Does flying REALLY dry your engine

Humidity After flight

Typical measurements of engine air temperature and relative humidity after flying


  • These measurements were taken after a one hour flight that started with a completely dry engine

    • This is a result of blow-by of combustion gases into the crankcase
      • For each gallon of fuel consumed, approximately 1.2 gallons of water is produced as a by-product
      • Approximately 10% of this is blown into the crankcase
    • While the engine is running this very moist blow-by gets mixed with the oil, driving the oil to its moisture saturation point
    • When the engine is shut down, this hot, humid blow-by air is left inside the crankcase, and the oil is left at its maximum moisture saturation point


  • Oil absorbs moisture directly from the air. This research paper describes this interaction and the effects it can have on engines and bearing life.

MOISTURE – The Second Most Destructive Lubricant Contaminate


  • Air at a dew point of 127º F contains approximately 10 times the amount of moisture than air at a 60º F dew-point


Dew Point Chart002


  • If this hot humid air is removed from the engine immediately after flight, the dry air inside the crankcase will result in the moisture dissolved in the oil to slowly evaporate back into the air until an equilibrium is reached with the dry air.