Visual Descent Point
The point that you can reasonably expect to see the runway and start down from MDA with a standard 3 degree descent to touchdown at
the end of the runway - The Missed Approach Point (MAP) timing will take you to approximately the end of the runway at the MDA
and leaves you to make a landing from the height of the HAT. For large aircraft this may not allow you to land on the runway remaining.
V MDA (MSL)
VDP (D nm from end of runway) = HAT/300
Total
Timing
is to here HAT (ft)
30
D (nm)
Destination Altitude (MSL)
The derivation:
[HAT (ft) x 1(nm) / 6000(ft)] / D (nm) = tan 3 0 where tan 30 = 0.0523
D (nm) = [HAT (ft) x 1(nm) / 6000(ft)]/tan 3 0 = HAT/(6000)(0.0523) = HAT / 313.8 nm ~ HAT/300 nm
Now figure the GS in nm/min. GS/60 nm/min the distance from the end of the runway is then D divided by this GS in
nm/min. gives the time in minutes that must be subtracted from the MAP to yield the VDP timing from FAF.
Some folks like to simply subtract HAT/10 from the time to MAP as an approximation.
Typical calculations:
1. Flying an approach at an IAS of 110 kts at an MDA that is 6500' with a corresponding HAT of 400'. The final approach course is 042
and the winds are given at the airport are 100 at 20 kts. The timing box has ground speed vs. timing.
70 90 100 120 140
6:52 5:54 5:22 4:29 3:33
2% per 1000' is (.02)(6.5) = .13
TAS = 110(1.13) = 124 kts
100-42=58 which is very close to 60 degrees off the nose. The cosine is therefore slightly greater than 0.5. Use 0.5 and if any rounding is
needed, round to the high side giving (.5)(20) = 10 kts HW
so GS = 124 - 10 = 114 kts
interpolating: 6/20 = x/53 --> x = 16 sec
timing is 4:29 + 0:16 = 4:45 to MAP
the HAT / 300 = 400/300 = 1.3 nm
114 kts groundspeed is 1.9 nm/min
1.3nm / 1.9 nm/min = .685 min or 41 sec
VDP should be 4:45-0:41=4:04 (some just subtract HAT/10 from the MAP time -- in this one 40 sec. just 1 sec. off)
2. Flying an approach at an IAS of 90 kts at an MDA that is 2200' with a corresponding HAT of 500'. The final approach course is 214
and the winds are given at the airport are 080 at 12 kts. The timing box has ground speed vs. timing.
70 90 100 120 140
6:33 5:45 5:01 4:11 3:12
2% per 1000' is (.02)(2.2) = .04
TAS = 90(1.04) = 94 kts
A Tailwind. 212-180=32 and 80-32=48 degrees off the tail (which is close to 45 degrees). The cosine is therefore slightly less than .707
Use .707 and if any rounding is required, round to the low side, giving (.707)(12)= 8 kts of TW.
so GS = 94 + 8 = 102 kts
interpolating: 2/20 = x/50 --> x = 5 sec
timing is 5:01 - 0:05 = 4:56 to MAP
the HAT / 300 = 500/300 = 1.7 nm
102 nm/hr = 102/60 = 1.7 nm/min
1.7 nm / 1.7 nm/min = 1min or 60 sec
VDP should be 4:56-1:00 = 3:56 (some just subtract HAT/10 from the MAP time -- in this one 50 sec. Or 10 sec. off)
See the MAP/VDP Calculator for practice.