Why uses RA instead of DH in ILS CAT 2 approach ?
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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.
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.
LOWW ILS 29 chart
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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.
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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 ?
Edit: EASA (part SPA) "The operator shall only conduct CAT II operations if the DH is determined by means of a radio altimeter"
Last edited by oggers; 11th Aug 2020 at 17:02.
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
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?
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?
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?
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?
Last edited by Sepp; 13th Aug 2020 at 09:07. Reason: Missed the "I" out of "AIP"!
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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.
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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.
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
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.
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.
FAA recommendation document 1/2
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.
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.
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
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.
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.
FAA recommendation document 1/2
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.
Last edited by LegiossTypeH; 15th Aug 2020 at 07:29.
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According to the LOWW ILS 29 chart, there are 3 numbers for CAT 2:
1. RA 97'
2. DA 700'
3. DH 100'
1. RA 97'
2. DA 700'
3. DH 100'
My company's manuals quote the words "DH is determined by radio altimeter" only, no mention about RA.