pilot officer vs second-in-command

pilot officer

noun

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

second-in-command

noun

  • Somebody ranking next below a commander. 

  • Someone ranking next below the person in charge. 

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