A typical IFR clearance from a non-towered airport will usually go something like this: “N12345 is cleared to Podunk Airport via runway heading to 2000 then as filed. Climb and maintain 5000; expect 10,000 one-zero minutes after departure. Contact center on 123.45; squawk 1234. Time now:1620. Clearance void if not off by 1630. If not off by 1630, advise intentions by 1640. You’re released.” That gives you 10 minutes to get airborne. Is that enough? What does “off” mean? What if it takes you 12 minutes to take off after being released? What will ATC do?
The FAA’s Aeronautical Information Manual (AIM) says a clearance void time is “[u]sed by ATC to advise an aircraft that the departure clearance is automatically canceled if takeoff is not made prior to a specified time.” Of course, there’s no way ATC can know when we take off from a non-towered field. Otherwise, we wouldn’t need a void time in the first place. And once we do take off, we’ll still need a few more minutes to get a couple of thousand feet beneath us, materialize on someone’s radar screen and advise ATC we’re airborne. That eats into the additional 10 “advise intentions by” minutes, but ATC expects it.
If you’re in the run-up block, call for clearance when the pre-takeoff checklist is complete and there’s no traffic to worry about, 10 minutes is plenty. If you’re still in the FBO and have to load passengers, etc., it’s not enough (for us, anyway). Ask for a later void time, depart VFR if you can or call back when you’re mo’ ready.
The “advise intentions by” portion of the release provides you and ATC a buffer between the time you’re supposed to takeoff and when you’ve done so. Given our imaginary clearance above, if you don’t advise airborne by 1630, you’re still within the clearance’s parameters as long as you can do so by 1640. Call it 1639. We’d prefer 1635.