In case you haven't noticed, the specification and approval process for aviation RNAV/GPS navigation systems is a mess. Peruse the recent changes to the Aeronautical Information Manual section on GPS and RNP and you'll see what I mean. The main reason that things are a mess has to do with the fact that much of the certification process has been made transparent to the pilot.
By transparent I mean pilots must know some pretty detailed stuff about their GPS before they can use it under IFR. It used to be sufficient to 1) check the aircraft you were going to fly for an Approved Flight Manual supplement that stated the GPS receiver could be used under IFR, 2) verify the GPS database had not expired, and 3) perform a RAIM prediction. Contrast that with using a VOR receiver where the only regulatory requirement is that you perform and log a successful VOR check every 30 days. There is no operational test available to pilots to ensure proper glide slope receiver operation - I guess we're just supposed to accept that on faith. The bar has apparently been set much higher for GPS receivers and the databases they use.
Designing RNAV departure, arrival, and approach procedures that will keep pilots from running into stuff is a complicated business, which helps explain why the certification process for aviation GPS receivers is pretty darn complicated, too. But that complexity should be opaque as far as the pilot is concerned, the way it is for ILS, LOC, LDA, SDF, VOR, and NDB procedures. Instead of hiding the complexity for RNAV/GPS procedures, the FAA has published some of the GPS certification data in the AIM (cf. Table 1-1-5) and apparently they expect pilots to understand the various Technical Specification Orders (TSO) and Letters of Approval (LOA). Just what we pilots needed: More trivia, more acronyms, and more chances to break regulations without realizing it.
If you haven't already guessed why I'm grinding away on this topic, it's because AOPA (among others) recently realized that an update to AC 90-100A made in March of this year contains a bombshell - the only GPS receivers widely available for GA aircraft that are now approved for substitution for DME and ADF are the Garmin 400, 500, and G1000 series. Suddenly all the old Apollo units, the GNS 480, the Chelton Flight Systems units, and many of the King GPS units that thousands of pilots have been using as a substitution for DME and ADF are suddenly no longer approved for such uses. Oh, and these units can't be used to fly T-Routes either.
The affected GPS unit presumably still comply with the original TSO under which they were certificated, but they no longer comply with AC 90-100A. And let's not forget about all the Approved Flight Manual Supplements out there that say these GPS units are approved for these uses. One would assume that they are now incorrect. Talk about The large print giveth and the small print taketh away!
A reasonable person has to wonder "Why did it take until the end of May for this to become common knowledge?"
I've spent considerable time trying to parse AC 90-100A and AC 20-153 and the only thing that is crystal clear to me is that the FAA isn't making it easy for us pilots. Nor are the manufacturers of GPS receivers. It seems that the reason that the majority of GPS receivers certificated under TSO 129 have been rendered non-compliant is that the manufacturers do not have a Type 2 Letter of Approval (LOA) for the aviation databases that they generate for said GPS receivers. And if there is no Type 2 LOA, then the GPS receivers' use is restricted until such time as an LOA is granted by the FAA. For some GPS receivers, the LOA is pending while for others there are no plans to ever obtain an LOA, hence they will forever be limited.
The interesting thing here is that the FAA has recognized the importance of valid data for GPS receivers. I appreciate this as I had to point out to Garmin on at least one occasion that their database did not have all the initial approach fixes defined on a plate for an ILS approach to an airport in Northern California. Equally interesting is that a GPS that complies with the original requirements specified in the TSO years ago can still be rendered less useful after the fact by an advisory circular.
While you're contemplating that cascading chain of dominos, consider this spreadsheet which lists the compliance of virtually all GPS, RNAV, and FMS from Boeing to Rockwell. That spreadsheet was apparently created by someone at the FAA at the end of 2006.
So did the FAA not notify the manufacturers of the non-compliant hardware until a few weeks ago, even though they apparently knew this in December of 2006 and the updates to AC 90-100A were released in March of this year? For their parts, Garmin and Rockwell-Collins, posted their LOAs right around the time AC 90-100A was updated in March. But did the manufacturers notify their potential customers, customers who purchased their products, not to mention the pilots who don't own but use their equipment? I don't know the answers to these questions, but I wish I did.
With the push for ATC user fees, the FAA keeps talking about providing better service to their customers. Based on the state of FSS under Lockheed-Martin, this GPS fiasco, and the fact that virtually all Boeing, Airbus, Embraer, Dassualt, and Gulfstream equipment already seem to be in compliance with AC 90-100A, one has to wonder just who are these customers the FAA keeps talking about?