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The nature of the minima’s, MDA(H) or DA(H):
Part 1 - minimum is MDA(H) - CDFA or non CDFA
The MDA(H) being a minimum descent altitude, no altitude loss below the MDA(H) is allowed during the approach and go-around; this implies to either:
− Level-off at the MDA(H) - step-down / dive-and-drive technique - until visual references are acquired:
− Initiate the go-around above the MDA(H) - constant descent-angle technique - if no visual references are acquired, in order not to “duck under” the MDA(H).
Part 2 - minimum is DA(H) - only CDFA
This is obviously not required when the applicable minima is a DA(H), which is a decision altitude; if no visual references are acquired when reaching the DA(H), a go-around must be initiated at DA(H)
... and more info from JAA
Issues related to DA/H and MDA/H
The application of the CDFA technique requires all NPA operations, to be flown with a decision altitude/height (DA/H). When determining the applicable DA/H, the operator must take account of the missed approach point (MAPt) and the minimum descent altitude/height (MDA/H). While it is quite clear that a missed approach must be initiated not later than at the MAPt, the question of MDA/H is a different matter. Quite a few operators use MDA/H as the DA/H with no height add-on; in fact this is the case with a majority of the largest European operators. This modus operandi has
raised concern that the unavoidable height loss below the MDA/H during a go-around might introduce a safety risk, even if the height loss can be minimised by the use of appropriate operational procedures (call-outs, high degree of on-speed/on-path discipline, training). In order to evaluate the safety of the use of MDA/H as DA/H, the AWOSG has compared the obstacle protection for this type of approach with the obstacle protection for ‘traditionally’ flown non-precision approaches as well as with the protection for approaches with vertical guidance (APV) using the criteria contained within ICAO PANS OPS. The AWOSG is convinced that using the MDA/H as a DA/H offers adequate obstacle protection. Another comparison between the CDFA technique and the ‘traditionally’ flown non-precision approaches indicates that the latter involve several safety traps, such as:
- Early descent with a prolonged flight close to obstacles;
- Multiple step-downs possibly inside the FAF;
- An approach which is, by definition, destabilised;
- Temptation to make a late and steep descent from MDA/H towards the threshold;
-Risk of descending early from the MDA/H;
While there are no records of accidents related to the use of the CDFA technique during approach operations, there are several accidents attributable to the risks listed above.
The matter of using the MDA/H as a DA/H is progressing in the ICAO OCP and OPSP. Until there exists a final result of the work in ICAO, it must be left to the discretion of each Authority to make decisions on the matter. Since the benefits of the CDFA technique are generally acknowledged, the decision is typically whether to require an add-on to the MDA/H to ensure that the height loss does not lead to flight below the related MDA/H during a go-around, based on formal or other reasons. It is not the intention of this NPA to imply that an add-on to MDA/H must be required, but rather an acknowledgement that it is an option. In order to facilitate the decision-making process, the evaluations referred to above can be made available in the form of Power Point slides and working papers. In addition, a short paper by Mr Theo van de Ven (KLM) outlining the safety arguments in favour of using MDA/H as a DA/H is attached at the end of this Explanatory Note (See Attachment A).
At the latest meeting of the ICAO OCP (WG of the whole in Singapore in November 2006, it was recognised that the use of DH in connection with the CDFA technique, could be acceptable from an obstacle clearance point of view and that future work should be referred to the ICAO OPSP, The CDFA uses a DA(H). The DA(H) figure is derived from the OCA(H) for the associated procedure plus any buffer decided by the operator or the authority. MDA(H) is the lowest altitude (height) for the level portion of an approach flown using the traditional dive-and-drive technique, not the CDFA technique. Use of MDA(H) would undermine the philosophy of the CDFA. The AWOSG has undertaken several studies which conclude that the use of MDA as DA is safe (attachment A to this document, WP 9 to OCP WGHL, March 2006 by Mr T v d Ven, NLR-report CR-2000-451 by Mr H.W. Kleingeld and in “A statistical look at the safety of CANPA procedures”, 11 June, 2002 by Capt J Meijer, RLD). If an operator or an authority thinks that mitigation is needed anyway, this can be done by means of a buffer and/or procedures and training. NPA OPS 41 does not prescribe how to decide, but leaves this to the NAAs. Use of the CDFA technique is considered as a significant safety improvement. (Ref to ALARP). It is also important to note that the NPA does not suggest anything that is not already in widespread use by European operators.