Don't mean to dig up an old thread, but I have some comments on the OP's situation. This is based on my experience as an air driver since 1990.
If you and this other air driver with less seniority both work preload or "inside the building" in some fashion, then you're not bid air drivers. In other words, this Saturday air work is considered "extra work." In my opinion, you guys didn't sign a Saturday "bid sheet," you signed an "intent list," so if your center only uses 1 air driver on Saturday then it should be you, because you have more building seniority than him (2006 vs. 2009).
We use about 15-18 air drivers for Saturday air where I work. When someone wants to become an air driver they sign an intent list (or contact someone in Human Resources and tell them they're interested in becoming one if there isn't a list posted). If they qualify, they're slotted into the qualified air drivers list list based on seniority.
So here's a scenario: someone has been with the company 5 years and has been air driving (as extra work) for a few years. Someone else has been with the company 10 years and just recently qualified as an air driver. The 10 year seniority person gets slotted into the qualified list and is now ahead of the other guy who has already been an air driver for 3 years. There is no "air driver seniority" when performing air work as extra work.
If you think that seems unfair, look at the parallels in this situation: say they offer extra work or double shifting inside the building during peak season, and that someone with 5 years seniority has been doing it all 5 years he's worked there (either because he signed an intent list they put up during peak season or because a supervisor went around and asked people if they were interested in extra work during peak). Then during the next peak season, a guy who's been there 10 years and has never been interested in extra work during peak decides he wants to pick up some extra hours, so he signs the list for extra work. The 10 year guy gets offered the extra work first. The 5 year guy doesn't have 5 years of "extra work seniority" over the 10 year guy, just like this 2009 seniority guy doesn't have any "air driver seniority" over the OP, Dnking.