14 CFR § 121.243 - Engine breather lines.
(a) Engine breather lines must be so arranged that condensed water vapor that may freeze and obstruct the line cannot accumulate at any point.
(b) Engine breathers must discharge in a location that does not constitute a fire hazard in case foaming occurs and so that oil emitted from the line does not impinge upon the pilots' windshield.
(c) Engine breathers may not discharge into the engine air induction system.
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