This probably thread worthy in Cleared For The Approach. My first guess was it has something to do with that 1216’ obstruction about a mile or so on final. And that descent below DA is figured into the calculations because when you make the ‘decision’ you’re going to descend a little further while the engine spools up and you get up and outta there, while MDA is a ‘hard deck’ so to speak. But then there’s the RNAV(GPS) Approach having much lower minimums that seems to defuse that logic. And then
@masloki brought up the ‘coupled approaches’ not authorized thing.
@RussR , when ya get some time, will ya chime in here?
Without looking this specific one up, I have a best guess.
Re: why the RNAV and ILS are different:
LPV and ILS final evaluations are the same, in area, obstacle clearance, DA calculation method, etc. But what is NOT, is the missed approach. On an ILS or an LPV (or an LNAV/VNAV or an RNAV (RNP)), if there is an obstacle in the missed approach that cannot be routed around, the only solution is to raise the DA so that 1) the missed approach starts sooner and therefore has more time to climb to clear the obstacle, and 2) the starting altitude is higher, helping to clear the obstacle.
GPS missed approaches are based on the GPS signal (duh), but ILS missed approaches are based on a nearby VOR signal, or like in this case, an NDB signal. Since GPS is so much more accurate than those others, the GPS missed approach evaluation area is much narrower and often can avoid obstacles that would impact an ILS missed approach.
In addition, the GPS missed approach can go anywhere, whereas the ILS missed approach design is severely limited by the surrounding NAVAID structure. Meaning, there are lots more options for a GPS and very limited options for an ILS. In this case, the GPS goes straight out, which is the easiest of course. But the ILS turns back around to return to the NDB. This means the whole turn area has to be evaluated too, which is pretty large, since a Cat D aircraft doesn't turn nearly as quickly as a Cat A.
Therefore, it's my guess that the obstacles causing the higher DA are either the 1732 tower or the 1714 tower, or both.
Re: Why the LOC is lower than the ILS.
This happens sometimes with LPV and LNAV as well. Basically, for an obstacle in final, the DA allows you to break out and see the offending obstacle in front of you. So if the geometry is just right (or wrong), the increase caused by an obstacle can be greater for the LPV/ILS than the LNAV/LOC.
Also, in this case as I suspect the missed approach is where the issue is, the design criteria for ILS and LOC is pretty drastically different actually. The LOC allows an immediate turn at the MAP, whereas the ILS allows for height loss and a straight section before turning. Since the evaluated areas gets wider at the MAP, this can cause situations where the ILS missed pulls in many more obstacles off to the side than the LOC missed. Hard to explain, if anybody wants I can probably find a picture.
Re: the autopilot coupled NA note:
This has no effect on DA. The DA is determined through evaluation of the obstacles, and then flight inspection determines if the signal is usable by autopilots all the way down. Some autopilots cannot deal with the signal getting a little squirrelly whereas the human brain can if hand-flying.
www.nbcnews.com
