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I have known two types IFR clearance that a pilot may request with flight plan filing.

  • Pop-up clearance: A pilot on VFR requests an IFR clearance to destination airport. ATC grants a clearance on a work-load permit basis.
  • IFR Climb to VFR-on-top: A pilot may obtain this type of clearance from ground clearance delivery and the clearance limit is usually a nearby Nav aid.

The unlikely clearance without pre-filed plan is,

  • Airport-to-airport: Ask ground clearance delivery for an IFR clearance to another airport.

What other types of IFR clearance else may I request ATC directly? We know that the flight plan checked-in time needs to 30 min prior to its start time.

Another example is

  • "IFR-descent-to-VFR-under-bottom": Is it possible to request an IFR descent through a cloud layer, say 8000 to 5000 through a layer 6000~7000? Is it just one special case of "pop-up clearance", or due to the descent nature, ATC won't grant it?

Are there any concrete regulations or general conventions that a pilot may follow?

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    $\begingroup$ Any IFR clearance that is requested from ATC without having pre-filed is, by definition, a pop-up. $\endgroup$ – rbp Apr 6 '16 at 22:45
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    $\begingroup$ Also, if you're flying a TEC route from airport to airport, you don't need to file before calling for clearance. $\endgroup$ – NathanG Apr 7 '16 at 2:48
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The theoretical answer to your question is that you can always get an IFR clearance directly from ATC over the radio, and filing a flight plan in advance isn't required (but it is strongly advised to avoid delays).

In reality, however, you should file in advance if at all possible. This is from the AIM 5-1-8 (my emphasis):

Instrument flight plans may be submitted to the nearest FSS or ATCT either in person or by telephone (or by radio if no other means are available). Pilots should file IFR flight plans at least 30 minutes prior to estimated time of departure to preclude possible delay in receiving a departure clearance from ATC.

The IFR flight plan system is there for good reasons: it helps ATC manage busy airspace and it avoids the need for lengthy radio discussions. If you don't file a flight plan in advance then you'll probably annoy ATC and you may be delayed because they can't take your request directly.

Pop-up clearances by definition don't have a full flight plan filed in advance, but they do have an abbreviated IFR flight plan:

An authorization by ATC requiring pilots to submit only that information needed for the purpose of ATC. It includes only a small portion of the usual IFR flight plan information. In certain instances, this may be only aircraft identification, location, and pilot request. Other information may be requested if needed by ATC for separation/control purposes. It is frequently used by aircraft which are airborne and desire an instrument approach or by aircraft which are on the ground and desire a climb to VFR-on-top.

Weather isn't predictable and a VFR flight may have to become IFR for safety reasons, so there needs to be some flexibility for aircraft that are actually in the air. A pop-up clearance is just a different, simpler way of requesting an IFR clearance; once you have it, you're IFR and operating in exactly the same way that you would if you had filed in advance.

Descending to VFR conditions is an interesting idea but ATC can't clear you to descend below the minimum IFR altitude for the area. You could request the lowest altitude ATC can give you, then cancel IFR if you break out. But descending without a specific altitude restriction seems very risky: the temptation would be to keep going lower to "have a look", and that could end up as a CFIT incident.

Finally, at least in my experience the 30-minute 'rule' is really a guideline to avoid delays. I've never had an issue filing IFR and departing 15 minutes later, but then I'm usually flying between class C or D airports and the airspace isn't particularly busy. A class B airport is probably a different story.

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