PDA

View Full Version : CAT II with DH>200ft?


EnxAero
28th Aug 2015, 22:01
Hi,

Im getting confused with a CAT II chart that I have just found. This airport in Spain (EU) used to have a CAT II approach. Recently, there have been a few changes to the runway and also new obstacles have appeared, so the charts have been updated.

It shocks me to see this ILS CAT II with DH greater than 200' (between 233' and 276') . How is this possible? I thought both in the US and in EU CAT II meant 100'<DH<200'.

This implies that the pilot must get visual contact when s/he is around 1km from the threshold, so at that point there is a valley that is more than 240' deep and RAs are around 500'; is the radioaltimeter reading accurate in that case?

https://dl.dropboxusercontent.com/u/79211130/CATII.JPG

Many thanks
EnxAero
P.S. I am not a pilot, but an airport engineer, so I would move the thread to a different forum if necessary.
308331336

Astra driver
28th Aug 2015, 22:16
Higher than typical Cat II minimums are due to the missed approach climb requirements; if you notice A/C that can perform a 5% missed approach climb gradient are allowed to descend lower Vs those that can only make 2.5%

Citation2
28th Aug 2015, 22:19
From Eu OPS: "Where the decision height (DH) and runway visual range (RVR) do not fall within the same category, the RVR will determine in which category the operation is to be considered."

DH could be limited to a higher height due higher OCH , however RVR is always the governing factor to determine the ILS category.

inbalance
28th Aug 2015, 23:26
http://static.panoramio.com/photos/large/2887839.jpg

EnxAero
28th Aug 2015, 23:56
Astra driver,
Indeed. One the modifications was the displacement of the threshold, so now the beginning of the runway is closer to the most critical obstacle in the missing approach. My surprise comes from the fact that even with a 5% gradient the DH > 200 ft.

Citation2,
I have found the sentence you mention in the context of determining whether it is CAT IIIA or CATIIIB.
However, for CAT II the definition seems to be:

‘Category II (CAT II) operation’ means a precision instrument approach and landing operation using ILS or MLS with:
(a) DH below 200 ft but not lower than 100 ft; and
(b) RVR of not less than 300 m.

http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2008:254:0001:0238:EN:PDF

Condition (b) is met, (a) is not.

Furthermore, since there are only 420 m of lights, I guess at least 700 m of RVR would be required.
253257563

RAT 5
29th Aug 2015, 09:08
Is there a CAT 1 ILS on this runway? If so what is the DA & RVR? The only parameter that meets CAT 2 is <550m RVR. It is an odd ball. CAT 2 ground installations have other specifications than a CAT 1. It seems a lot of bother for 100m reduction in RVR. However, the CAT 1 RVR may be much higher than 550m.

CaptainProp
29th Aug 2015, 11:13
As RAT 5 suspected, CAT I RVR varies between 1300 and 2400m depending on aircraft category and missed approach climb gradient.

CP

EnxAero
29th Aug 2015, 11:36
RAT5,
There is a CAT I equipment with DH ~550 ft. That is a funny one as well: for categories B, C & D the DA of the LOC approach is lower than with ILS CAT I. (In fact the ILS CAT I DA is even higher than the DA of the VOR approach).

I have no idea where the RVR of 450 m for the CAT II is coming from, as this is clearly not enough: When an aircraft reaches the DH of 233' (best case scenario for a category A aircraft), the distance to the threshold is (233-54)*0.3048 / tan(3º)= 1041 m.
Since there are only 420 m of lights before the threshold (IALS), even a RVR of 550m would not be enough, since 420+550= 970 < 1041
https://www.dropbox.com/s/4n2cb1shulz8o5c/CATI.jpg?dl=0https://www.dropbox.com/s/4n2cb1shulz8o5c/CATI.jpg?dl=0http://https://dl.dropboxusercontent.com/u/79211130/CATI.jpg
https://www.dropbox.com/s/6izbj709x632b0j/vor.jpg?dl=0
442452456https://dl.dropboxusercontent.com/u/79211130/vor.jpg460464

EnxAero
29th Aug 2015, 20:41
Having had a careful look at the AIP I believe this should be a “Lower than Standard Category I Operation” (that is "A Category I Instrument Approach and Landing Operation using Category I DH, with an RVR lower than would normally be associated with the applicable DH."). The aircrafts would need to be approved for CAT II and capable of a climb gradient of 5%, but the DH would be determined by baro altitude.

But let's say that I am wrong and this is a CAT II. There is no inner marker and the RA for a category C aircraft is 514'. Are GAs going to be performed the first time the radio altimeter reaches 514', which should be ~1.6 nm (~3 km) before the threshold? If that is the case the ILS CAT I and even the VOR procedure would let aircrafts get a bit closer to the threshold.

One final doubt: If the aircraft is capable of only a missed approach climb gradient of 2.5%, when do we start the GA? There is no RA for that case.

436264

No Fly Zone
29th Aug 2015, 21:54
Sir,
I'm not able to identify the location of this image, but what a joy it is to view. It looks much like SW Germany. If you don't want to share the details in public, a PM would be sincerely appreciated. What a beautiful place it is! Standing by for PM if that is your choice. Thank you. NFZ:D

EnxAero
29th Aug 2015, 22:08
@No FLy Zone
The pic is from the NW of Spain. That is the view from the cockpit approaching the runway 21 of LECO (A Coruña, Spain), the airport whose charts are mentioned in the thread. The pic shows runway 22, but it was renamed a few years ago.
162166170174

peekay4
1st Sep 2015, 03:38
What I can't get my head around though is the increased Baro Alt minima for a regular CAT I approach, for which the decision point is not determined by reference to the Rad Alt...
That typically means there is an obstacle somewhere along the ILS flight path, at an altitude below the LOC MDA but penetrates the protection surface of the 3.00 degree ILS.

Remember that while the LOC MDA is a "hard deck", the ILS has to provide protection even if the aircraft is flown below the glide slope (up to full deflection), and below the DA when a missed approach is initiated. So near the runway the ILS protection is actually below the MDA in this case.

Additionally, the ILS design has to take into account that a real-life glideslope doesn't precisely signal the ideal flight path but may have some errors up and down due to reflections, etc.

When there are obstacles penetrating the ILS surface, the two basic choices are: 1) raise the DA(H); and/or 2) raise the glideslope angle beyond 3.00 degrees.

Derfred
1st Sep 2015, 14:03
Quote:
Remember that while the LOC MDA is a "hard deck", the ILS has to provide protection even if the aircraft is flown below the glide slope (up to full deflection), and below the DA when a missed approach is initiated.

Hi FlyerGuy, sorry if this is thread drift but your quote in bold is interesting to me... This has been the source of several discusions I've had with no resolution. Does the ILS really guarantee protection in the missed approach if the missed approach is initiated below DA? What about engine out? And do you have a reference for this? Cheers.

EnxAero
1st Sep 2015, 21:23
If the missed approach were initiated below the DA the protection in the missed approach would be reduced, as you are starting the segment lower and closer to the obstacle.

I guess what peekay4 means is that if you started the missed approach at precisely DA, the aircraft still loses a bit of altitude until you it starts to climb, and it goes slightly below DA. Those few ft are taken into account and the aircraft is protected.
7867

EnxAero
1st Sep 2015, 21:36
As a quick graph to give some clarity, I have drawn a few lines over the obstacle chart (from the AIP). The original numbers in black are in meters. My numbers in purple are in ft. The horizontal axis is distance in meters to the end of the runway (so 2188 is the threshold)
https://dl.dropboxusercontent.com/u/79211130/profile2.jpghttps://www.dropbox.com/s/agkjmfs9g31ypnj/profile2.jpg?dl=0http://

Because of the deep valley (~260'), the radio altimeter height around the DH point is higher than when flying over the hill
9397

peekay4
2nd Sep 2015, 00:32
Hi EnxAero I'm not sure that is the right chart to use. That "Type A" chart is mainly used for take-offs. So that particular chart is actually for RWY 3 departures instead of RWY 21 arrivals. The maximum heights shown on that chart may (or may not) actually be on the relatively narrow localizer path.

There is another ICAO chart called the "Precision Approach Terrain Chart" for use with CAT II and III approaches.

This "PATC" chart is used to asses any terrain which may impact the Radio Altimeter, but only to a maximum distance of 2000 meters. So basically, anything over 2000 meters gets ignored.

PATC for LECO RWY 21: http://www.aena.es/csee/ccurl/341/104/LE_AD_2_LECO_PATC_en.pdf.

underfire
2nd Sep 2015, 02:52
If the missed approach is initiated below the DA the protection in the missed approach is reduced

ummmm....MA below the DA?

EnxAero
2nd Sep 2015, 08:48
peekay4,
In general you are right, but for this particular case it more or less works, as in both cases the profile is done along a straight line.

http://www.enaire.es/csee/Satellite?blobcol=urldata&blobheader=application%2Fpdf&blobheadername1=Content-Disposition&blobheadervalue1=attachment%3B+filename%3D28%2F527%2FLE_AD_2 .pdf&blobkey=id&blobnocache=true&blobtable=MungoBlobs&blobwhere=1237568177691&ssbinary=true

AOC is not as precise, and the horizontal distances are to the threshold of runway 21, but it was a schematic drawing to give people an idea of the terrain before the threshold.

The "issue" with the PATC in this case is that, although it shows the deep valley at around 1000 m for threshold, it does not show the peak of the hill 3000 m before. 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.


4049515661

EnxAero
2nd Sep 2015, 08:56
ummmm....MA below the DA? Sorry underfire, I was trying to reply Derfred hypothetical scenario.

My sentence should have read:
"If the missed approach were initiated below the DA the protection in the missed approach would be reduced"

Clearly, that shouldn't be done.

I have edited my reply. Sorry for my mistakes with English grammar.
93100102

roulette
2nd Sep 2015, 09:47
Re Peeka4y's response:Quote:
Remember that while the LOC MDA is a "hard deck", the ILS has to provide protection even if the aircraft is flown below the glide slope (up to full deflection), and below the DA when a missed approach is initiated.

It's still rather strange that the ILS Cat I DAs for Cat BCD are higher than the LOC Only MDA!
Perhaps if they lopped off the top of some of those trees in the final segment (as shown in the "Southern Germany" lookalike pic) the CRM analysis for determining the minima for the precision approach? Or, perhaps those who designed the approaches only used the OAS (Obstacle Assessment Surfaces) method and didn't do a CRM (Collision Risk Method) analysis, where the latter may in fact give lower DAs that are equally safe.


Re EnxAero's response to Defred:Quote:
I guess what peekay4 means is that if you started the missed approach at precisely DA, the aircraft still loses a bit of altitude until you it starts to climb, and it goes slightly below DA. Those few ft are taken into account and the aircraft is protected.
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).
And so as EnxAero indicated in one reply, in such cases the design provides protection in the missed approach (for normal ops ... ie, not necessarily EO conditions).

Re the Cat II APCH and the relatively unusual large RAs due to the valley in the 1k are prior to the THR - yup, it is a bit special! Especially if the RA at around 1.6NM (3k) is less than the RA at the 0.5NM (1k) mark!
Possibly that's why that Cat II APCH is a special authorisation required (ref chart below) procedure?

Hmmm, interesting diversion.

peekay4
2nd Sep 2015, 23:49
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 (http://155.178.201.160/d-tpp/1509/00346I16R.PDF) at 2031 ft AGL is a whopping 460 ft (!) higher than the LOC MDA (http://155.178.201.160/d-tpp/1509/00346L16R.PDF).

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
3rd Sep 2015, 05:29
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
3rd Sep 2015, 06:36
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 :O?

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
3rd Sep 2015, 08:05
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!

http://i59.tinypic.com/jrfz9z.jpg

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
3rd Sep 2015, 14:26
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
4th Sep 2015, 00:56
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.
118291

oggers
4th Sep 2015, 14:00
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
15th Sep 2015, 20:28
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]
114122130299