PDA

View Full Version : Calculate height/distance

bobbyboo
28th Feb 2011, 10:08
Hi, I have been looking at the approach plates for ACE and see that on the VOR for 21 there is not height/distance information for the descent. Can anyone explain how you calculate this yourself? The platform is 3600' it's a 3.54 degree slope and the descent starts at 9.7 dme.

Thank you!!

PBL
28th Feb 2011, 10:21
bobby,

It's as well to memorise that a 3° glide slope is about 1 in 20.

A 3.54° glide slope is about 1 in 16.

The way to calculate this yourself is to put your cheap calculator in "degrees" mode, type in "3.54", then press the "tan" key, then the "1/x" key.

Or you can do it without a calculator as follows. You can remember, as above, that 3° is 1 in 20, note that 3.54° is 18% higher (54/3 = 18), and take 18% - let's say 20% - away from 20 to get 16, for your 1 in 16.

This simple arithmetic is not mathematically-bomb-proof accurate, but it's good enough for aviation. You can use it for glide slopes, but after about 8-10° you'd need to start putting in second-order corrections.

PBL

rudderrudderrat
28th Feb 2011, 14:19
Hi Bobbyboo,

Determine the altitude to be lost, divided by the horizontal distance to be flown expressed as X ft per nm. (approx 360 ft / mile in your example). Plot your table with distance from VOR versus Altitude using X ft per mile. (From your known start point.)

ROD on 3 deg slope at 140 kts ground speed = approx. 700 ft / min (half the GS trick * 10) Your 3.54 slope is about 20% steeper so add 20% to get required ROD from 3 deg trick. (700 + 140 = 840 ft per min)

Green Guard
28th Feb 2011, 15:15
The platform is 3600' it's a 3.54 degree slope and the descent starts at 9.7 dme.

I am not sure what do you mean by platform.
But "3.54 degree slope and the descent starts at 9.7 dme" is enough :

Ht diff = Slope deg x NM x 106.05 (here you get 3642 feet !)

if it is an ILS GS then

Ht diff = ( GS deg + NM / 120 ) x NM x 106.05

If you need a rate of descent then:
Rod ( '/min)= 1.767 x GrdSpd x Slope deg

and any time during descent a quick 3.14 deg slope NM = ( Alt - Elev ) x 3 /1000