What is the english term for "vol en ficelle"?
Update:

I found this translator-help page, which suggests: 


  [low altitude] constrained flight path




Perhaps nap-of-the-earth flying (Wiki) is what you're looking for.

According to wikipedia:


  Nap-of-the-earth (abbreviated NOE) is a type of very low-altitude
  flight course used by military aircraft to avoid enemy detection and
  attack in a high-threat environment.


It also mentions other terms such as (emphasis mine)


  Other, mostly older terms include "ground-hugging", "terrain masking", "flying under the radar" and "hedgehopping".




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