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Paul_Smith

JS41 FMS TOD calculation

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Does anybody know how to get the FMS to re-calculate the TOD mid-flight? My flight was from NZWN, DAMBO, CC, BELEE, ODISI, DIVSU, NZCH (ILS RW20). When I settled in and cruising from CC to BELEE, the FMS VNAV page shows the TOD about the same distance as the DME shows NZCH. I used the DirectTo button to go direct to ODISI but the VNAV TOD does not update (it is no longer visible on VNAV page 2). As I expect to reach ODISI at 3,000 ft, that makes descent planning difficult. Possible Update. When about 60NM from ODISI, the VNAV page updated itself and gave me the correct descent plan, but I don't know if I did something to force it.


Paul Smith.

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Paul, I think the TOD is recalculated if you reenter a constraint on the VNAV fix list page or the cruise altitude on the data screen.PS: Robert, who flew these, has said they seldom used the VNAV function but did the distance calc in their head (use 300ft/nm as an easy descent angle).


Dan Downs KCRP

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Guest Jimmatc

Hey Paul,As Dan mentions, it seems to be related to altitude constraints not being in place. If you tell it Christchurch is at 123' it seems to recalculate. I tend to go for the manual calculations though: TOD = FL/3 (e.g F150/3 = 50nm). So, taking the arrival into account I'll start a descent from F160 into NZCH from the north from about 55 CH DME for runway 20, and about 35 CH DME for runway 02. Of course this aircraft is a little less forgiving than other turboprops if you end up high. But that's all part of the fun!!

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Hey Paul,As Dan mentions, it seems to be related to altitude constraints not being in place. If you tell it Christchurch is at 123' it seems to recalculate. I tend to go for the manual calculations though: TOD = FL/3 (e.g F150/3 = 50nm). So, taking the arrival into account I'll start a descent from F160 into NZCH from the north from about 55 CH DME for runway 20, and about 35 CH DME for runway 02. Of course this aircraft is a little less forgiving than other turboprops if you end up high. But that's all part of the fun!!
Well I think you need to add that your VS should be 5x GS and then you will end up right on the spot.

Happy flying!
Alexander M. Metzger

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Guest Jimmatc

Quite right!

Well I think you need to add that your VS should be 5x GS and then you will end up right on the spot.

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Well I think you need to add that your VS should be 5x GS and then you will end up right on the spot.
Could you explain that with an example please? Am I right in the following? I am cruising at 18000. I want to be at an intersection at 3,000 with a normal GS of 3 degrees.18000 - 3000 = 15,000ft of descent = FL150Distance = FL/3 = 150 / 3 = 50 NMVS = 5xGS = 5x3 = 1500ft/per minute (is this correct?)So to get from 18,000 to a 3,000 I need to start a 1,500ft/per minute descent 50NM from target? (How does speed affect this?)

Paul Smith.

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Ground speed directly impacts verticle speed if you are descending on a constant angle.A 3-deg slope is equivalent to 318ft/nm (simple trig). To descend at 318ft/nm your verticle speed depends on your ground speed. If you have a ground speed of 300kts, that is 5nm per minute (300/60) so you need to descend (5*318) 1592 ft/min. If your ground speed is 240kts, the required descent rate is 1274 ft/min and note that as you descend into denser air your ground speed will decrease given a constant attitude/power (ignoring the increase in %torque due to denser air).The alt(100's)/3 and the 5xGS rules of thumb are handy tools for quick in the head calculations.Hope this helps.


Dan Downs KCRP

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Ground speed directly impacts verticle speed if you are descending on a constant angle.A 3-deg slope is equivalent to 318ft/nm (simple trig). To descend at 318ft/nm your verticle speed depends on your ground speed. If you have a ground speed of 300kts, that is 5nm per minute (300/60) so you need to descend (5*318) 1592 ft/min. If your ground speed is 240kts, the required descent rate is 1274 ft/min and note that as you descend into denser air your ground speed will decrease given a constant attitude/power (ignoring the increase in %torque due to denser air).The alt(100's)/3 and the 5xGS rules of thumb are handy tools for quick in the head calculations.Hope this helps.
Is the GS in 5xGS = Ground Speed and not Glide Slope then?That would change the above to: I am cruising at 18000. I want to be at an intersection at 3,000 with a GS of 250knots.18000 - 3000 = 15,000ft of descent = FL150Distance = FL/3 = 150 / 3 = 50 NMVS = 5xGS = 5x250 = 750ft/per minute (Not the 1,500ft per minute I had last time, is this correct?)So to get from 18,000 to a 3,000 at 250knots I need to start a 750ft/per minute descent 50NM from target?

Paul Smith.

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Actually, 5*250 is 1250 (or more precisely 250/60=4.2 and 4.2*318= 1327 fpm).


Dan Downs KCRP

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