From: mpaton@oakhill.sps.mot.com (Michael J Paton)
Subject: DH determination (was Re: GPS and IFR Approaches)
Date: Tue, 5 Jan 1993 16:43:08 GMT
Reply-To: mpaton@tempest.UUCP (Michael J Paton)
Organization: Motorola Inc., Austin, Texas

Like Kerry, I apologize for the length of included postings. Originally, I
discuss the effect of DH determination by baro altimeter on the SYSTEM
accuracy of a CAT-I ILS, and get the folowing reply postings:-

In article <1993Jan5.004619.322@pony.Ingres.COM> kerry@Ingres.COM (Kerry Kurasaki) writes:
>In article <greg.726184317@saltydog> greg@saltydog.dpsi.com (Gregory R. Travis) writes:
>>
My blurb deleted...
>>Yes, but consider that there are often one to three "backups" to the altimeter
>>determination of DH:
>>
>>1.  Using a LOC/DME, DH can be determined by DME distance to the localizer
>>    antenna.
>>2.  Using the middle marker as a reference, DH can be determined by activation
>>    of the middle marker light*.
>>3.  Using a radar altimeter, DH can often be determined by radar altitude
>>    above terrain**.
>>
>>In a typical CAT-I ILS, I would tend to value the information from #1 above
>>the most highly.

None of this stuff appears to me to have sufficient accuracy, precision and
availability to serve as backup except in an emergency. A DME may very well not
be available, or may not be usable in the location you want it, or may go out
or saturate with excess interrogations when you want it. (eg our local 
Austin, TX DME has a request made in the A/FD that you turn your DME off
while on the ground. I don't know anybody who does this). The DME displayed
precision appears to be +- 0.1nm which translates to +- 30 ft assuming a 
3 degree glideslope and my math being correct, which seems like enough.
Seems may well not be good enough.

>>
>>*#2 suffers because: a) MM transmitters are often, but not always, co-located
>>with GS DH intercept, b)  The size of the MM beam also makes EXACT DH
>>determination difficult, AND c) MM in most countries (the USA included) are
>>going away.

Again, and as you mention, a Middle Marker is not always a required component
of a CAT-I ILS.

>>
>>**#3 is very good for approaches in which the terrain at DH is the same
>>elevation as the runway threshold elevation.  Obviously an approach to a
>>runway whose threshold began abruptly atop a 300' cliff would be a poor
>>candidate for DH determination via radar altimeter.  But, if I knew I
>>were making a tight approach into a airport on land as flat as a pooltable,
>>I think I would rather use the RA to determine DH over all else.  Certainly
>>if my RA said I was at DH and my GPS told me otherwise, I would go with the
>>RA.
>>
I can tell you've considered the obvious problems with this, so what follows
is going to look like a flame. It's really not meant to be, but as Kerry says
below, someone may read this wrongly. This practice is potentially
EXTREMELY DANGEROUS. For a period after LRRAs or Low Range Radio Altimeters
became cheap enough to be available in GA aircraft there were some accidents
where pilots had used the Radio Altitude and descended across a shallow
slope ditch and then were too low to the runway when the ditch ended. They
cleared the ditch, but got the approach lights. 

What you would be doing is inventing your own CAT-II approach. If you want to
do this, do it where some kind soul has implemented a CAT-II approach. 

>
>
>Sorry Greg, but I have to disagree with your choice of words.  There is
>no such thing as a "backup".  There is only one way to determine DA(H)
>with a CAT I approach, and that's with the altimeter on the GS.
>
>A better choice of words is "cross-check".  To the casual observer,
>it seems like any of the three methods you describe would work, and
>practically, they probably will, but in my training and understanding
>of the FARs, only the altimeter is legal.
>
>I just want it make absolutely clear that the examples you cited above
>may NOT be substituted for the DH.  Someone might read your post wrong.
>

They might indeed and frequently do. Personally, I distinguished

	"backup"

from

	 backup

but I do find it hard to communicate accurately on the net without sounding
like a pedant.

>It is also my understanding, and someone can correct ME if I'm wrong,
>that the 200' DH for CAT I approaches includes a possible 75' error
>in the altimeter.  There is that much safety margin, provided the
>approach is flown properly (on or above the GS, etc.).
>

I also believe this is so, and it makes me much happier to continue
believing this.

and taken out of order, Kerry adds-

>Now you may suspect that your altimeter is whacko due to the improper
>altitude readout at the outer marker.  Some of your choices are to
>use another approach, or the LOC (GS out) minima.
>
I believe some European air regulating bodies believe the altimeter is
enough of a problem to require 2 of them for IFR flight. Perhaps someone
can explain their rationale for this requirement, and if separate static
systems are required for the 2 altimeters.

I'm not sure 2 would make me much happier or more secure. I'm usually in
something of a quandry on a local VOR/DME approach when my 2 VOR's which are
both in tolerance say different (but in tolerance things). I know I'm supposed
to pick one and use it, but I keep trying to remember which one would be best.

Michael Paton
mpaton@tempest.sps.mot.com

Newsgroups: rec.aviation.ifr