Vdp

[Jack Mehoff]

been a while and I was asked by a former student how do calculate a VDP. Whats the quick formula? for dist. and time?

 

[Amish RakeFight]

The Old VDP Question....

DISTANCE:

HAT / 300 = VDP (miles)

ex. HAT = 600'/300 = 2 miles.
VDP is 2 miles prior to MAP




TIME:
10% of HAT = VDP (time)

ex. HAT = 600' x .10 = 60 seconds.
VDP is 60 seconds less than the published time for category.

 

[DCAinstructor06]

I use this for distance.

HAT is 600 right? Multiply times 3 and drop the zeros.
600*3=1800 so the real answer is 1.8miles.

Now for the bonus question! Take a WAG why 3 is used?

For time, I use my GROUNDSPEED and do the math.

 

[Amish RakeFight]

I use this for distance.

HAT is 600 right? Multiply times 3 and drop the zeros.
600*3=1800 so the real answer is 1.8miles.

Now for the bonus question! Take a WAG why 3 is used?

For time, I use my GROUNDSPEED and do the math.

Bonus Answer: 3 is the glideslope angle.

VDP isn't an exact science and the difference between 2.0 and 1.8 miles is small and places you closer to the runway. Keep in mind though that the closer you are to the runway, the steeper the descent rate required. The HAT/300 offers a slightly larger margin in distance to descend although they both work out pretty much the same.

As far as timing goes, there is no need to know what the groundspeed is in determining VDP in time. The time derived from multiplying HAT by .10 (10%) is a set amount of time that is deducted off the timing provided by the different ground speeds on the plate. Whether your GS is 90 knots or 120 knots, you take the time from the appropriate category for you GS and subtract the time you arrived at from multiplying HAT by .10.

 

[Doc Holiday]

This is semantics, but should be noted. A VDP cannot be calculated. It is published on an approach chart.

What these previously posted formulas give you is commonly referred to as a PDP (planned descent point) or TTS (time to see). What these formulas provide is rule-of-thumb planning, they are not set in stone. However, they are very usefull, especially when doing a non precision approach in low visibility (which alters your depth perception). In other words, what these formulas give you is an estimate of a position you should be in to execute a normal approach and landing.

On the other hand, a VDP is published and evaluated for terrain. It is a KNOWN position.

Just be aware that calculating a descent point on your own is not the same as a VDP, and may not always provide the same level of terrain seperation.