It looks like for all instances of "Air", that's an incorrect word for the usage context.
e.g. We have track_wt -> "Train", road_wt -> "Truck", air_wt -> "Air".

That doesn't work. Perhaps "Aircraft" as a generic word for airborne vehicle? (Needs to cover airship, plane, helicopter, ...)
There's also get_waytype() in get_waytype.cc that links air_wt to "air" still. Hence the broken chain "air" -> air_wt -> "air_wt" -> dbg->fatal("get_waytype()","invalid waytype
If get_waytype() is changed to "air_wt" to maintain consistency, then every .dat file for aircraft in every pakset needs updating, not just speedbonus.tabs
I'd suggest reverting to "air" and change "Air" to something else...