Believe it or not, I've actually heard this question from someone who was interested in joining MAF!
You see, one of the requirements to join MAF is that a candidate (both husband and wife if married) must have 12 hours of college level Bible credits (or equivalent Bible training). Some Missionary Aviation organizations don't have this requirement, but I for one am glad that we do.
Because it's not about airplanes - well at least not primarily. Our FIRST name is Mission while our MIDDLE name is Aviation. And to me that's the right order. We are all missionaries FIRST and then Pilots (or Mechanics, or Teachers, or Computer Specialists, or Business & Finance folks, etc.).
And to be a good missionary, to be an authentic incarnational presence of Jesus, you have to have a good grasp of the Bible. You have to understand that we were Created, that we Sinned, and that God has a magnificient plan to Reconcile all of us back to Himself through the Person and work of His Only Son, Jesus Christ.
Not only do you have to have a good grasp of the basic doctrines of the Bible, you also have to be able to explain those to someone else - to be able to clearly share the gospel message (by your life and actions, to be sure; using words if necessary). Now transfer all that to a second language! If your knowledge and understanding is a bit shaky in your mother tongue, what will it sound like in Spanish (or French, Russian, Indonesian, etc., etc.)?
I also tell prospective candidates that even if you never go to the Mission Field, formal Bible training will prepare you to better live the Christian Life in a God-honoring way, will deepen your faith, and will exercise your mind.
So those are some of the reasons why we require the Bible credits. If you have any questions about that I'd be glad to talk to you - just drop me a line. Until then, happy studying!