(8)possession of any explosive, which is not for sale and is required solely for the navigation of aircraft, when kept in an aircraft for use therein, or for distribution to other aircraft or to aerodromes or at an aerodrome for use there or for distribution to aircraft or to other aerodromes:Provided that the maximum quantity so possessed shall not exceed twenty five kilograms when carried in an aircraft and fifty kilograms when kept at an aerodrome; and
