pilot officer vs sub-lieutenant

pilot officer

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

sub-lieutenant

noun
  • A commissioned officer of the Royal Navy ranking above a midshipman and below a lieutenant. 

  • A commissioned officer of the navy ranking above an acting sub-lieutenant and below a lieutenant. 

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