Visual Descent Point

[jergar999]

Is there some standard method of calculating a visual descent point when one is not published? I always hear people talking about this as an interview question, but have never actually seen it in texts.

 

[jergar999]

Thanks for the numbers. 120 works perfect in the King Air, so I don't even have to do any math.

 

[flx757]

I use an alternate method than the one that gunfyter posted. It basically arrives at the same numbers (the exact same numbers for the rate of descent) and a number within .1 or .2 miles on the distance from the runway, but for me the mental math is a little easier, and with my 6th grade education, I need all the simplification I can get.

To calculate the distance from the runway for the VDP, I take the HAT and multiply times 3. For example: HAT = 500 ft. 500 x 3 = 1500. Stick the decimal point in the logical place, and descend 1.5 miles from the runway. The key is a little advance planning, so that you are actually descending at the calculated point, not waiting until you reach the point, then start thinking about descending. By the time you achieve your desired rate of descent, you are serveral tenths of a mile past this point.

To calculate the required rate of descent, I take half the groundspeed, and add a zero. To use the previous example of 120 kts, half of 120 is 60. Add a zero, and the required rate is 600 fpm. For a groundspeed of 140 kts, half of 140 is 70, so a 700 fpm descent rate is required, and so on. Again, I use this method simply because the mental math is easier for me.

 

[DoinTime]

Say HAA is 450. 450/300 = 1.5. So begin Descent 1.5 miles from the runway.

Now if you only had a good way to determine where 1.5 miles from the touchdown zone was.

If you can't see the vasi or papi and don't have the depth perception yet to make a normal approach yourself you should think twice about leaving MDA. I've been in a couple situations where the other crewmember botched the VDP calculation or didn't have a good way of measuring it and we wound up dangerously low. VDP's aren't published for many approaches for a reason. Becareful when you think your smarter than the system.

 

[flx757]

Doin Time,

No one said anything about being smarter than the system. The typical reason VDPs are actually published on approaches is for obstacle clearance reasons, and when published, are mandatory.

What we're talking about on this thread is calculating a point from which a stabilized, 3-degree glidepath to the runway can be made when transitioning from the MDA to the visual portion of a non-precision approach. You mentioned if you can't see the vasi or papi and don't have the depth perception to make a normal approach, you should think twice about leaving MDA. If I were at or past what I had calculated as the VDP, and the conditions you describe exist, I wouldn't at all think twice about leaving MDA. I would leave MDA and execute the missed approach procedure. This is a far better option than continuing along at the MDA for some distance past my calculated VDP, then attempting a de-stabilized, excessive rate of descent dive for the runway.

A botched VDP calculation or not having a good way of measuring it is simply poor planning. With very few exceptions, any non-precision approach will have a final approach fix and a missed approach point, and a method for calculating either by time or distance.

The FAA is advocating, and many companies are adopting, the "constant path angle" method of flying non-precision approaches, instead of the old "dive and drive" method, as a way to reduce exposure to CFIT risks. With this constant path angle type of non-precision approach, you descend from the FAF at a constant 3-degree (or whatever is published on the plate, if different) glide path, either by calculating what descent rate will give you the desired glidepath, or by dialing in the desired flight path angle in the autoflight system on aircraft that are so equipped, such as the A-320 or B-717, for example. You then treat the MDA as a "psudo" decision altitude, where you either have the required visual references to continue the approach in your already stabilized configuration, or if not, execute the missed approach. This method insures the aircraft is configured and stablized, and remains so throughout the final approach segment. It also makes the calculations of VDPs unnecessary. The MDA becomes the VDP.

Obviously, this applies only to straight in approaches, and not for circling maneuvers.

 

[Full of LUV]

If you are using an approach that uses timing from the FAF to the end of the runway, take 10% of the HAA and subtract that from the timing to MAP, that will give you a timing to VDP. For example:

ADF approach with HAA of 550' and timing from FAF to MAP calculated at 3 minutes.

Take 55 seconds (10% of 550) and subtract it from the 3 minutes to get 2 mins 5secs from FAF to VDP.

If you get past 2 mins in timing on the approach, you will be well above a 3degree glide slope if you elect to continue.

 

[skyboat]

Gotta love V-Nav, even if it only is advisory only.

Gunfyter- rate of descent formula works well, but another way to skin the cat is to take your ground speed, divide by 2 and add a zero. Sometimes that fancy math is too much for me....