What is the air date when the U.S. viewers was 5.50 million?
CREATE TABLE "table1_17467447_1" (
  "series_num" real,
  "episode_num" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "u_s_viewers_million" text,
  "original_airdate" text,
  "production_code" text
);
SELECT "original_airdate" FROM "table1_17467447_1" WHERE "u_s_viewers_million"='5.50';