Friday, February 21, 2014

Approach procedure design considerations - example

During my interview with the Stuck Mic AvCast, we discussed a few of the approaches I designed while at the FAA.  I was asked several questions about what kind of considerations go into approach procedure design, so I wanted to go into more detail using an example.  For this, I'll discuss the St George, Utah, KSGU (formerly KDXZ) LDA/DME RWY 19 approach.  

This airport was one of my favorite projects.  The complexity of the terrain, the fact that it was at a brand-new airport, and the unique orientation of the localizer posed some interesting challenges.

This is an LDA approach, not a "localizer", that also has a glideslope installed.  Note the position of the antenna, at the approach end of the runway (normally a localizer is oriented at the far end of the runway, to provide signal guidance all the way down to rollout in some cases).  Also, the antenna itself is physically aimed about 7 degrees to the east instead of directly in line with the runway.  This siting was necessary in order to aim the final approach course between two mountains.  Had it been a standard straight-in alignment, the final would have come too close to the 10024 MSL peak.  So it's essentially an offset ILS.

This procedure has quite a few stepdown fixes, especially in the intermediate segment before the Final Approach Fix of PAYLR.  Determining the location and number of these fixes took a lot of manual evaluation of the terrain contours starting from HOPEB inbound.  You can see the rising terrain on the east side of final, but one of the most troublesome areas was actually the medium tan terrain on the northwest abeam WATLA and IPPOD.  It's higher and rises steeper on that side.  Fix placement is a balancing act - move the fix further south ("down" the mountain) and maybe you can get a lower altitude, but then the distance to the next fix is reduced, so that might require an excessive descent.  Move it further north ("up" the mountain), and you get more distance to descend but also a higher altitude to descend from.  Depending on the exact geometry of the situation, you may have to try several times to get just the right combination, as I did in this case.

One last thing - notice the note in the profile view "Loc unusable inside 0.7 DME".  What does that mean?  On any localizer, the signal gets narrower as you get closer to the antenna.  Usually, the localizer antenna is at the far side of the runway, so it's at least a mile away once you get to the missed approach point - getting too close to the antenna is not a factor.  On this procedure, however, the antenna is right there at the approach end of the runway.  So, at 0.67 nm from the antenna, flight inspection deemed the signal was too narrow to follow anymore, hence the restriction at 0.7 DME.  Notice this is also the location of the MAP when not using the glideslope.  Also, the DA for the LDA/GS line of minima is calculated to put you right at that 0.7 DME point when you reach 3170 MSL (286 feet above the touchdown zone elevation).

Those are just a few of the various types of considerations that go into an approach procedure like this. Generally, the more complicated the terrain, the more factors must be considered. Sometimes it can seem like a great big puzzle!

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