If the passengers are 15,505,566, what is the iata code?
CREATE TABLE "table1_18047346_4" (
  "rank" real,
  "airport_name" text,
  "location" text,
  "iata_code" text,
  "passengers" text,
  "pct_chg_2009_10" text
);
SELECT "iata_code" FROM "table1_18047346_4" WHERE "passengers"='15,505,566';