What is the Tenure of the Officer who died in a helicopter accident with Badge/Serial Number 16805?
CREATE TABLE "officers" (
  "rank" text,
  "badge_serial_number" text,
  "tenure" text,
  "date_of_death" text,
  "cause_of_death" text
);
SELECT "tenure" FROM "officers" WHERE "cause_of_death"='helicopter accident' AND "badge_serial_number"='16805';