When a pilot operates an aircraft, he chooses one of two sets of flight rules under which he'll operate. One option is to fly under visual flight rules (VFR). When operating under VFR, the pilot generally may fly any route he chooses and fly at any altitude below 18,000', without contacting air traffic control. The pilot is required to see other aircraft and avoid them. To ensure he can see other aircraft, the pilot must meet certain inflight visibility requirements and remain certain minimum distances from clouds. Contact with ATC is required to operate in certain classes of airspace; these will be covered in other lessons.
A second option for the pilot is to operate under instrument flight rules (IFR). To operate under IFR, the pilot must obtain a clearance from air traffic control. The controller is required to ensure that all IFR aircraft are safely separated from other IFR aircraft. Because of the separation provided by the controller, there are no visibility or cloud clearance requirements for IFR pilots. With few exceptions, IFR aircraft are not separated from VFR aircraft. When visibility permits, IFR pilots are expected to see and avoid VFR aircraft.
VFR flights are never required to file a flight plan.
Flight plans are required for most IFR flights. At the controller's option, IFR flights that are local in nature need not file a flight plan. Also, aircraft flying between participating airports in southern California need not file a flight plan if the two airports are covered by a TEC route. TEC routes are discussed in more detail in the clearance delivery lesson.
When issuing a clearance, it is important to make sure the flight plan stored on the Vatsim servers exactly matches the clearance issued, so that subsequent controllers are clear on the route issued and other details of the flight.
To create a common system for controllers to know the navigation capabilities of an aircraft, equipment suffixes are used. When checking the flight plan for an IFR aircraft, ensure that an equipment suffix has been entered. If not, ask the pilot what the appropriate suffix is. A complete list of suffixes can be found in the 7110.65, chapter 2, section 3. Equipment suffixes are not required for VFR aircraft.
{Reference: FAAO 7110.65 2-3-8}