Friday, April 25, 2008

How Low Can You Go?

Continuing with the IFR minutiae of my last few posts, a regular reader asked a question about the new GNSS MEA (Global Navigation Satellite System Minimum En route Altitudes) that are starting to be depicted on low altitude en route charts. So what are these new MEAs, why do they exist, and when would you use them?

Traditionally, a minimum en route altitude on an airway was the lowest altitude at which an aircraft could be operated under IFR and still have adequate ground-based radio navigation reception and two-way radio communication with ATC. There sometimes is a lower altitude published that can be used - the Minimum Obstruction Clearance Altitude - but you need to be within 22 miles of the ground-based navigation station that defines the route segment and no guarantee of two-way radio communication with ATC is provided.

"Special MEAs" were first developed in Alaska under the Capstone project (the precursor to the NextGen system that will supposedly change the National Airspace System). The purpose of these Special MEAs was to allow aircraft to fly at lower altitudes on an airway to stay out of icing conditions while still providing two-way communication with ATC and adequate obstruction clearance. The new GNSS MEAs seem to be an extension of this concept to the lower 48 states in the U.S.

You may be wondering why these new MEAs are called "GNSS MEAs" (Jeppesen uses the term "GPS MEA"). GNSS is an international standard and the U.S. GPS/WAAS is just one implementation of that standard, or at least it will be when all the international requirements are met. As we used to say in the software world - "The nice thing about standards is that there are so many of them." One reason this distinction is important is that WAAS coverage in the U.S. GPS system, which depends on ground-based reference stations, is not designed to provide world-wide coverage. So while you may be able to use your GPS in Nepal, don't expect to have the additional accuracy that differential GPS (WAAS) would provide if you were located within its service volume.

To use these new, lower GNSS MEAs, it's not clear if your aircraft must be equipped with an appropriate, IFR-certified GPS/WAAS receiver (TSO C145a and TSO C146a), which would include the Garmin WAAS-enabled G1000, GNS 530W/430W. Older, non-WAAS GPS receivers with RAIM capability (TSO C-129), including the non-WAAS G1000 and the GNS 530/430 may also be okay since there are no regs I can find saying they aren't okay for this lower MEA. The latest version of the Instrument Procedures Handbook doesn't provide any information or guidance on GNSS MEAs. Jeppesen airway manuals use different terminology and contain only a brief description.
GPS MEAs are supplemental to and lower than the regular MEA. GPS MEAs are not established for every route, or for every route segment. The absence of a GPS MEA means one has not been provided and the regular route MEA applies. A GPS MEA may be higher than, equivalent to, but not lower than a Minimum Obstruction Clearance Altitude (MOCA) associated with a given route segment.

GNSS MEAs are depicted in blue with a G suffix, such as 4000G for a 4000 foot MEA for GPS/WAAS-equipped aircraft. The IFR Chart User's Guide published by NACO offers a brief description.

For their part, AOPA is lobbying for GNSS MEAs on one T-route in Oregon that would only provide adequate obstruction clearance without two-way radio communication with ATC. The idea is to provide the absolute lowest altitudes in an area where airframe icing is particularly prevalent, but doing so would introduce a new-to-me concept: An airway MEA that could only be used by one aircraft at a time, similar to the way class E airspace around a non-towered airport is reserved for one IFR aircraft at a time.

While there is a WAAS service model for the U.S. that is used to predict degradations in WAAS service that may affect RNAV approaches, only the larger airports have WAAS NOTAM service. This means that if you plan to fly an RNAV approach into an airport without WAAS NOTAM service or if you plan to use a GNSS MEA, you'd best be using an approved Fault Detection/Exclusion program during your preflight planning. And you best have a plan B, too. Right now, how WAAS outages might affect the use of GNSS MEAs during en route navigation does not seem to be very well thought out.


FAAFollies said...

FWIW... I work as a controller at ZSE, Seattle Center. We noticed the new GNSS MEAs show up on our charts a few months back.

Naturally, nobody knew what they were or when they had appeared or what the deal was with them.

(Thank the FAA's "run like a business" mentality, which means they keep cutting staff back so we don't get properly briefed or taught about stuff.)

When we asked, the word was that we should NOT use them until they're fully briefed and double-checked and blah blah blah.

So be warned- in many facilities you might not be allowed to use the GNSS MEAs. If you ask me for it at ZSE, I'll deny it, based on the guidance of my management.

John said...


Thanks for that information! It's always good to know what is going on on the other side of the frequency.

It's a shame that one arm of the FAA is creating procedures that another arm of the FAA doesn't know how to support, especially since the concept is pretty simple.

It's certainly not the individual controller's fault.

eric said...

John, as always, thanks for a great overview on this. I'm glad to see that my initial reaction ("looks like it needs WAAS!") was correct, based on the description in the low enroute legend, and I really appreciate the overall information you've wrapped into one article.

faafollies - I operate out of KPAE as an instructor and we often run into similar GPS situations. Our GPS RWY 34 approach is verboten ("sorry, we'd have to shut down Sea-Tac") at all times. The same kind of bureaucratic changes are seen in the last revisions of KCLM's ILS approach, which force pilots to fly 20-30 nm past the airport at 6000, picking up ice the entire time.

Anyway, I'd like to extend a grateful thank-you to everyone at ZSE for dealing with our incessant practice approach requests and the overall great job you guys do for us!