Mental Math

[Hankpilot2002]

I am gearing up for a Netjets interview and I see that they like to stress some mental math problems; in which I am severely challenged.

I have been computing a VDP by: (MDA - TDZE)/2 such as:

MDA 800 - TDZE 200= 600/3= start your descent 2 miles out for a 300 FPM descent rate.

I suppose this could be expressed as HAT/3=VDP in miles as well.

Is that even a little bit accurate?

 

[Crobar]

td=200'
mda=800'

600' differance, 3 degree glidepath is 300 feet per mile, 2 miles from the threshold is your vdp, either time it or use dme

 

[jet600]

Mental math

As far as VDPs - the easiest way I have found is to take your height above touchdown and divide it by the G/S you want to use...(example...on a non - precision, straight in - lets say HAT= 450' and you want to use a 3 degree G/S. 450 / 300 = 1.5.) Thats 1.5 dme from approach end of Rwy...assuming you have dme ( you may have to add or subtract that number to a current dms already given....clear as mud?) You can also calculate VDPs using time.

 

[gangdev]

From what I understand, VDP is the depicted point on the profile view of an approach plate. The PDP (planned descent point) is more what you are talking about, it can be calculated for time or distance. For distance, use HAT/300. If your HAT is 450, then 450/300 = 1.5(nm). Your PDP is 1.5 nm from the TDZ/runway end. This is based on a 3 degree descent path, which I believe is 318ft/nm, but 300 is close enough for the mental math.

For time, use HAT/10. If HAT is 380, 380/10 = 38, or :38 seconds. If the approach plate has a time (for your category) of 1:45 minutes from FAF to MAP, then just subtract :38 from 1:45 to get :67 seconds from FAF to PDP. This is a very approximate method, and the 10 is based on a certain ground speed. Anyone correct me if I am wrong.

 

[Hankpilot2002]

Crobar thats the easiest way that I have found to do the VDP. Now if I can only figure out descent planning math....VNAV has made me lazy.

 

[Wankel7]

As far as VDPs - the easiest way I have found is to take your height above touchdown and divide it by the G/S you want to use...(example...on a non - precision, straight in - lets say HAT= 450' and you want to use a 3 degree G/S. 450 / 300 = 1.5.) Thats 1.5 dme from approach end of Rwy...assuming you have dme ( you may have to add or subtract that number to a current dms already given....clear as mud?) You can also calculate VDPs using time.

Wait what? Where do you put the G/S in?

 

[JokerFuel]

3 degree descent formula. Take the amount of altitude you have to lose, and multiply that number by three. That sum will give you the distance at which you must begin your descent in DME. Then divide your groundspeed by two and add a zero to it. That will give you your rate of descent. Example: Your aircraft is at FL400. You must cross JFK at 10000 feet, and your groundspeed is 400kts. You have to lose 30000 feet. 30 x 3 = 90. Your descent must begin at ninety miles from the JFK VOR. 400/2 = 200. So you must descend at 2000 fpm

 

[learflyer]

what ever happened to just landing the f*&^ing airplane?

 

[a319stud]

3 degree descent formula. Take the amount of altitude you have to lose, and multiply that number by three. That sum will give you the distance at which you must begin your descent in DME. Then divide your groundspeed by two and add a zero to it. That will give you your rate of descent. Example: Your aircraft is at FL400. You must cross JFK at 10000 feet, and your groundspeed is 400kts. You have to lose 30000 feet. 30 x 3 = 90. Your descent must begin at ninety miles from the JFK VOR. 400/2 = 200. So you must descend at 2000 fpm

does that really work? been doin this stuff for quite awhile and thats the easiest way i have heard. anybody else heard of this?

 

[JokerFuel]

It absolutely works, I learned it on the Brasillia many years back. Next time you have an assigned descent just use this formula to back up the Auto/VNAV and you will see.