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 Part 135 and 121 which deals with the certification and operation of flight charter companies, airlines, and commercial aviation businesses Part 91 deal primarily with the pilot and the pilots rules and regulations for most aspects of flight. Perhaps one of the most import rules in Part 91 is FAR 91.03 which determines whom is the pilot in command of an aircraft.
Part 91 ranges from the certification of a private pilot to commercial pilot and 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 alone with 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 pilots limitations but it 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 subpart F and G the minimum equipment and ownership of large aircraft is 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 whether to operate under Part 91 or 61. The differences a slim but they are existent none the less. Part 61 deals with more certification of pilots where as 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 of a person convicted of a felony involving drugs or alcohol. Whether you do your training under Part 61 or 91 all pilots should be familiar with both and understand the rules and regulations put forth in both sections.