BDL GPS-6 sillyness.

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

http://www.airnav.com/depart?http://...6/00460RY6.PDF
http://www.airnav.com/depart?http://...6/00460RZ6.PDF

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

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?

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.

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?

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.

In article , wrote:
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.

Wow, this really blows my mind (a bad state to be in when initiating a
missed approach). I can just imagine some CVR tape having: "Not in
sight, going around, power up, pitch up, gear up, positive rate, hey,
why the f*** is the GPS doing that!?" on it.

In these two procedures, we've seen three different ways of spelling "go
to". There's "direct", "course", and "track". And there's two
different modes the GPS can be in, "fly leg" and "OBS". How do those
three words correspond to the two modes?

If I understand you right, here's what should happen on my CNX-80 on the
Z approach (assuming it's in the database to begin with!):

1) I load the approach, and the box will sequence through PENNA, HUNEE,
and RW06, at which point it will go into SUSP mode. Up until this
point, my CDI indications will ignore how I've got my OBS set (although
good practice says to set it to 058, just like on an ILS).

2) When I decide to go missed, I hit the SUSP button, and it will
sequence to FARIL, but it will be in OBS mode instead of "fly leg" mode.
If I had followed good practice earlier and set 058 on the OBS, I'm
fine. If not, I'll suddenly get a chance to use some excellent pilot
vocabulary.

3) Since OBS mode (at least on the CNX-80) prevents automatic
sequencing, when I get to FARIL, I'll once again have to hit SUSP to get
guidance to ERICS. That should be good for a few more WTF's.

In the "Y" procedure you are free (and actually expected) to use the
OBS mode all the way to ERICS.

Ummm, the Y version is the one that says "direct ERICS". Why would I
want to be in OBS mode instead of "fly leg" mode?

Roy Smith wrote:

In the "Y" procedure you are free (and actually expected) to use the
OBS mode all the way to ERICS.

Ummm, the Y version is the one that says "direct ERICS". Why would I
want to be in OBS mode instead of "fly leg" mode?

I could have worded it better: you are expected to use OBS or direct-to once
you're ready to navigate the missed approach. Direct-to is a legs mode in the
sense it builds a leg on the fly from present position to the selected waypoint;
but it is not a procedural leg.

In article , wrote:

Roy Smith wrote:

In the "Y" procedure you are free (and actually expected) to use the
OBS mode all the way to ERICS.

Ummm, the Y version is the one that says "direct ERICS". Why would I
want to be in OBS mode instead of "fly leg" mode?

I could have worded it better: you are expected to use OBS or direct-to once
you're ready to navigate the missed approach. Direct-to is a legs mode in
the
sense it builds a leg on the fly from present position to the selected
waypoint;
but it is not a procedural leg.

I guess I'm still confused. From a TERPS point of view, what you're
saying is kind of interesting. But, from a pilot's point of view, all I
really want to know is what buttons I gotta press. And from a pilot's
point of view, I can't see that either the Y or Z procedure should
require me to do any more than hit the SUSP button once.

It seems to me that the TERPS guys have made this far more complicated
than it has to be. For all practical purposes, both missed procedures
boil down to "go in a straight line from RW06 to ERICS". Adding in
intermediate waypoints, multiple modes, and confusing phrasology is just
making things complicated with no practical advantage. Something has
gone amuck here.

Roy Smith wrote:

It seems to me that the TERPS guys have made this far more complicated
than it has to be. For all practical purposes, both missed procedures
boil down to "go in a straight line from RW06 to ERICS". Adding in
intermediate waypoints, multiple modes, and confusing phrasology is just
making things complicated with no practical advantage. Something has
gone amuck here.

It has indeed gone amuck. But, I am just the messenger.

Having said that, from my perch, there is *some* justification for what they are
doing, but it is all moving way, way too fast.

If you operated out of some deep mountain valley airports in the West, you would
like this stuff to differentiate between LNAV at Bradley and LPV at Rifle,
Colorado.