To be eligible for a flight navigator certificate, a person must --

(a) Be at least 21 years of age;

(b) Be able to read, write, speak, and understand the English language;

(c) Hold at least a second-class medical certificate issued under part 67 of this chapter within the 12 months before the date he applies; and

(d) Comply with §§63.53, 63.55, and 63.57.