Another Perspective
Just think for a moment about the physical properties of a Methane Gas cloud, The bubble or cloud will difuse at at a rate proportional to the ambient Temp and inversely proportional the the molecular mass of the gas, in addition to natural mixing by dissapation in air currents. As CH4 is lighter than air (at a given temperature) any release of the gas will rise and disapate . You can imagine that you would need an enormous release to reach the flight levels flown by modern aircraft.
CAT III