redwave

Pls help with info to figure out vdp (dme and timing) and where is info on second and third segment climbs? thx REDWAVE

TonyC

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):

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.
Combine the two equations above:

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:
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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JayDub

Holy $h*t! This is the short version??? I read this one and my lips are tired!

Just fooling on ya Tony. Welcome to this board.

Respectfully,

JayDub

Typhoonpilot

Good stuff Tony.

For the second part of the question. 2nd Segment is from the gear up height ( usually 35 feet ) to the height at which acceleration to the Flap Retraction speed begins.

JARs and FARs all stipulate that the Minimum Flap Retraction Height will be 400 feet and the maximum is 1500 feet. Many operators use a standard 1000feet with some special airport exceptions.

Throughout the Second Segment, the landing gear is fully retracted and the speed maintained at V2. Maximum Take Off Thrust is used ( within AFM limitations ).

The minimum climb gradient are 2.4% for a two engine jet, 2.7% for a three engine jet, and 3.0% for a 4 engine jet.

The Third Segment is the "clean up" segment during which the aircraft is accelerated to the Flap Retraction Speed, at which point the flaps are retracted, climb speed established, and the power reduced to the climb or max continuous setting. It is usually flown level or in a shallow climb.

Once the flaps are up, speed is set, and climb or continuous power is established we enter the Final Segment. Careful not to confuse the terms for Third Segment and Final Segment. In the Final Segment a twin must meet a 1.2% gradient, trijets a 1.4% gradient, and four engine aircraft a 1.5%gradient.


TP

KiloAlpha

I am well aware that you know more than me TP, so dont take this the wrong way. I dont believe your climb gradients are correct.

The 2.4, 2.7, 3.0 are for single engine climb with gear retracted 25.121

And when the aircraft reaches 400' agl, the climb gradient may not be less than 1.2, 1.5, 1.7 percent 25.111

I may be wrong though, please correct me if I am. This is a question I have also been trying to get answered... I am so confused

loudgarrettdriver

Remeber this is called a PDP. A VPD is shown on the plate.

Take 10% of the (HAT) subtract it from time listed on the plate.

Ex. Time on the plate is 2 minutes. (HAT) is 400ft.
10 into 400 is 40. Right. Now- subtract 40 secs from 2 minutes which gives you 1 minute 20 secs. At 1:20 if you don't have the vis go missed.

This is for a 3 degree glide slope.

Typhoonpilot

I think we agree on the second segment gradients. Just a little confusion on where second segment ends and third segment begins. I think your numbers on final segment are correct, I took the ones I used from a book I have on performance from the UK so they could be UK CAA numbers versus FAR numbers.

Here is some more info I found:

First Segment: begins at lift off and ends when the landing gear is fully retracted. The climb requirement in first segment is a positive gradient, out of ground effect, for two engine aircraft and 0.3% for three engine aircraft. The rotation speed, Vr, must be selected so that V2 is achieved by the time the aircraft reaches 35 feet in the air.

Second Segment: begins at the end of first segment and is continued to not less than 400 feet above the airport elevation. The climb requirement in second segment is 2.4% gradient for two engine aircraft and 2.7% for three engine aircraft. Second segment is usually, but not always the most limiting of the segments within the takeoff flight path.

Third Segment: begins at the end of second segment and ends when the aircraft reaches the speed for final segment. While third segment is usually flown in level flight, the available gradient must be at least equal to that required in final segment. During third segment the high lift devices are retracted.

Final Segment: begins when the aircraft reaches the final segment speed and ends when the aircraft reaches 1500 feet above the airport elevation. The climb requirement in final segment is 1.2% gradient for two engine aircraft and 1.5% for three engine aircraft. At the beginning of final segment, the power is reduced to maximum continuous. Each segment must be flown at a constant power setting and the end of the acceleration segment is often coincident with the end of the five minute limitation on Takeoff thrust.

These numbers pertain only to turbojet/turbofan aircraft not recip/turboprop aircraft.


TP

TonyC

Call it whatever you like, the same principles apply. (I think you meant VDP, not VPD. )


The technique you give was cited above, along with its other major limitation - - it's only "accurate" for 120 Kts groundspeed.



[EDIT: wrong site ]

loudgarrettdriver

Or you could just look out the window.

TonyC

That's right, because in a "real man's world" there are no visual illusions, and no requirement to be smart.









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