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.
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
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.
