Why is it generally assumed that hot water on an airplane is kosher? Is the urn that warms up water on an airplane strictly for water use? Or do they alternate and it can be used sometimes for other things? My question is also: is that urn ever washed with other traife kaylim (nonkosher utensils)?
I have been flying nearly every week for 20+ years. I cannot recall having seen the hot water urns on planes used for anything else than hot water, tea or coffee (note many airlines prepare coffee directly into the urns). They often have a spout that would only work for liquids anyway.
Regarding the kashrut of hot water on planes (the title of your question), many sources allow drinking tea and coffee on planes, e.g.,
- dinonline (here, #65) writes "a hot water urn or teakettle may be used without kashering. You may presume it was used only for water and is thus permissible, unless you know for sure otherwise."
- R Tsvi Heber from COR (kashrut hekhsher) writes "Urns and kettles may be used year-round ... year-round [but not on Pesach] it is permitted to drink unflavoured coffee that has already been prepared for the office"
- OU Kosher writes "Rav Belsky said, in general there is no concern that the utensils that cooked the coffee were used with non-kosher. The coffee pot is usually rinsed out and reused, and is not sent through the dishwasher. Rabbi Schachter added that there would be reasons to be lenient even if the coffee pot was sent through the dishwasher."
- R Yaakov Goldstein doesn't worry about the kashrut of the water but writes here regarding those who are stringent because of bishul akum
See also this related question on MY.
Even if the utensil was indeed used for non-kosher liquids, or washed together with non-kosher utensils, it could only affect the use of hot water put inside afterward if it was used with non-kosher within 24 hours of the current hot water use.
See Shulchan Aruch YD 122:6 Stam keilim einam bnei yomam - If we don't know, we assume that utensils were not used [for non-kosher] within the past 24 hours.