Functional Check Flight:A Functional Check Flight is a description normally given to a non revenue flight which has the sole purpose of checking for the correct airborne function of certain aircraft systems.
These flights can be conducted for a number of reasons, such as:

after a single significant maintenance action.
after a major Base Maintenance Check such as a 'C' Check or a 'D' Check
in association with requirements for the acceptance or change of possession of an aircraft;
as required according to organization, local, state regulations.

The term does not include formal test flights carried out by trained test pilots employed by an aircraft manufacturer or National Aviation Authority (NAA) for purposes such as customer acceptance of new aircraft or the renewal of a Certificate of Airworthiness, or any type of flight requiring trained test pilots.