FYI, the fx_N_gtrail effect is tied to altitude through AnKor's shaders so I have the speed range set fairly large. It turns on progressively as you drop below 5,000 ft where the moisture levels are higher, and switches off when you drop below stall speed at touchdown.
The texturemagic.ini entries that go with these is:
[HighAltSprite]
tr_condenstrail_basis1.dds=7500|500
tr_condenstrail_basis2.dds=7500|500
tr_condenstrail_basis3.dds=2500|-1500
helowashdust.dds=250|-150