Why uses RA instead of DH in ILS CAT 2 approach ?
 

Below is LOWW ILS 29 chart.
When shooting ILS 29 CAT 2 approach, we need to insert 97' as decision height in RADIO entry field of APPR panel in FMS, but, why don't we insert 100', which is DH, in stead of the RA 97' ? I mean what regulation (FAR, AIM, or ICAO something, whatever ) stands for that ? I've looked for it couples days but can't find it, and people just say "If there's RA, you insert RA, not DH, in the RADIO ". The RA xxx' only shows in CAT 2.


https://cimg7.ibsrv.net/gimg/pprune....6d4f2e8584.jpg
LOWW ILS 29 chart

The CAT 2 minimum is measured by the radio altimeter. As you are typically not above concrete runway surface yet when passing through 100 ft DH your charting provider has to calculate the RA minimum considering the (possibly uneven) terrain contour in front of the runway threshold. Depending on whether it is higher/lower than the runway threshold elevation it will show a RA value lower/higher than the DH.

And to give an extreme example have a look at the Luton (EGGW) Cat2 minima

If no inner marker is installed there must be an RA minimum which you are required to use for 100' Cat II. It is not permitted to use the baralt for this minimum unless there is an inner marker. Equally, if the inner marker is installed and operational there is no general prohibition on using the baralt to fly to the DH, but as with most of these questions, whatever is in your company manual is the law for you. Sorry I don't have the time to cite the regulatory basis right now.

Edit: EASA (part SPA) "The operator shall only conduct CAT II operations if the DH is determined by means of a radio altimeter"

https://cimg0.ibsrv.net/gimg/pprune....69f9fe25b0.jpg

66 - I asume you accept DA(H) as OCH? In that case, in the example given for LOWW, do you set 97 RA or 100?

"You should use RA for Cat II, so 97 ft.". But 97 is less than 100, is it not? Lost's example has RA > DA(H).

Set 97Ft RA.

At the point where you reach the DH, 97Ft RA, you will be 100Ft above the landing runway. (There is obviously a 3ft difference in the height of the ground at that point to the end of the runway, ARTE)

I hope that helps

That’s because EGGW is on a big hill and the ground falls away sharply at both ends of the runway, so 100’ ARTE 1/3 of a mile before the threshold is a lot more than 100’RA.

Roj - "At the point where you reach the DH, 97Ft RA, you will be 100Ft above the landing runway. (There is obviously a 3ft difference in the height of the ground at that point to the end of the runway, ARTE)

I hope that helps"

Not really - slightly irrelevant? It disregards the wording quoted by 66. I repeat, with emphasis:

A Category 2 decision height must be identified by reference to radio altitude and must not be less than
100ft arte or the OCH

What do you reckon the OCH is?

So, your 'quoted quote' is not legally binding, then? Un-confuse me.

AIP Austria chart LOWW AD 2.24-6-4 states that the OCH for a Cat C a/c conducting a Cat II ILS rwy 29 at LOWW is 91ft. Does this allay your worries?

Perhaps this picture can help clarify

https://images.app.goo.gl/5mEjCw7snK7uyYPo9

Thanks, Sepp - at least you understood.

Baro. alt. has a greater variation in true altitude, compared to rad. alt. The closer proximity to the ground for Cat II compared to Cat I means the approach design is predicated on the more accurate vertical positioning provided by rad. alt.

Many thanks all replies above !!
but my question just derives from the EASA regulation:
"The operator shall only conduct CAT II operations if the DH is determined by means of a radio altimeter"
Same words are in CAT 3 (as below), so the words can't solve my question.
https://cimg6.ibsrv.net/gimg/pprune....870c99927e.jpg
CAT 3 regulations in EASA


According to the LOWW ILS 29 chart, there are 3 numbers depicted in CAT 2:
1. RA 97'
2. DA 700'
3. DH 100'

We know how the RA 97' comes from ( just as mentioned above), and DA 700' is advisory only which is applicable in some exceptional cases,
but, how about the DH100' ?
Why can't we setup 100' in FMS radio altitude field then rely on radio altimeter callout ?
If I do that, do I violate any regulation, expectation or something? fail in checkride ? <-- This is my real question.


Initially I thought it is about provider's legend, because there's no mention about RA 97' in LOWW's officially chart.
https://eaip.austrocontrol.at/lo/200..._24-6-4_en.pdf
https://cimg7.ibsrv.net/gimg/pprune....a5d02a7851.jpg
LOWW 29 profile and minimums
( It also shows another question about the OCH for Category C in CAT 2 is 91' some guy mentions above. )


So I check the JEPPESEN's legend, but only get the words "Radio Altimeter height, associated with CAT 2 precision approaches". That's all, no mention about what I'm looking for like "RA numbers are prior to DH numbers" or "DH numbers in the chart is nothing if RA numbers are published" at all.
https://cimg6.ibsrv.net/gimg/pprune....c3cd906d8d.jpg
Jeppesen's label about DA(H) and RA


Then I google it, and find the story why RA numbers depicted in CAT 2 approach chart.
https://cimg1.ibsrv.net/gimg/pprune....95b6b5760f.jpg
FAA recommendation document 1/2
https://cimg6.ibsrv.net/gimg/pprune....03295e45c0.jpg
FAA recommendation document 2/2


====
I summarize my finding and question:

In old time, CAT 2 approach uses radio altimeter and the associated DH depicted in the chart, but since the format is easy to confuse pilot to use DA, men in FAA suggest to change CAT 2 minima depiction to what we see now, a RA XX' been added.

Here is my question: The DH is still depicted in the chart, not been erased, so if no regulation mentions which has priority, or DH is advisory only, or something like that, how can we say RA is prior to DH to been used ? EASA regulation says only DH, never RA.

PS.
My company's manuals quote the words "DH is determined by radio altimeter" only, no mention about RA. But some peaky checkman could ask me this question someday.

All those 3 represent the exact same point on the glideslope. With perfect equipment you could use 700 QNH, 100 QFE, or 97 AGL. But the baralt is not accurate enough for these low minimums, therefore the reference you use is the radalt which happens to be 97 feet at that point on the glideslope. As for what goes into the kit, your aircraft also needs to have the correct reference as determined by the approval process. As for regulation, in the USA at least, you can in theory use an inner marker instead of the radalt - which would be the case if you saw "RA NA" (not authorised), but not in Europe.

...using the published RA puts you at the DH. That is what it means.