What is the average total number of passengers of airports that are associated with aircraft "Robinson R-22"?
return airports ;return aircraft #1 are associated with  ;return #1 where #2 is  Robinson R-22 ;return passengers of #3 ;return number of  #4 ;return sum of #5 for each #3 ;return average of #6
['select', 'project', 'comparative', 'project', 'aggregate', 'group', 'aggregate']