TheAlchemist
Well-Known Member
Why isn't it depicted in all non precision approach plates? Some will have it depicted and some won't. Why?
Thanks.
Thanks.
VDP. Why it isn't it depicted in all charts?
- Thread starter TheAlchemist
- Start date
germb747
Well-Known Member
I'm pretty sure they're only depicted if they're required to meet TERPS requirements.
You can use the memory aid GuS wears a HAT to calculate your own:
HAT
----
GS
where HAT is Height Above Touchdown and GS is desired glidepath, usually 3 degrees, or 300 in this formula. The resulting number is distance from the runway you want to start down; you have to look at what's on the chart to convert that to a meaningful DME. If the procedure is based on timing, there's a formula for that too but I don't recall what it is off the top of my head.
You can use the memory aid GuS wears a HAT to calculate your own:
HAT
----
GS
where HAT is Height Above Touchdown and GS is desired glidepath, usually 3 degrees, or 300 in this formula. The resulting number is distance from the runway you want to start down; you have to look at what's on the chart to convert that to a meaningful DME. If the procedure is based on timing, there's a formula for that too but I don't recall what it is off the top of my head.
tgrayson
New Member
Two common reasons are
- Remote altimeter settings
- Lack of DME source to detect VDP
SIUav8er
Narcosis
you can use time to calculate VDP.
VDP= Missed Approach Time – (10% of HAT [Height Above Touchdown])
example:
The HAT at MDA is 450 FT.
The MAP is 2:45 from the Final Approach Fix at 90 kts.
450 ÷ 10 = 45
2:45 – 45 = 2:00
2 minutes from the FAF is the VDP.
probly has nothing to do with why its not depicted on the chart, but this works great if you dont have DME
If you do have DME, just divide the HAT by 300.
example:
HAT=600
600 ÷ 300 = 2 VDP is 2 DME from MAP
NJT916
Well-Known Member
How much do you actually use this?
SIUav8er
Narcosis
Well, i never really use the time method cause ive always got DME or GPS.
I calculate VDP on every approach which doesnt have one depicted. Figuring out when you are supposed to be descending is really important especially when you start flying larger aircraft and jets. you cant just chop the power, jam the props forward, and dive.
In the jet we use the 3:1 descent rule even when were doing a visual approach. for every mile form the runway, you should be about 300 feet above the TDZE.
ex. 5 mile final = 1500 feet AGL (comes out the same as the VDP calculation)
Another good one is top of descent. Take the altitude to lose ÷ 1000 X 3 to get TOD in miles.
ex. your at FL230. the airport elevation is 1000. You need to lose 22,000 feet. 22x3 is 66, which is the distance you need to start a descent. You need to constantly be thinking about this so you can remind ATC to let you descend. Sometimes you will end up annoyingly high at the airport if you dont query ATC about starting a descent when you reach your TOD
I calculate VDP on every approach which doesnt have one depicted. Figuring out when you are supposed to be descending is really important especially when you start flying larger aircraft and jets. you cant just chop the power, jam the props forward, and dive.
In the jet we use the 3:1 descent rule even when were doing a visual approach. for every mile form the runway, you should be about 300 feet above the TDZE.
ex. 5 mile final = 1500 feet AGL (comes out the same as the VDP calculation)
Another good one is top of descent. Take the altitude to lose ÷ 1000 X 3 to get TOD in miles.
ex. your at FL230. the airport elevation is 1000. You need to lose 22,000 feet. 22x3 is 66, which is the distance you need to start a descent. You need to constantly be thinking about this so you can remind ATC to let you descend. Sometimes you will end up annoyingly high at the airport if you dont query ATC about starting a descent when you reach your TOD
seagull
Well-Known Member
Divide your groundspeed by two and mulitply by 10 will give a ballpark for your vertical speed for a 3 degree g/s. Not exact, but close enough. So, if your final approach speed is 160 kts you should be descending at about 800 ft/min.
All of these methods are fine, and I do them too, but just remember that if no VDP is published, once you leave MDA, you have no guarantee that there is no obstruction between you and the runway. Keep those eyeballs peeled!
SIUav8er
Narcosis
maybe thats why there isnt a VDP depicted on all approach plates... The TERPSters cant guarantee obstacle clearance in the descent from MDA unless youre visual to see and avoid. Just a guess.
Another weird thing ive noticed is that Jepp has removed the VDP from some approaches. The rest of the procedure is unchanged
tgrayson
New Member
8260.3 United States Standard for Terminal Instrument Procedures (TERPS)
253. VISUAL DESCENT POINT (VDP) (applicable to straight-in procedures only). When dual minimums are published, use the lowest minimum descent altitude (MDA) to calculate the VDP distance. PUBLISH A VDP FOR ALL STRAIGHT-IN NONPRECISION APPROACHES except as follows:
• Do not publish a VDP associated with an MDA based on part-time or full time remote altimeter settings.
• Do not publish a VDP located prior to a stepdown fix.
• If the VDP is between the MAP and the runway, do not publish a VDP.
a. For runways served by a VGSI, using the VGSI TCH, establish the distance from THR to a point where the lowest published VGSI glidepath angle reaches an altitude equal to the MDA. Use the following formula:
VDP Distance = [MDA - (TCH + THR Elevation)]/Tan(VGSI Angle)
b. For runways NOT served by a VGSI, using an appropriate TCH from table 18A, establish the distance from THR to a point where the greater of a 3° or the final segment descent angle reaches the MDA. Use the following formula:
VDP Distance = [MDA - (TCH + THR Elevation)]/Tan(*Angle)
c. Marking VDP Location.
(1) For Non-RNAV SIAP’s, mark the VDP location with a DME fix. The DME must be collocated with the facility providing final approach course guidance (USN/USA/USAF NA). If DME is not available, do not establish a VDP. Maximum fix error is ± 0.5 NM.
(2) For RNAV SIAP’s, mark the VDP location with an along track distance (ATD) fix to the MAP. Maximum fix error is ± 0.5 NM.
(3) If the final course is not aligned with the runway centerline, use the THR as a vertex, swing an arc of a radius equal to the VDP distance across the final approach course (see figure 14-12). The point of intersection is the VDP. (For RNAV procedures, the distance from the point of intersection to the MAP is the ATD for the VDP.)
And here is the issue that seagull mentioned:
b. Obstacle Clearance. Two obstacle identification surfaces (OIS) overlie the visual area with slopes of 20:1 and 34:1, respectively. When evaluating a runway for circling, apply the 20:1 surface. When evaluating a runway for an approach procedure satisfying straight-in alignment criteria, apply the 20:1 and 34:1 surfaces. Calculate the surface height above threshold at any distance “d” from an extension of the area origin line using the following formulae:
20:1 Surface Height = d/20
34:1 Surface Height = d/34
(1) If the 34:1 surface is penetrated, take ONE of the following actions:
(a) Adjust the obstacle height below the surface or remove the penetrating obstacles.
(b) Limit minimum visibility to ¾ mile.
(2) In addition to the 34:1 evaluation, if the straight-in runway's 20:1 surface is penetrated, take ONE of the following actions:
(a) Adjust the obstacle height below the surface or remove the penetrating obstacles.
(b) Do not publish a VDP, limit minimum visibility to 1 mile, and take action to have the penetrating obstacles marked and lighted.
(c) Do not publish a VDP, limit minimum visibility to 1 mile, and publish a note denying the approach (both straight-in and circling) to the affected runway at night.
(3) If the 20:1 surface is penetrated on circling runways, mark and light the penetrating obstacles or publish a note denying night circling to the affected runway.
seagull
Well-Known Member
Hmm. Guess it depends on what you fly, and where. I've flown 121 a fair bit longer than you have, and I have used it a fair number of times.