Can be bought from the Elitas Travel website for $300,000 (GTA Online only).
The term Mammatus is used to describe a certain type of pouch-shaped cloud which can be seen commonly before storms. Ironically, the word Mammatus derives from the word mamma meaning breast, which could be an obscene easter egg.
Given the fact that the Dodo is a very similar aircraft it is unknown why Rockstar chose to make a new model/name.
Dodo, a similar looking aircraft from past GTA games.
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