DA vs DH Confusion

[TK211X]

Ok, so from what I understand for a precision approach we have our minimums DH/DA and non precision approach we have an MDA.

As I understand......

Left side digits = (DA) and that's MSL
Right side smaller digits = (DH) and that's AGL

I was looking at the king course and when explaining a precision approach Martha pointed to the digits and identified them as stated above. However in the next video she says for a precision approach your altitude is a "Decision Height" and for a non precision approach you have a MDA.

That confused me a bit, but I feel like saying that myself when looking at those numbers myself at a glance. Was she referring to decision height as a blanket term or was she referencing the DH/AGL?

Also and So.... reaching DH/DA marks your MAP. However for a MDA you determine your MAP by NAV reference?

Just wanted someone to clarify - Thanks in advance.

 

[Cpt_Kirk]

DH/DA (precision approach) is a point is space that is defined both laterally and vertically. It can't change. If you reach your DH without any of the essential visual identifiers needed to land, you are required to go missed. I probably shouldn't, but I use DA/DH interchangeably.

MDA (non-precision approach) only satisfies the vertical portion of the requirements needing you to identify the lateral point via a fix. You can essentially descend quickly to your MDA, "hang out" there and then wait until you reach the necessary MAP before you go missed. You can descend visually and land using normal maneuvers from different points along the MDA.

The small numbers are just listed in AGL for both types of approaches.

 

[dmspilot]

However in the next video she says for a precision approach your altitude is a "Decision Height" and for a non precision approach you have a MDA.

That confused me a bit, but I feel like saying that myself when looking at those numbers myself at a glance. Was she referring to decision height as a blanket term or was she referencing the DH/AGL?

She meant only to distinguish DA/DH from MDA. I am guessing it was an older segment. Back in the day there was no such term as DA. DH used to be in MSL, or sometimes AGL depending on context, which is obviously confusing which is why they renamed it.

However for a MDA you determine your MAP by NAV reference?

Yes, on a nonprecision approach, the MAP is a fix identified by a NAVAID or dead reckoning (time).

 

[Harold Rutila]

I am not aware of any charts that still use Decision Height. As far as I am aware, Decision Altitude (DA) is now used instead.

The video you're referencing was likely made in the 90s and is outdated with respect to its use of DH. Otherwise the King program is a good one.

 

[BigBadLou]

:nod:
The guys are right. DH is an "outdated" term because somebody decided it was confusing pilots. So they started using DA instead. DH is still found in older teaching materials.

 

[flyingron]

It's not "outdated." It just needs to be used appropriately. Altitude refers to MSL, Height refers to above the reference for the approach (typically the threshold elevation). Both are listed.

By the way, the numbers in parens on the FAA charts are military use only.

 

[BigBadLou]

By the way, the numbers in parens on the FAA charts are military use only.

Not many people know this, as I found out.
Especially when a CFII quizzed me before my IR checkride, I gave him the right answer and he seemed puzzled. He asked multiple times "Really?" and no, he was not trying to sway me, he was genuinely puzzled. I think he learned something new that night.

 

[jordane93]

Use Jepp charts and you won't be confused!

 

[aterpster]

It's not "outdated." It just needs to be used appropriately. Altitude refers to MSL, Height refers to above the reference for the approach (typically the threshold elevation). Both are listed.

By the way, the numbers in parens on the FAA charts are military use only.

Minimums are based on TDZ elevation. TCH is based on threshold elevation. There was a brief time recently where minimums became based on threshold elevation. But, it violated the regulations, so it had a fairly short life.

 

[John Collins]

Minimums are based on TDZ elevation. TCH is based on threshold elevation. There was a brief time recently where minimums became based on threshold elevation. But, it violated the regulations, so it had a fairly short life.

I brought up the THRE instead of TDZE on approach charts at the ACF. The switchover to THRE had been underway for a while and was a result of an ICAO harmonization effort. The fellow that pushed for it retired just after it became the standard for US charts. About 4000 approach charts had been transitioned to THRE instead of TDZE. My understanding is that the last 400 procedures are just being updated in the next update cycle.

I asked the question, how can a pilot comply with 91.175 without TDZE available on the chart as TDZE can never be lower than THRE, but it can be higher by as much as 20 feet. Whoops? As an emergency fix, the FAA published all of the TDZE values in a separate document.

 

[jaybee]

DA is on Cat 1 approaches, is measured in height above Mean Sea Level and doubles as your Missed Approach Point.

DH is used on Cat 2 approaches and is measured in height above ground with a radar altimeter and is also your MAP.

MDA is used on non precision approaches and MAP is as charted.

Now with that said - it is charted as DA(H) i.e. 213(200). For all intents and purposes the DH is useless unless you are an airline pilot. (disclaimer - this does not cover every possible instance of every possible thing just trying to give a simple break down that may be useful to you.)

Lastly, in addition to what you posted in others have said, you're MAP could be based on timing also.

 

[flyingron]

That's not quite true. Both the DA and DH are listed on CAT I approaches though most of us do use the DA.
You can waive the RA requirement for CAT II. CAT II approaches list both the RA decision height and a both DA and DH for other ways (you don't strictly have to have a RA for CAT II, it can be waived). Some CAT II's that arrive over uneven terrain only have baro DA/DH's provided.

Since the FAA has boogered up the definition of precision approaches, it's more correct to say that MDAs apply to approaches without vertical guidance (i.e., not an ILS/GS, LPV, or LNAV/VNAV approach).

 

[midlifeflyer]

Your confusion is mostly because of the foibles of human language usage. DA and DH do mean different things. But, despite the fact most were actually talking about DA, in the past, the phrase "decision height" was commonly used to refer to the MAP of an APV (top of the hat to @flyingron). Might still be.

King's language "slip" was probably just a matter of old habits being hard to break.

 

[aterpster]

Then there is AH for airline pilots.

 

[luvflyin]

Then there is AH for airline pilots.

?????

 

[jaybee]

?????

Alert Height, used on CAT 3

 

[aterpster]

Alert Height, used on CAT 3

Picking at nits: Used on fail-operational auto-land CAT III.

 

[SbestCFII]

That's pretty much right, but DH is informational. It is a height above the ground at DA, but not relevant to flying an approach unless you have a radar altimeter (which most of us don't). The DA is what you read off a properly set altimeter and that is when and where you make the decision to either descend to land or climb away from the ground. Unfortunately, it's hard to break people of improperly saying "decision height" when they mean decision altitude. I find myself correcting IFR students a lot. It's sort of a pet peeve.

The MDA is simply the lowest altitude to which may safely descend and remain until you reach the designated missed approach point (MAP). If you cannot identify elements of the runway environment and safely initiate a landing using "normal procedures" at the MAP, then you are required to execute the missed approach... unless of course your intent to perform a circling approach at or above the published circling minimums.

 

[flyingron]

Scott, please back up and read the information I presented earlier. The only difference between DA and DH is one is MSL and the other is above some reference. There are CAT II approaches that use BARO DH's. Radar decision altitudes are spelled out as Radar Altitude (oddly it's a height but they call it Altitude) on the minimal table. You don't want to be using your RA with the baro DA/DH especially at fields that don't have flat terrain leading to the threshold.

And for circling (you knew this but it was stated awkwardly), you still need to see the airport while doing the circling (even if it's not the runway end environment that you need to descend below the MDA). The circling minimum is, in fact, another MDA for that kind of approach operation.

 

[dmspilot]

It's not "outdated." It just needs to be used appropriately. Altitude refers to MSL, Height refers to above the reference for the approach (typically the threshold elevation).

Yes, it is. Or more precisely, it's not the term that is outdated, but the definition.

The 1980 edition of the Instrument Flying Handbook, which was not superseded until 2001, states:
"DH – “Decision Height”, with respect to the operation of aircraft, means the height at which a decision must be made, during an ILS or PAR approach, to either continue the approach or to execute a missed approach. This height is expressed in feet above mean sea level (MSL), and for Category II ILS operation the decision height is additionally expressed as a radio altimeter setting." (Emphasis mine.)

I would check the AIM but I couldn't easily find old editions.

Martha's video segment was probably one of the original ones produced in the 1980s.

 

[flyingron]

The 1980 edition of the Instrument Flying Handbook, which was not superseded until 2001, states:
"DH – “Decision Height”, with respect to the operation of aircraft, means the height at which a decision must be made, during an ILS or PAR approach, to either continue the approach or to execute a missed approach. This height is expressed in feet above mean sea level (MSL), and for Category II ILS operation the decision height is additionally expressed as a radio altimeter setting." (Emphasis mine.)

Well, that is wrong for today . Hopefully they fixed it in the interim. DH is not used for radar altimeter minimums on CAT II ILS approaches. That altitude is specifically called an RA (Radar Altitude). As I stated, if you see a DA/DH on a CAT II approach it's abaro DA/DH.

 

[luvflyin]

If they'd of just gotten rid of DH when they changed it to DA this wouldn't be such a hassle. The height above the runway always was and is still there. Changing what used to be HAT and giving it a new name, DH, a name that used to mean something else was doomed to cause confusion.

 

[flyingron]

I guess it's there to appease the QFE crowd.

 

[luvflyin]

I guess it's there to appease the QFE crowd.

QFE???

 

[flyingron]

Altimeter setting that when you dial it in, puts the field elevation at zero on the altimeter.
What we normally use is signified by QNH (puts zero on the altimeter at 0MSL).

 

[SbestCFII]

Scott, please back up and read the information I presented earlier. The only difference between DA and DH is one is MSL and the other is above some reference. There are CAT II approaches that use BARO DH's. Radar decision altitudes are spelled out as Radar Altitude (oddly it's a height but they call it Altitude) on the minimal table. You don't want to be using your RA with the baro DA/DH especially at fields that don't have flat terrain leading to the threshold.

And for circling (you knew this but it was stated awkwardly), you still need to see the airport while doing the circling (even if it's not the runway end environment that you need to descend below the MDA). The circling minimum is, in fact, another MDA for that kind of approach operation.

You're right of course and I missed reading all of the relies before writing my 2-cents. My peeve is that pilots say DH when they mean DA. Though they are the both the same point above the ground... The DH is informational and not a useful number in the cockpit.

 

[Greg Bockelman]

You're right of course and I missed reading all of the relies before writing my 2-cents. My peeve is that pilots say DH when they mean DA. Though they are the both the same point above the ground... The DH is informational and not a useful number in the cockpit.

Unless you are talking cat II or Cat III.

 

[citizen5000]

Many fly mainly at or near sea level and their charts do not show much difference between DA(H). Take a look at plates at KRNO or KDEN or KAPA and you will see thousands of feet difference between them. https://skyvector.com/files/tpp/1704/pdf/05715R35R.PDF

 

[RotorDude]

Back in the day there was no such term as DA. DH used to be in MSL, or sometimes AGL depending on context...

I've been flying IFR for decades, and I've never seen a definition of DH that is either AGL or MSL. It's always been, and is today, the height above threshold elevation in feet. Nothing to do with sea level. Anything to the contrary is just a mistake.

 

[dmspilot]

I've been flying IFR for decades, and I've never seen a definition of DH that is either AGL or MSL. It's always been, and is today, the height above threshold elevation in feet. Nothing to do with sea level. Anything to the contrary is just a mistake.

It's defined the way I said in multiple places in the 1980 FAA Instrument Flying Handbook. It's not a mistake.

 

[RotorDude]

It's defined the way I said in multiple places in the 1980 FAA Instrument Flying Handbook. It's not a mistake.

I had been flying by then several years of IFR, including many (single pilot) low approaches to minimums in pretty poor weather, and if there is one thing I focused on when sniffing for asphalt, it was the exact definition of "DH". I can assure you that that definition has not changed over the last 40+ years, and I suspect ever since the term was first introduced. If you have a hard copy document with a typo or mistake in it, it would not surprise me at all, since in those days getting a new version out was very time consuming. I suspect that if you check the definition in the FAR's and/or AIM from that era (where it really counts), you'd agree.

 

[dmspilot]

I had been flying by then several years of IFR, including many (single pilot) low approaches to minimums in pretty poor weather, and if there is one thing I focused on when sniffing for asphalt, it was the exact definition of "DH". I can assure you that that definition has not changed over the last 40+ years, and I suspect ever since the term was first introduced. If you have a hard copy document with a typo or mistake in it, it would not surprise me at all, since in those days getting a new version out was very time consuming. I suspect that if you check the definition in the FAR's and/or AIM from that era (where it really counts), you'd agree.

Dude, multiple pages of the 1980 IFH refutes what you say and agrees with what I wrote. It is absolutely NOT "a typo". Do you want me to scan them and post them?

 

[RotorDude]

Dude, multiple pages of the 1980 IFH refutes what you say and agrees with what I wrote. It is absolutely NOT "a typo". Do you want me to scan them and post them?

No need to post, I trust you that it says that. So a government publication had a bug in it. But the bottom line is still that the definition that counts, in the FARs and AIM, as well as glossaries, has always been the same: Decision Height is the height above the threshold.
I can assure you that in the multitude of sources I had when studying for my IR in the 70's there was never any doubt or discrepancy in the definition of that term and it was drilled into our heads repeatedly by CFIIs and respected authors like Richard Collins. "Height" always meant above the runway, not above the ocean.

 

[azblackbird]

• Decision altitude (DA) is referenced to mean sea level and decision height (DH) is referenced to the threshold elevation.

Thank you @RotorDude for making me look that up. It now goes into the 'ole memory bank for future reference when I start prepping for my IFR.

 

[dmspilot]

No need to post, I trust you that it says that. So a government publication had a bug in it. But the bottom line is still that the definition that counts, in the FARs and AIM, as well as glossaries, has always been the same: Decision Height is the height above the threshold.
I can assure you that in the multitude of sources I had when studying for my IR in the 70's there was never any doubt or discrepancy in the definition of that term and it was drilled into our heads repeatedly by CFIIs and respected authors like Richard Collins. "Height" always meant above the runway, not above the ocean.

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.

 

[RotorDude]

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

Well, your example is clearly a "bug", as you seem to imply. Goes to show you, I guess.
There is no bug in my memory, however. If you disagree, find one IFR rated pilot from the 70's (or earlier, I think Richard Collins started flying single pilot IFR in the 50's) on, who flies IFR/IMC to low approaches regularly and thinks there is any doubt whatsoever as to the definition then or now of the term "DH" when it comes to instrument flying.

 

[Clip4]

Ok, so from what I understand for a precision approach we have our minimums DH/DA and non precision approach we have an MDA.

As I understand......

Left side digits = (DA) and that's MSL
Right side smaller digits = (DH) and that's AGL

I was looking at the king course and when explaining a precision approach Martha pointed to the digits and identified them as stated above. However in the next video she says for a precision approach your altitude is a "Decision Height" and for a non precision approach you have a MDA.

That confused me a bit, but I feel like saying that myself when looking at those numbers myself at a glance. Was she referring to decision height as a blanket term or was she referencing the DH/AGL?

Also and So.... reaching DH/DA marks your MAP. However for a MDA you determine your MAP by NAV reference?

Just wanted someone to clarify - Thanks in advance.

Well kinda. The MDA is an altitude in which you may not descend below until you have the required visual elements as listed in the regulations. An MDA has a missed approach point that is identified by some navigational fix.

A DA is an altitude where you make a decision if you can see the required elements. You do not level off at the DA, you look up and make a decision. If the elements are not seen, you go missed. You may not continue the approach below the DA without seeing the required elements, but your track while executing the missed may take you below the DA. Also an approach light system is part of the approach and the ALS may be used to descend to 100 ft above the touch down zone elevation without the pilot seeing the runway in some cases.

 

[dmspilot]

Well, your example is clearly a "bug", as you seem to imply. Goes to show you, I guess.

That sentence doesn't even make sense. I didn't imply the written test supplement was a bug. I only stated it was older.

In the test bank from 1998, there are several written test questions which refer to DH as being in MSL.

The 1980 Instrument Flying Handbook; the written test questions from the 90s (and maybe newer—I didn't check); and the current computer test supplement -- all say DH is in MSL. They can't all be "bugs."

Fact is, the term "decision altitude" did not appear in FAA lexicon until relatively recently, and you are in denial.

There is no bug in my memory, however. If you disagree, find one IFR rated pilot from the 70's (or earlier, I think Richard Collins started flying single pilot IFR in the 50's) on, who flies IFR/IMC to low approaches regularly and thinks there is any doubt whatsoever as to the definition then or now of the term "DH" when it comes to instrument flying.

The colloquial definition may have been different from the FAA's definition, and I'm going to trust written paper before a 47-year-old memory engram. It is known that memory is unreliable, why do you think we use checklists in aviation

 

[Kritchlow]

And I think the above confusion is exactly why they now distinguish DA from DH.

But I do remember when it was DH only.

 

[RotorDude]

That sentence doesn't even make sense.

In the test bank from 1998, there are several written test questions which refer to DH as being in MSL.

The 1980 Instrument Flying Handbook; the written test questions from the 90s (and maybe newer—I didn't check); and the current computer test supplement -- all say DH is in MSL. They can't all be "bugs."

Fact is, the term "decision altitude" did not appear in FAA lexicon until relatively recently, and you are in denial.

The colloquial definition may have been different from the FAA's definition, and I'm going to trust written paper before a 47-year-old memory engram. It is known that memory is unreliable, why do you think we use checklists in aviation

I am not referring to DA. That is clearly a new term, introduced a few years ago. I don't think that there is any doubt that DA = MSL altitude at DH.

But I think I have this issue figured out. In the old days (as now for that matter), "DH" was the point in space where you were supposed to make the decision whether to land or perform the MAP. The approach plate provided that physical point in two forms, MSL (which is what you'd see on your altimeter), and HAT (height above threshold).
In other words, the DH was a spatial concept, a physical point in space, not a value in any specific unit system, although it could be expressed as either MSL or HAT. Because of possible confusion, the DA term was added recently, and after that DH could always be expressed as HAT, while DA is always above sea level.

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.