On a low-altitude IFR chart, the T-airways (GPS) have (for the most part) an MAA of 15,000 feet. Why is this?

Here's a link about T-Routes. They are Area Navigation IFR Terminal Transition Routes:

http://www.aopa.org/whatsnew/newsitems/2005/050929...

Here's a link about "T" routes to Puerto Rico being redesignated as "Y" routes:

http://209.85.173.104/search?q=cache:UTcDoiEdOnYJ:...

The "T" is for Terminal. That is the reason for the altitude limitation.

As for high altitude use of GPS on published area nav routes, check out the "Q" routes. Have flown them over the Gulf of Mexico at FL 390 and FL 400.

I could very well be wrong about this, and if I am please correct me...but doesn't the "T" in T-Airway stand for "Terminal?"

That would mean that in order for that routing to vbe assigned, it would be given by a TRACON, rather than an ARTCC, as most IFR clearances are assigned. The TRACON may simply have other traffic flying over those airways at higher altitudes (i.e. SIDs and STARS) and need to limit TEC traffic to lower altitudes than those the enroute traffic is using.

Terminal Enroute Clearances (TECs) are assigned by the local RADAR approach controller (TRACON) and do not require approval from Center controllers (ARTCC). These routes are typically designed to keep you seperated from other IFR traffic, but since the clearance does not go through the ARTCC, it can be handed out with no knowledge of other IFR traffic. For that reason, the routes and altitudes assigned to TEC clearances are different than those assigned to other IFR traffic. It may well be that there is an agreement between whatever ATC facilities you are dealing with that they are allowed to assign altitudes only up to 15,000'. That could be reflected on the Low Altitude chart as an MAA simply to avoid confusion.

Low Enroute Chart

Low Enroute Chart Legend

I follow your reasoning and MAA is usually either terrain or NAVAID reception driven. Since the GPS reception is not masked by terrain (enroute), the sectional must have a bunch of terrain. What's the MAA's on the nearby Vicotr routes? It' might actually be higher if the terrain masked the VORTAC.

EDIT: Hmmm, that's really strange. It's got to be some sort of procedural deconfliction thing then.

. RNAV terminal transition routes, referred to as Tango or “T” routes, allow Global Positioning System (GPS) equipped, instrument flight rules (IFR) operations to efficiently fly around or through Class B and Class C airspace areas.

An additional Tango route was established in the Outer Banks, North Carolina, area to replace an airway that became unusable with the decommissioning of a supporting nondirectional beacon..

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The number in the box is the total distance between the VOR's. The smaller number is the distance of an intermediate route segment. Look closer, you'll find the fixes that define it the 63nm segment. The intermediate segment distances added up should equal 114nm. P.S. Read the chart legend. It's explained there.

Where are you looking? I looked at several low-altitude en-route charts and T airways are scarce, and the few I saw had no MAAs specified.