How to calculate a VDP?
- Thread starter Alchemy
- Start date
seagull
Well-Known Member
If no VDP is published, most pilots like to find the point that would give them a nominal 300'/mile descent path from the MDA. Dividing the MDH by 300 would give an approx distance from the touchdown point in miles. The 1000' downfield would be met by the delay in your reaction time, so not something to worry about, just subtract if from the end of the runway. How you determine that point would depend on what type of approach, what equipment you had, etc.
Having said all that, if a VDP has not been established, then there is NO guarantee that you do not have obstacles between the MDA and the runway that project into the nominal path. There is no requirement on a non-precision to ensure that it is obstacle free, so unless it's also served by an ILS, you are taking a bit of a chance during the visual segment. A "roll you own" VDP can work, but you have to be very careful with it for that reason.
Having said all that, if a VDP has not been established, then there is NO guarantee that you do not have obstacles between the MDA and the runway that project into the nominal path. There is no requirement on a non-precision to ensure that it is obstacle free, so unless it's also served by an ILS, you are taking a bit of a chance during the visual segment. A "roll you own" VDP can work, but you have to be very careful with it for that reason.
[ QUOTE ]
Can anyone explain how to calculate a VDP? I know VDP stands for visual descent point and it's a point on a non precision approach where you may begin descending if you have the runway in sight. Other than that, I know nothing. Any help?
[/ QUOTE ]
GuS wears a HAT. Take the desired glidepath you wish to calculate for (ie 3 degrees) and divide it into the Height Above Touchdown. Take that figure as miles, and count back from the beginning of the runway to get where your VDP would be.
Easy example: You have an airport where the HAT is 600', and your desired glidepath is 3 degrees. Take 3 degrees (300) and divide it into the HAT of 600. That gives you 2. Lets assume that the missed approach point depicted is at the beginning of the runway at, for example, 7 DME (with the FAF being the VOR for example). Your VDP would be 2 miles short of this, or 5 DME......which would make sense since if you desire a 300 to 1 descent profile, it would take 2 miles to descend 600 feet.
That's a very simplified example, but the equation works well when you have DMEs (miles) compute to things like 1.7, 2.4, etc.
Clear as mud? /ubbthreads/images/graemlins/smile.gif
Can anyone explain how to calculate a VDP? I know VDP stands for visual descent point and it's a point on a non precision approach where you may begin descending if you have the runway in sight. Other than that, I know nothing. Any help?
[/ QUOTE ]
GuS wears a HAT. Take the desired glidepath you wish to calculate for (ie 3 degrees) and divide it into the Height Above Touchdown. Take that figure as miles, and count back from the beginning of the runway to get where your VDP would be.
Easy example: You have an airport where the HAT is 600', and your desired glidepath is 3 degrees. Take 3 degrees (300) and divide it into the HAT of 600. That gives you 2. Lets assume that the missed approach point depicted is at the beginning of the runway at, for example, 7 DME (with the FAF being the VOR for example). Your VDP would be 2 miles short of this, or 5 DME......which would make sense since if you desire a 300 to 1 descent profile, it would take 2 miles to descend 600 feet.
That's a very simplified example, but the equation works well when you have DMEs (miles) compute to things like 1.7, 2.4, etc.
Clear as mud? /ubbthreads/images/graemlins/smile.gif
Alchemy
Well-Known Member
Okay, so:
MAP - [(HAT)/(Descent Angle)]/100 = VDP.
Let me try a sample.
Say the MDA for the approach is 800 HAT and you want a 2.5 degree descent angle. The MAP for the approach is 6 DME.
6 - (800/2.5)/100 = VDP
6- 320/100 = VDP
6-3.2 = VDP
VDP = 2.8
So your VDP is 2.8 DME.
Sound right?
MAP - [(HAT)/(Descent Angle)]/100 = VDP.
Let me try a sample.
Say the MDA for the approach is 800 HAT and you want a 2.5 degree descent angle. The MAP for the approach is 6 DME.
6 - (800/2.5)/100 = VDP
6- 320/100 = VDP
6-3.2 = VDP
VDP = 2.8
So your VDP is 2.8 DME.
Sound right?
[ QUOTE ]
Okay, so:
MAP - [(HAT)/(Descent Angle)]/100 = VDP.
Let me try a sample.
Say the MDA for the approach is 800 HAT and you want a 2.5 degree descent angle. The MAP for the approach is 6 DME.
6 - (800/2.5)/100 = VDP
6- 320/100 = VDP
6-3.2 = VDP
VDP = 2.8
So your VDP is 2.8 DME.
Sound right?
[/ QUOTE ]
Correct. And you can cross-check this also by the logic test (ie, does it make sense?) In your example, assuming the navaid (VOR) is the FAF and the DME is increasing as you get to the MAP, for a 3 degree glidepath, you'd subtract 2.6 miles from the 6 DME for a VDP of 3.4 DME. That makes sense since with the shallower 2.5 degree GP you're wanting, you have to descend earlier (sooner....that is, at 2.8 DME), than with the 3 degree GP (3.4 DME). So it passes the logic test. /ubbthreads/images/graemlins/smile.gif
The biggest thing you have to remember is "how do I apply the answer I come up with?" Do you add it? Subtract it? You really need to cross-check the beginning of the runway and go from there. For example, your 6 DME MAP you mention may start 1.0 miles from the beginning of the runway. In that case, you'd have to take that extra 1 mile into account, and your VDP would actually be located at 3.8 DME, or 3.2 miles from the beginning of the runway. Make sense? To get this info accurately, look at the profile view of the instrument approach, and see where it is that you're computing from........"walk" back from the beginning of the runway with your figure that you come up with, then match it to the correct DME point for you to use as your cockpit reference.
Okay, so:
MAP - [(HAT)/(Descent Angle)]/100 = VDP.
Let me try a sample.
Say the MDA for the approach is 800 HAT and you want a 2.5 degree descent angle. The MAP for the approach is 6 DME.
6 - (800/2.5)/100 = VDP
6- 320/100 = VDP
6-3.2 = VDP
VDP = 2.8
So your VDP is 2.8 DME.
Sound right?
[/ QUOTE ]
Correct. And you can cross-check this also by the logic test (ie, does it make sense?) In your example, assuming the navaid (VOR) is the FAF and the DME is increasing as you get to the MAP, for a 3 degree glidepath, you'd subtract 2.6 miles from the 6 DME for a VDP of 3.4 DME. That makes sense since with the shallower 2.5 degree GP you're wanting, you have to descend earlier (sooner....that is, at 2.8 DME), than with the 3 degree GP (3.4 DME). So it passes the logic test. /ubbthreads/images/graemlins/smile.gif
The biggest thing you have to remember is "how do I apply the answer I come up with?" Do you add it? Subtract it? You really need to cross-check the beginning of the runway and go from there. For example, your 6 DME MAP you mention may start 1.0 miles from the beginning of the runway. In that case, you'd have to take that extra 1 mile into account, and your VDP would actually be located at 3.8 DME, or 3.2 miles from the beginning of the runway. Make sense? To get this info accurately, look at the profile view of the instrument approach, and see where it is that you're computing from........"walk" back from the beginning of the runway with your figure that you come up with, then match it to the correct DME point for you to use as your cockpit reference.
Similar threads
