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4 Answers

CDA Continuous Descent approach (confused)!!

Asked by: 5834 views Commercial Pilot, FAA Regulations, Flight Instructor, Instrument Rating

hey dear Captains i have searched everywhere on the internet couldn't find any  useful answer .all they'r saying is what it is , and not how it works.!!! Its about CDA . i know many airliners use it because it has many benefits compared to Step-down FIX descend . so my question is simple ! . What's the formula?? how can we calculate when and with what rate to descend from initial approach fix (IAF) to FAF continuously , and after FAF, descend continuously to 50ft TCH Best regards.  

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4 Answers



  1. Mark Kolber on Jan 06, 2016

    Read AC 120-108 Continuous Descent Final Approach. It discusses the technique – a stabilized descent from the FAF to the runway for non-precision approaches – as well as the calculation, which is essentially using the VDA published on the chart to calculate the rate of descent based on your GS.

    (I’m assuming some basic knowledge)

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  2. miladmaz on Jan 06, 2016

    dear mark thanks for the feedback . i studied what you mentioned. very useful
    just got one question : I’ve heard there is a new technique in performing Approaches which the pilot can start a continuous descent even before FAF , it could be from En-route flight or arrival-FIX or Initial-approach-Fix until FAF, see the following link

    https://www.eurocontrol.int/sites/default/files/publication/files/2011-cd-brochure-web.pdf

    it’s called CDA . i was wondering if you know about it. is it calculated like the CDFA or a totally different story?

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  3. Russ Roslewski on Jan 07, 2016

    The calculation is no different. It’s one of those examples of a simple concept that can often be really hard to execute.

    The considerations are: altitude to lose, distance to lose it in, desired glidepath angle and groundspeed.

    Appendix 1, Figure 3 in the AC mentioned above is still useful.

    Let’s say you want to descend from 10,000 feet to a runway at 1000 feet elevation, then you have to lose 9000 feet. Suppose you want to descend at a 3 degree glidepath.

    From the Figure 3, 3 degrees is 318 feet per nm.

    9000/318 = 28.3 nm, so you need to start your descent 28.3 flying miles from the runway (if you’re not straight-in, you have to include turns in here, which is tricky).

    What rate do you descend at? Depends on your groundspeed. Let’s say you’re maintaining 150 knots GS. From the same chart, that means you have to descend at 797 feet/minute.

    There are lots of “rules of thumb”. Notice, for example, that this descent takes about 3 nm for every 1000 feet you have to lose. so the quick calculation would be starting descent at 27 nm from the runway. Close enough.

    The tricky thing is that the wind and therefore the groundspeed, changes as you descend. So might your configuration – an airplane doing an enroute descent at 150 probably doesn’t fly final at 150. So there’s a LOT of “slop” in this determination, especially if you’re doing it in your head, or by hand.

    Advanced avionics will of course do these calculations for you.

    Hope this helps.

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  4. miladmaz on Jan 09, 2016

    tnx russ , very helpful . just like the Top of descend formula which i have on my E6B – computer
    plus we must calculate the changes and arrival routes

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