What is the proportion of active aircraft from CASL (commercial aircraft sales and leasing)?
According to The Economist data in 2012 showed that:


  Over a third of the world's airline fleet is now rented (see chart)
  and the proportion is likely to keep growing.


The article has some interesting background about the companies involved and the advantages of leasing vs. buying, which one Delta source compares to housing:


  [Delta's fleet planner] argues that big airlines are better off buying planes and keeping
  them for their full lifespan of 30 years or so. Like houses, then,
  buying is cheaper than renting in the long term.