LHR CDA Calculator

Input
Current Altitude ft.
 
Distance to TD kts.
 
Ground Speed kts.
 
Output
VS to target ft/min
Angle to target deg
 
Setup
Target Altitude ft.
Target distance nm.

Discussion

Caution:

1. This is highly experimental. I have not used it yet. This is not endorsed by anybody. Use at your own risk.

2. I calculate the geometry of the situation. You may want to use less than the geometrically calculated descent rate in order to avoid an early level off.

Official source for descent rates

Airway Manual 3.2.8.1 has a profile table. Search for "CDA".

My personal thoughts on technique

I usually avoid LHR for a variety of reasons, but the CDA is not one of them. The CDA descents originally seemed overly complicated, but I found that they are not bad after doing a couple. This is not a precision maneuver. I simply use the chart in the airway manual and fly a descent rate a little less than the table value. Once I approach the localizer, I then have some guidance so I reduce my descent rate to make sure that it is above 200 fpm and fly into the glideslope.

I have not been high. However, I use the trick of dividing the groundspeed by 2 to get an idea of the 3 to 1 descent. For example a 200kts groundspeed would be about 1000 fpm for a 3 to 1 glide slope. If the chart has a higher rate of descent than 3 to 1, my thinking is that I want to descend by at least the Airway Manual table value and may end up intercepting the glide slope from above. However, I have not yet run into this scenario.

Why I created this calculator

My original thought was to create a tool like this calculator. However, after flying a few of these, I thought that it was overkill. I then read an article in the June 2022 330 Fleet Newsletter and another mention in the July 2022 330 Newsletter. Since this calculator was not hard to develop, and I thought it was interesting, I thought that I would create this. I also like that I can split screen with Jepp and use easier than the Airway Manual in AeroDocs.

Inputs

Note that the calculator gives the VS required to reach the FAF which is defaulted to 2500 ft. at 7.5 nm from touchdown. The input distance is the distance to touchdown which is what approach gives.

Comparison with Airway Manual table

I find that my calculator give higher descent rates than the table. I have crosschecked my calculations with a seperate spreadsheet and I beleive my calculations are accurate. I speculate that the difference is that I just give the geometry of the situation and the AM table gives a descen rate less than the geometry would indicate so that you do not level off prematurely.

Feedback

I would enjoy your feedback. I would be curious as to weather you find this useful, prefer the Airway Manual, or simply find that a little mental math is adequate. thejohnbell@gmail.com.