What is the aircraft for 21 crews?
CREATE TABLE "tactical_organization" (
  "type_of_unit" text,
  "type_of_aircraft" text,
  "number_of_aircraft" text,
  "number_of_crews" text,
  "men_per_crew" text,
  "total_personnel" real,
  "officers" real,
  "enlisted" real
);
SELECT "type_of_aircraft" FROM "tactical_organization" WHERE "number_of_crews"='21';