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M20K down in Saarbrücken (CDA v. D&D etc)

ROBERTL18C wrote:

Test standard is +/- 100 feet at the DME/Altitude references, and you are expected to adjust rate of descent to be on target at the next DME checkpoint

JasonC wrote:

And test or not, if you don’t hit the DME altitudes accurately, you aren’t flying a CDFA.

Of course one should not disregard the reference altitudes. What I meant is, that there is no issue if you remain constantly 50ft high during the course of the approach, because your calculated rate of descend doesn’t match exactly. In regards of the wording my view is, that CDFA is more about the omitted level flying segment on the end, which can be achieved better with constant means of checking.

bookworm wrote:

the descent usually requires zero or at most one level off between FAF and MAPt. … In trimmed level flight at MDA, it is much easier for a single pilot to look out and find the runway than in a few seconds approaching DA.

And exactly this scares me compared to CDFA. Retrimming and power changing at low level, followed by low level flying in IMC. Compared to this a CDFA reduces the low level flying time to a minimum and follows the same procedure (become visual or go-around) as precision approaches. Of course you might miss a chance of a landing, but this is not my main concern when it comes to safe operation.

But unlike an ILS glideslope or APV, there is no continuous readout, so it is much more difficult to assess trends as you would on ILS/APV using not only the position but the motion of the vertical CDI.

And this is an equipment issue. Unfortunately only with advanced avionics an advisory glideslope is available on NPAs. And clearly that is something that makes it easier.

Last Edited by TobiBS at 31 Oct 07:55
P19 EDFE EDVE EDDS

JasonC wrote:

Compare that to a calculated descent with check alttitudes and V/S adjustments.

Like Jason, I also find the conventional method (ICAO’s choice of expression for D&D) easier than CDFA. In terms of the conceptual complexity, the descent usually requires zero or at most one level off between FAF and MAPt.

Non-precision approaches are called “non-precision” for a reason. If you go back to the days of timed teardrop NDB approaches, no one would contemplate flying one as a CDFA, because the precision of knowing where you are along the 2D trajectory is too poor to use that as a guide to the target altitude. Throw in DME and that changes the game. That allows regular assessment of altitude vs range. But unlike an ILS glideslope or APV, there is no continuous readout, so it is much more difficult to assess trends as you would on ILS/APV using not only the position but the motion of the vertical CDI.

The other thing I find easier about the D&D method is the decision. In trimmed level flight at MDA, it is much easier for a single pilot to look out and find the runway than in a few seconds approaching DA. This is much more significant, of course, in conditions of low cloud base (where level determines when you become visual) than low visibility (when horizontal distance does).

RobertL18C wrote:

Test standard is +/- 100 feet at the DME/Altitude references, and you are expected to adjust rate of descent to be on target at the next DME checkpoint.

And test or not, if you don’t hit the DME altitudes accurately, you aren’t flying a CDFA.

EGTK Oxford

I learned, that the regular 1nm step altitudes are for reference only, they are a help to not reach the decision altitude early

Test standard is +/- 100 feet at the DME/Altitude references, and you are expected to adjust rate of descent to be on target at the next DME checkpoint.

Oxford (EGTK), United Kingdom

JasonC wrote:

Compare that to a calculated descent with check alttitudes and V/S adjustments. I just find it easier.

OK, that explains to me, what part you perceive more complicated. I learned, that the regular 1nm step altitudes are for reference only, they are a help to not reach the decision altitude early. Only if there are mandatory altitudes charted in between you have to obey them and otherwise the the approach can theoretically be continued, until reaching the decision altitude. So I see them as an aid, while on the Dive and Drive I am a bit lost in between the FAF and the MAPt.

But to say again, i think CDFA is better and that is how I fly.

I never questioned this, I hope this was clear! In general my questions and comments are meant to get different views on a topic.

I doubt it is equipment driven.

This was just an example of issues that might be between your and my view of the things. Sometimes the exact same tasks with the exact same tools look hard to one person, while they look easy to another person.

Last Edited by TobiBS at 30 Oct 21:10
P19 EDFE EDVE EDDS

TobiBS wrote:

Well OK, that is hard to argue with of course. But it would still be interesting to know, why you perceive it that way? The reasons could be manifold, from the equipment you have, the technique you have been taught or anything else that really makes it easier for you. This is what I am interested in.

I really dont know. I was taught D&D as everyone was in the 90s. At the FAF chop the throttle, get to the MDA and look for the runway. Pretty straightforward.

Compare that to a calculated descent with check alttitudes and V/S adjustments.

I just find it easier. But to say again, i think CDFA is better and that is how I fly.

I doubt it is equipment driven.

EGTK Oxford

Peter wrote:

150ft actually.

The Oxford Aviation Academy ATPL manual says 50 ft. I haven’t had the opportunity to try it for myself.

ESKC (Uppsala/Sundbro), Sweden

JasonC wrote:

It is my opinion based on my experience of both.

Well OK, that is hard to argue with of course. But it would still be interesting to know, why you perceive it that way? The reasons could be manifold, from the equipment you have, the technique you have been taught or anything else that really makes it easier for you. This is what I am interested in.

Airborne_Again wrote:

I disagree! I find CDFA much easier to fly.

At least I am not alone. ;-)

Aviathor wrote:

Most people tend to take acronyms for granted and do not realise they are exclusive.

Not only that, most people also think they are unique. So if people from different somains are communicating it can be a real mess to find out, what they are talking about.

Last Edited by TobiBS at 30 Oct 15:58
P19 EDFE EDVE EDDS

My friend who is certified to fly these says that starting at 200ft, the signal of the radio altimeter is integrated with gradually increasing gain until 50ft, from where the GS isn’t factored in any more.

150ft actually.

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Shoreham EGKA, United Kingdom
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