Can you depart IFR from an airport without an approach?
There's no takeoff clearance needed without a tower, but until you get the OK, the release, from approach, you can't depart IFR. So you'll then readback that clearance and they'll say “Cessna 9334H readback correct advise ready for departure.” And they may ask you which runway you're going to depart from.
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So, as a general answer, in the U.S., flight at altitudes at and above 18,000 feet (5.49 km) above mean sea level requires a flight plan (an Instrument Flight Rules flight plan - known as IFR) and joyriding/offroading as you put it, is not permitted.
What is the 3 1 rule IFR? In aviation, the rule of three or 3:1 rule of descent is a rule of thumb that 3 nautical miles (5.6 km) of travel should be allowed for every 1,000 feet (300 m) of descent.
Class G airspace is Uncontrolled Airspace. In class G airspace, operations may be conducted under IFR or VFR. ATC has no authority, but VFR minima are to be known by pilots. Traffic Information may be given by ATC as far as is practical in respect of other flights.
In theory, a pilot can operate in sub-VMC conditions under IFR without an ATC clearance in Class G airspace. However, to enter or operate in controlled airspace under IFR, an ATC clearance is required.
The benefits of operating under IFR are numerous. Often the equipment on board allows for very precise flight along a route that is more direct than the twists and turns that might be required to dodge airspace and some weather under VFR.
Class A. IFR flights only are permitted, all flights are provided with air traffic control service and are separated from each other. Class B. IFR and VFR flights are permitted, all flights are provided with air traffic control service and are separated from each other.
When ATC issues an IFR clearance to pilots of airborne aircraft, pilots must respond by reading back those parts containing altitude assignments or vectors and any part requiring verification. This is mandatory.