Varies widely from location to location. Around here it is nearly impossible to "become" an air driver if you already have a job with UPS. Air drivers are typically based at facillities at or near airports, all of the ones I've talked with were new hires, and nearly all new and vacant jobs are posted in that building only, and if not filled, will be filled with a new hire. With a combo job you might be able to do some air driving for part of the day, although the shfts will be split between buildings, sort at one drive at another etc... Look for ads(online or otherwise) at airport facilities.