Descent out of MDA, visual or VDP?

[regionalcap]

I agree. Use the HAT.

 

[TonyC]

Just base your descent on the profile 300 feet per nautical mile. If you have to, set up a ratio of 300:1 to your height over "n" (VDP distance).

For example, let's say your MDA is 550 feet:

300 x 550
..1..... n

300n = 550

n = 550/300

n = 1.83


So your VDP is 1.8 miles. So start your descent at 1.8 miles and you are fine. I'm sure there are other methods but this one is quick and easy.

Good luck...

You've got the right idea. If field elevation (more specifically, touchdown zone elevation) is zero, then what you said is correct. If it's not, then you need to use HAT (Height Above Touchdown) in your ratio.

Also, in setting up your ratio, you used "x" (times) intead of "=". For 300 ft/NM, the correct ratio would be:

300 = HAT
...1.......n..

which after cross-multiplication and dividing both sides by 300 yields: n = HAT / 300

just use the HAT (height abv tdz) /300...

therefore: MDA 800', TDZE 200' = HAT 600' and MAP is 1.0 DME

600/300=2 nm so 3.0 DME is your VDP

for a quick descent rate at 300 fpnm or a 3 deg GS, remember 120 kts will give 600fpm (2 nm/min), 90kts will give 450 (1.5 nm/min), 60 kts will give 300 (1nm/nmin) so you can estimate from there

Exact same math, expressed differently. Both methods give a distance from runway. The "units" is NM (not necessarily DME; there may BE no DME), and the distance is measured from where the descent is begun to the point of where the planned descent path intercepts the runway (touchdown if you do it carrier style). If you calculate distance from the END of the runway, pass the VDP in level flight and begin your descent at the planned descent rate you will be "aimed" at a point just beyond the end of the runway (can't descend instantaneously) and be situated to flare and land in the touchdown zone.

If you plan on a descent gradient other than 300 ft/NM, substitute that number in the VDP = HAT/ Gradient (ft/NM) formula. 300 ft/NM gives approximately a 3 degree descent angle (the "60:1 rule" is an approximation, not an equivalent) but given the other inaccuracies mentioned above, it's close enough.

Determining when you actually ARRIVE at the VDP is another story entirely. There may be a VOR/DME, VORTAC, or TACAN located along runway centerline that you can use to determine distance. [If the VOR/DME is, for example, located halfway down the 6,000' runway, and you're using a VDP of 1.5 (450' HAT/ 300 ft/NM) your VDP would 0.5 + 1.5 = 2.0 DME. If, on the other hand, that VOR/DME is located 5 miles prior to the runway approach end (maybe it's the FAF?), your VDP would be 5 - 1.5 = 3.5 DME.] One of these (VOR, etc.) located off to the side somewhere (perhaps on a LOC or NDB approach) would be of no value in determining the VDP. There may be an ILS-DME that you can use. You may be forced to use some sort of timing calculation to determine where the VDP is. Whatever the method, the usage is the same.

I need to know the VDP before I begin the approach, because I need to make sure I'm at the MDA at or prior to the VDP. I need to determine the distance to travel from the FAF or step-down fix and the altitude to lose in order to plan a sufficient descent rate to arrive at the VDP at the MDA. Arriving early is no problem - - I level off and wait until the VDP to continue the descent. Arriving late is a problem - - I may be too high to safely continue the descent to the touchdown zone, so it's time to execute the missed approach procedure. NOW I wish I had planned better!


A Visual Descent Point is the point where you begin a descent from a given altitude on a predetermined descent path or angle. Knowing this point ahead of time is useful for planning your descent TO that altitude (i.e., descent to an MDA) and for planning your descent FROM that altitude, i.e., to the runway.

The whole point of descending AT the VDP is that it will establish you on that pre-planned descent angle, "glide-path" if you will, to arrive at a predetermined point on the runway. Descending (at the predetermined rate) BEFORE the VDP ensures you will be below that planned descent gradient. Descending (at the predetermined rate) AFTER the VDP ensures you will land beyond the planned landing point.

Apply this to a Part 91 C-172. If your VDP is planned to take you to the touchdown zone of the runway, descending prior to the VDP puts you low on your descent. In order to re-establish that descent rate to the TDZ, you must level off or decrease your descent rate to intercept the planned descent path. Seems like a lot of work to me, and I can't think of many advantages. On the other hand, descending after the VDP leaves you with the choice of increasing descent rate to re-intercept the planned descent path, or accepting a long landing. I would highly discourage the former; the latter would be dependent on the runway length.

Same scenario in a transport category aircraft has more serious implications. Descent early will put the airplane low, which is undesirable if not illegal. Late descent could result in landing beyond the touchdown zone, which is not allowed.


Bottom Line: If the approach does not have a published VDP, calculate one yourself, and use it to plan your approach.


Oh, and when you descend out of the MDA at the VDP, you're performing a visual maneuver.

 

[Swerpipe]

The ALS is closer than the runway

Keep in mind that many VDPs are greater than one mile from the runway, yet many non-precision approaches have one mile or 1.5 miles visibility minimums.

Therefore in order to shoot a non precsion approach you have to fly a steep glide path.. ie not stabilized... ouch.... what is a pro pilot to do?

Your VDP distance from the rwy is actually your visibility minimums....

Remember, on an instrument approach it is likely that you will see the Approach Lighting System first, not the runway. Once you have the ALS you can start the descent from MDA or DH to 100 above HAT or HAA as applicable except that to descend below 100 you have to have the runway environement in sight. It can happen that after you start the descent to 100 feet you realize the beginning o the runway is fogged in past the ALS. I can not think of any approaches where if the visibility is at minimum it is impossible to fly the approach and land. Of course the big factors are the crew proficiency, familiarization with the approach and airport and many others.
The VDP that is calculated by the pilot is often called the PDP or planned descent point. If the VDP is charted the the pilot can not start the descent until reaching even if he has the runway in sight.

 

[TonyC]

If the VDP is charted the the pilot can not start the descent until reaching even if he has the runway in sight.

Just out of curiosity, do you have a reference for that handy?

 

[Swerpipe]

FAR AIM book

5-4-5. Instrument Approach Procedure Charts

e. Visual Descent Points (VDP's) are being incorporated in selected nonprecision approach procedures. The VDP is a defined point on the final approach course of a nonprecision straight-in approach procedure from which normal descent from the MDA to the runway touchdown point may be commenced, provided visual reference required by 14 CFR Section 91.175(c)(3) is established. The VDP will normally be identified by DME on VOR and LOC procedures and by along track distance to the next waypoint for RNAV procedures. The VDP is identified on the profile view of the approach chart by the symbol: V.





​

1. VDP's are intended to provide additional guidance where they are implemented. No special technique is required to fly a procedure with a VDP. The pilot should not descend below the MDA prior to reaching the VDP and acquiring the necessary visual reference.


2. Pilots not equipped to receive the VDP should fly the approach procedure as though no VDP had been provided.


TonyC,
This is section 5-4-5 on the FAR AIM book. It is somewhat a "new" concept that has gained popularity with FAA examiners due to a few CFIT accidents. It was a question posed to me by an FAA inspector (formerly known as Captain America). LOL BTW the additional note on not descending below VDP has not always been there but it is now. Hope this helps, take care...

​

 

[TonyC]

FAR AIM book

5-4-5. Instrument Approach Procedure Charts



e. Visual Descent Points (VDP's) ... 1.



​

BTW the additional note on not descending below VDP has not always been there but it is now. Hope this helps, take care...

Thanks, Swerpipe, that's exactly what I was interested in. I reckon I oughtta get a more current AIM!

CANPA (Constant Angle Non Precision Approach) is the way to go.

 

[Swerpipe]

WAAS is here

Tony,

I totally agree with you. There are way to many non-precision /A,B,C still around. Hopefully the WAAS with the LPV(LNAV/VNAV) and TAWS will give us an added level of safety. Done too many of those drive and drop approches on the Metro at night