Abstract: During winter in Northeast China,a certain aviation auxiliary power unit frequently fails to start up due to freezing in the start-up oil supply pipe. The analysis shows that the reasons are forced cooling of the helicopter down-wash stream,the partial pressure rise of water vapor in the air after pressurization and a small amount of mixed gas. Water vapor condenses and freezes on the inner wall of the oil supply pipe. When the ambient temperature is low,the condensed water in the start-up oil supply pipe mainly comes from the mixed gas,and the freezing is not closely related to the environmental humidity. The higher the ambient temperature,the higher the proportion of water vapor in the ambient atmosphere in the condensate water of the start-up oil supply pipe,and the greater the impact of humidity. Due to the inability to completely insulate the start-up oil supply pipe,adding insulation sleeves can reduce the frequency of freezing,but the condensed water may still occur. By using a start-up oil drain solenoid valve instead of a start-up cleaning valve,the problem of freezing in the start-up oil supply pipe was solved.

Key words: auxiliary power unit, oil supply pipe, freezing, humidity