So the target is in the weapon's arc, outside of the hex the VTOL is actually in, you're just at Lv.0?
Blaze away.
Battletech doesn't care if the weapon is mounted side-saddle on the nose like the cannon on late-model Hinds today, under the nose like the Apache, in stub-wings, or directly under the pilot's nose- if it's in arc, it can be fired, airborne or not. In fact, if your rotor gets damaged, this is the only way a VTOL can continue to fight. (Note that a landed WiGE can also perform this action)