From: dfr@usna.navy.mil (PROF D. Rogers (EAS FAC))
Newsgroups: rec.aviation.ifr
Subject: DME/Radar rquired--why--Answer
Keywords: DME, Radar, Answer, IFR
Date: 25 Jan 93 17:17:53 GMT

G'day,
 
The orginal question was:
 
	>Here's an interesting ifr approach question. It's
	>mostly for those in the northeast who might have
	>the approach book or for readers of IFR magazine.
 
	>In a recent issue of IFR they discuss the ILS 7 approach
	>to Provincetown Muni (PVC) in Mass. (It's on page 233 of
	>the NE-1 approach book). The approach plate says
	>Radar/DME required, yet this is not an ILS/DME approach.
 
	>The editors comment that they don't understand the reason
	>for the Radar/DME requirement. I admit that I didn't until
	>I'd thought about it a bit.
 
	>How about it? Why is Radar/DME required for this approach?
 
	>The clue is to think like the approach DESIGNER and not as a pilot.
 
an additional clue was:
 
    >What happens if the aircraft is equipped with only ONE NAV?
 
A number of people indicated that the reason DME/Radar was
required was that the crossing radial from HYN was not accurate
enough. This is not correct. The TERPS says (p 32 sec. 286c)
"For a fix to be satisfactory for use as a final approach fix,
the fix error shall not exceed plus-or-minus 1 mile (see Fig
31), ......" Fig 31 shows that the error is +- 1 nm along the
centerline of the localizer. The HYN VOR is approximately 19 nm
from the FAF which yields an error of less than +- 1 nm (it
could be as much as 30 nm (approx) away and still be within +-
1 nm).
 
Phony is the final approach fix and the crossing radial from
HYA is accurate enough to identify Phony. However, think like
the approach designer. First what is the FAF. It is a point in
three space defined by the vertical plane containing the
localizer/GS, the horizontal plane containing the `approach'
altitude at GS interception and one OTHER piece of
information.  What can that other information be: a crossing
radial, a DME distance, an OM, a Radar position, .....
 
Now suppose you have only ONE NAV in the aircraft (this is quite
legal for IFR), then you do not want the pilot SWITCHING the
NAV for the localizer/GS frequency to identify the FAF using a
crossing radial--he will loose the localizer and more
importantly the glide slope function and it is no longer a
precision approach! So, since there is no OM, the designer has
one of three choices; make it an ILS/DME appraoch, require dual
NAV/COMs or require Radar in order to identify Phony. The least
restrictive is Radar required--but what if the Radar is out?
Then if you have DME you can shoot the approach anyway. No DME,
and no Radar you are legally out-of-luck.
 
This allows a very minimally equipped IFR bird (or a wounded
bird) to shoot the approach.
 
Makes perfect sense from the approach DESIGNERS point-of-view.
Remember he is looking for the minimum requirements.
 
However, I would have stated DME/Radar/Dual Nav required which
would cover even more situations but this may not be allowable
by regulation--at least I have never seen one like it.
 
Dave Rogers
 
P.S. Note that for an ILS the FAF is identified by GS
intersection at a particular altitude. Look closely at the IAP
and note that there is an arrow with the notation of 2000
pointing to a short line segment to the LEFT of the X indicating
the FAF fix. Also note the indication of 1979 above the FAF. This
is the altitude plane defining the FAF. The number 1979 is really
the only way that you have of checking the accuracy of the GS
receiver WHILE on the approach.

Organization: Sponsored account, Carnegie Mellon, Pittsburgh, PA