Part 91 of the Federal Aviation Regulations, or FARs, is one of two main parts dealing with the certification of pilots, their starting point until being hired by an airline or charter company. Unlike Parts 135 and 121, which deal with the certification and operation of flight charter companies, airlines, and commercial aviation businesses, Part 91 deals primarily with the pilot and the pilot's rules and regulations for most aspects of flight. Perhaps one of the most important rules in Part 91 is FAR 91.03, which determines who is the pilot in command of an aircraft.
Part 91 ranges from the certification of a private pilot to a commercial pilot and includes weather minimums and requirements. It gives pilots guidelines to follow when operating VFR (visual flight referencing) or IFR (instrument flight referencing) and also gives them minimum equipment requirements for operating in those situations. Part 91 also defines airspace in the current National Airspace System. It defines prohibited areas and uncontrolled areas and the requirements for flying in both. More importantly, to pilots, it defines the experience they need in order to stay current. A pilot that is not current is restricted to certain flights. For example, a non-current commercial pilot cannot fly with passengers. Currency for pilots is stated in FAR Part 91, subpart A.
Not only does Part 91 define a pilot's limitations, but it also defines the limitations of aircraft and aircraft companies operating under it. Fractional ownership and waivers are only a few of the sections that apply to the operations of an aircraft. In subparts F and G, the minimum equipment and ownership of large aircraft are defined, along with the procedure minimums for flights over water. Special flight operations and foreign aircraft operations and ownership are also stated under Part 91.
Most flight schools have a choice of whether to operate under Part 91 or Part 61. The differences are slim, but they are existent nonetheless. Part 61 deals with more certification of pilots, whereas Part 91 also includes rules and restrictions. Certain FARs in Part 91 apply to all pilots, like FAR 91.17, which states the procedures and requirements for a person convicted of a felony involving drugs or alcohol. Whether you do your training under Part 61 or Part 91, all pilots should be familiar with both and understand the rules and regulations put forth in both sections.