How would one estimate the number of commercial airplanes bought in the US every year? I am not sure where to start with this question. Do you start by estimating number of major airports? How many airplanes decommissioned/and new ones bought?
 A: it rather depends on what information you have available! one simple approach might be to estimate the total size of the fleet, and the average number of years in service for a plane. when talking of number of planes the skewed distribution of plane size means that passenger numbers are not so helpful, though you could assume an approximately lognormal or gamma distribution, for example. to estimate the size of fleet in operation you could try various approaches. one would be based on the fact that there are readily-available airport statistics like "one plane takes off every 4 minutes". here the skewness works to your advantage, as you could get a good initial estimate by just looking at a small number of airports with the largest traffic flows. other relevant data come from the economic stats. get an estimate of how much Americans spend per year on air travel, then try and partition this into different categories of costs. for cars you will find that the amount spent on vehicle purchase is the same order of magnitude as the amount spent on fuel, so that could be a starting point, coupled with information on the distribution of new plane prices, though here the skewness factor would tend to work against you (unless quantitatively modeled using the distributions already suggested). a further line of approach might be based on examination of the employment statistics for flight crew personnel 
