Descend Planning

[Olendirk]

Hey guys!

i have a question concerning descend planning. Could you give us unexperienced pilots some hints how to do a good descend planning? I mean like you have to cross a vor at altitude....., you are .... miles out, how is the descend rate etc....

Thank you very much indeed!

oD

[GusHoneybun]

I use the DME and a benchmark ROD of about 1000 ft/min. By using the DME time to go function, this not only tells you how many minutes till you reach the VOR, but also, how many thousands of feet you can lose.
So if I need to lose 6000 ft, I will start the descent at 6 minutes to go. Not exact, but good enough for government work

[redsnail]

I use as a guide the 3x profile.
The explanation is based on the aerodrome being at sea level. If you want to level off etc or aerodrome is higher, adjust.

Take a look at the altimeter. eg, 8000'.
To commence a descent on a calm day multiply 8 by 3.
That gives 24.
Therefore on a calm day to arrive at the aerodrome using a rough 3 deg slope, you'll need to commence your descent by 24 miles out.
How fast should you be descending? Monitor your distance versus height profile. eg, by 7000' you should be 21 miles from the aerodrome.
A good guide for your rate of descent is the same as the one you use for the ILS. Half the groundspeed X 10. eg 200kt g/s = 1000 fpm.

If you have nervous passengers and in an unpressurised aircraft, try a 5 x profile. The rate of descent will be less.

[Hobo]

With no FMC type machine to do it for you:

Multiply height in 1000's ft to be lost by 3 and add on an allowance to slow down if appropriate (typically 10 miles to slow from 330kts to 210kts). Descend at Groundspeedx10/2 thousand ft/min.


eg. At FL340, need to be level at a VOR at FL100, G/S 460kts.

You will need (34-10)x3nm = 72nm @ 2300fpm


If you need to be at 210kts at the VOR then start at 82nm and if the G/S drops drops off adjust the RoD accordingly, eg 420kts needs 2100fpm.

Just idle the throttles at T/D and wind in the appropriate RoD in the V/S.

Works every time.

[Olendirk]

hey!

so am i thinking right? Im out 40NM at 25000ft and I want to touch down on an aerodrome with an elevation of 4000ft. So i should start descend 70NM ahead of airport for a rough 3 glide path?

Right?

OD

[redsnail]

Yeah, that'll work. It'll give you some space if required to slow down to gear/flap config speeds too.
25,000-4000' = 21,000'.

21 x 3 = 63nm. Therefore 70 nm is fine.

(Not sure about the 40nm though?)

[Olendirk]

isnt it right to devide 21000 by 3?

Thanks!

Dirk

[High Wing Drifter]

The one I have memorised is from RANT:

NMs before BoD = (Height in 000s * (GS/60))/RoD in 000s

Looks messy, but you will know the GS/60 before you fly.

If you want to know what RoD is required to meet ATC BoD request then just transpose to

RoD in 000s = (Height in 000s * (GS/min)) / NMs before BoD.

This KTs/min is used in many mental math calcs so it is worth memorising cardinal points like 90kts, 120, 180, 240, etc.

If you need to mentally divide by say 2.5, then I find it easiler to double both operands first to remove the decimal.

Easy peesy :)

[Number Cruncher]

On a 3 degree glide slope, you will loose 3nm/18,258ft vertically for every 60 nautical miles travelled horizontally. Therefore divide 60 by 18,258, multiply it by 21,000 (25,000-4,000) and you will get 69 miles.

[High Wing Drifter]

NumberCruncher,

I see you're on telly tonight. Ch5 21:00 :8

[Number Cruncher]

Hey HWD

And who siad the ATPL'S didn't create 'Incredible Brains' amongst us!! ;)