Page 51 of Total Warfare allows VTOLs to fly up to elevation 500, which is approximately 3,000 meters. Per the Low Altitude table on page 81, that is Altitude 9. As Weirdo pointed out in Ground Combat, there is currently no mechanism for VTOLs to be tracked on the Low Altitude map. This is by no means a finished product, and comments are welcome.
I propose allowing any VTOL suicidal enough to climb above Nap of the Earth (NOE), about 9 elevation levels, to be tracked on the Low Altitude map if it so desires (and, of course, the Low Altitude Map is in play). Low Altitude map hexes are roughly one map sheet in size, so any VTOL that can muster 17 hexes of movement on a regular map sheet can move one hex per turn. If a VTOL can achieve 34 hexes of regular movement, it can move two hexes. VTOLs pay no movement points for facing changes on the Low Altitude Map, but must track their facing for purposes of weapons fire.
VTOLs are notoriously slow climbers compared to true aerospace units, and must pay 9 MP to climb or descend between Altitude Levels 2 and 3, 9 MP between 3 and 4, 18 MP between 4 and 5, 30 MP each between levels 5, 6, 7 and 8, and a total of 120 (over multiple turns of course) to reach Altitude Level 9. When climbing or descending between levels that require more than one turn, the VTOL may not engage in weapons fire or spotting (to include TAG). The crew is too busy pushing the machine to its aeronautical limits.
For purposes of ground-based fire, VTOLs on the Low Altitude Map are considered to be in the middle of the map sheet represented by the Low Altitude hex they are in (hex 0808 on a "normal" map sheet) at an elevation 11 levels above the highest terrain on the map sheet. VTOLs that moved 1 Low Altitude hex receive a +4 Target Movement Modifier (for 17 hexes) and those that move 2 receive +6 (for 34). While flying this high and fast, VTOLs do NOT receive the +1 to be hit from being airborne.
For purposes of fire from other units on the Low Altitude Map, VTOLs receive no movement modifiers (they are above Nap of the Earth, after all), nor do they receive the +1 to be hit from being airborne.
Climbing to the Low Altitude Map requires a VTOL to climb to 10 elevation levels the highest terrain on the ground map sheet, and declare they are leaving the ground map. If they are already at elevation level 10 prior to moving, they can declare they are switching maps and move on the Low Altitude Map. Making this transition precludes weapons fire or spotting (to include TAG) during the turn of the transition.
Returning to the ground map requires a VTOL to not move on the Low Altitude Map (i.e., a VTOL may not move to another ground map sheet and descend in the same turn). A VTOL returning to the ground map this way starts in the middle of the map sheet (hex 0808 on a "regular" sheet) and spends MP to descend at least 1 elevation level from 10 elevation levels above the highest terrain on the sheet before moving. This also precludes weapons fire or spotting (to include TAG) during the turn of the transition.