You can start the CDFA after the procedure turn if you are established on the final approach track, not before.
Ibra wrote:
Why you can’t descend on CDFA after finishing the timed procedural turn?
Of course you can, but the navigation uncertainty would be great enough that it’s unlikely that you’ll arrive in approximately the right spot at minima.
At least in Europe the IR practical test standard is CDFA. There are non DME, NDB non precision approaches still, and IR examiners have been known to fail the DME if the candidate did not identify it, and seeing if the candidate can fly a timed CDFA.
In the UK you can now use overlay GPS up and until the FAF (so an NDB only the GPS will identify the FAF) but then the final track is flown on the published NDB.
On a timed CDFA (somewhat of an oxymoron) you would be expected to maintain a stable rate of descent with the usual NDB limits, and initiate the missed approach at MDA ie not drive at MDA until the time on the plate, or go around at the planned time if not yet at MDA.
In the former case you can’t use CDFA, unless you “cheat” using a GPS fix
Why you can’t descend on CDFA after finishing the timed procedural turn?
Just curious, not that I am flying it anytime soon using a stopwatch (I fly these on LNAV+V using GPS with AP hooked, I only need to know the legal visibility for planning)
For a piston aircraft such as my Bonanza, pilots can easily fly a 6 degree descent, still needing some power. It has not been demonstrated that CDFA is safer than DND for piston aircraft.
Likely true on long runways and pilots who can fly steeper visual segments?
My experience of DnD on 800m runway in calm winds with 500ft OCH was ugly but CDFA seems to work fine but needs more visbility…
requires significantly less visibility
That makes sense, since at each SDF you are obstacle-safe to descend to the next level immediately, and that means, after the last SDF has been passed, you are flying to the start of the runway at a lower level, and visibility is going to be better when you are closer to the ground.
This situation is immediately obvious when doing this for real, down to minima, and in bad vis.
However it may not be statistically true if the conditions are well above minima. In that situation one might expect other factors cause accidents, and some form of “total screwup” is more likely if doing D&D.
Airborne_Again wrote:
He is right that you are more likely to get into the airport in marginal conditions using DnD but you do this by sacrificing safety.
I did not read Rod’s article, but the FAA did an analysis using MITRE (an engineering company that does a lot of government reports) to evaluate DND vs CDFA and CDFA showed a clear safety improvement for turbojet aircraft, but was unsubstantiated for piston aircraft, in fact DND had a better safety record although it was not statistically significant. DND allows aircraft to get much closer to the runway and requires significantly less visibility. For a piston aircraft such as my Bonanza, pilots can easily fly a 6 degree descent, still needing some power. It has not been demonstrated that CDFA is safer than DND for piston aircraft.
Bathman wrote:
How does CDFA work for a timed approach?
Are you referring to approaches without a FAF (timed outbound leg) or approaches with a timed MAPt?
In the former case you can’t use CDFA, unless you “cheat” using a GPS fix (of your own or provided by Jeppesen) as FAF. In the latter case, you use elapsed time rather than distance in the advisory altitude table. I’ve seen Jepp charts like that.
Dan wrote:
FWIW, no airline I know of uses the D&D method… this disappeared some years ago as CDFA stats showed it to be much safer, whilst GPS accuracy and avionics vastly improved
CAT are not allowed to use DnD except by approval by the competent authority in each individual case. (CAT.OP.MPA.115)
Same as for CDFA. Timed approaches are horribly inaccurate and needs loads of protected area. And if your stopwatch runs out, you die 
How does CDFA work for a timed approach?
