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Originally Posted by redwave
Pls help with info to figure out vdp (dme and timing) and where is info on second and third segment climbs? thx REDWAVE  |
I composed a lengthy post on a different website to answer a question about calculating a VDP using DME. Rather than repeat that effort, I hope it will be OK to include here a link to that thread. In addition to my post you'll find several other opinions, some of which are even valid.
Calculating a VDP? Continuous descent Non-Precision Appch?
As for calculating VDP using timing: To precisely calculate this value, you will begin with the same principles that apply to the DME problem. VDP based on DME is independent of groundspeed; VDP based on timing is not. Once you've computed the VDP using the above method, you need to determine how long it will take you to travel from the VDP to the MAP (Missed Approach Point), ASSUMING that the MAP is the end of the runway, and subtract that time from the published timing from the FAF (Final Approach Fix) to the MAP. Of course, if the MAP timing is not published on the approach plate as part of the IAP (Instrument Approach Procedure), then this calculation is not possible or meaningful.
First, the distance from the runway for the VDP can be computed by dividing the HAT (Height Above Touchdown) by 318 ft/NM. (This will give a 3 degree descent gradient, which is most common. For other descent gradients, substitute the altitude lost (in feet) per NM of distance covered during the descent. You might ask, "Why 318 feet instead of 300 feet?" Well, since I assume you're going to be using a calculator for this, we might as well use the actual value instead of the approximation given from the application of the "60-to-1" rule. If you're using mental math, make it 300 and you'll be close enough.) The value derived from this calculation will be a distance in NM.
I'll try to present this as a formula using the Code feature and a equal-spaced font, (vBulletin software doesn't seem to like "extra" spaces):
Code:
Height Above Touchdown
DISTANCE = -----------------------------
Altitude lost per Distance traveled
HAT (ft)
DISTANCE = ------
318 ft/NM
Second, determine the amount of time that will be required to travel this distance. If you begin with a groundspeed in knots, you need only make a couple of conversions to compute the equivalent groundspeed in NM/sec. For example. if the groundspeed is 120 knots, or 120 NM/hr, multiply by (1 HR / 60 min) and then by (1 min / 60 sec) to get a groundspeed of 1/30 NM/sec, or 0.0333 NM/sec.
Using the distance derived in the first step, and the groundspeed just derived, only a simple division operation will yield the time needed to travel from the VDP to the runway. Take the distance computed (NM), and divide by the groundspeed (NM/sec) and you'll get a time, in seconds, that will be required to travel from the VDP to the runway.
Code:
DISTANCE DISTANCE (NM)
TIME = -------- = ---------------
RATE Groundspeed (NM/sec)
Combine the two equations above:
Code:
HAT(ft) NM 1 hr 60 min 60 sec
VDP = ------- x ----- x ------ x ------ x ------
318ft GS(NM) hr min
Now, once you have that time, simply subtract that time from the published time from the FAF to the MAP, and you'll have the time from the FAF to the VDP. When you pass over the FAF at the calculated groundspeed, and maintain that groundspeed for the calculated time, you should be over the VDP when that time transpires. If you do not at that point have adequate visual reference with the runway environment, and cannot begin a descent using a normal rate of descent and normal maneuvers, you should consider executing a missed approach no later than the MAP. (You can begin the climb at any time, but don't make any turns prior to the MAP.)
Now, let's see if we can't condense all that into one formula:
Code:
HAT (ft) 1 3600 sec
VDP (in seconds) = ---------- x ---------- x ----------
318 (ft/NM) GS (knots) 1 hr
-or-
HAT (ft) 3600 sec
VDP (in seconds) = --------- x -----------
GS (NM/hr) (318 ft/NM)(hr)
-or-
HAT (ft)
VDP (in seconds) = --------- x 11.3207547 (sec NM)/(ft hr)
GS (NM/hr)
OK, so let's take a look at an approach where the MAP timing is 2:01, the HAT is 450 feet, and the groundspeed is 120 knots. As the saying goes, plug and chug:
(450/120) x 11.3207547 = 42.5 seconds
Subtract 42 seconds from 2:01, and your VDP by timing is 1:19
This method is very precise, and very useful, if you have the time to break out the calculator and do the math. This should be part of your preflight planning, not something you remember to do as you're approaching the FAF. Since you won't know your exact groundspeed until you're flying the approach, you should make the calculations for several speeds that are in the "neighborhood" of your planned approach speed. Keep in mind, the winds at altitude could be considerably different from surface winds.
A quicker, but less accurate shortcut is to subtract 10% of the HAT from the MAP timing. In this case, 10% of the HAT is 45, which is very close to the 42 seconds derived from the calculation. This method assumes a groundspeed of 120 knots, and is less accurate with slower or faster groundspeeds. A faster groundspeed would result on being closer to the runway at the computed time, and might result in a steeper descent gradient. A slower groundspeed would result in being farther away from the runway, and might tempt you to drag in the approach to landing.
Climb segments... I'm not sure what your question is, and my fingers are tired.
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VDP and segment climbs
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