What effect would a release of natural gas into the atmosphere have on
engine performance if a helicopter flew through a large amount of it without any prior warning of its presence
Depends on the gas concentration, a large ammount of gas could be ignited or the O2-concentration is not high enough to get a good combustion in the engine.
I remember an accident in germany some time ago, where a AS 350 encountered a flameout during aerial construction, caused by the hot gases leaving that chimeney they were working at.
skadi