DME/Radar rquired--why--Answer
PROF D. Rogers (EAS FAC)
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.
Skip Guild
(PROF D. Rogers (EAS FAC)) writes:
...
Your answer is interesting but one issue is still left unresolved. As I
pointed out in my answer, the logic you apply to this approach would appear
to be applicable to the Portland ILS RWY 29 approach (Portland, Maine) which
is almost identical to the Provincetown approach (except for distances,
altitudes, and angles of course) in that it is an over-water approach
without OM. However the Portland approach does not require DME or radar.
Why?
Kerry Kurasaki
>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.
>
This also means that the FAF on an ILS can change based on the
altitude you are cleared to intercept the GS. Example, assume the
depicted intercept altitude is 2000, but ATC instructs: "Maintain
two thousand five hundred until GS interception."
or, alternatively,
"Maintain one thousand five hundred, cleared for the approach".
The FAF is thus moved "out" or "in" respectively.
I've actually heard this at O'Hare over the aircraft radio:
"United 123, maintain seven thousand, intercept the glideslope,
cleared for the approach."
I'm not familiar with the ORD approach, but I assume that the GS
intercept point is along the final approach segment.
Comments?
Kerry
Lars-Henrik Eriksson
Technically, the poinmt at which you intercept the glide on an ILS
is called final approach *point* (FAP).
I believe that the FAA uses the term "FAF" for both the FAF and FAP.
The FAP may or may not
coincide with the FAF, and in fact an FAF is not necessary for
every approach (as pointed out earlier).
This is stressed by the fact that in our (European, based on ICAO) rules
for setting up a MDA for a non-precision approach, it is *explicitely*
considered whether the approach has a FAF or not; in the first case,
a margin of 75' is added, in the latter case this becomes 90'.
A good example of an approach without a FAF is an NDB or VOR (*not*
VORDME) approach where the facility on located on, or very close to,
the airport and there is no other suitable facility that can be used
to define a FAF.
Usually an NDB approach would use a locator at approximately 4 miles
from the runway as both the IAF and the FAF. The MAP would be
determined by timing from the FAF. If the NDB is too close to the
runway, it cannot be used as the FAF, but it can be used as the MAP.
--
Lars-Henrik Eriksson Internet: l...@sics.se
Swedish Institute of Computer Science Phone (intn'l): +46 8 752 15 09
Box 1263 Telefon (nat'l): 08 - 752 15 09
S-164 28 KISTA, SWEDEN Fax: +46 8 751 72 30
Wolfgang Diestelkamp
[ FAF changes, if cleared altitude is different from standard ]
>
> Comments?
Technically, the poinmt at which you intercept the glide on an ILS
is called final approach *point* (FAP). The FAP may or may not
coincide with the FAF, and in fact an FAF is not necessary for
every approach (as pointed out earlier).
This is stressed by the fact that in our (European, based on ICAO) rules
for setting up a MDA for a non-precision approach, it is *explicitely*
considered whether the approach has a FAF or not; in the first case,
a margin of 75' is added, in the latter case this becomes 90'.
--
Wolfgang Diestelkamp
w...@cs.tu-berlin.de
wolf...@first.gmd.de
John R. Gersh
>
>...
>
>Your answer is interesting but one issue is still left unresolved. As I
>pointed out in my answer, the logic you apply to this approach would appear
>to be applicable to the Portland ILS RWY 29 approach (Portland, Maine) which
>is almost identical to the Provincetown approach (except for distances,
>altitudes, and angles of course) in that it is an over-water approach
>without OM. However the Portland approach does not require DME or radar.
>Why?
Indeed, and it's even more curious. I just did a quick scan through
the NOS NE-1 approach book. There are eight approaches in the category
of interest (ILS overwater approach with no outer marker, intersection or
DME determination of the FAF of the associated localizer-only
approach):
Bridgeport, CT (BDR) ILS-6
Groton, CT (GON) ILS-5
Hyannis, MA (HYA) ILS-15
Martha's Vineyard, MA (MVY) ILS-24 (My most-frequently-flown
approach!)
New Haven, CT (HVN) ILS-2
Portland, ME (PWM) ILS-29
Portsmouth (Pease), NH (PSM) ILS-34
Provincetown, MA (PVC) ILS-7 (The one under discussion)
(I made only a quick scan; I might have missed some.)
Of these approaches, only the HYA and PVC approaches require radar or
DME, the rest do not, so it seems like there may be something other
than the need for single-nav operation involved here.
A very interesting case, however, is the Portsmouth approach (Pease
International Tradeport, used to be Pease AFB). There are two
approaches, ILS-34 and ILS/DME 34. As far as I can tell, these two
approaches are identical (a straightforward ILS approach, but with no
marker beacons, nothing odd about the miss), except that the ILS/DME
approach has a 50' lower DH! (And lower a lower visibility minimum for
cats D,E.)
So what _is_ going on here??
---------------------------------------------------------------------
John Gersh John_...@jhuapl.edu
The Johns Hopkins University Applied Physics Laboratory
Johns Hopkins Rd., Laurel, MD 20723 (301) 953-5503
PROF D. Rogers (EAS FAC)
!In article <26...@usna.NAVY.MIL! d...@usna.navy.mil
!(PROF D. Rogers (EAS FAC)) writes:
!Your answer is interesting but one issue is still left unresolved. As I
!pointed out in my answer, the logic you apply to this approach would appear
!to be applicable to the Portland ILS RWY 29 approach (Portland, Maine) which
!is almost identical to the Provincetown approach (except for distances,
!altitudes, and angles of course) in that it is an over-water approach
!without OM. However the Portland approach does not require DME or radar.
!Why?
How about a different designer and I am not kidding. Don't forget the
human factor.
Dave Rogers
Les Niles
>...
>A very interesting case, however, is the Portsmouth approach (Pease
>International Tradeport, used to be Pease AFB). There are two
>approaches, ILS-34 and ILS/DME 34. As far as I can tell, these two
>approaches are identical (a straightforward ILS approach, but with no
>marker beacons, nothing odd about the miss), except that the ILS/DME
>approach has a 50' lower DH! (And lower a lower visibility minimum for
>cats D,E.)
>So what _is_ going on here??
MM. With no MM installed, that penalty would always be there on the
ILS-34, but the ILS/DME-34 could use a DME fix instead of the MM.
Now that the 50 ft penalty has gone away, the distinction between the
ILS-34 and ILS/DME 34 approaches *should* disappear, but since
they're considered different approaches, the ILS-34 probably requires
recertification at the lower DH -- it doesn't get taken care of by a
global change to the inop components table.
-les ni...@parc.xerox.com
Skylane N2461G
Lee Cooprider
>How about a different designer and I am not kidding. Don't forget the
>human factor.
Also, TERPS change over time, and are subject to different interpretations.
The same person or a different person might interpret the same situation
differently.
An approach, once published, says set until someone decides it needs to be
changed. Perhaps the FAA pilots who check out approaches chose to raise a
concern about one but didn't necessarily look through the charts for all other
approaches that might have the same problem (e.g. a nuisance for a single NAV
pilot). Or pilots who frequent Providence complained about that approach, but
no one called in about the one in Portland (or where ever).
I expect that consistency among approach design minutia is found in main, but
not in the specific.
--
Lee W. Cooprider PP-ASEL/IA
Lee_Co...@vos.stratus.com Skylane N2908F "Wiley"
Kerry Kurasaki
>>...
>>A very interesting case, however, is the Portsmouth approach (Pease
>>International Tradeport, used to be Pease AFB). There are two
>>approaches, ILS-34 and ILS/DME 34. As far as I can tell, these two
>>approaches are identical (a straightforward ILS approach, but with no
>>marker beacons, nothing odd about the miss), except that the ILS/DME
>>approach has a 50' lower DH! (And lower a lower visibility minimum for
>>cats D,E.)
>>So what _is_ going on here??
>Until a few months ago, there was a 50 foot DH penalty for an inop
>MM. With no MM installed, that penalty would always be there on the
>ILS-34, but the ILS/DME-34 could use a DME fix instead of the MM.
>
>Now that the 50 ft penalty has gone away, the distinction between the
>ILS-34 and ILS/DME 34 approaches *should* disappear, but since
>they're considered different approaches, the ILS-34 probably requires
>recertification at the lower DH -- it doesn't get taken care of by a
>global change to the inop components table.
>
Uhhhh, the only substitute for an inop MM is either a compass locater,
or precision RADAR (PAR). Legal substitutes for the OM include RADAR,
DME fix, or compass locater. Not looking at the approach, it is
still a mystery to me.
James Olsen
>I just did a quick scan through the NOS NE-1 approach book. There are
>eight approaches in the category of interest (ILS overwater approach
>with no outer marker, intersection or DME determination of the FAF of
>the associated localizer-only approach):
> ...
>Of these approaches, only the HYA and PVC approaches require radar or
>DME, the rest do not, so it seems like there may be something other
>than the need for single-nav operation involved here.
The HYA ILS RWY 15 approach is easily explained: the FAF intersection can
only be identified by DME or radar; no cross-radial fix is depicted, so
DME or radar are the only ways to identify the FAF.
For the PVC approach, I hope you realize that the HYA VOR is not a happy
camper. It has ten different sectors published as 'unusable'. Since one
of those sectors includes the approach route from WOMEK to PHONY (the FAF),
I don't think we need look any farther to explain the DME/radar
requirement on that apporach.
As to Dave Rogers' single-nav-requirement answer: since we have five
cases that contradict that answer, and one case (the PVC approach) that
can be explained by other factors, count me as a skeptic.
--
Jim Olsen ol...@cag.lcs.mit.edu "Tache d'etre heureux." -- St-Exupery