Calculating VDP

[Skyboy722]

Freightdog,

Sweet Fancy Moses!

 

[bigsky]

I use HAt minus 50 feet( for threshhold crossing) and divide by 300
Hat= 830 minus 50 = 780/ 300 = 2.6
for widebody ac on 3 bar vasi it is normally 325 feet per minute so now approximately 2.4 miles.....
also if you are talking a large transport ac it reacts a little slower so normally lead the descent or pushover by abot .2 nm. so now the 2.4 +.2= 2.6 just like the other fromula.

For timing, dividing by 10 is really set up for an approach speed of 120
400/10 = 40 seconds from end of ry.
AT higher speeds (150) I use .8 and multiply by hat..400X.8 = 32seconds.

 

[JFReservist]

According to the 217 (AF guys & gals)

8.5.4.6. Visual Descent Point (VDP). The VDP is a defined point on the final approach course of a non-precision straight-in approach procedure from which a normal descent (approximately 3°) from the MDA to the runway touchdown point may be commenced, provided visual reference with the runway environment is established. The VDP is normally identified by DME and is computed for the non-precision approach with the lowest MDA on the IAP. A 75 MHz marker may be used on those procedures where DME cannot be implemented. VDPs are not a mandatory part of the procedure, but are intended to provide additional guidance where they are implemented. A visual approach slope indicator (VASI) lighting system is normally available at locations where VDPs are established. Where VASI is installed, the VDP and VASI glide paths are normally coincident. If VASI is not installed, the descent is computed from the MDA to the runway threshold. On multi-facility approaches, the depicted VDP will be for the lowest MDA published. Therefore, on an approach with a higher MDA, the published VDP will not be correct and must be computed by the pilot. No special technique is required to fly a procedure with a VDP; however, to be assured of the proper obstacle clearance, the pilot should not descend below the MDA before reaching the VDP and acquiring the necessary visual reference with the runway environment. The VDP is identified on the profile view of the approach chart by the symbol “V” (Figure 8.16).
8.5.4.6.1. In some cases a published VDP may be absent from an IAP due to an obstacle that penetrates a 20:1 surface. In addition, there was a period of time where the FAA did not place any emphasis on publishing VDPs on IAPs. As a result, many IAPs were designed without published VDPs. The problem is that when a IAP is published without a VDP, there is no way for the pilot to know if it is due to an obstacle penetration, or because the TERPS specialist just did not publish it. If performing a non-precision approach to an unfamiliar field at night (or very low visibility) without a published VDP, and no visual or “normal” electronic glide path guidance to that runway is available, use caution when departing the MDA, as there could potentially be an obstacle penetrating the 20:1 surface. See Chapter 15, Visual Glide Slope Indicators (VGSI) for more information on obstacles in the 20:1 surface.

that being said, gus wears a hat, gs/2 for vvi and a bit of TLAR.

 

[JFReservist]

And more...

14.2.1.2.6.8. In many cases, the minimum visibility required for the approach will not allow you to see the runway environment until you are beyond the VDP. This accentuates the need to compute a VDP and determine a point along the approach when you will no longer attempt to continue for a landing. A common error is to establish a high descent rate once the runway environment is in sight. This can go unnoticed during an approach without visual glide path guidance and may lead to a short and/or hard landing. Caution should also be used to avoid accepting a long touchdown and landing roll.



6.8. Calculating a Visual Descent Point (VDP). The first step to computing a VDP is to divide the Height Above Touchdown (HAT) from your approach procedure by your desired descent gradient. Most pilots use a 3° (300 feet/NM) glidepath for landing. Here’s the formula to use:
HAT / Gradient (normally 300) = VDP in NM from end of runway

6.8.1. Now that you know how far the VDP is from the end of the runway, you may add this distance to the DME at the end of the runway to get a DME for your VDP. Armed with this information, it is easy to compute the distance from the FAF to the VDP. This distance is important in computing the climb gradient necessary for final approach.
6.8.2. Using the FAF altitude, the MDA, and the distance from the FAF to the VDP, you can compute a descent gradient from the FAF to the VDP along with a target VVI to ensure you are meeting the desired descent gradient.
Example: Use the following information to determine the descent gradient from the FAF to the
VDP:
HAT = 420 feet, MDA = 840 feet MSL, DME at the end of the runway = 0.5 DME, FAF =
6 DME, FAF altitude = 2,500 feet MSL, desired landing gradient = 300 feet/NM, Approach
airspeed = 150 KTAS, no wind.
VDP = HAT/Gradient =420/300 = 1.4 NM
VDP DME = DME at end of runway + VDP distance = 0.5 DME + 1.4 DME = 1.9 DME
Descent Distance = FAF DME - VDP DME = 6.0 DME - 1.9 DME = 4.1 DME
Altitude to lose = FAF altitude - MDA = 2500 - 840 = 1,660 feet
Descent Gradient = altitude to lose / distance = 1660 / 4.1 = 405 feet/NM (4° descent gradient)
VVI = Angle (NM/MIN X 100) = 4 (2.5 X 100) = 1,000 feet/MIN
6.8.3. With this information you can depart the FAF maintaining a 4° descent gradient (400 feet/NM).
Your target VVI is 1,000 FEET/MIN. Each mile you should lose 400 feet. At 5 DME, you should be at
2,100 feet, at 4 DME, 1,700 feet, etc . . . Continue this descent gradient until reaching VDP at 840 feet
MSL. Hopefully, at the VDP, you’ll have the runway in sight. Adjust your descent to a 300 feet/NM
gradient and pick up your normal aim point.

 

[freightdogfred]

And more...

14.2.1.2.6.8. In many cases, the minimum visibility required for the approach will not allow you to see the runway environment until you are beyond the VDP. This accentuates the need to compute a VDP and determine a point along the approach when you will no longer attempt to continue for a landing. A common error is to establish a high descent rate once the runway environment is in sight. This can go unnoticed during an approach without visual glide path guidance and may lead to a short and/or hard landing. Caution should also be used to avoid accepting a long touchdown and landing roll.



6.8. Calculating a Visual Descent Point (VDP). The first step to computing a VDP is to divide the Height Above Touchdown (HAT) from your approach procedure by your desired descent gradient. Most pilots use a 3° (300 feet/NM) glidepath for landing. Here’s the formula to use:
HAT / Gradient (normally 300) = VDP in NM from end of runway

6.8.1. Now that you know how far the VDP is from the end of the runway, you may add this distance to the DME at the end of the runway to get a DME for your VDP. Armed with this information, it is easy to compute the distance from the FAF to the VDP. This distance is important in computing the climb gradient necessary for final approach.
6.8.2. Using the FAF altitude, the MDA, and the distance from the FAF to the VDP, you can compute a descent gradient from the FAF to the VDP along with a target VVI to ensure you are meeting the desired descent gradient.
Example: Use the following information to determine the descent gradient from the FAF to the
VDP:
HAT = 420 feet, MDA = 840 feet MSL, DME at the end of the runway = 0.5 DME, FAF =
6 DME, FAF altitude = 2,500 feet MSL, desired landing gradient = 300 feet/NM, Approach
airspeed = 150 KTAS, no wind.
VDP = HAT/Gradient =420/300 = 1.4
VDP DME = DME at end of runway + VDP distance = 0.5 DME + 1.4 DME = 1.9 DME
Descent Distance = FAF DME - VDP DME = 6.0 DME - 1.9 DME = 4.1 DME
Altitude to lose = FAF altitude - MDA = 2500 - 840 = 1,660 feet
Descent Gradient = altitude to lose / distance = 1660 / 4.1 = 405 feet/NM (4° descent gradient)
VVI = Angle (NM/MIN X 100) = 4 (2.5 X 100) = 1,000 feet/MIN
6.8.3. With this information you can depart the FAF maintaining a 4° descent gradient (400 feet/NM).
Your target VVI is 1,000 FEET/MIN. Each mile you should lose 400 feet. At 5 DME, you should be at
2,100 feet, at 4 DME, 1,700 feet, etc . . . Continue this descent gradient until reaching VDP at 840 feet
MSL. Hopefully, at the VDP, you’ll have the runway in sight. Adjust your descent to a 300 feet/NM
gradient and pick up your normal aim point.

All that cipherin and you're 1/10 of a mile different....

 

[Jonny Sacko]

I'm sorry fella's but the winner must go to:


"If you can see it, you can make it"


Too funny, I still bust out everytime I read that. Nice job.

 

[JFReservist]

Yeah, I know...

 

[Amish RakeFight]

Amish in your example VDP is 2 miles from MAP is only true if the MAP is at the end of the runway. VDPs have nothing to do with MAP they are a calculation for reaching the Threshold at 50' HAT. I am sure you meant that but I just wanted to clarify. All your other poop was spot on.

Yep. Thanks for clarifying my poop. My assumption was that the MAP would be the threshold on a non-precision as is many times the case. As you stated, it may be displaced form the threshold and the calculated numbers may have to be reworked slightly (plus or minus the figure).

 

[nosartoofar]

More Math

Multipy the Height Above Touchdown (HAT) for the approach minimums to be used by 3, then divide by 1000 (or just move the decimal place over three places). To limit mathmatical gymnastics, round up the HAT in 50 ft intervals (HAT 327 to 350, 365 to 400, etc.) Add or subtract the distance to the DME MAP to obtain a 3 degree GS.VOR/DME Approach with the FAF over the VOR and MAP @ 4.5 DME. HAT (always AGL)is 300 ft. 300 X 3 = 900 or 0.9 DME - Start descent @ 3.6 DME for a 3 degree GS.

 

[flyboyike]

Just fly a CANPA and be done with it. Never liked dive'n'drive.