Direct fire and Flak Artillery rules are on pg 185 TacOps.
Ground units attacking Airborne Aerospace units, determining range. pg 107 TW
One of the key phrases is in the direct fire rules,
"the target hex must be no more then seventeen hexes away"
This isn't referencing range directly, instead its setting up a sphere of affect.
The reason I bring this up is because Megamek 47.7 is including altitudes as part of the range, an interpretation different from my own. This is actually the source of a number of my questions this year.