Just saw it a 2nd time, just dawned on me 1 of 2 things happened & realistically prolly both.
1 - Bruce fixed the auto pilot 6 months prior to the ending
2 - He dropped the bomb in the ocean & flew away from the blast
1- u just realizing that upon 2nd viewing. it was the recurring theme throughout the film.
2- he didn't drop the bomb in the ocean. that's why he had the autopilot. he ejected himself prior to the BAT crossing through the building and over the bridge and on towards the ocean. (u may need a 3rd viewing).