Database: aircraft
Question: What is the average total number of passengers for all airports that the aircraft "Robinson R-22" visits?
SQL:
SELECT avg(T3.Total_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"