Ping Sam Spade: DA versus DH on an ILS?

[Peter R.]

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

[Mark Hansen]

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

[kevmor]

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

[Ron Natalie]

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.

[kevmor]

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.

[Sam Spade]

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. ;-)

[Sam Spade]

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.

[Mike Young]

"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.

[Peter R.]

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

[Sam Spade]

Peter R. wrote:

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

>

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

[Peter R.]

On 2/28/2007 10:11:34 AM, Sam Spade wrote:

> Peter R. wrote:
>
>>
>> Sorta like the transistion from SID to DP....
>>
>
>>
>
> You mean SID to DP to SID in FAA-land. ;-)

Ah, sheesh... I didn't realize they are going back to SIDs. Just when you
think you have this stuff down.

--
Peter

[Gordy]

FAA-H-8261-1
page 5-18

DA is currently used on RNAV approach charts with vertical descent guidance.
DA will replace DH for Category I precision IAPs. MDA and DA are referenced
to MSL and measured with a barometric altimeter. CAT II and III approach DHs
are referenced to AGL and measured with a radio altimeter.

[Sam Spade]

Gordy wrote:

> FAA-H-8261-1
> page 5-18
>
> DA is currently used on RNAV approach charts with vertical descent guidance.
> DA will replace DH for Category I precision IAPs. MDA and DA are referenced
> to MSL and measured with a barometric altimeter. CAT II and III approach DHs
> are referenced to AGL and measured with a radio altimeter.
>
>
>

CAT II is DH/DA. Unrestricted CAT III Is Alert Height, measure in MSL.
Radar altitude is not used as a determination of Alert Height.

DA/DH is used in restricted CAT III operations.

[Peter R.]

On 2/28/2007 10:38:03 AM, "Gordy" wrote:

> FAA-H-8261-1
> page 5-18
>
> DA is currently used on RNAV approach charts with vertical descent guidance.
> DA will replace DH for Category I precision IAPs. MDA and DA are referenced
> to MSL and measured with a barometric altimeter. CAT II and III approach DHs
> are referenced to AGL and measured with a radio altimeter.

Hmmm, I just looked at the online Instrument Procedures Handbook at page 5-18
and don't see that quote there:

http://www.faa.gov/library/manuals/aviation/instrument_procedures_handbook/

I even searched the entire chapter for a string from your quote and didn't
run across it. What am I missing? Are you quoting from a paper copy? If so,
is it one to a few years older?

--
Peter

[Gordy]

The publication used was from 2004, FAA-H-8261-1. I was not aware it has
been updated.
The new FAA-H-8261-1A publication is dated 2007. I need buy the new version.


"Peter R." > wrote in message
...
> On 2/28/2007 10:38:03 AM, "Gordy" wrote:
>
>> FAA-H-8261-1
>> page 5-18
>>
>> DA is currently used on RNAV approach charts with vertical descent
>> guidance.
>> DA will replace DH for Category I precision IAPs. MDA and DA are
>> referenced
>> to MSL and measured with a barometric altimeter. CAT II and III approach
>> DHs
>> are referenced to AGL and measured with a radio altimeter.
>
> Hmmm, I just looked at the online Instrument Procedures Handbook at page
> 5-18
> and don't see that quote there:
>
> http://www.faa.gov/library/manuals/aviation/instrument_procedures_handbook/
>
> I even searched the entire chapter for a string from your quote and didn't
> run across it. What am I missing? Are you quoting from a paper copy? If
> so,
> is it one to a few years older?
>
> --
> Peter

[Sam Spade]

Peter R. wrote:
> On 2/28/2007 10:38:03 AM, "Gordy" wrote:
>
>
>>FAA-H-8261-1
>>page 5-18
>>
>>DA is currently used on RNAV approach charts with vertical descent guidance.
>>DA will replace DH for Category I precision IAPs. MDA and DA are referenced
>>to MSL and measured with a barometric altimeter. CAT II and III approach DHs
>>are referenced to AGL and measured with a radio altimeter.
>
>
> Hmmm, I just looked at the online Instrument Procedures Handbook at page 5-18
> and don't see that quote there:
>
> http://www.faa.gov/library/manuals/aviation/instrument_procedures_handbook/
>
> I even searched the entire chapter for a string from your quote and didn't
> run across it. What am I missing? Are you quoting from a paper copy? If so,
> is it one to a few years older?
>


It is there:

MINIMUM DESCENT ALTITUDE,DECISION ALTITUDE,AND DECISION HEIGHTDAis
currently used on RNAV approach chart swith vertical descent guidance.
DAwill replace DHfor Category I precision IAPs. MDAand DA are referenced
to MSL and measured with a barometric altimeter. CATII and III approach
DHs are referenced to AGL and measured with a radio altimeter.

And, it is just plain wrong as to CAT III. It is misleading as to CAT
II because CAT II DA/DH is expressed in both MSL (Da/DA) and AGL for RA
(at RA site).

[Gordy]

The new 1A publication contains the same information. Chapter 5, page 18.

MINIMUM DESCENT ALTITUDE, DECISION ALTITUDE,

AND DECISION HEIGHT

MDA and DA are referenced to MSL and measured

with a barometric altimeter. CAT II and III

approach DHs are referenced to AGL and measured

with a radio altimeter.

"Gordy" > wrote in message
...
> FAA-H-8261-1
> page 5-18
>
> DA is currently used on RNAV approach charts with vertical descent
> guidance. DA will replace DH for Category I precision IAPs. MDA and DA are
> referenced to MSL and measured with a barometric altimeter. CAT II and III
> approach DHs are referenced to AGL and measured with a radio altimeter.
>
>
>

[Peter R.]

On 2/28/2007 11:22:50 AM, "Gordy" wrote:

> The publication used was from 2004, FAA-H-8261-1. I was not aware it has
> been updated.
> The new FAA-H-8261-1A publication is dated 2007. I need buy the new version.

I guess I do, too. :)

Thanks for the information, although as Sam pointed out, some needs to be
taken with a grain of salt. Considering that I will most likely never fly a
CAT II or CAT III approach, it is not that big a deal.

--
Peter

[Sam Spade]

Gordy wrote:

> The new 1A publication contains the same information. Chapter 5, page 18.
>
> MINIMUM DESCENT ALTITUDE, DECISION ALTITUDE,
>
> AND DECISION HEIGHT
>
> MDA and DA are referenced to MSL and measured
>
> with a barometric altimeter. CAT II and III
>
> approach DHs are referenced to AGL and measured
>
> with a radio altimeter.
>
> "Gordy" > wrote in message
> ...
>
>>FAA-H-8261-1
>>page 5-18
>>
>>DA is currently used on RNAV approach charts with vertical descent
>>guidance. DA will replace DH for Category I precision IAPs. MDA and DA are
>>referenced to MSL and measured with a barometric altimeter. CAT II and III
>>approach DHs are referenced to AGL and measured with a radio altimeter.
>>
>>
>
I guess they didn't do much of a review. ;-)

[Gordy]

You will not see me flying any CAT II or III approaches either.

One of my instrument instructors had me land while under the hood. It
started out as a no gyro approach simulation and continued to a full stop
landing. It is really hard not to peek when you are descending at 100/200
feet per minute on the gauges and making small corrections as directed by
your instructor. I'm not sure how much help he gave me, he said none, but I
can guarantee he was hanging on.

"Peter R." > wrote in message
...
> On 2/28/2007 11:22:50 AM, "Gordy" wrote:
>
>> The publication used was from 2004, FAA-H-8261-1. I was not aware it has
>> been updated.
>> The new FAA-H-8261-1A publication is dated 2007. I need buy the new
>> version.
>
> I guess I do, too. :)
>
> Thanks for the information, although as Sam pointed out, some needs to be
> taken with a grain of salt. Considering that I will most likely never fly
> a
> CAT II or CAT III approach, it is not that big a deal.
>
> --
> Peter