Vdp Calculation On Airbus
 

Can someone help me in calculating the vertical descent angel for a non precision approach? I have heard several ways of doing this but I'm still confused.
The Jeppesen general flight supplement booklet has a table in the terminal section pages 3 and 4. However using the method I am familiar, still does not work out to the same figures.
E.g. FAF height 1900 AGL, distance from FAF to Runway Threshold 5.4 NM gives an angle of 3.2.
1. Is this angle the same as the descent gradient that you select on the A-320?
2. Is there a way to calculate the distance you should commence your descent to maintain a 3 deg gradient if you know the FAF height and the TCH or MDA?
Any help or comment?
sing

Sing

Your numbers look correct. 3.2 deg = 339'/nm.

Don't forget that normally you should allow the 50' TCH when calculating. However not sure if Jepps have factored this in to their table without looking but I suspect not.

To find the DP for a 3deg slope divide the height (-TCH) by 320 which will give the nm to the threshold. To be super accurate use 318'/nm!!!

Also don't forget to add 0.3nm or so to permit the a/c to actually begin descent.

Agree
 

Sing330,

As IDG said, the 3 deg slope is good to use across the board for all aircrafts and places(even though its a NPA), works a treat, except in certain circumstances where the surveyed approach path is more than 3 deg (usually published due terrain or other reasons)

Commencing the descent 0.3nm prior to the final approach fix, for the 320 is spot on all the time.

With the allowance for TCH, if operating within the US there is no need to factor it in ....so u would enter the published minima into the MDA field when setting it up in the MCDU. When operating outside of the US u would enter MDA+50 as the minima.

Ta

Thanks
 

So if I say to calculate the distance that you need to commence the descent if you take FAF height in AGL - TCH and divide the answer by 320 the distance would work out for all approaches?

Then you must add .3 NM to initiate the descent to be on the profile right from the start.

Is this correct?

Thanks.

Sing

Not Quite
 

Sing,

U would divide it by 320 if u want a 3.2 deg glide slope path, it works like this if u want to maintain a 3 deg slope divide the height to loose by 300, if u want to maintain a 3.5 deg or 4 deg slope you would divide it by 350 or 400 resp to give u the distance and so on

U mentined the Jepps, these days in all the approaches in the profile view everything is there (the angle the distance etc)

Bare in mind there will be minor adjustments if reqd to stay on profile by either inc or dec FPA or Vspeed. The above formula if u like will enable u to quickly calculate u'r profile altitude for a given distance mentally, thus can also avoid chasing it and doing what I call the snake dance

Ofcourse on the 320 u don't have to waste u'r mental energy so much (no that its hard) cas u have the brick which indicates u'r VDev and is pretty much like a glide slope indicator

yes. Adding the .3nm, is a rule of thumb. Like any aeroplane u always have to be ahead of the aeroplane this rule of thumb in this case helps do exactly that as IDG mentioned it a takes a lil time from the time u command it for it to happen, same with the turns, from the time u initiate the roll command it takes a lil time for the aerolplane to turn, so if u do not anticipate it in this case u do the horizontal snake dance.

Hope it helps

Ta