Where did the second bat plane come from and how did he get into it, from the one that was carrying the bomb? What I mean is he was inside the bat plane that had a bomb strapped to it, and flew out of Gotham. There was no second bat plane, and he couldn't have got into it while midflight in the first one. He could have only done it by, firstly getting into the bat plane, saying "bye" to Gordon, then going round the corner, landing, setting it to autopilot and getting out. Which seems a lot of fuss considering the bomb is moments from killing everyone in the city. If he'd have done that, somebody would have also seen him flying away from the city in another bat plane. And we know he was in a bat plane, because we saw an interior shot of him inside one just before the bomb went off. So either he snuck off, out of the city, in a second bat plane that nobody spotted, or he was still inside the first one.
I just can't see any realistic way in which he survived it. I think it's much more likely that he gave Gotham everything he had left (i.e. his life), like he told Selina he wanted to, then handed the Batcave over to Blake. Then Alfred went to the same cafe he'd always wanted to see Bruce Wayne sitting at, and in his grief, imagined him sitting there with a beautiful girl, just like he'd always imagined. Having him survive an impossible situation, nullifying the emotional impact of his actions, then having Alfred and him give a knowing nod to each other, just seems far too hokey of a move for Nolan.