The IFR flight plan and the ATC system

An IFR flight plan can be filed with the nearest Flight Service Station or air traffic control tower, in person, by phone (1-800-WX-BRIEF), by computer, or by radio if nothing else is available. Filing at least 30 minutes before the estimated departure time is the standard advice, specifically to avoid a delay while waiting for a clearance -- a flight plan filed at the last minute is a flight plan ATC may not have processed yet.

Before relying on any of the resources used to plan the flight, it helps to know what each one actually covers: the AIM for procedures, the Airport/Facility Directory for airport, NAVAID, and frequency data, the Notices to Airmen Publication for NOTAMs and Flight Data Center restrictions, and the aircraft's own POH/AFM for performance and limitations specific to that airplane.

A flight plan does not have to originate on the ground. It can be filed airborne under two conditions: a flight outside controlled airspace that expects to enter IFR conditions in controlled airspace, or a VFR flight expecting to encounter IFR weather en route. Either way, the plan is filed with the nearest FSS or directly with the ARTCC.

Filing through FSS means relaying the normal preflight information plus present position and altitude, since "point of departure" no longer applies; FSS then forwards it to the ARTCC, which issues a clearance from the present position or a specified fix. Filing directly with ARTCC skips that relay -- but traffic saturation can prevent a Center from accepting flight plans by radio at all, in which case the nearest FSS is the fallback.

An IFR flight plan can be canceled any time the flight is in VFR conditions outside Class A airspace, simply by telling the controller or air-to-ground station "cancel my IFR flight plan." At a towered airport, landing cancels the flight plan automatically. At a non-towered airport, cancellation is the pilot's own responsibility -- by phone after landing if no other means exist, or, carefully, while still airborne in VFR conditions if radio contact allows.

Canceling promptly matters beyond paperwork: ATC separation and information services end the moment a flight plan is canceled, and other IFR traffic cannot use that airspace until it is freed up.

Four kinds of facility share the work of separating IFR traffic: the Flight Service Station, usually a pilot's first contact, handling briefings, flight plans, clearance relays, and NOTAMs; the Air Route Traffic Control Center (ARTCC), responsible for separation in the en route structure; Terminal Radar Approach Control (TRACON), working arrivals and departures around busy terminal areas; and the airport traffic control tower.

Center airspace, like terminal airspace, is divided into sectors, and much of it is further divided by altitude into high and low sectors. Each sector has its own dedicated controllers and radio frequencies, reachable through a network of remote transmitter/receiver sites -- which is why a pilot can keep talking to what is functionally the same controller while being told to change frequencies several times over a long flight. Where two facilities' airspace meets, a Letter of Agreement between their managers, not published in any pilot resource, quietly determines exactly where and at what altitude a handoff happens.

A typical IFR departure from a towered airport moves through a predictable sequence of facilities, each one handing the flight to the next.

Position reports at compulsory reporting points (a solid triangle on the chart) are required over every one used to define the route, regardless of altitude -- reporting points shown as an open triangle only require a report if ATC specifically asks. Once ATC confirms 'radar contact,' position reporting can stop, and resumes only if ATC advises 'radar contact lost' or 'radar service terminated.'

Beyond position reports, several events must be reported to ATC without a specific request: leaving any assigned altitude for a new one; being unable to climb or descend at least 500 feet per minute; missing an approach; a true airspeed that varies from the filed value by 5 percent or 10 knots, whichever is greater; and any loss of navigation, approach, or communication equipment capability, along with its effect on the ability to continue under IFR.