copilot vs touch down

copilot

verb
  • To serve as relief or assistant pilot. 

noun
  • A backup or assistant pilot of an aircraft. 

touch down

verb
  • To land an aircraft or spacecraft. 

  • To score a try. 

  • To score a touchdown 

How often have the words copilot and touch down occurred in a corpus of books? (source: Google Ngram Viewer )