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DH in MDA field for non precision approaches in A320

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DH in MDA field for non precision approaches in A320

Old 14th May 2020, 06:27
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DH in MDA field for non precision approaches in A320

Hello,

can someone please explain the reason why it is necessary to put the decision height in the MDA field when using a non precision approach (e.g. RNAV)? I think the chosen value is then called DA instead of DH.
I do understand the reason behind DH and MDA and why to use DH on Cat III and MDA on Cat I landings. However I do not clearly understand the reason for taking the DH for the MDA field for an RNAV approach.

Thank you!

Best regards Andi
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Old 14th May 2020, 10:22
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Hmm. You do not fly a Cat I to an MDA you fly to a DA or DH. Cat I is a precision approach, MDAs are for non-precision. Not sure where you got this idea from.

When you ask 'why put a DH in an MDA box', I think you might be referring to one of two things. It could be the MDH which has always been there for those operating on QFE. Or it could be the DA/H for a Continuous Descent Final Approach (CDFA has always been an option for NPAs but the difference here is how you treat the minima). That is my answer.

It used to be that precision approaches were all flown to DA/H, and non-precision approaches to MDA/H. Precision approaches were ILS mainly plus MLS and PAR. Non-precision was everything else. However, in this century the powers-that-be have really pushed the CDFA over the 'dive and drive', which has led to the application of the DA/H concept to the non-precision minima. Also, GPS has bridged the gap in 'precision'. All good of course but the jargon is now quite confusing.
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Old 14th May 2020, 11:13
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Ok..then I definitely misunderstood something here. Just to make sure that I understood it correct now.

Precision Approaches (e.g. ILS):
DA/DH is used
Minima can be underflown slightly
Named Height (below 200feet; usually for Cat II and III)
Named Altitude (above 200 feet; usually Cat I)

Non Precision Approaches (e.g. RNAV, VOR, NDB):
MDA/MDH is used
Minima can not be underflown!
Named Height (below 200feet)
Named Altitude (above 200 feet)

One question....if the set minimum is 150feet (either DH or MDH) will the "minimums" call out come from the radio altimeter measurement?
And if the set minimum is 300 feet will the callout come from the barometric altimeter?
Or will it always be the barometric altimeter, which is responsible for minimums callout?

Thank you!
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Old 14th May 2020, 14:39
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Sorry it's probably my fault for not explaining in detail.

Firstly Cat I is altimeter (it could either be a DA or DH depending on whether QNH or QFE is set). Cat II/III is radalt. I will try and clarify the rest by editing your post:

Precision Approaches (e.g. ILS):
DA/DH is used [yes always]
Minima can be underflown slightly [I know what you are saying so yes basically. In a geometrical sense you dip below the minima if you make the decision at the DA itself but in a legal sense you have not busted it because the requirement is to make the decision and intiate the missed, and of course we never bust minima]
Named Height (below 200feet; usually for Cat II and III) [yes]
Named Altitude (above 200 feet; usually Cat I) [minima may be down to 200' above TDZE either height or altitude, but higher is not uncommon]

Non Precision Approaches (e.g. RNAV, VOR, NDB):
MDA/MDH is used
Minima can not be underflown! [Basically correct. It used to be always be the case but now certain operators may use the MDA/H as a DA/H. However, you know if that applies to you because it is in the Ops Manual and you will have been trained for it. The general procedure now is to fly to 50' above the minima and use that as a DA/H (referred to as a Derived DA, DDA) and thereby still not dip below the MDA/H. Hope that makes sense]
Named Height (below 200feet) [No. There are no NPAs with minima below 250', certainly that used to be the case with VOR/DME and SRA (non-precision radar) sometimes getting you that low. I am not 100% sure that with all the new developments and re-purposing of terminology that something which is not technically a "precision approach" could have a lower minima than 250' but that is a grey area that should not distract from the big picture.
Named Altitude (above 200 feet) [see above]
The disitinction in all this is not whether you fly to a height or altitude. You can use either for both precision and non-precision approaches (the exception being Cat II/III is radalt height only).
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Old 14th May 2020, 18:07
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Anything put in the DH field is referenced to the Radio Altimeter (RA), anything put in the MDA box is referenced to the baro altimeter. The RA shows the height above the ground, directly measured, it requires a specific terrain profile to be usable. Since higher decision altitudes are further out, they are usually outside the airport perimeter and therefore not under the control of the authority, which means they are not controlled and can be altered, and the terrain is usually not completely flat from the runway out to that point.

The baro altimeter is independent of the ground beneath the airplane, but is less precise, which is OK for higher minima, but not at all for CAT II/III operation.

The question whether we fly a DA or MDA is independent of the used reference. And for quite some time now even non-precision approaches, at least in EASAland, are flown to a DA, not a MDA anymore. So we usually just use the DA system in europe, in other legislations it is different. With a DA the decision to continue or go around has to be taken latest at the DA and in case a go around is needed the first action should come simultaneously, however, the aircraft will of course descend slightly below the DA before it starts to climb away.

With the MDA you are not allowed to descend below the MDA without the required visual reference, which means either adding a certain amount of altitude to the MDA if you want to fly it as DA-style CDFA (Continuous Descent Final Approach), or levelling off at the MDA.
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Old 14th May 2020, 19:47
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Originally Posted by 737Andi View Post
can someone please explain the reason why it is necessary to put the decision height in the MDA field when using a non precision approach (e.g. RNAV)? I think the chosen value is then called DA instead of DH.
I do understand the reason behind DH and MDA and why to use DH on Cat III and MDA on Cat I landings. However I do not clearly understand the reason for taking the DH for the MDA field for an RNAV approach.
Hello again, Andi. Please do not forget to add every now and then, what is the reason behind the question. I.e. what airplane you fly.

The old naming in the Airbus FMS as displayed also on the PFD FMA is "MDA" and "DH". Newer machines say "BARO" and "RA" and that is miles easier to understand. RA is used only for CAT II and III approaches, based on radio-altimeter. All the other possibilities are using BARO values, DA or MDA (assuming you do not operate using QFE). The naming convention of minima used for CDFA technique during NPAs (RNAV being one of them) gets a little confusing. Let's not go there just yet, the core behind your dilemma seems to be elsewhere.

Check this FAA publication, I am sure you'll find a great number of carefully explained topics there:
https://www.faa.gov/regulations_poli...aviation/phak/
https://www.faa.gov/regulations_poli...t_handbook.pdf

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Old 15th May 2020, 07:30
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I think I got it now

Only this is confusing me....
Originally Posted by Denti View Post
Anything put in the DH field is referenced to the Radio Altimeter (RA), anything put in the MDA box is referenced to the baro altimeter.
I thought for Cat I approaches, which are precision approaches, we are setting the DA value (assumed we use QNH) into the DH field of the Perf page. And Cat I is baro altimeter in contrast to Cat II/III, which are radio altimeter.

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Old 15th May 2020, 08:25
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Using QNH (and avoiding confusing terminology for CDFA 2D approaches!):

On the new Airbus kit, you set the DH into the RA field for CAT II/III.
On the new Airbus kit, you set the DA into the BARO field for CAT I.
On the new Airbus kit, you set the MDA into the BARO field for non-precision approaches such as VOR-DME or NDB.

Read further only if perfectly clear.

On the older Airbus models, the RA field is called DH and that is where you set DH for CAT II/III.
On the older Airbus models, the BARO field is called MDA and that is where you set DA for CAT I.
On the older Airbus models, the BARO field is called MDA and that is where you set MDA for non-precision approaches.

Why did Airbus originally decided to call the BARO field MDA? Good question. Maybe DA/MDA was too long, or (M)DA confusing. Nevertheless that is where you put DA for CAT I ILS. There is no DA prompt at all on Airbus, old and new.
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Old 15th May 2020, 09:31
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Everthing is perfectly clear now...thank you guys!
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