calculating Vertical Speeds

[EHM-1343 Jonathan]

ok, here is the scenario, i am on the STAR for Manchester, the MIRSI 1A arival.



In the STAR designator, and descent planing, saying be fl200 10NM before MONTY.

Say i'm at FL300 and 30NM before MONTY, how would i know what vertical speed is required to get me down to that FL, within the restrictions.

[EHM-1001 Robert]

Hi Jonathan,

well, you can expect a -2000 fpm descent from FL300. So the question is not how much descent rate, but how far from the desired point to start ?

It depends on your speed. With -2000 fpm, to descend 10000 ft takes 5 minutes (0,083 hours) During this time, you take distance depending on your speed:
- at 500 kts GS you take (s=v*t) > distance = 500*0,083 =  41,5 nm
- at 450 kts the distance = 450*0,083 = 37 nm
- at 400 kts the distance = 400*0,083 = 33 nm
- at 350 kts the distance = 350*0,083 = 29 nm
- at 300 kts the distance = 300*0,083 = 25 nm
... and so on

Afterall, I would suggest start your descent safely about 40 nm before MONTY

[EHM-1617 Iain]

Right:

FL300 (where you are) - FL200 (where you're going) = 10000ft (to lose).

Divivde that by 1000, to give 10 in this case, then times that by 3, to leave 30nm. The descent at a proper "1000ft in 3nm" will take 30nm. This is a standard descent rate for all aircraft.

So you will need to start the descent at 40nm inbnd to MONTY to be at FL200 by 10nm.

To calculate the VSI:

Groundspeed x 5.5 (6 if youre lazy or rubbish at maths) = proper VSI for descent.

Check it every 1000ft.

E.g
FL300 40nm
FL290 37nm
FL280 34nm
...
FL250 25nm
...
FL200 10nm

Adjust your descent rate a couple of hunded FPM in the proper direction if the plane starts getting out of the pattern, and recalculate VSI if you change speed or the wind changes big time or anything like that.

In short:

(Altitude to lose / 1000) x 3 = distance needed for descent.
GS x 5.5 = VSI.

Obviously this isnt going to work if its at 500GS and wants a 3000fpm descent.

So you would usually calculate a distance for a standard desent and then a VSI for a standard descent, not a given distance and then a VSI to match.

Bit much expecting to lose 10000 in 20 miles wasnt it?

Edit:OMG thats an essay and a half lol

[EHM-1343 Jonathan]

yup, thanks a lot guys, these formulas should come in handy
@Iain, that would get you an A* in my school hehehehheheehehe