Ping Sam Spade: DA versus DH on an ILS?

Sam and others, I have about 1,100 hours now with about 700 of those flown
after getting my instrument rating in the US back in 2003. Since then I have
flown IFR almost twice per week.

On a web-based aviation forum there is a general disagreement of the terms DA
(decision altitude) versus DH (decision height) when talking about an ILS
approach.

For those approaches that have published altitude minimums (typically ILS CAT
I or CAT II in the US), I was taught that the point at which one would begin
the missed approach is the decision height, or DH. However, in that aviation
forum there are a few who insist that this point is called the decision
altitude, or DA, and that DH is not valid for aircraft not equipped with
radar altimeter.

I did note that the current Pilot/Controller's glossary defines both DA and
DH as the point at which a missed does begin on a precision approach, but the
DA definition also includes the note about it being an ICAO definition.

Is this a case of semantics? Are DA and DH interchangeable when discussing
the point at which a missed begins? Or is there a more important distinction
between the two and I was simply taught inadequately?

--
Peter

On 02/27/07 15:06, Peter R. wrote:
Sam and others, I have about 1,100 hours now with about 700 of those flown
after getting my instrument rating in the US back in 2003. Since then I have
flown IFR almost twice per week.

On a web-based aviation forum there is a general disagreement of the terms DA
(decision altitude) versus DH (decision height) when talking about an ILS
approach.

For those approaches that have published altitude minimums (typically ILS CAT
I or CAT II in the US), I was taught that the point at which one would begin
the missed approach is the decision height, or DH. However, in that aviation
forum there are a few who insist that this point is called the decision
altitude, or DA, and that DH is not valid for aircraft not equipped with
radar altimeter.

I did note that the current Pilot/Controller's glossary defines both DA and
DH as the point at which a missed does begin on a precision approach, but the
DA definition also includes the note about it being an ICAO definition.

Is this a case of semantics? Are DA and DH interchangeable when discussing
the point at which a missed begins? Or is there a more important distinction
between the two and I was simply taught inadequately?


I was taught that for instrument charts, "Altitudes" refer to MSL altitudes
and "Heights" refer to height above the ground. This means that DA refers to
the MSL altitude where the decision is made.

I think I read in the Jeppesen materials (or maybe it was a Rod Machado book)
that this terminology is the direction where things are going, but not everyone
agrees with this (yet). For example, the NACO Terminal Procedures Publication
still uses DH.

If you look at the Jeppesen charts, they use DA(H) to indicate that
this is the Decision Altitude MSL, with the height above the ground
shown in parenthesis.

I don't know from whom you can get a definitive answer.

Hope that helps,

--
Mark Hansen, PP-ASEL, Instrument Airplane
Cal Aggie Flying Farmers
Sacramento, CA

I think I've heard that DH is the newer term for DA and they are for
the same thing...

On Feb 27, 3:06 pm, "Peter R." wrote:
Sam and others, I have about 1,100 hours now with about 700 of those flown
after getting my instrument rating in the US back in 2003. Since then I have
flown IFR almost twice per week.

On a web-based aviation forum there is a general disagreement of the terms DA
(decision altitude) versus DH (decision height) when talking about an ILS
approach.

For those approaches that have published altitude minimums (typically ILS CAT
I or CAT II in the US), I was taught that the point at which one would begin
the missed approach is the decision height, or DH. However, in that aviation
forum there are a few who insist that this point is called the decision
altitude, or DA, and that DH is not valid for aircraft not equipped with
radar altimeter.

I did note that the current Pilot/Controller's glossary defines both DA and
DH as the point at which a missed does begin on a precision approach, but the
DA definition also includes the note about it being an ICAO definition.

Is this a case of semantics? Are DA and DH interchangeable when discussing
the point at which a missed begins? Or is there a more important distinction
between the two and I was simply taught inadequately?

--
Peter

kevmor wrote:
I think I've heard that DH is the newer term for DA and they are for
the same thing...

DH is AGL.
DA is MSL.

DH is actually the older terminology and people tend to use it
for both concepts.

Would you use DH with a radar altimeter?

On Feb 27, 4:08 pm, Ron Natalie wrote:
kevmor wrote:
I think I've heard that DH is the newer term for DA and they are for
the same thing...

DH is AGL.
DA is MSL.

DH is actually the older terminology and people tend to use it
for both concepts.

Peter R. wrote:

Is this a case of semantics? Are DA and DH interchangeable when discussing
the point at which a missed begins? Or is there a more important distinction
between the two and I was simply taught inadequately?


There is no difference between DA and DH.

DH is old FAA

DA is ICAO.

DA is new FAA. ;-)

kevmor wrote:

Would you use DH with a radar altimeter?

On Feb 27, 4:08 pm, Ron Natalie wrote:

kevmor wrote:

I think I've heard that DH is the newer term for DA and they are for
the same thing...

DH is AGL.
DA is MSL.

DH is actually the older terminology and people tend to use it
for both concepts.



DH is MSL just as is DA. The only altitude you will see that is AGL is
RA on a CAT II ILS and that is AGL above the DA/DH point.

"Mark Hansen" wrote in message
...
On 02/27/07 15:06, Peter R. wrote:

Is this a case of semantics? Are DA and DH interchangeable when
discussing
the point at which a missed begins? Or is there a more important
distinction
between the two and I was simply taught inadequately?


I was taught that for instrument charts, "Altitudes" refer to MSL
altitudes
and "Heights" refer to height above the ground. This means that DA refers
to
the MSL altitude where the decision is made.

I think I read in the Jeppesen materials (or maybe it was a Rod Machado
book)
that this terminology is the direction where things are going, but not
everyone
agrees with this (yet). For example, the NACO Terminal Procedures
Publication
still uses DH.

If you look at the Jeppesen charts, they use DA(H) to indicate that
this is the Decision Altitude MSL, with the height above the ground
shown in parenthesis.

I don't know from whom you can get a definitive answer.

In the TPP frontal matter, of course.

Landing minima are given as DA, decision altitude in feet MSL; RVR in 100's
of feet or whole and fraction statute mile; HAT (feet height above
threshold); and parenthetic equivalents for military. Circling minima
specify HAA (feet height above airport) rather than HAT. All four elements
are always present in NACO TPP landing minima.

On 2/27/2007 9:38:00 PM, Sam Spade wrote:

There is no difference between DA and DH.

DH is old FAA

DA is ICAO.

DA is new FAA. ;-)

Sorta like the transistion from SID to DP....

Thanks, Sam.

--
Peter

Peter R. wrote:


Sorta like the transistion from SID to DP....




You mean SID to DP to SID in FAA-land. ;-)