double the set rate, the required fps will keep increasing. Any idea what I'm doing wrong? I even looked in the manual for the real-world unit, and that didn't help.

 

[ark 1,288]

1 hour ago, eslader said:

Hoping someone has some insight. I'll set the vnav page up, and it will tell me X time until I should start descending. The first odd thing is that the countdown moves at 3x real time. So, every one second, about 3 seconds pass on the countdown.

Then I'll get to where I'm supposed to start the descent, and I'll descend at the programmed rate, but the required rate display keeps increasing. For instance, if I set vnav up for an 800fpm descent, and then descend at 800 when it tells me to, the required descent will soar well over 2,000 by the time I get to the waypoint, and I'll be high. Even if I descend at double the set rate, the required fps will keep increasing. Any idea what I'm doing wrong? I even looked in the manual for the real-world unit, and that didn't help.

 

What sim are you using, and what GPS version are you using? Also, some screen shots of your VNAV setup would be helpful I think. What other plane have you used the same VNAV setup on where it worked well?  The reason I ask this is because my initial thought is this seems to be more of a GPS setup issue than a particular plane issue.

Al

Edited June 11, 2019 by ark

[eslader 1,783]

Sorry. P3dv4 and I'm using the GPS unit that comes with the plane - I have made no addons or modifications to any of the systems the plane ships with.

I haven't used this VNAV setup before. I usually fly tubeliners these days, and the FMS handles VNAV very differently than the little Garmin units.

 

[ark 1,288]

15 hours ago, eslader said:

Sorry. P3dv4 and I'm using the GPS unit that comes with the plane - I have made no addons or modifications to any of the systems the plane ships with.

I haven't used this VNAV setup before. I usually fly tubeliners these days, and the FMS handles VNAV very differently than the little Garmin units.

 

Unfortunately I have not used that GPS so I am not familiar with its VNAV capabilities. Hopefully someone who is will provide some help here.

Al

Edit: If you go to the FSW Product page at  https://flysimware.com/website/FALCON_50.html you can download the Flysimware GNS 530 manual which has VNAV instructions.

Edited June 12, 2019 by ark

[Guevorkyan 69]

GPS VNAV is not at all like an FMS VNAV. 

It only takes into consideration your ground speed, altitude to lose and distance remaining to the fix in question.

TOD is calculated at your present GS, but that might and will vary as you descend. Therefore, all the other variables will as well.

It's a dynamic excercise where you need to manage your GS accordingly, and know that if you start to go slower, less FPM will be required, whereas if you go faster, you need to sink faster.

GTN is no diferent, that's the dynamic of a GPS VNAV. It is mainly an aid to calculate a descent profile.

A built in FMS, depending on its complexity, will also take into account aircraft performance (guidance), winds aloft (met) and speed or altitude restrictions along the path.

None of that comes with a GPS VNAV.

Edited June 14, 2019 by Guevorkyan

[eslader 1,783]

That's good to know. I'll play with that some more. Would you expect ground speed to be the problem when, say, vnav is calling for an 800fpm descent, and I initiate a 2500fpm descent and vnav's required descent keeps increasing anyway?

And yeah, I did read the manual for the 530 from FSW, but it didn't explain why I'm having this problem. I even read the real-life 530 manual which didn't help.

Edited June 14, 2019 by eslader

[Guevorkyan 69]

I wouldn't know about that. Wouldn't be logical for it to be like that, IF the GPS calculations are correct.

 

[somiller 694]

On 6/14/2019 at 11:40 AM, eslader said:

That's good to know. I'll play with that some more. Would you expect ground speed to be the problem when, say, vnav is calling for an 800fpm descent, and I initiate a 2500fpm descent and vnav's required descent keeps increasing anyway?

And yeah, I did read the manual for the 530 from FSW, but it didn't explain why I'm having this problem. I even read the real-life 530 manual which didn't help.

Just tonight I was working on descents to approach fix altitude constraint, and I would say indeed ground speed is the problem you are dealing with. If after starting your descent you reduce power to maintain a constant airspeed, you'll see the vnav calculated rate of descent remain relatively constant. If you let the speed increase, you'll need a higher rate of descent to hit the target altitude at your waypoint. If you reduce your speed, you'll see the calculated rate of descent start to drop.

Kind Regards,

[Cool 27]

There is an easy rule of thumb to determine the TOD:

subtract the current altitude from the target altitude, 
ex: 30000ft - 10000ft = 20000

divide the result by 1000 and multiply with 3 and you will get the miles
20000 : 10000 = 20 x 3 = 60 miles

so you have to start the descent 60 miles away from the point where you plan to be at 10000

During the descent:

• descent rate is ~2500 ft/min but not less than 2000 ft/min
• speed is M 0.8 until crossing 300kts then 300kts until FL100 and 250kts below

Pascal

[ark 1,288]

11 hours ago, Cool said:

There is an easy rule of thumb to determine the TOD:

subtract the current altitude from the target altitude, 
ex: 30000ft - 10000ft = 20000

divide the result by 1000 and multiply with 3 and you will get the miles
20000 : 10000 = 20 x 3 = 60 miles

so you have to start the descent 60 miles away from the point where you plan to be at 10000

During the descent:

• descent rate is ~2500 ft/min but not less than 2000 ft/min
• speed is M 0.8 until crossing 300kts then 300kts until FL100 and 250kts below

Pascal

If you calculate your TOD distance (NM) as outlined above by Pascal [ TOD = altitude to be lost (in 1000 of ft) x 3 ] then using a vertical descent rate (ft/min)  VS = 5 x Ground Speed will give you very close to a 'standard' 3 degree descent. The GPS will give you the GS. In practice I find multiplying the GS by10 and then dividing by 2 is usually easier if doing the VS calculation in my head.

If during the descent you GS goes up or down by a significant percentage, adjust your VS by the same percentage.

Al

 

Edited June 20, 2019 by ark

[Guevorkyan 69]

2 hours ago, ark said:

If you calculate your TOD distance (NM) as outlined above by Pascal [ TOD = altitude to be lost (in 1000 of ft) x 3 ] then using a vertical descent rate (ft/min)  VS = 5 x Ground Speed will give you very close to a 'standard' 3 degree descent. The GPS will give you the GS. In practice I find multiplying the GS by10 and then dividing by 2 is usually easier if doing the VS calculation in my head.

If during the descent you GS goes up or down by a significant percentage, adjust your VS by the same percentage.

Al

 

Correct!

I myself calculate descent rate as GS/2, then add a zero.

For example, if I'm doing 120kt, then 120/2= 60. Add a zero, 600fpm. For 450kt, then 225, add a zero, 2250fpm.