16384x256 is a really stupid dimension, why would anyone do that? I mean it is already heresy to use non 1:1 ration textures, but luckily Torque3D is able to tolerate those, but 16384x256 is probably stretching the limit too far.
A mipmap is the texture divided by 2 to create the next lower res version, now try dividing 256 by 2 over 13 times.
256, 128, 64, 32, 16, 8, 4, 2, 1
As you can see you cannot even divide those 13 times, it is already over at 8 and even if you could, it would make no sense at all, on the other hand the 16384 can and has to be divided a lot more.
My guess is the flight gear developers do not use mipmaps or they are amateurs that have no clue how to do things or there is a secret absurd reason why it has to be that way.
Textures should always be in .dds format, which already includes the pre-made mipmaps inside the image and improves performance by a lot and atlas textures should be in 1:1 ratio or 1:2.