An IFR clearance is required for flight in IMC into controlled airspace, which is Class A, B, C, D, or E. An IFR clearance is required for Class A airspace, even when VMC. This means if you can maintain VMC or outside controlled airspace, you have the option of picking up your clearance after departure. Many pilots choose this option when departing from non-towered airports to save time on the ground. If you choose this option, you must be able to remain VMC or outside of controlled airspace until you receive your clearance. You need to be in a situation where there is no time limit for obtaining your clearance. The controller may have something going on and respond to your call with “standby”. The frequency may be busy or inoperative. It would be fine to depart VFR and have to level off under the clouds at 6,500 feet instead of cruising at 8,000 feet like you would prefer. But, it’s entirely different to depart VFR, level off at 1,500 feet heading towards terrain, hoping you’ll get your IFR clearance quickly. You’ll also need to be able to handle the additional workload involved in obtaining your clearance while flying the airplane.
If you are already airborne and have not filed a flight plan, it is still possible to obtain an IFR clearance. In this case, you would call the flight service station
on the radio to file your flight plan. Then, you would call approach or center to receive your clearance. While it is possible for a center or approach controller to
file the flight plan for you, the practice of calling center or approach to "air file" is discouraged. This request will only be accepted by a center or approach controller
when workload permits. So, you could get a reply from the controller indicating he or she is too busy followed by instructions for you to file with the flight service station.
Once you have your IFR clearance, it is valid for all the Class A, B, C, D, or E airspace you’ll be going through on your flight, all the way to your clearance limit, which is usually your destination airport. Air traffic control will reroute IFR traffic around prohibited, restricted, active military airspace, and TFRs. You should still be aware of these things and not rely on ATC. Also, your planning will be off.
Your clearance will be given in the following format: clearance limit, departure procedure (if any), route of flight, initial altitude, departure frequency, transponder code, and then any release or void times. ATC may also issue what is referred to as an abbreviated clearance. Even in this case, your clearance will always contain the instrument departure name, number, and transition, if appropriate, after a the statement that you are cleared to you destination airport "as file". Here are a few examples of typical clearances:
From a Class B airspace:
Your call: “International clearance, Cessna seven zero eight golf lima to Kirksville with alpha”
Clearance Delivery: “Cessna seven zero eight golf lima, international clearance delivery, cleared to Kirksville, Missouri airport via radar vectors napoleon, then as filed. Climb and maintain 3000, expect 7000 one zero minutes after departure. Departure frequency one one niner point one five. Squawk one seven five one.”
From a Class C airspace:
Your call: “Springfield clearance, Cessna seven zero eight golf lima to Rolla with juliet”
Clearance Delivery: “Cessna seven zero eight golf lima, springfield clearance, cleared to the rolla missouri airport via direct. Climb and maintain 7000. Departure frequency one two seven point zero. Squawk six two four four.”
From a Class D:
Your call: “Downtown ground, Cessna seven zero eight golf lima to Clinton with uniform”
Ground: “Cessna seven zero eight golf lima, downtown clearance, cleared to the clinton missouri airport as filed. Climb and maintain 3000, expect 5000 one zero minutes after departure. Departure frequency one three two point zero seven. Squawk two four zero three”.