DA and DH confusion on GPS and ILS approach chart.

[John777]

I reviewed the above definitions from IPH chapter 2 and I still have stuffs to figure out.
DA, DH is mostly the same thing but DA is expressed in MSL where DH is in AGL.
When I looked at both GPS and ILS chart(Jepp), every decision altitude was expressed as DA(H), DH expressed in parenthesis. So what is the reason that the FAA is having two different terms even though they are meant to be same thing?

Next, We know LPV and LNAV/VNAV approach are not the precision but act like precision approach.
On FAA IAP, DA is denoted on minmums table for both. DA is defined as" specified altitude 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." They are not the precision approach, then why FAA put DA on the chart?

Secondly, what is the real difference between DA and DH? they are meant for the and only difference was that, for DH, it is the height at which a decision must be made during ILS, MLS, or PAR IAP..." Why is DH for only three types of approach?

 

[fiveoboy01]

I reviewed the above definitions from IPH chapter 2 and I still have stuffs to figure out.
DA, DH is mostly the same thing but DA is expressed in MSL where DH is in AGL.
When I looked at both GPS and ILS chart(Jepp), every decision altitude was expressed as DA(H), DH expressed in parenthesis. So what is the reason that the FAA is having two different terms even though they are meant to be same thing?

Well, one is read off the altimeter, the other is an AGL height which you can use to determine if, based upon the reported ceilings, you can shoot the approach and break out to see the runway environment. But you don't really need the AGL height for that, if you know the airport elevation.

Next, We know LPV and LNAV/VNAV approach are not the precision but act like precision approach.
On FAA IAP, DA is denoted on minmums table for both. DA is defined as" specified altitude 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." They are not the precision approach, then why FAA put DA on the chart?

Because you need a reference point to either continue to land or go missed. A WAAS GPS provides vertical guidance with LPV or LNAV/VNAV service, that GPS-generated glideslope acts just like an ILS.. even if the approach isn't labeled precision, there's still a missed approach point. There has to be a DA.

Secondly, what is the real difference between DA and DH? they are meant for the and only difference was that, for DH, it is the height at which a decision must be made during ILS, MLS, or PAR IAP..." Why is DH for only three types of approach?

I'm not exactly sure what you're asking here. Read the approach plate. If there's a DA, use it. I'm not familliar with Jepp plates but the FAA ones show the AGL decision height... I use the term "DA" and DH" interchangeably referring to MSL even if that's not technically correct. I'm not concerned with the AGL height, because I'm reading the altimeter, not doing the math in my head as I'm descending.

 

[MauleSkinner]

I reviewed the above definitions from IPH chapter 2 and I still have stuffs to figure out.
DA, DH is mostly the same thing but DA is expressed in MSL where DH is in AGL.
When I looked at both GPS and ILS chart(Jepp), every decision altitude was expressed as DA(H), DH expressed in parenthesis. So what is the reason that the FAA is having two different terms even though they are meant to be same thing?

All approach minimum altitudes, whether MDA or DA, are expressed in both MSL and AGL altitudes. As stated above, DA is read from the altimeter...it gives you the direct answer to "when do I reach minimum altitude?" in flight.
DH, on the other hand, gives you a direct answer to "where will the airport be when I get there?" Without having to do math in the cockpit. If I break out at 200 feet above the ground, I'm going to need to do things sooner Than if I break out 400 feet above the ground. (I think the highest DH I've flown was about 600 feet...way different! )

Next, We know LPV and LNAV/VNAV approach are not the precision but act like precision approach.
On FAA IAP, DA is denoted on minmums table for both. DA is defined as" specified altitude 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." They are not the precision approach, then why FAA put DA on the chart?

Keep in mind that GPS approaches are relatively new...there are places where the FAA hasn't merged all of their regulations and definitions properly. This is one of them.

Secondly, what is the real difference between DA and DH? they are meant for the and only difference was that, for DH, it is the height at which a decision must be made during ILS, MLS, or PAR IAP..." Why is DH for only three types of approach?

Are you asking about the difference between DA & MDA here? DA/DH are used for approaches designed to require glide path indications that put you in a specific spot relative to the runway, and therefore have different obstacle criteria and a different method of use than an MDA. Where an MDA can't be penetrated unless you see approach lights or other runway indications, a DA can be penetrated briefly during a missed approach.

 

[somorris]

All that was done to confuse pilots, John .

My understanding of why RNAV/GPS APV approaches are not called precision approaches is because of an issue with ICAO. Others here know more than I do, though.

The DA versus DH is probably another issue like that. Don't you know we have to be in alignment with Europe! For a pilot like me, DA versus DH means the same thing. Go missed!

 

[fiveoboy01]

My understanding of why RNAV/GPS APV approaches are not called precision approaches is because of an issue with ICAO. Others here know more than I do, though.

That's what I recall reading as well, though I don't know any details either. Makes sense... Many times an LPV approach will get you just as low and has the same course width sensitivity as the ILS.

 

[Everskyward]

My understanding about why they got away from using DH and went to DA is the the DH is AGL above the threshold, but when you reach DA, you are not above the threshold yet, so your radar altimeter (if you have one) could be reading something other than DH.

 

[somorris]

Makes sense... Many times an LPV approach will get you just as low and has the same course width sensitivity as the ILS.

One drawback, though, is that the LPV approach is considered non-precision so if you are looking at alternates, you have to use the 800 and 2 instead of 600 and 2. Most of the time this wouldn't make much difference, but with my luck the only alternate I could make it to would be forecast <800 .

 

[Doggtyred]

I reviewed the above definitions from IPH chapter 2 and I still have stuffs to figure out.
DA, DH is mostly the same thing but DA is expressed in MSL where DH is in AGL.
When I looked at both GPS and ILS chart(Jepp), every decision altitude was expressed as DA(H), DH expressed in parenthesis. So what is the reason that the FAA is having two different terms even though they are meant to be same thing?

Next, We know LPV and LNAV/VNAV approach are not the precision but act like precision approach.
On FAA IAP, DA is denoted on minmums table for both. DA is defined as" specified altitude 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." They are not the precision approach, then why FAA put DA on the chart?

Secondly, what is the real difference between DA and DH? they are meant for the and only difference was that, for DH, it is the height at which a decision must be made during ILS, MLS, or PAR IAP..." Why is DH for only three types of approach?

One is read with a baro altimeter (altitude above sea level)
One is read with a radar altimeter (if so equipped, as height above ground).

Decision altitude is the point on a precision approach glideslope (or defined descent path) where you look up and if you see the field you can commit to landing, or must go around. Alternatively if you see the approach lights you can descend a bit further to a lesser decision altitude to try and safely acquire the runway (example, on an ILS at 200 ft AGL you see the approach light system, but not the threshold or runway lights. You might be permitted to go to 100 ft AGL at which point you either see the runway and can land, or you dont and must go around)

Nonprecision approaches have an "minimum descent altitude". before vertical guidance was adapted to many of these approaches, the accepted practice was to cross the final approach fix, descend promptly to this MDA, and fly the final approach course at that MDA until you either 1) saw the runway and descended for landing, or reached the missed approach point and executed a go around/missed approach. Modern nonprecision approaches might have some vertical guidance, but you cannot descend below the MDA without the runway environment in sight.

So.. precision approach = Decision altitude/height. Nonprecision approach = Minimum descent altitude or MDA.

 

[Doggtyred]

My understanding about why they got away from using DH and went to DA is the the DH is AGL above the threshold, but when you reach DA, you are not above the threshold yet, so your radar altimeter (if you have one) could be reading something other than DH.

Would make sense for the RA to read the height above ground at the decision point and have that be the decision height value, not the height above threshold or airport reference point.... Now, whether that's the case, I do not know.

 

[RussR]

Would make sense for the RA to read the height above ground at the decision point and have that be the decision height value, not the height above threshold or airport reference point.... Now, whether that's the case, I do not know.

That is the case on Cat II approaches (which are designed for use with RA), yes.

 

[fiveoboy01]

Alternatively if you see the approach lights you can descend a bit further to a lesser decision altitude to try and safely acquire the runway (example, on an ILS at 200 ft AGL you see the approach light system, but not the threshold or runway lights. You might be permitted to go to 100 ft AGL at which point you either see the runway and can land, or you dont and must go around)

100 feet above TDZE, not AGL. They might not be the same.

 

[fiveoboy01]

One drawback, though, is that the LPV approach is considered non-precision so if you are looking at alternates, you have to use the 800 and 2 instead of 600 and 2. Most of the time this wouldn't make much difference, but with my luck the only alternate I could make it to would be forecast <800 .

Excellent point!

Brings up a question from me. The 800/2 and 600/2 rule for alternates, depending on what approaches are available... That's simply for the purpose of filing the alternate, and in practice, if you need to go to the alternate, you can legally shoot whatever approach you wish, correct?

My guess is yes. Obviously with the caveat that the approach gets you low enough to break out at the right place.

 

[MauleSkinner]

One is read with a baro altimeter (altitude above sea level)
One is read with a radar altimeter (if so equipped, as height above ground).

Decision altitude is the point on a precision approach glideslope (or defined descent path) where you look up and if you see the field you can commit to landing, or must go around. Alternatively if you see the approach lights you can descend a bit further to a lesser decision altitude to try and safely acquire the runway (example, on an ILS at 200 ft AGL you see the approach light system, but not the threshold or runway lights. You might be permitted to go to 100 ft AGL at which point you either see the runway and can land, or you dont and must go around)

Nonprecision approaches have an "minimum descent altitude". before vertical guidance was adapted to many of these approaches, the accepted practice was to cross the final approach fix, descend promptly to this MDA, and fly the final approach course at that MDA until you either 1) saw the runway and descended for landing, or reached the missed approach point and executed a go around/missed approach. Modern nonprecision approaches might have some vertical guidance, but you cannot descend below the MDA without the runway environment in sight.

Of course, the definition of "runway environment" must include approach lights...they're still legal to take you to 100 AGL from an MDA.

 

[John777]

Thank you for your participation everyone, and I am learning a lot from you all !

Thanks again,

John

 

[Doggtyred]

Excellent point!

Brings up a question from me. The 800/2 and 600/2 rule for alternates, depending on what approaches are available... That's simply for the purpose of filing the alternate, and in practice, if you need to go to the alternate, you can legally shoot whatever approach you wish, correct?

My guess is yes. Obviously with the caveat that the approach gets you low enough to break out at the right place.

Correct, for alternate filing purposes the airport must have the required approaches and required weather... Once filed, IF you divert there (or anywhere else) you can accept and fly any approach you are capable of accepting.

 

[Doggtyred]

Of course, the definition of "runway environment" must include approach lights...they're still legal to take you to 100 AGL from an MDA.

Agreed... but the approach lights on a non-precision runway are not required to be anywhere's close to what you would find on a category 1(or better) ILS. At DH/DA you are over the functioning light array at 200 feet. At MDA on a nonprecision, you might be 400 vertical and who knows how far laterally from the lights.

What little actual IFR training I did was about punching through a layer and having the field in sight before reaching DA or getting to the MAP.. I've never done "really" hard IFR in fog/soup all the way to landing...

 

[MauleSkinner]

It's loads of fun, trust me.

 

[whattauser]

DH is measured only with a radio/radar altimeter. It can be quite a bit different from the HAT that you might find on a precision DA. WAAS LPV approaches are "precision-like" and are treated the same as an ILS in procedure. LNAV/VNAV, in the other hand are simply non-precision approaches with some vertical guidance. If you set your altimeter incorrectly on one of those, you can be on "path" and fly straight into the ground.

 

[midlifeflyer]

Excellent point!

Brings up a question from me. The 800/2 and 600/2 rule for alternates, depending on what approaches are available... That's simply for the purpose of filing the alternate, and in practice, if you need to go to the alternate, you can legally shoot whatever approach you wish, correct?

My guess is yes. Obviously with the caveat that the approach gets you low enough to break out at the right place.

Yes. Heck, you don't even need to go to the filed alternate. Head anywhere the weather is better and has an sufficient minimums for the approach you decide to use.

All the rules about alternates are for flight planning and filing purposes. There are no alternate rules to follow once your wheels leave the ground.

 

[denverpilot]

Yes. Heck, you don't even need to go to the filed alternate. Head anywhere the weather is better and has an sufficient minimums for the approach you decide to use.

All the rules about alternates are for flight planning and filing purposes. There are no alternate rules to follow once your wheels leave the ground.

And the assembly said, AMEN! Hallelujah. Pass the Tylenol.