Roy Smith wrote:

Windsor Locks (BDL) has two different GPS RWY 6 approachs, Y and Z:

The procedures are almost identical, and I'm scratching my head trying to
figure out the differences.

Lots of places have the same situation. But, try loading the Y approach.
It's not in your RNAV IAP database. The Jeppesen Y chart tells you this,
but the NACO chart does not. Nonetheless, not in the database, no can use.

Most of the avionics in use today cannot support two of the same approach
charts to a given runway end. The box hiccups. So, what you're seeing is a
tug-of-war between the FAA on the one hand and Jeppesen and the avionics
vendors on the other hand.

The first difference is that Z has LNAV/VNAV minima published, instead of
Y's step-down fix. Oddly enough, the step-down version gives you
lower minima. Why?

It isn't odd at all. The LNAV "Y" approach has a stepdown fix to achieve a
lower LNAV MDA. Until a recent change in criteria, stepdown fixes were not
permitted on procedures with VNAV minimums; thus the reason for the higher
LNAV MDA in the "Z" procedure. Because of a lot of adverse user feedback
from AOPA and others, this is being changed. Jeppesen and the big-player
avionics vendors have been running this show rather than the FAA. And,
these folks are lap dogs for the airlines and heavy iron; and didn't want
any "pesky" non-precision stepdown fixes messing up (in their view) their
VNAV final approach segment.

As to the VNAV visibility minimums being higher than the LNAV visibility
miniums, that is nothing new; the same occurs with ILS approaches with high
DAs. It's the trade off for the safety of a precision approach. The MAP is
always the DA with a precision approach, so where the height above touchdown
gets higher, the MAP moves further away from the runway threshold. And, so
up goes the visibility to provide reasonable assurance the runway or ALS
will be sighted not later than DA. You have several ILS approaches around
the country with the same result.

The missed procedures are equally perplexing. Y is a straight-forward
"climb to 3000 direct ERICS". Z has the more complicated "Climb to 3000
via 058 course to FARIL WP then via 060 track to ERICS". This brings up
two questions.

First, what's the difference between flying a course and flying a track?
I've always considered the two words to be synonyms. The fact that they
use two different words makes me think there's some subtle difference I
should be aware of.

Track is what you do to make good a course. In conventional navigation that
is a distinction without a difference. But, with RNAV you get into some
pretty subtle differences because of the nature of the normal means of
navigation; i.e., the legs or automatic mode. In legs mode the equipment
knows only one thing; the track between Waypoint A's LAT/LON and Waypoint
B's LAT/LON. In the pure sense this has nothing to do with either true or
magnetic course. It is a great circle route, tied together by two geodetic
positions. Course, on the other hand, is defined by reference to either
true or magnetic north.

And, you say, so?

The practical implication is that in the "Z" procedure you are free to use
the OBS mode from the MAP to FARIL, but you must use the legs mode from
FARIL to ERICs. Why is this all so much more complex with the "Z" procedure
than the "Y" procedure? Because the "Z" procedure has VNAV DA minimums,
thus the missed approach criteria used is much more complex than the missed
approach criteria used in the LNAV-only "Y" procedure. These difference in
criteria might not make much difference at Windsor Locks, but they will at
some other locations. Missed approach criteria for DAs is much more complex
than for MDAs. And, it is getting more so now that we not only have ILS, we
have VNAV and LPV.



Second, why complicate things with FARIL at all? The two procedures
have you flying almost exactly the same ground tracks as each other (if
my math is right, the Z version moves you about 600 feet to the left).
The only nearby obstacle is the 314 tower east of the airport, but
you're already 300 feet above that before you even begin the missed.
What's the deal here? What bit of TERPS trivia does FARIL satisfy?

From a containment area standpoint, the differences come into play at FARIL,
because a lot less lateral airspace is consumed with the FARIL-ERICS leg,
than with the Runway 06-ERICS *course* in the "Y" procedure. In the "Y"
procedure you are free (and actually expected) to use the OBS mode all the
way to ERICS. Not so with the "Z" procedure. Again, none of this matters
much at Windsor Locks, but could at some high mountain airport. So, if they
design the procedure to VNAV specs, they must use a VNAV missed approach
procedure.

What is so complex and confusing about all of this are; 1. The criteria are
evolving and changing; 2. No one is doing an adequate job of explaining all
of this to the users, 3. Some avionics vendors aren't even getting the
intended implementations loaded correctly in their databases.