This doesn't really clarify what he used in the studio, but...
From:
http://www.geocities.com/magikflea/fleainbassplayer.html
There are a bunch of different styles mixed together in the bass line of "Aeroplane." How did you come up with the slap part in the verses?
I was sitting in my garage with a bass Louis Johnson gave me-a Treker Louis Johnson Signature 4-string-and I started playing that '70s funk line. The bass had light strings on it and had that whacka-whackita sound. It's kind of a "been done" groove, but it's nice and Anthony liked it. The chorus part was one of those things where we were stuck; sometimes when we're looking for another part, I'll have no idea what I'm going to do, but I'll say, "What about this?" I went [mimics zooming sound up neck]; it's all sliding on the E string. [Ed. Note: See page 45 for more on the song.]
Actually, "Aeroplane" was the only song I was worried about-I thought it sounded like another stupid white boy trying to be funky! [Laughs.] I put it out anyway, but it's the one thing I'd go back and fix. When I played it live in the studio with the band, the bass didn't record right for some reason, so it was one of the few things I had to overdub. The part kept feeling stiff to me, as if it wasn't my day; I wasn't flowing with the drums. I wanted to redo it, but Rick said, "It's cool."