Q: How often do airplanes crash?

Q: How often do airplanes crash?
A: Just once.

The humor in this airplane joke comes from the fact that the listener assumes that the jokester is asking "how many airplanes crash per day/year?", when in fact they are asking "how often does any given airplane crash?"

The question that they asked is not actually parseable in the way that the jokester wants it to be, so the humor requires the listener to accept that the setup is pretty weak for this to be funny. Honestly this ruins the joke for me.