In the case of a flight requiring FEX operations, when can it operate under Part 91?

Flight operations under Part 91 can be conducted in a variety of scenarios, but they typically do not include operations that are intended for hire or compensation. Test flights, which are conducted to verify the airworthiness of an aircraft after maintenance, modifications, or repairs, are a scenario where operations may fall under Part 91. These flights are not for compensatory purposes but are designed to ensure that the aircraft meets safety standards and is fit for future flights.

The other scenarios such as scheduled flights, charter services, and operations following maintenance checks can often require adherence to more stringent regulations, such as those outlined in Parts 121 or 135, which govern commercial operations. These parts have specific requirements for operational readiness, crew qualifications, and maintenance standards that exceed those outlined in Part 91, which is more focused on private and personal flying. Thus, the operations performed during test flights represent a unique and acceptable condition under Part 91.