Straybow
SOC-14 1K

If flying a plane is riding a bicycle, flying a helo is juggling flaming, razor-sharp, double-ended daggers on a unicycle balanced at the top of a long flight of stairs. Except that unicycle skill would help significantly in riding a bicycle the first time, whereas helo experience doesn't help untrained fixed wing flying.
A helo pilot is only marginally better at flying a fixed wing than somebody who has played lots of MicroSoft Flight Simulator on the computer; either might be able to land a plane without killing anybody as long as the tower talked them through the procedure. Fixed wing pilots do not attempt to fly helos. If they're in one and something happens to the pilot, prayer is more productive than grabbing at controls they won't know what to do with (and that's not a joke).
What you need is two classes of helos. One is the fly-by-wire, AI assisted models of the future that safely interpret fixed wing craft controls (which might be called "Roto-planes" or "Ornithopters" or something). These would cascade with fixed wing skills. The other is cyclic/collective helocopters outside the fixed wing cascade.
•In addition, you need one-way cascading. Our primitive helo pilot skills would cascade to the advanced Ornithopters or whatever (the AI interpreted controls might even include options to mimick the cyclic/collective controls of older helos). Pilot skill on the advanced types would not cascade to primitive choppers unless their instructors forced them to practice with alternate controls.
•Jet [Harrier] and prop [Osprey] VTOL craft are distinct non-cascading classes as well, both with fixed wing pilot as a prerequisite. Advanced AI assisted models (call it mixed mode flight) might cascade with fixed wing of the same type (jet or prop), while jet or prop VTOL will cascade with any kind of advanced mixed mode.
•I would argue that regular jet and prop skills don't truly cascade either. Expert jet pilots make stupid prop flying mistakes that have cost their lives, and vice-versa. Skill of 2+ in one type gives level 1 skill maximum for the other type (they know the basics well enough). In the future many societies might not have prop planes at all, except for the eccentric hobbyist. That could complicate the picture.
•If an additional class of winged aircraft that utilize thrusters other than prop/jet is included (some variant of AG, repulsors, poofda-tech, etc), other fixed wing skills would cascade to winged poofda-craft. Mystery tech craft skills of 2+ would confer level 1 max for jet or prop (assuming the pilot has ever seen these primitive things, otherwise she's clueless).

If the character takes at least one skill in the lesser, then the character's greater skill can be considered to cascade. The only benefit then is if the character has at least skill 3 of one type. This could be noted on a character sheet as, eg, "Pistol cascade" below the long arm skill or "long arm cascade" below the pistol skill. Then on, it is assumed the character practices enough with the cascade type to maintain cascade skill.