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The general qualifications for an Instructor Rating require you have a combination of experience, knowledge and skill. Title 14 of the Code of Federal Regulations (14 CFR) part 61 requires that pilots must be able to read, write, speak, and understand the English language. If you cannot meet these requirements of English fluency, an airman certificate cannot be issued. For medical reasons, an appropriate limitation may be placed on the certificate.
You must pass the Fundamentals of Instructing Knowledge Test and a Flight or Ground Instructor Knowledge Test specific to the instructor rating sought. You may take these tests on the same day, and you do not have to take them in any particular order. An instructor rating obtained by passing the Military Competence Instructor (MCI) exam does not require the candidate to take the Fundamentals of Instructing (FOI) exam.
When applying for any additional instructor rating, you are not required to take the Fundamentals of Instructing Knowledge Test again. Once you have acquired a Flight Instructor Certificate, you are eligible to give ground instruction required for a Pilot or Instructor Certificate or rating, based on the ratings on your Flight Instructor Certificate. Because of this, it is not necessary to obtain a separate Ground Instructor Certificate since you would already have these privileges.