Height loss factored into Decision Altitude

that even if we fly a CDFA we need to factor in a margin so that we never descend below MDA after initiating a go-around.

That is what taught on commercial IR courses. DA is DA under CDFA under part.NCO, and the decision has to be taken to go around before DA as in a precision approach. So there is an element of train for test going on here. There isn’t a minus zero tolerance on a CDFA under part.NCO.

Most ATOs add 50 feet to the MDA to take into account the inertia of the aircraft, but this is an SOP item not a regulatory requirement.

Some ATOs will add 50 feet to a precision DA for PEC (pressure error correction) unless the AFM states a different PEC. Again the decision in a single crew operation has to be announced before DA, ie do not announce after you have a CAS minimum announcement.

My brief is to countdown to the SOP DA, in this case DA plus fifty feet for PEC, and as you pass twenty feet to go announce throttles moving and go around. You are not expected to look up as this is a train to test, where the examiner will have briefed you are going around. She just wants to see you respect minima. The FAA are more realistic for single crew and expect a look up for the decision sequence, in EASA they regard the look up as peeking – go figure. But then 99% of EASA new IRs are going straight to multi crew where pilot flying would stay on the instruments.

Inertia taking you through DA is understood, a 747-8 on a special CAT 1 might even briefly touch down having gone around legally. What is important is initiating and verbalising the decision before, but close to minima.

For DA, why having an add-on to what has been already factored in the approach design on top of system accuracy

Weight-Aerodynamics inertia is already factored in the category (only speed matters here)

Engine-Weight inertia is not factored but I suppose the DA design is tailored conservatively to the heavy category on rolls-royce engines, no?

TERPS accounts for the momentum in the missed approach initial segments. The first 1460 feet past the DA location use the same OCS as the final approach segment, then the in next 8401 feet, the OCS rises at 28.5 to 1 slope. For a standard 3 degree slope, the OCS for the final approach course is 34 to 1. So 1460/34 is approximately 43 feet. Obstacle penetrations will cause adjustments.

TobiBS wrote:

Hi forum,
had a discussion lately with a CFI and wanted to ask for some sources, as I couldn’t find any. The point of discussion was the difference between MDA and DA. For the MDA it is undisputed, that even if we fly a CDFA we need to factor in a margin so that we never descend below MDA after initiating a go-around.

That is certainly not undisputed! The EASA Air Ops regulation is currently contradictory about this, but with a coming revision it will be made clear that it is perfectly ok to go below the MDA on a CDFA as long as the go-around is initiated at the MDA. Check GM8 to NCC.OP.100 in this document. local copy (Part-NCO will not be revised at this time, but the exact same reasoning applies.)

However the decision altitude should have that factored in already, as here it is the altitude at which we initiate the go-around and hence there will be a height loss. Unfortunately neither in the ICAO documents I found, nor in the FAA TERPS I was able to find, how much height loss is taken into account for the DA.

The height loss margin is found in PANS-OPS volume II. For approach category A aircraft it is 130 feet and also includes the altimeter error margin.

Since the minimum obstacle clearance at the MDA for a 2D approach with FAF is 246 feet, it is obvious that there is no safety issue with height loss at the point of going around. It could be an issue in case of a missed approach if you’re below the MDA at the MAPt, however it is not difficult to see that with any reasonably designed procedure this is extremely unlikely. Again, see the EASA document referred to above.

To add to AA’s post, which I agree with, there is, in EU implementing rule, no MDA for a non-precision approach flown with CDFA. A DA is used instead.

NCO.OP.111 Aerodrome operating minima — NPA, APV, CAT I operations
(a) The decision height (DH) to be used for a non-precision approach (NPA) flown with the continuous descent final approach (CDFA) technique, approach procedure with vertical guidance (APV) or category I (CAT I) operation shall not be lower than the highest of:
(1) the minimum height to which the approach aid can be used without the required visual reference;
(2) the obstacle clearance height (OCH) for the category of aircraft;
(3) the published approach procedure DH where applicable;
(4) the system minimum specified in Table 1; or
(5) the minimum DH specified in the AFM or equivalent document, if stated.

(b) The minimum descent height (MDH) for an NPA operation flown without the CDFA technique shall not be lower than the highest of:
(1) the OCH for the category of aircraft;
(2) the system minimum specified in Table 1; or
(3) the minimum MDH specified in the AFM, if stated

The contradictory AMC will be removed in the next revision.

FWIW, I had hoped that we’d get the revisions to Part-NCO into the same package as the rest of the AWO, but time is running out and the NPA hasn’t been published, so it may have to wait.

There is DA, and then there is DA, which is something else.

For an ILS, DA has factored in a height loss that occurs after “decision”. That is the sink NCYankee describes above, only his numbers are for TERPS. Looking in ICAO 8168 Vol II, interpretation of table II-1-1-2 gives an height loss of 42 ft for Cat A airplanes. Those numbers are comparable.

For non-precision approaches, it is different. Jeppesen started drawing most procedures as continuous descent (CDFA) instead of “dive-and-drive” when that (CDFA) become the thing some years ago. On any continuous descent, precision or non, Jeppesen writes DA for minimums, not MDA(H). So they changed the designation, but not the numbers – no height loss was ever factored in when they removed the “M” from MDA/H.

This is why whenever you look at a Jeppesen plate and read the DA, you have to add a height loss margin if it is a non-precision approach, because the DA is really an MDA that you cannot go below unless you see things outside. If it is a precision approach (ILS), there is a little room for you below to stop sinking, and you don’t have to add anything to the DA.

It’s a mess, and hence a very good question by @TobiBS.

Edit: I did not read input by AoA and bookworm until after I posted. It seems the mess is being cleaned up.

huv wrote:

Looking in ICAO 8168 Vol II, interpretation of table II-1-1-2 gives an height loss of 42 ft for Cat A airplanes.

42 ft is the height loss margin for Cat II approaches where a radio altimeter is used to determine the DH. (That’s why your number is different from the 130 ft I quoted.) I guess your interpretation is that 42 ft is the assumed maximum actual height loss while the difference up to 130 ft (88 ft) is the altimeter margin?