"If not off by XXXX"

[turby47]

Hello,

When flying out of a nontowered airport with IFR clearance, I always hear the provision "if not off by...".
I always wonder why they void the clearance after certain period of time.
Can anybody give some explanation or source of documents explaining the exact reason for that?

Thanks in advance,

KC

[Shawn Goldsworthy]

The reason for a clearance void time is because we can only allow one aircraft in or out of an uncontrolled field at a time. So If you have a release, we can't clear another aircraft into or out of that airport until you check in with the approach controller. So for obvious reasons, we can't issue an unlimited clearance void time because someone else could be waiting to get out of that airport, or someone could be holding waiting to get into that airport. Make sense?

[turby47]

Now it makes sense to me. Thank you for your kind explanation.
BTW, how can ATC define the area or airspace around the airport in which only one IFR flight is allowed?

Thanks,

KC

[Keith Smith]

This is also the reason why you need to report cancellation of IFR at non-towered fields, either in the air or on the ground.

In terms of the airspace that needs to be covered, in the real world it's governed by how much ground you'll cover prior to reaching solid radar coverage, as well as the proximity of other airports. For example, at N07 where I fly, radar coverage begins at ~1200ft AGL. As a result, they treat MMU, CDW and N07 as a single airport in terms of departures.

They won't release someone out of CDW if they already have an IFR departure/arrival at N07.

[Kyle.Sanders]

7110.65 4-8-1.f
"Except when applying radar procedures, timed or visual approaches, clear an aircraft for an approach to an airport when the preceding aircraft has landed or canceled IFR flight plan."

[turby47]

Thank you so much for all the replies!
PilotEdge is great.

Best,

KC