When the temperature is below 32F, what should the windshield washer reservoir be filled with?

When the temperature is below 32°F, using a mixture of 50% water and 50% isopropyl alcohol in the windshield washer reservoir is effective for preventing the liquid from freezing. This mixture balances the need for effective cleaning while lowering the freezing point, ensuring that the windshield washer fluid remains operable in cold temperatures.

Isopropyl alcohol has a lower freezing point than water and serves to prevent the formation of ice in the washer reservoir and the lines. This ensures that the fluid can be sprayed onto the windshield without the risk of clogging or freezing, which could impede visibility. Using 100% water or relying entirely on alcohol might not achieve the same protective properties against freezing, while other mixtures lacking sufficient isopropyl content could lead to issues when temperatures drop. This understanding is critical for maintaining the safety and functionality of the aircraft's systems during colder weather operations.