DA vs DH Confusion

RotorDude said:
So as bottom line, conceptually pilots have always flown the ILS approach in terms of height above threshold, since that was the idea of 200 and 1/2, but had to translate the 200 into some value that showed up on the altimeter at minimums, referenced to sea level. Now with DA, there is less potential for confusion.
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Well, conceptually, not "always". Before the concept of DH you may recall that landing minimums were specified in terms of ceiling and visibility. In 1967 they coined MDA & DH. Until approach charts were all updated, MDA & DH were derived by adding the ceiling minimum (AGL) to the airport elevation, thus the result was an MSL value. The definition of DH is defined in AC 90-1A, effective 4/10/68, as "...expressed in feet above mean sea level (MSL)". On the other hand, CAT II DH is "..expressed as a radio altimeter setting".

EDIT: FWIW an ILS, being a straight-in approach, uses HAT where the "T" stands for "touchdown zone" not "threshold".

You're welcome.

dtuuri
 
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dtuuri said:
Well, conceptually, not "always". Before the concept of DH you may recall that landing minimums were specified in terms of ceiling and visibility. In 1967 they coined MDA & DH. Until approach charts were all updated, MDA & DH were derived by adding the ceiling minimum (AGL) to the airport elevation, thus the result was an MSL value. The definition of DH is defined in AC 90-1A, effective 4/10/68, as "...expressed in feet above mean sea level (MSL)". On the other hand, CAT II DH is "..expressed as a radio altimeter setting".

EDIT: FWIW an ILS, being a straight-in approach, uses HAT where the "T" stands for "touchdown zone" not "threshold".

You're welcome.

dtuuri
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Sounds right.
I think the key here is the semantic difference between "Decision Height" and ""Minimum Descent Altitude". Government lawyers (and lawyers in general) use words carefully, so the question is (pre "Decision Altitude", which is a modern term) why make the distinction between "height" and "altitude"?
The way textbooks at the time (70's) explained it, is that "altitude" conveys a general flight level, as in most phases of flight, and refers to the level flight during which you look for the runway, and you execute the MAP if you can't find it by a certain point (or time). OTOH, "decision height" introduces two distinct concepts relative to MDA. One is "Decision": you don't linger at an altitude waiting for the runway to appear, but must make an instant DECISION as soon as you get to the specified point. That point is defined as a "height" because it's conceptually referenced to the height along the glide slope above its end point, i.e. the runway elevation (originally touchdown zone), although practically for most pilots that would rely on a sea level referenced barometric altimeter reading.
Because the above is inherently confusing, the DA concept was recently introduced to help clarify the distinction, so now DH is strictly "feet above threshold" and DA is strictly "feet above sea level", both at the "decision" point.
 
At the risk of being too picky, but for the students' sake, from TERPS (my em):

Section 3-2. Establishing Minimum Altitudes/Heights

3-2 -1. Establish minimum altitudes/heights for each authorized approach category.
Minimum altitudes/height types are:

a. Decision altitude (DA). A DA is a specified minimum altitude (feet MSL) in a PA or
APV instrument approach procedure at which a decision is made to either continue the approach
or to initiate a missed approach. Determine the DA using the appropriate criteria and specify in a
one-foot increment (for example, 234.10 rounds to 235).

b. Decision height (DH). A DH serves the same purpose as a DA for CAT II ILS, but is
expressed as a radio altimeter height above terrain. ​

So DH is the same as in 1967, but now only with respect to CAT II ILS procedures and is still above terrain or as defined in Part 1.1 "above ground", i.e., straight below the aircraft not as you said "feet above threshold".

FWIW, the formula for DA from TERPS is (Formula 10-2-10.):
DA = HAT + TDZE​

So, the word "threshold" doesn't technically apply in either definition. I think you're confusing Threshold crossing height (TCH) with TDZE.

dtuuri
 
RotorDude said:
The current FAA glossary says:
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Ah, but that's an ICAO definition. I guess their CAT II radio altimeters can project ahead to the threshold? :rolleyes: Anyway, IMO, I'd say Part 1.1 trumps it:

Decision height (DH) is a specified height above the ground in an instrument approach procedure at which the pilot must decide whether to initiate an immediate missed approach if the pilot does not see the required visual reference, or to continue the approach. Decision height is expressed in feet above ground level.​

dtuuri
 
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dtuuri said:
Ah, but that's an ICAO definition. I guess their CAT II radio altimeters can project ahead to the threshold? Anyway, IMO, I'd say Part 1.1 trumps it:

Decision height (DH) is a specified height above the ground in an instrument approach procedure at which the pilot must decide whether to initiate an immediate missed approach if the pilot does not see the required visual reference, or to continue the approach. Decision height is expressed in feet above ground level.​

dtuuri
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It might be ICAO, but it is part of the FAA's current official glossary. I agree that "above the ground" is somewhat contradictory to the glossary, in that the ground location is unspecified, given that for non radar altimeter equipped aircraft it has always meant "above the runway" (touchdown or threshold), whereas simply stated "AGL" could mean above the ground at the current aircraft location, which of course could be way off the HAT.
 
The FAA glossary is incorrect. There was a brief period where the FAA changed from DA being referenced to TDZ elevation to, instead threshold elevation, but it was determined that was not in compliance with FAR 91.175 l (4), so they reverted to TDZ. Most DAs have since been converted back to being predicated on TDZ elevation, but there are still some to be changed back.
 
RotorDude said:
It might be ICAO, but it is part of the FAA's current official glossary. I agree that "above the ground" is somewhat contradictory to the glossary, in that the ground location is unspecified, given that for non radar altimeter equipped aircraft it has always meant "above the runway" (touchdown or threshold), whereas simply stated "AGL" could mean above the ground at the current aircraft location, which of course could be way off the HAT.
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B,b,but... don't CAT II approaches require a radio altimeter? Since DH only applies to CAT II approaches (CAT III also, I assume), the ICAO definition makes no sense applied to us (US). The non-ICAO definition of DH in the glossary, OTOH, does makes sense.

dtuuri
 
Again, a radar altimeter minimum is specifically called out as a RADAR ALTITUDE (RA) on the plate. It is not a DH. There are a few CAT II's that do have Baro DH's that are not radar altitudes.
 
flyingron said:
Again, a radar altimeter minimum is specifically called out as a RADAR ALTITUDE (RA) on the plate. It is not a DH. There are a few CAT II's that do have Baro DH's that are not radar altitudes.
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Correct, places with a irregular terrain prior to, and usually lower than the threshold. KPIT 10R comes to mind.

Also, a fully fail-operational CAT III approach uses Alert Height (AH).
 
aterpster said:
The FAA glossary is incorrect. There was a brief period where the FAA changed from DA being referenced to TDZ elevation to, instead threshold elevation, but it was determined that was not in compliance with FAR 91.175 l (4), so they reverted to TDZ. Most DAs have since been converted back to being predicated on TDZ elevation, but there are still some to be changed back.
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I guess this is one of those situations where your avatar is very apt. :)
 
flyingron said:
Again, a radar altimeter minimum is specifically called out as a RADAR ALTITUDE (RA) on the plate. It is not a DH. There are a few CAT II's that do have Baro DH's that are not radar altitudes.
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It might be called an RA, as opposed to baro or inner marker, but isn't it still a DH? I must confess, I have never been CAT II qualified nor much interested in it because the expense and complexity seemed not worth the advantage.

dtuuri
 
Interestingly, the current legend for the FAA approach plates lists both "DA" and "DH" in the list of abbreviations (no definitions given) (pdf page 7). The ILS minimums in MSL are labeled "DA" and the minimums in AGL are labeled "HAT". "DH" does not appear anywhere at all in the entire front matter except the aforementioned abbreviation list.
 
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aterpster said:
Correct, places with a irregular terrain prior to, and usually lower than the threshold. KPIT 10R comes to mind..
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KPIT 10R specifies a RA of 100 feet, DA 1235, and TDZE of 1135? So that doesn't seem to be an example of the discrepancy @flyingron brought up.
 
dtuuri said:
It might be called an RA, as opposed to baro or inner marker, but isn't it still a DH? I must confess, I have never been CAT II qualified nor much interested in it because the expense and complexity seemed not worth the advantage.

dtuuri
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At my shop we call all minimums an MDA except for CATII approaches which we call DH. Or if RA is not authorized, we call CATII minimums an MDA
 
jordane93 said:
At my shop we call all minimums an MDA except for CATII approaches which we call DH. Or if RA is not authorized, we call CATII minimums an MDA
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Maybe you mean DA?
MDA is non-precision only, AFAIK. Or am I losing it?
 
jordane93 said:
At my shop we call all minimums an MDA except for CATII approaches which we call DH. Or if RA is not authorized, we call CATII minimums an MDA
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Do I have this right? You call DA an MDA? Call an RA a DH? If you can't call an RA a DH, then it's called MDA? :confused: EDIT: I think I got it.

dtuuri
 
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dtuuri said:
Do I have this right? You call DA an MDA? Call an RA a DH? If you can't call an RA a DH, then it's called MDA? :confused: EDIT: I think I got it.

dtuuri
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Every approach ILS (CAT I), VOR, RNAV we call an MDA. Every CAT II is a DH unless the plate says "RA Not Authorized"
 
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jordane93 said:
Every approach ILS (CAT I), VOR, RNAV we call an MDA. Every CAT II is a DH unless the plane says "RA Not Authorized"
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Yes. Same at my shop. Which is why I posted my original post as I did, for simplicities sake. No reason for a Private Pilot getting his Instrument to get wrapped up in all this minutia that they will not use. You'll get the appropriate training to use it when you fly something that has the capability. In fairness though they are correct - It is utilized the way you are saying because that is how the Air Data Reference panel is set up on the CRJ.
 
jordane93 said:
Every approach ILS (CAT I), VOR, RNAV we call an MDA. Every CAT II is a DH unless the plate says "RA Not Authorized"
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I can see their rationale. They decided that "altitude" is barometric MSL, whether on the GS or not, while "height" is a direct measurement of feet above TDZ (although it could be off before the runway). So from their perspective, everything non-RA is "non-precision", since it's based on a barometer.
I guess if you are big enough, you can afford to redefine the language to your own liking. :)
 
jordane93 said:
Every approach ILS (CAT I), VOR, RNAV we call an MDA. Every CAT II is a DH unless the plate says "RA Not Authorized"
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So do you add feet to the DA to come up with the MDA or what? How do you account for altitude lost during execution of a missed approach at the DA? I don't get it.
 
You round up DA but not because of altitude lost but because the Air Data Reference panel is only capable of inputting digits to the 10s.

He's saying MDA because there are two selections on the ADR panel - MDA and DH, so while it is charted one way it can only be input into the computer system one way also.

DA is a point in space that a decision needs to be made and has nothing to do with altitude lost after making that decision. Wheels touching during a go around is not only acceptable but expected.
 

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jaybee said:
DA is a point in space that a decision needs to be made and has nothing to do with altitude lost after making that decision. Wheels touching during a go around is not only acceptable but expected.
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That's exactly my point. He said they don't use DAs for category 1, but use it as an MDA. You can't go below an MDA, period. So they must have to add a cushion to it?
 
dmspilot said:
So do you add feet to the DA to come up with the MDA or what? How do you account for altitude lost during execution of a missed approach at the DA? I don't get it.
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It's just the name. Minimums are the same. What you would call a DA for an ILS, we just call MDA
 
dmspilot said:
That's exactly my point. He said they don't use DAs for category 1, but use it as an MDA. You can't go below an MDA, period. So they must have to add a cushion to it?
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Its still a DA, it can only be input into the system as a MDA.
 
This is why understanding the concept and why is more important than the terminology used and also why these pedantic arguments are useless.

Its like being "Dead Right" about who had right of way on traffic - I'd rather be alive then right...

Fly a CRJ and this is their terminology, fly something else or at a different airline could be different. I don't give a crap what its called, when I get to that altitude I'm going around or landing.
 
Well, 50 years ago the FAA decided that the concepts were different enough to warrant different names, and, with the exception of the altitude vs. height debate in another thread, they pretty much mean exactly what you'd expect from plain English. Shame on the CRJ manufacturer for mucking it up.
 
dmspilot said:
Well, 50 years ago the FAA decided that the concepts were different enough to warrant different names, and, with the exception of the altitude vs. height debate in another thread, they pretty much mean exactly what you'd expect from plain English. Shame on the CRJ manufacturer for mucking it up.
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It's those darn French Canadian engineers.
 
dmspilot said:
Nope, it's not a "bug". The only bug is your memory.

In fact, the FAA has still not updated some of its publications with the newer terminology. The current instrument rating computer test supplement still has an older approach chart legend in it. The DH arrow points to the MSL altitude.

View attachment 52750
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I'm with you. I learned to fly instruments in the AF in the early 90s, and DH was always in feet MSL. In fact, I was an AF instructor in the late 90s, and we taught DH as MSL. It was in all our manuals as such and also the FAA manuals. It wasn't until I got to the airlines that the switch was made to calling the MSL altitude a "DA."
 
dmspilot said:
You can't go below an MDA, period. So they must have to add a cushion to it?
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Not entirely true. Not to make this discussion go even more into the weeds, but there are times when (using the right Ops Spec) that you can treat the MDA like a DA and descend below it, even if momentarily while executing a missed.
 
dmspilot said:
KPIT 10R specifies a RA of 100 feet, DA 1235, and TDZE of 1135? So that doesn't seem to be an example of the discrepancy @flyingron brought up.
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Not until you look at the Jepp chart and the FAA source documents. Sort of scary, isn't it.
 

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dmspilot said:
Well, 50 years ago the FAA decided that the concepts were different enough to warrant different names, and, with the exception of the altitude vs. height debate in another thread, they pretty much mean exactly what you'd expect from plain English. Shame on the CRJ manufacturer for mucking it up.
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Eh, I find it a lot simpler personally. As I outlined in my first post - just forget the concept of DH until you are flying a CAT2 capable aircraft. As you said different concept, different names. So in my KISS world - DA=Cat1, DH=Cat2.

A "DA" still is an "MDA" the only difference is when you make your land/go around decision. On a DA you make that decision when you get to that altitude (which with vertical guidance is coincidentally a VDP if you think about it...), on a MDA you also make that decision when you get to that altitude and a predetermined point such as a VDP or as we use at my shop a PDP (which coincidentally if you flew a constant descent angle...). In either case, its briefed beforehand and already decided before you make the approach. So its like this - you set up your cockpit with the "MDA" in the window and you brief your approach - Precision: we will descend to 230ft, if we don't see the runway we'll commence the missed or Non Precision: we will descend to 600 feet and upon reaching 2.5 DME if we don't see the runway we will commence the missed. So in both cases they are both minimum descent altitudes...

As said earlier, its not that the pedantics are wrong by any means. I consider it more of a failure of the FAA for over teaching to the Private Pilot. You are not going to fly a CAT 2 approach unless the aircraft is equipped with a certain set of equipment and as a pilot you won't fly that aircraft until you've had training on that equipment - so why are we teaching a PP spam can driver about stuff they won't even use. I know the argument is "you might someday" - well, yes... and when that day comes you will get trained on that equipment.
 
aterpster said:
Not until you look at the Jepp chart and the FAA source documents. Sort of scary, isn't it.
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So why does the FAA chart say RA 100, is it a mistake or am I interpreting it wrong?
 
dmspilot said:
So why does the FAA chart say RA 100, is it a mistake or am I interpreting it wrong?
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It says RA NA. Not authorized so you'd use the DA of 1235 as the minimums (we would call it an MDA at my shop).
 
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