Fly a pair of RNAVS: can two RNAV approaches be better than one? A close look at TERPS reveals why the answer may be a resounding "yes.".
Likewise, it was "deja vu all over again" when looking at two RNAV approaches at Charlottesville, Va., (KCHO) for Runway 21. Why would this runway need two RNAV instrument approach procedures (IAPs)? I'm accustomed to seeing one approach with a variety of minima to choose from. Here, on the other hand, were two separate IAPs with only one set of minimums each. It was going to take some study to determine which should work best and why.
Why Ask Y?
I'll bet a Coke that the designers built the RNAV Y first. The most basic, and least precise, is the lateral navigation (LNAV) approach. LNAV minimums assume that your GPS predates the added precision of the Wide Area Augmentation System (WAAS) and only provides lateral guidance. LNAV is a non-precision, "dive and drive" approach. Because these procedures are the most basic, they can be flown by any level of IFR-approved box.
FAA Orders 8260.38A and 8260.48 lay out the TERPS expectations for GPS procedures and RNAV approaches. These texts provide the templates used when designing an approach. Just like sliding the paper lock stencil over the door to see where it would fit best, I imagine the TERPS gurus slip their obstacle identification template around on a detailed terrain chart looking for the best compromise between providing the most direct line to the runway and the lowest minimums.
Unless you enjoy words like "ellipsoidal" and "geodetic," don't read the aforementioned FAA orders. I'll cut to the chase: The TERPSters evaluate an area shaped somewhat like a paper airplane for obstacles and terrain. This shape is roughly one mile wide near the threshold and fans out from there. Just as the wings of a paper airplane angle up and away from the plane's centerline, so do the areas investigated.
Not having a 1:24,000 hiker's map handy, I looked to the Washington sectional for some clues. Ahha! Neatly tucked under the northern edge of the Class D circle was Piney Mountain, no doubt a high spot. The TERPSter probably slid the template west until the terrain was safely quarantined. This would explain the 1219 elevation just east of course and the final's 14 or so degrees of offset from the runway. With Piney Mountain safely to one side, the minimums for the approach popped out of the various formulas. The end result was the best anyone could hope for: KCHO ended up with an RNAV route that met straight-in criteria with a height above touchdown (HAT) of 752 feet.
The added precision of WAAS GPS made the LPV approach (localizer precision with vertical guidance) possible and another volume was written, FAA Order 8260.50, solely for LPV IAPs. In it, the trapezoids and ellipsoids used to identify obstacles look amazingly similar to those for an ILS. Not only does it have a tight, 800-foot sector near the threshold, it uses the concept of a glidepath qualification surface (GQS), which is an imaginary ramp below the intended glidepath that is checked for obstructions.
Using this completely different template, the TERPSters found that an LPV approach could be built on centerline with the runway because Piney Mountain would be avoided by way of the GPS-derived glidepath. Remaining on the slope is so critical, however, that a note says that even circling aircraft must use the RNAV glidepath--no dive and drive here. Flying the black line on this beauty, though, rewards the pilot with a low, 250-foot, straight-in DH.
Until RNAV (GPS) approaches become old hat, there will be instances like this that will challenge our perceptions of what is normal. In this case, a traditional GPS user is stuck with the RNAV Y approach and a WAAS flyer could choose from both. The WAAS pilot, though, needs to respect the additional capability and understand that if GPS fault-detection flags the LPV, there's no reverting to what we'd think of as "localizer-only" minimums. It's a missed approach, plain and simple.
Both approaches work; they just bring different pros and cons to the table, similar to the locks I picked up. Just be glad that we pilots aren't responsible for TERPSing our own procedures. We merely evaluate the plate to ensure we can meet its expectations. Instead of protractors and templates, preflight planning and pilot discipline are the tools required to deliver the ostensibly magic performance of GPS RNAV.
No longer locked out of his hangar, Ken Holston is flying WAAS at every opportunity.
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|Title Annotation:||APPROACH CLINIC; area navigation, terminal instrument procedure|
|Date:||Feb 1, 2008|
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