What is the gender of an aircraft?

The most common way to refer to an airplane is it.

If one were to decide to infer gender onto an aircraft, female would be the obvious choice for airships (eg Zeppelins), and by extension probably the logical one for smaller aircraft like planes.

However, I would advise against doing any such silly thing. Getting rid of gender for inanimate objects is one of the main features of English I like.

For automobiles, there really isn't a standard. The most common usage is again "it". My mom had an odd tendency to name her cars, but they were generally neutral names like: "The Blue Blump". Probably the most popular personified cars in media are Herbie the Love Bug (male), Christine (female), and the Cars cartoon series (both).

Aircraft are usually referred to as "she", just like ships. Automobiles follow the same principle.