Database: aircraft
Question: What is the average number of transit passengers for all airports that the aircraft "Robinson R-22" visits?
SQL: select avg(t3.transit_passengers) from aircraft as t1 join airport_aircraft as t2 on t1.aircraft_id  =  t2.aircraft_id join airport as t3 on t2.airport_id  =  t3.airport_id where t1.aircraft  =  "robinson r-22"