MDA

[bookworm]

My understanding is that the idea behind CD NPA is to try and make the execution of these as similar as possible to an ILS.

The flaw is that it pretends a little too much that the non-precision approach is an ILS , i.e. a precision approach.

If the onboard equipment (e.g. BaroVNAV) allows you to fly to a point in space with high vertical and horizontal precision, then you might as well treat it like a precision approach. And because you dip (because you're treating it like a DH) below the MDH/OCH at that precise point, where there are no obstacles, there's little risk of hitting something even if you're in still cloud.

But if you're using raw data, selecting a rate of descent with mile-spaced DME crosschecks or worse, there's a significant variability in the distance from the runway at which you hit the MDH/OCH, and your dip of 50 ft eats directly into the margin above obstacles on the approach. I would love to see the risk assessment that demonstrates that dipping 50 ft below the OCH in these circumstances is within tolerable risk.

The progression towards CDFA for high-inertia, multicrew commercial air transport with appropriate avionics is clearly a step forward in safety. But the one-size-fits-all attitude of EU-OPS is not.

[aterpster]

bookworm:

But if you're using raw data, selecting a rate of descent with mile-spaced DME crosschecks or worse, there's a significant variability in the distance from the runway at which you hit the MDH/OCH, and your dip of 50 ft eats directly into the margin above obstacles on the approach. I would love to see the risk assessment that demonstrates that dipping 50 ft below the OCH in these circumstances is within tolerable risk.

No matter how much you'd love to see them, they don't exist.

[Sciolistes]

Bookworm,

I don't understand your assertion that you would eat into the OCH on a CDA.

[bookworm]

I don't understand your assertion that you would eat into the OCH on a CDA.

If you use the OCH as a DH rather than an MDH, then you will dip below it while making the decision to go around and changing the attitude of the aircraft -- typically 50 ft is considered normal. The OCH may be just 246 ft above obstacles or terrain, so by going below it by 50 ft you're eroding that minimum obstacle clearance (I previously used the word "margin", which in PANS-OPS-speak has a different meaning. I meant MOC.)

[Sciolistes]

Bookworm,

But the use of a CDA is predicated on MDA+50 as the minina, is has to be. Where did OCH=DH come into scope?

[Tee Emm]

Quote: During my JAA IR I was told to add 50' on top of the DA for an ILS which I never understood. Unquote..Yet, in a Cat3A ILS where the DH is 50 feet, do you now have to add another 50 ft to ensure you don't go below 50 ft agl? The Boeing 737 FCTM states it is possible the wheels will touch the runway if an automatic GA is made at the DH of 50 feet. So obviously there is no problem if the wheels do touch on a GA. So it seems a bit superfluous that your JAR testing officer says add 50 ft to the normal ILS DH for an ILS GA. Methinks the testing officer is adding personal opinion rather than hard data?

[Sciolistes]

Already been explained further up Tee Emm, it is trained as standard practice for barometric altimeters give such altimeters have an allowable error of +50'

Of course, such a low DH approach in a 737 (for example) references the radio altimeter.

[eckhard]

Punk666 and others,

During my JAA IR I was told to add 50' on top of the DA for an ILS which I never understood.
Yeah I'm pretty certain we taught the same but I really can't remember the reason behind it. I remember it made perfect sense though when I was told....Bollocks this is going to do my head in now till I find out!!!

The only guess I could make (which I don't think is the real reason for it) is that I think the limitations were +100 -0 so maybe adding 50ft to DA was a safety buffer so the student didn't go below DA before making a decision?...on second thoughts that can't be the reason!!

Anyone able to help?

I think I can answer this one. I spent about 1000hrs training guys and gals for the CAA initial IR test. The criteria for the test required (amongst other things) that the candidate initiate GA from DH -0' +50'.

We used to recommend that candidates THINK about commencing the GA at DH +50' to ensure that they didn't COMMENCE it below DH. Of course, it was (and still is) acceptable to dip slightly below the DH during the GA manoeuvre, provided that the GA was commenced between DH and DH +50'.

PEC had nothing to do with it, as this was a value which was tabulated in the AFM and could be applied if necessary. From memory, the light twins that we used had negligable PEC at SL airfields and approach config/speeds.

For the NPA, the candidate had to level off at MDH and then maintain MDH -0' +100'. For this reason, we recommended adding 50' to the MDH and using this as the 'target'. This gave the candidate + or - 50' to play with while he flew towards the MAPt, tracking the NDB (which by now was behind him/her) on the RBI and working out how long to run to the threshold from the stopwatch which he/she had forgotten to start.

If in doubt, when the examiner asked' 'how long to run?', we used to say, 'just tell 'em 40 seconds. It'll be about right'.

All of this was in the late 1970s and early 1980s, using hand flying (without FD) and using QFE for the approaches.

Also, we used 'dive and drive' for the NPA.

Happy days!

Eck

[bookworm]

But the use of a CDA is predicated on MDA+50 as the minina, is has to be. Where did OCH=DH come into scope?

No, it's not, not with the "New" EU-OPS scheme. See post #10 above.

The EUOPS reference that 172 refers to is the latest legislation. Note there is now no requirement to add 50 ft for any reason.

The Old Appendix split NPAs with an MDH from precision approaches with a DH. The New Appendix does not:

Appendix 1 (New) to OPS 1.430
(b) Category I, APV and non-precision approach operations
...
2. A non-precision approach (NPA) operation is an instrument approach using any of the facilities described in Table 3 (System minima), with a MDH or DH not lower than 250 ft and an RVR/CMV of not less than 750 m, unless accepted by the Authority.
...
4. Decision height (DH). An operator must ensure that the decision height to be used for an approach is not lower than:
(i) the minimum height to which the approach aid can be used without the required visual reference; or
(ii) the OCH for the category of aeroplane; or
(iii) the published approach procedure decision height where applicable; or
(iv) 200 ft for Category I approach operations; or
(v) the system minimum in Table 3; or
(vi) the lowest decision height specified in the Aeroplane Flight Manual (AFM) or equivalent document, if stated; whichever is higher.

While EU-OPS 1.430(d)2 is quite emphatic about the need to fly NPAs as CDFA, it doesn't do a good job of explaining that such CDFA NPAs use a DH, not an MDH. But that is the way it is expected to be.

[PEI_3721]

Aterpster, - risk assessments, #24.
IIRC such an assessment was undertaken by a European research organisation – NLR ??
With fading memory, a conclusion was that was a slightly increased risk in using MDA as DH, i.e. accepting a small transgression below MDA. The risk was quantified and compared predominantly with the risk / benefit of ‘dive and drive’ NPAs (higher risk) vs a continuous descent/DH (reduced risk). I also recall some reference to the ICAO collision risk model.
I did have a copy of the document / presentation – searching my archives after a house move.

The issue was debated by JAA, including AWOSC, who (failing memory) drafted a change to JAR OPS. One aspect was to delegate approval to local authorities. However, these proposals appear to have been lost / dropped / found to be in error, or just not in accord with ICAO, such that they have not been used subsequently.
At the time, as I understood the argument, based on balancing the risks, it was reasonable to equate MDA/DH as an ALAR initiative
and thus I was surprised that this has not been progressed.

One possible concern was that there are differences between the PANS-OPS and TERPS obstacle allowances, which I think still apply in the current debate.
Lest anyone should think that the above is a case for deviating from current regulation / guidelines, don’t. In retirement I’m out of touch with current thoughts and memory fails me.

TM - #28; there is a difference in approach construction with obstacles on an approach (NPA), and a Cat 3 operation with an obstacle free zone.

[Zeffy]

Galaxy Flyer

GF

As this issue has been discussed many times at recurrent (Global Express), is there an express prohibition on FAR 91 operators using the MDA as DA, IAW the note on the Jepp chart? Where is it?

The fact that the profile note does not exist on equivalent FAA charts tells you something.



If you can bear a visit to the sausage factory, the technical issues related to the approval are a work in progress.
http://www.faa.gov/about/office_org/...004-02-258.pdf

The essence, extracted:

"...It is apparent to AFS-410 that certain technical aspects of this Bulletin are flawed. Specifically, the Bulletin does not provide satisfactory guidance to operators on the method of determining if a visual segment assessment has been made by the FAA in which no penetrations to the 34/1 surface were identified, or the appropriate criteria for industry assessment of the visual segment. In addition, it allows carte blanche application of the operational concept of using the MDA as a DH in cases where the underlying non-precision approach may not be suitable. Finally, the Bulletin provides no authority for Part 91 operators to use this capability..."

Also note that the above FAA chart does not depict the shaded feather in the profile view, indicating that the final is not 34:1 clear.