The IFR clearance

A clearance provides separation from other known traffic within controlled airspace along the route and altitude it specifies. Separation itself comes in four forms: vertically, by assigning different altitudes; longitudinally, by controlling time spacing between aircraft on the same course; laterally, by assigning different flight paths; and, where available, by radar, which can substitute for or reinforce all three.

ATC does not provide separation for a flight operating outside controlled airspace, or for one cleared with 'VFR-on-top' instead of a specific altitude, or at any time the flight is in VFR conditions -- because uncontrolled VFR traffic may share that same airspace. None of that changes what the pilot owes the flight: obstacle clearance, performance, and weather judgment are never delegated to ATC.

Every departure clearance is delivered in the same order, which is why the copying technique C-R-A-F-T exists: write five blank lines before the radio call, then fill them in as the controller talks instead of trying to remember everything at once.

A worked example makes the order concrete. A flight files from Seattle to Sacramento via Victor 23 at 7,000 feet. Clearance delivery reads it back with a departure procedure attached. Copying it with CRAFT: write 'SAC' after C, because the clearance limit is the destination; write the departure name and 'V23' after R, because that is the route; write '70' after A for the 7,000-foot altitude; write the departure control frequency after F; and leave T blank, since the transponder code is usually generated by computer and read last.

A second, fully worked clearance shows the same order with every blank filled in: 'Cleared to Skyline airport via the Crossville 055 radial, Victor 18, maintain 5,000. Clearance void if not off by 1330.' Limit: Skyline. Route: Crossville 055 radial to Victor 18. Altitude: 5,000. The frequency and transponder, and the void time, would follow in the same call.

'Cleared as filed' means the route on the flight plan is reissued in full; the controller states only the departure procedure, if any, and any changes, so the pilot needs the filed route in hand to reconstruct the rest. A 'cruise' clearance is different again: it authorizes any altitude from the minimum IFR altitude up to and including the stated altitude, with climbs or descents within that block left to the pilot's discretion. Once a pilot reports leaving an altitude within that block, though, returning to it needs a new clearance.

Reading a clearance back is the system's only error-detection mechanism. The controller listens to the readback specifically to confirm that the trajectory reserved and the trajectory understood are the same thing, which is why altitude, transponder code, and any restriction must come back word for word, not paraphrased.

A clearance that was not understood gets a request to repeat it, without hesitation. A clearance the pilot cannot actually fly -- for lack of a chart, the right equipment, or the performance to comply -- gets a request for an alternative instead of a silent acceptance and a hope it works out.

At an airport with neither a tower nor an FSS, a pilot phones the flight plan to the nearest ATC facility at least 30 minutes before the planned departure. If conditions allow, departing VFR and picking up the clearance airborne is one option; if not, ATC issues a clearance by phone with a time restriction, since a controller has to protect airspace for an airplane it cannot yet see.

A clearance void time example from real phraseology: 'Clearance void if not off by 0900.' That means airborne before 0900, full stop -- not wheels rolling, not lined up on the runway, actually off the ground. If the takeoff does not happen, the pilot must call ATC back; without word within roughly thirty minutes of the void time, a search-and-rescue response can be triggered.