DC8 Flyer said:There are two ways to find a VDP as well, DME and Timing.
If you have an approach that uses DME it, simply divide your MDA by 300, ie a 600' MDA would equal a 2 mile DME VDP. The tricky part comes in where you have to figure out where the DME source is and how much to "add" onto that 2 miles to get 2 miles from the end of the runway. For example you are doing a LOC only approach to a 6000' long runway with a 600' MDA. So your distance from the end of the runway for a 3 degree slope would be 2 miles, but you have to add 1 mile for the length of the runway. So at 3 DME you would be at your VDP.
Second way is if you only have timing for your approach, off field NDB for example. Take your MDA (600 feet again) and drop the zero, or divide by 10, does the same thing. So we get 60 (60 seconds). Subtract that from your timing, say 3:00. So your VDP would be 2:00 This is off a 600 Foot/min assumption so if you are faster than 120kts GS (5 * GS = descent rate for a 3 degree glideslope), say 160 knots = 800 FPM for 3 degree slope, you have to start down 13 seconds earlier, so 1:47. I got that from 600 FPM = 100 feet/10 seconds, increase by 200 FPM (600 to 800 FPM) 800 FPM = 130 Feet/10 seconds.
That looks like a lot of math mumbo jumbo but just use 600 FPM as a baseline of 100'/10 seconds, so 1200 FPM is 200'/10 seconds.
180KTS GS equals 900 FPM descent rate, equals 150'/10 seconds = 15 seconds earlier VDP.
CLECA said:Excellent post I never thought about a calculated VDP , I just assumed he meant published VDP......and you know what happens when one assumes