front line vs leading edge

front line

noun
  • A front, or a boundary between opposing positions. 

  • A low level. 

  • attack, collectively the attackers or forwards. 

  • A site of a conflict, effort, or controversial matter of any kind. 

  • The site of interaction with outsiders, such as customers. 

leading edge

noun
  • The frontmost edge of a wing or other airfoil of an aircraft; sometimes contains slots or slats 

  • At the forefront of practice or technology, contrast with bleeding edge 

  • The foremost part of a trend etc; a vanguard 

How often have the words front line and leading edge occurred in a corpus of books? (source: Google Ngram Viewer )