Originally Posted by
DesertNomad
I am going to guess that 75% of them are in the air at any given time
I think that number is high. The average hours in the air per plane per day is nowhere near 18. Even CO - which pushes their aircraft to the edge in terms of scheduling - doesn't get much above 16 hours on average from their planes. Between maintenance, turn time and general downtime I'd guess that the average across the world is closer to 12-14 hours. That is still a lot of planes in the air and a lot of people, but I think that the 75% number (18 hours) is high.
You also have to consider load factors (~75-80%) and the glut of CRJs and ERJs in the world, among other manufacturers. There are a few Yak-42s still flying every day, though they represent a small number of total passengers.