CAT II with DH>200ft?

[peekay4]

EnxAero:

You can see the terrain going up 2000 m before the threshold but the chart ends before the peak is reached. 2000 m before the threshold, the terrain is still below the airport elevation. I guess it is not common to have a relevant feature 3km before the threshold...but on the other hand, I guess it is not common to have a RA of 500' at DH either.

The 2000m limit is required by ICAO rules.

Another way to look at it: from ICAO's perspective for the purposes of using RA to determine DH, anything over 2 km from the threshold should not be considered relevant.

There could be rising terrain under the approach 3 km away, 5 km away, right below the FAF, etc., but as long as they are below the protection surface, they should not be considered for evaluating the DH RA.

roulette:

It's still rather strange that the ILS Cat I DAs for Cat BCD are higher than the LOC Only MDA!

It is not very common, but does happen. For another example look at the runway 16R approaches into Reno, Nevada (KRNO). There the ILS DA at 2031 ft AGL is a whopping 460 ft (!) higher than the LOC MDA.

As mentioned earlier this can happen when there are obstacles penetrating the ILS protection surface.

For non-precision approaches, the approach design can use step-down fixes (including the MDA floor) to basically "clear" any obstacles.

For precision approaches, the design must either raise the DA or raise the approach angle beyond 3.00 degrees.

Since protection must be extended below the glide-slope and below the DA in case of missed approaches, the raised DA could well be above the MDA.

(For LECO, a hint here is the LOC CDA angle being 3.10 degrees while the ILS angle is 3.00 degrees). In this case, the choice of raising the DA vs. raising the ILS angle is unknown to me.

[underfire]

Design of Approaches using Vertical Guidance, where the minima is expressed as a DA/DH (and even RA), always takes into account the sink rate factor that's going to occur if the Decision is made to GA AT or ABV the DA/DH (MA not initiated after/BLW).

That is not correct. You cannot initiate a GA at the DA. A 50 foot momentary descent was already calculated in when the DA was set. The pilot must know their own sink rate and initiate accordingly to avoid going below the DA. For most ac, the momentary descent is far more than 50 feet, given response time, configuration, and thrust settings.

[roulette]

Underfire:

I don't want to get into an endless debate here, but a DA does mean that the decision to GA must be made no lower than that Decision ALT, and the procedure design criteria allows for the fact that the aircraft will sink below that alt when performing the GA. Obviously this is different from an MDA for NPAs where the decision to GA must be made well prior so that the aircraft does not bust the Min Desc ALT.

Peekay4:

Thanks - I know it does happen on the odd occasion due to the specific set of circumstances and confluence of the terrain and obstacle environment. In this case based on what I see from the charts in this thread (without investigating in more detail, which I'll never find time to do) I still find it a little strange.
For example, I can see that the LOC only has two DFs, but it's only the one at D4.0 LCO that facilitates the MDA to be down to 820ft (the fix at D1.1 LOC is beyond the MDA on glide @ 3.1°), so I cannot see an obstacle prior to 4NM being the cause of the relatively high DAs for the ILS (even with a slightly lower GS angle of 3.0° - which is a difference of 0.175% of 10.6ft/NM (so the additional margin of 40ft over 4NM or 50ft over 5NM may be influential?)).

Considering the Cat II APCH has a minimum CG of 5% for the missed, and the NPAs (LOC only and VOR) both have MAPts ≥1NM prior to the THR, I suspect that it's actually obstacles in the early phase of the missed that could be influential on the Cat I ILS minima (even without raising the GS by 0.1°).
Of course without knowing the specifics I could be laughably wrong ?

Regardless, if this was in my area of control, I'd seriously look at the feasibility and possible safety benefits (not to mention perhaps better Cat I ILS minima) of redesigning and recalibrating all procedures and landing aids (ILS, PAPI) so that the GS of all approaches was consistent - even at 3.1°.

And I'll take a look at KRNO one day out of curiosity.

Cheers

[underfire]

You are correct, I had been confused with MDA. Sorry.

DA uses 50 foot momentary descent after the DA for the start of the climb profile. Of course, ac go from descent to climb at an angle!



The pilot needs to know that the DA includes only a 50 foot momentary descent. Most aircraft cannot go from approach config to GA and only descend 50 feet.

[aterpster]

Underfire:

The pilot needs to know that the DA includes only a 50 foot momentary descent. Most aircraft cannot go from approach config to GA and only descend 50 feet.

I think the airplanes that would go more than 50 feet below DA are the exception rather than the rule. I base this on the premise that the rule requires the decision to be complete at DA, not to start the assessment at DA. Having said that the only penalty for going below DA is an increased climb gradient for the early stage of the missed approach.

[EnxAero]

peekay4:
The hill 3km before the threshold is well below the protection surface of the approach. If that were the only obstacle, a CAT III approach will be possible.

The PATC may end at the 2000m mark, but the hill will still be there, at 3km.

So let's say that the you are flying an A320 and the RVR is 2200m. You should start the MA at a radio altimeter height of 514'. Three km before the threshold you fly over the hill and get a radio altimeter height of 460' (approx 840' baro alt) but the runway is not insight. What do you do? Do you say "my dme is 1.6 nm, I am not there yet, so I can keep going" or do you start the MA at 3 km from the threshold and therefore you would be better off by flying the ILS CAT I and putting this CAT II in the bin?
This was a bit the concern from my first post.
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[oggers]

The pilot needs to know that the DA includes only a 50 foot momentary descent. Most aircraft cannot go from approach config to GA and only descend 50 feet.

The design criteria for the approach and published minima are based on the certification requirements that the aircraft has met. Therefore ALL aircraft being operated 'normally' can achieve the minimum obstacle clearance on a go around initiated at the correct DA(H).

[EnxAero]

I have been told that the answer to my doubt is in the Appendix 1 to EUR-OPS 1.455 (b) 2. (ix), where it states the requirement for all height calls below 200 ft to be based on the radio altimeter.

So [I have been told that] even if Jeppesen published the chart as "CAT II", it seems it is still legal to perform the whole approach using the baro altimeter as a reference, since all the height calls before becoming visual are greater than 200 ft. [But I still have doubts about this]
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