first lieutenant vs pilot officer

first lieutenant

noun
  • The second lowest rank of a commissioned officer in the United States Army, Air Force, or Marine Corps, ranking above a second lieutenant and below a captain. The rank of first lieutenant is equivalent to the naval rank of lieutenant junior grade. 

  • An equivalent rank in other military forces. 

pilot officer

noun
  • The lowest rank of a commissioned officer in the RAF, equivalent to an Army ensign or coronet 

How often have the words first lieutenant and pilot officer occurred in a corpus of books? (source: Google Ngram Viewer )