In the course of our operations we operate both VFR and IFR, often switching between the two regimes, sometimes more than once, in the course of a single flight. VFR stands for Visual Flight Rules, and the rules are simple. As long as my company knows where I am, I don't need to file any flight plan, follow any specific route or maintain a particular altitude or speed. Under VFR I am not allowed above 18,000' in most places, and I need to plan a landing with thirty minutes of fuel remaining, and I need at all times to be able to navigate and maintain control of the airplane by looking out the window at visual features, e.g. rocks, lakes, roads, cities and the horizon. If I can't see where I'm going, I must file IFR.
IFR stands for Instrument Flight Rules. It requires an advance flight plan showing a precise routing, a clearance before departure, and permission to make any changes in altitude or heading. In any area, I'm not allowed below the minimum IFR altitude, even if I can see that it's perfectly safe. If I fly IFR at all during a day, I am limited to eight hours of any flying during that day, which may limit reaching an objective. I must fly IFR if the weather conditions anywhere along the route do not allow me to fly visually, or if I wish to fly above about 17,000'.
So you can see that if my company wanted me to leave an area of poor weather, do some low level flying in an area of good weather, and then land somewhere else covered in low clouds, I would need to be IFR then VFR then IFR. But if the task was to fly low over a city for thirty minutes and then climb up to flight level 220 (about 22,000') for three hours and then then land at a tiny airport in the mountains with no instrument approaches, I would be VFR then IFR then VFR. Such a plan is called a composite flight plan. This shouldn't be that complicated, except that the people in the IFR and VFR systems hate one another. As best as I can tell, the controllers that handle the stereotypical high speed, prestigious IFR traffic are contemptuous of the people in the system that handles the slower, lower, quirkier VFR traffic, and the VFR folks in turn resent the IFR controllers. They aren't really set up as two different systems. The Flight Service Specialists who track VFR flights also pass clearances to IFR traffic, but they get it from the people who will be managing the IFR flight after departure. Those same controllers handle many VFR flights, but in many cases they release us as soon as we are clear of controlled airspace, lacking radar coverage or time to pay attention to flights they have no legal obligation to provide coverage for.
Let's say I file an IFR flight plan out of Frog Valley, proposed to last two hours and terminate under VFR in Weasel Swamp. I can file this flight plan with either the predominately VFR FSS or with directly with the IFR data people. If I do the former, the FSS will simply forward the IFR portion to IFR data, where the planners will map it out to ensure that it doesn't interfere with other traffic or violate any rules or procedures. If I file with data they will lob the VFR portion off to the FSS. So imagine I depart on that flight, but after an hour company calls and tells me to fly to Ptarmigan Inlet. I tell the controller that we aren't going to Weasel after all, please amend our destination to Ptarmigan, estimated time enroute, three hours from now. I request descent out of controlled airspace, turn en route for Ptarmigan Inlet and then an hour out call up Flight Services to update destination weather. At least half the time I will encounter a controller who is on the edge of frantic, considering me an hour overdue into Weasel. The IFR controllers forget to propagate the change in destination and ETA through the system, even when I explicitly confirm with them that they will. Simultaneously changing flight rules, destination and ETA is an extremely common occurrence for our operation. I tried for a while explicitly telling the IFR data controller NOT to propagate the VFR portoin of the plan, that company flight following would handle it, but they didn't always comply with my request, so I still had to check, and I discovered that the FSS was annoyed at being circumvented. I now always close the flight plan with both the IFR controller and Flight Services, but often terrain, altitude or remote location prevents me from reaching an FSS right away. I've learned, however, that when I call late, telling the annoyed flight service specialist that I did amend the plan, but the IFR controller forgot to pass on the change immediately soothes the wrath. In fact, just about any error occurring in the course of a composite flight plan can safely be blamed on the part of the system you are not currently speaking to. A similar lack of communication, but not the outright hatred, applies across provincial and territorial borders, even on a flight plan that is not composite. A controller from one flight information region (FIR) was unfamiliar with the abbreviation PTD (proposed time of departure) that designated the content of a blank on a standard form from an adjacent FIR, and to work the edges of our national, integrated air traffic control system, it pays to have an intimate knowledge of regional and departmental preferences.