LHR CDA Calculator

GPS is unavailable.

Input
Distance to TD		nm.
Current Altitude		ft.
Ground Speed		kts.
Output
VS to target		ft/min
Angle to target		deg
Reference
Original Dist to TD		nm.
Current Dist to TD:		nm.
Ground run:		nm.
Setup
Target Altitude		ft.
Target distance		nm.
Note: To avoid confusion, all distances are positive.
Note: Negative altitudes are allowed to faciliate testing.

Discussion

Overview:

This is designed with LHR (London Heathrow) continuous descents angle initial approaches (CDA) in mind. When the controller gives you the ground track miles to go, enter it. If the GPS is not working, this is simply a calculator where you will have to enter your altitude and groundspeed. If GPS is working, then this will use the GPS values for speed and altitude to calculate the descent rate necessary to intercept the target. This should also use the GPS to count down the miles flown from the initial ground track miles entry.

This is not a precise maneuver. The GPS altitude may be slightly different from the baro altitude and the ground track distance given by the controller is probably not precise. The idea is to get down fast enough so you do not have to dive to catch the glideslope and to avoid gettting down so quickly you have to level off before descending via the glideslope. Generally, you will join final well above and outside the target (2500feet and 7.5 nm in LHR). Once you get close to final, the glideslope will help refine the necessary descent rate.

In general, I would think using an amateur web page for VNAV would be a really stupid idea. However, this is not a VNAV approach close to the ground. It is just a way of descending to catch the glide slope without having to level off.

I only use positive values for distances. Arguably, the distance to the target and touchdown should be negative like the FMS. There is no use in this calculator for distance on the other side of the target, so I just use absolute values so either logic should work.

It may seem strange that I allow negative altitudes. This allows me to test. When I am walking around my neigborhood looking like a wierdo with my iPad at 100ft MSL, I can put the Target Altitude at -2400 to check the calculations when using GPS. While there might be some risk of someone entering a negative distance using FMS logic, there should be no reason to accidentally enter a negative altitude.

Caution:

This is highly experimental. This is not endorsed by anybody. Use at your own risk.

Official source for descent rates

Undisclosed Air Line Airway Manual 3.2.7.1 has a profile table. Should this location change, search for "CDA" of "Continuous Descent Approach".

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 descent rate less than the geometry would indicate so that you do not level off prematurely.

Offline Use

If you are opening this from a shortcut that I shared with you, it will work offline. However, if you found this on my website, the trick to using it offline is to use the Reading List feature of Safari. Here is an Apple article on how to use this: link.

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.

Updates

2025/01/05	Fixed and issue where GPS speed was being ignored.
2024/12/23	Initial release of this version.