Airplanes that have retractable landing gear, flaps, and a controllable pitch propeller are considered complex. Sea planes (no retractable landing gear) and airplanes with FADEC (or other electronic engine control) are considered complex as well.
The complex endorsement is relatively easy to obtain, yet affords a pilot the ability to fly a wider range of aircraft). Unlike other certificates and ratings, the complex endorsement is conveyed solely by a Certified Flight Instructor.
The complex endorsement must be added to a certificate and therefore, the aviator must hold at least a private pilot certificate. There are no time requirements associated with a complex endorsement, but the aviator must exhibit complete competence in the operation of a complex aircraft. Upon the demonstration of proficiency, the Certified Flight Instructor will award the aviator the complex endorsement.