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#1
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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? |
#2
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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. |
#3
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#4
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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. |
#5
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#6
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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. |
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