How do you become a millionaire?
Be a billionaire and buy an airline.

Never invest in an airline:
  Massively capital intensive.
    New plane models rolled out frequently, obsolete quickly, maintenance, fuel, storage, external costs
    Lots of risks; volcanoes, terrorism, pandemics, etc. One mishap can destroy a brand.
      (Very expensive to be grounded).
    Lots of competition.
    Airports are monopolies (squeeze every $ from airlines).
    Travel is seasonal and a luxury people can do without (ie the first thing people cancel).