Complex sentence: The "Akron" was not certifies as airworthy again until later in the spring, and on April 28 cast off for a nine hour flight with Rear Admiral Moffett and Secretary of the Navy Adams aboard.
Split into simpler sentences: The "Akron" was not certified as airworthy again until later in the spring. Its next operation occurred on April 28 when the airship made a nine hour flight with Rear Admiral Moffett and Secretary of the Navy Adams aboard.