Decision altitude vs. Decision Height

[Freetime]

What is Decision Altitude? From what I can tell DH is just for precision and DA is for non-precision (GPS) approaches. If this is the case, why don't they call it an MDA for LNAV/VNAV or LPV approaches?

[monkeyspankmasterflex]

DA when the approach offers vertical guidance (LNAV/VNAV). It's an altitude you'll read off the baro with appropriate alt setting vs a height above ground as in DH. MDA for GPS when you have lateral guidance only (LNAV). My guess anyway.

[sidestick stirrer]

S'right: DA means the altitude is read off the barometric altimeter, while at DH is read off the radio altimeter.

[Doc]

You'd best pull out your CAP GEN and look that puppy up.

[2low]

Becauce it usually requires a 2.5 to 3.5 degree glide slope, from the FAF, basically making a non precision approach a precision approach,(GPS) therefore, it has to be a DA (read off the pressure altimeter) and not an MDA, since you don't level off at the required altitude. It is basically a DH. Hopefully that makes sence.....I've been at Arizona's and the Landing Strip all night

And that's straight out of my Navajo AOM. WERD

[duplicate2]

S'right: DA means the altitude is read off the barometric altimeter, while at DH is read off the radio altimeter.

I hope you're kidding.

[JigglyBus]

Seems pretty straight forward in the definitions...

MDA - Minimum descent altitude. Altitudes are always in feet above sea level. This is for all non precision approaches. It is not where/when you go around, it is the minimum altitude you can descend to while waiting to get to the fix/point (the MAP) where you do.

DA - Decision Altitude. The Altitude, again in feet above sea level, at which point you must make a Decision to continue or go around, on a precision approach. (Approaches with vertical guidance, such as ILS,MLS, or PAR's). It it the same thing as the MAP (missed approach point), for a precision approach.

DH - Decision Height. Height is measured in feet above ground. This is effectively the same point as the DA, but measured differently. DH will only be the same number as DA if the approach airfield is at Sea Level. Or, in other countries, if you have QFE set on the altimeter. It is still the MAP, for precision approaches.

With regards to the radio/radar alt issue, I was always taught that, somewhat like a stall, to overshoot on the first sign of DA/DH. For example if you reach 200 on the radar alt, before you reach your DA, to go around. I have no idea if this is legally the case, but it's certainly safer than just relying on the one gauge.

[complexintentions]

S'right: DA means the altitude is read off the barometric altimeter, while at DH is read off the radio altimeter.

I hope you're kidding.

Why do you hope he's kidding?

[KAG]

DH - when shooting an ILS (or PAR), at DH, you either see the runway and land or don't and go missed. It is not altitude instrument specific.

MDA - is used for all non precission approaches. It's the altitude you descend to after you pass the FAF (assumeing no step down altitudes) get the plane stable (gear, flap) and looking for the runway. Once you either pass a predetermined time or the fix itself and you have no visual you go missed.
One note with MDA, do not descend slowly to this height, get down to it now. The last thing you want is to break out of cloud 500' above ground over the threshold, try to be level and looking at least a couple of miles back.

[JigglyBus]

KAG,

If the DH is not "altitude instrument specific" then what instrument do you use to tell you height above ground? A rope? There are only two choices. A Radar Alt, or an Altimeter set to QFE, which is not used in Canada.

DH - is decision HEIGHT (above ground)
DA - is decision ALTITUDE (above sea level)
MDA - is Minimum Descent Altitude.

[Hedley]

Remember that a non-precision approach, by definition, does not provide vertical guidance, which a precision approach, by definition, does.

Keep in mind:

1) Do not descent below MDA. During a non-precision approach, you will level off at the MDA and maintain it, after the FAF.

2) DH is precisely that - it is the height, during a stablized descent during a precision approach, at which you will make the decision to continue the approach and land, or to miss. Some people think you aren't supposed to descend below DH, and that's just not the case. The aircraft has inertia - of course it's going to descend below DH as you commence the missed.

There are many stories, some of them possibly even true, of large aircraft (remember inertia?) descending after the missed was commenced, and touching down on the runway, and climbing away.

Moral of the story: keep the needle centered during the missed.

If you're interested in the subtle nuances (and history of the terms) between Decision Height and Decision Altitude, click on:

http://www.jeppesen.com/download/aopa/jul99aopa.pdf

When the TERPs criteria first went into effect, the minimum altitude on precision approaches was called a Decision Height (DH).

Technically this is not correct since the point is determined by barometric altitude - which measures altitude, not height. Jeppeson charts have been including the letters for DA(H) for decision altitude (height) with both figures since the mid-1980's to show both values. The FAA is gradually adopting the term decision altitude to replace decision height. All new WAAS and LAAS approaches will have minimums expressed as decision altitudes (heights).

I have no idea what the rest of you have been prattling on about - ropes, radar altimeters, etc.

[KAG]

Jigglybus... some were talking that DH was RAD ALT, and MDA was an altimeter. It doesn't matter what friggin source you get it from as long as you know at DH, land or go around. Jesus, it's not rocket science.

[Hedley]

Can someone please provide the CARs reference requiring the use of a radar altimeter to fly a vanilla CAT I ILS?

Heck, didn't Transport decommission most of the marker beacons around 15 years ago?

[complexintentions]

If you are doing a Cat II or Cat III approach you will most certainly be using radalt for your decision height.

As far as non-precision approaches, the prevailing philosophy is to use a Visual Descent Point to allow a stabilized descent to minimums. This may be MDA, or it may be higher...but in the airline environment, "dive n' drive" after the FAF went out of vogue long ago.

We do not "level off" at MDA, we descend at a constant rate on the entire non-precision approach until our VDP, at which point we either land or go missed...

A CAT I does not require a radalt, (unless your ops spec requires you to have one installed

KAG, you say it doesn't matter what source you get your info from, I disagree. A radalt won't even be useable on most non-precision approaches, given the MDA could be above it's useful range. And you better be clear for even a precision DH which one you're going to use, one is absolute above ground, the other is above sea level...kinda important distinction don'tcha think?! Maybe not in YVR but for sure in YYC...heheh

[JigglyBus]

KAG, that's what I'm saying. What instrument are YOU giong to use to tell you AGL. As in Height, as in Decision HEIGHT. The altimeter? or the Rad Alt?'

The altimeter is the perfect device for DA, or MDA, but not DH.

[monkeyspankmasterflex]

I think all Freetime was asking was why LNAV/VNAV have a DA and not an MDA.

DH=DA, who knew? For all intents and purposes they would be treated the same anyway, ie set rad alt and bug to temp corrected alt on baro.


DECISION ALTITUDE OR DECISION HEIGHT (DA/DH):
Means an altitude or height
specified in the Canada Air Pilot or the route and approach inventory at which a missed
approach procedure shall be initiated during a precision approach or an approach
procedure with vertical guidance, if the required visual reference necessary to continue the
approach to land has not been established.

[JigglyBus]

Interesting post Monkey... you're quite right... who knew?

Been a while since I studied the CAP. I think Transport needs to review that legend, and perhap revise it to international standards.

I don't know how they can refer to 767 as a "height". Height above what?

Then why aren't the other altitudes height's. Why not MDH?

Internationally, Jeppesen plates etc, would post that same minimum as
ILS DA(DH) 767(200)

What does the CAP call the 200 then? the HAT. As in Height.

So which is it, is the DH a Height, or is the HAT a Height. I don't see how they can both be.

I fully agree that DH and DA are the exactly same point, why Canada chooses to refer to an altitude as a 'height' I don't know.

Worse yet, why would it be DH for precision and the DA for VNAV. It's a decision point. Why is one an altitude and one a height?


Editted.... I found the ICAO defintion.....

DECISION ALTITUDE/DECISION HEIGHT [ICAO]- A specified altitude or height (A/H) in the precision approach at which a missed approach must be initiated if the required visual reference to continue the approach has not been established.

Note 1: Decision altitude [DA] is referenced to mean sea level [MSL] and decision height [DH] is referenced to the threshold elevation.

Note 2: The required visual reference means that section of the visual aids or of the approach area which should have been in view for sufficient time for the pilot to have made an assessment of the aircraft position and rate of change of position, in relation to the desired flight path.

[ahramin]

Just to make a point here for the newer guys. Beware of guys who know everything even though

Been a while since I studied the CAP

They probably didn't know it all when they stopped studying and they certainly don't know it all now.

The confusion arises from Decision Height being a misnomer. Before this was corrected with the term Decision Altitude, MDA was the altitude (asl) that you could descend to on a npa until you landed or got to your MAP and DH was the height (asl) that you either landed or did your MA. Both were predicated on a certain height above ground and then converted to height above sea level. Now there is no longer any need to use DH as MDA and DA work fine for both NPAs, PA, and GPS VNAV/LNAV. None of which require a radar altimeter.

However because DA has now become an acceptable term for precision approaches DH can now be used to describe the agl altitude which is set on the radar altimeter. Not quite straighforward but not as confusing as before either.

[monkeyspankmasterflex]

However because DA has now become an acceptable term for precision approaches DH can now be used to describe the agl altitude which is set on the radar altimeter.

I don't think that's right. I think DA is a product of RNAV approaches and if you're really into it you can probably find the distinction between the two in the TPH 308 (CH3). My guess is DH occurs along an X degree glide path provided by radio signals to the aircraft and that (assuming not false GP) glide path will always be X degrees regardless of +/-ISA. DA on the other hand is based on a MAWP with an associated altitude, in fact the whole approach is a series of waypoints with associated altitudes. As conditions fall above or below ISA, the vertical path angle will rise and fall. So if you're flying to a DA you must ensure obstacle clearance on decent to DA. If it's cold and your FMS isn't capable of temp correcting all the wayoints in the approach you may be flying a vertical decent angle lower than the approach was designed for and therefore not ensuring sufficient obst clrce. My take anyway, there is still alot of grey area with respect to VNAV/LNAV in my mind, I don't fly em but I think we all will in the not so distant future.

[ahramin]

Well yes. So far DA has only been used specifically for vnav GPS approaches. The definition being the same i think it is only a matter of time before we start using DA instead of DH for ILSs.

As for X degrees regardless of +-ISA, DH is a barometric altitude and must still be temperature corrected for cold weather. So it's still the same DA or DH, you treat them exactly the same way for every aspect.

[JigglyBus]

Ahramin,

Thank for the insult, didn't realize I was coming across as a know it all.

I also apologize for not studying the CAP in a while. I haven't shot an approach in Canada in about 8 years, so I hadn't seen the need to continue my subsciption.

Strange thing is, you belittle me, and then somewhat go on to agree with me. Quote "The confusion arises from Decision Height being a misnomer"

You also state that "this was corrected with the term Decision Altitude", but as the picture of the CAP above would suggest, DH remains, uncorrected. (if that's a current scan?). Also, you stated "Now there is no longer any need to use DH", but again that CAP scan would suggest otherwise.

Internationally (ICAO), outside of Canada, it is very clear. As defined in one of my previous posts. Decision altitude [DA] is referenced to mean sea level [MSL] and decision height [DH] is referenced to the threshold elevation. This definition can be found on the FAA's website at http://www.faa.gov/ATpubs/PCG/D.HTM#DEC ... HT%20[ICAO]

Also further clarification for DA(H) can be found on http://www.faa.gov/library/manuals/exam ... ga9912.doc. It states...Explanation of use of DA(H). The "Altitude" value is typically measured by a barometric altimeter (relative to mean sea level) and is the determining factor for minima for Category I Instrument Approach Procedures. The "Height" value specified in parenthesis is typically a radio altitude equivalent height above the touchdown zone (HAT) used only for advisory reference, and does not necessarily reflect actual height above underlying terrain.

So, when you state Radar Alt is not required, you are somewhat erroneous. Not required is true. But to determine the advisory value of DH, it would 'typically' be needed. Unless you were trailing a rope.

In the end, I don't know if personal attacks are really necessary but if it makes you feel better, carry on. I don't know it all. Didn't know any of us did....

Do you?

[JigglyBus]

After a little more investigation, it seems as though Canada has changed it, or at least is going to.

It will be defined exactly the same as ICAO.

From this webpage ..... http://www.tc.gc.ca/civilaviation/RegSe ... 002024.htm

"The term "decision height" or "DH" has been incorrectly used in Canada. All Category I precision approach procedures are developed to utilize a "decision altitude" or "DA" rather than a decision height. The approach minima are referenced to an "altitude" indication on the altimeter rather than a specific "height" above a surface. Introducing the term "decision altitude (DA)" into the Regulations matches the ICAO definitions.

More clarity here...
http://www.tc.gc.ca/civilaviation/Regse ... 000268.htm

""decision altitude or decision height (DA/H)". A specified altitude or height in the precision approach (PA) and approach with vertical guidance (APV)at which a missed approach procedure shall be initiated if the required visual reference necessary to continue the approach to land has not been established; (altitude de décision ou hauteur de décision (DA/H))"

I see the DA/H as exactly the same thing whether on a PA or APV. You get there, you either see and land, or don't and go missed.

But, like Ahramin said, don't believe me, go check it out for yourselves.

[A330]

Thanks very much for the headache,

DA or MDA, same thing, is the Minimum altitude referenced to an altimeter, so MSL, during precision ( Cat I ) and non-presision approaches.

DH is referenced to a radar altimeter and is therefore height above ground used during Cat II and Cat III operations.

Geez, keep it simple...

[Ryan Coke]

I'll put in my opinon as another know-it-all who hasn't studied the CAP for a while, but I had no idea there could be this much confusion over it.

MDA--NPA minima read from altimeter

DA--Precision app minima read from altimeter

DH--height above ground minima read off of rad alt (cat II and III)

[duplicate2]

S'right: DA means the altitude is read off the barometric altimeter, while at DH is read off the radio altimeter.

I hope you're kidding.

Why do you hope he's kidding?

I'll retract since sidestick stirrer may have been referring to CAT II/III ops. I was taking this as implying that he would make his landing decision at a predetermined rad alt reading for CAT I.