From Halopedia, the Halo wiki
At least two such aircraft carriers took part during the Battle of Mombasa on October 20, 2552. These vessels deployed F99 UCAVs to provide surveillance and air strikes against the Covenant, supporting scattered UNSC Marines and New Mombasa Police Department personnel around the Mombasa Quays.
During Halo 3s development, the UNSC was going to have a "wet NAVY" that included an aircraft carrier.
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