@wbardorf
You got it right.
Just file for FL70 and if necessary fly at 5000 feet.
If you fly below MEA, watchout for obstacle/terrain, military areas, navaid range (if you need any) just like you would do for a VFR flight.
As long as the difference between filed flight level and actual flight level is reasonable (2000 ft is ok) it will work just fine.
But if there is altitude restrictions on SID/STAR you can’t make, advise ATC as early as possible (ie when requesting startup if you can’t comply with SID altitude restrictions).
I would avoid (when possible) to switch from IFR to VFR back and forth.
Guillaume wrote:
As long as the difference between filed flight level and actual flight level is reasonable (2000 ft is ok) it will work just fine.
Hello @Guillaume, and thanks for your popping up.
What kind of trouble would it cause to ATC, if one filled IFR at FL70, and then requested a descent to 3000ft AMSL ? (flat terrain and restricted area permitting)
If 3000ft is in CAS, interfering traffic could prevent from flying at 3000 feet.
Otherwise, if 3000ft is OCAS you should be fine.
Ok
But I guess that in CAS, even descending by 2000ft, interfering traffic could limit the clearance.
BTW, if IMC, quite unlikely the OP meets traffic, in CAS or OCAS, between 3000 and MEA…
Yes, even descending 1000ft could be an issue in controlled airspace if there is traffic underneath it but in case of icing or weather (CBs!), ATC’s duty is to give priority to you and reroute other traffic around you if necessary. IFR separation rules mean that there should be enough room for ATC to maneuver.
Normally, a “due icing” or “to avoid weather” helps get the attention. If that doesn’t draw attention, I normally go for the phrasing “I require heading/flight level immediately due […]”. On a bad weather day (i.e. full with CBs/TCUs) in the tropics, one can hear a deviation request almost every minute on a busy radar frequency.
Giving ATC advance warning is part of the professional etiquette (normally possible with CBs/TCUs) although there have been cases where it was not possible (e.g. one case with unexpected medium icing) where I would not have hesitated to declare an emergency to get what I needed to ensure safety of flight (prerogative of the PIC) but ATC were receptive enough to pick up the urgency in my voice to react promptly.
Wolfgang
We have focussed on french system.
@wbardorf and @boscomantico, would you tell us how this request would be handled in other countries ? (UK, Germany, Italy for eg.)
boscomantico wrote:
And the interesting question is: why has this “problem” seemingly never been “picked up” by anybody in France? I mean, we know that the vast majority of “small GA” in France is at the aeroclub level, where 98% of flights are purely VFR. But still, with so many private pilots as there are in France, there must be a certain (not totally insignificant) number of pilots flying “real” A-to-B IFR flights, more or less all year round, in non-deiced, non-turbo SEP aircraft. Why have they never adressed this with the DSNA?
Occam’s razor comes to mind, and the most plausible explanation is a given 
I fly VFR only, but I do fly in and out of controlled airspace, flight plan or not. Why should this be more difficult flying IFR? G is G for everyone, in fact about 1/3 of the commercial airfields used by airliners are in G. No clearances are given in G, only information. Your controlled flight stops at the boundaries, and it doesn’t matter if it’s VFR in a tiny SEP or a IFR in a 737.
As for SERA, giving a further look, it appears that transposition into French regulation (SERA —> FRA), implied some light interpretations / adaptions.
All in all, it’s almost the same.
About switching from VFR to IFR in-flight, I found this: Arrêté du 21 avril 2017 relatif aux règles et procédures pour les services de la circulation aérienne rendus aux aéronefs évoluant selon les règles de la circulation aérienne générale
on a Z flight plan, the pilot should ask 10 minutes before reaching thefiled waypoint where the flight rule is to change, to get the initial clearance
while VFR (with or without FP), and facing unforcast conditions, the pilot must give his IFR FPL over the radio. As this is unforcast, he must account for a delay, depending on trafic density, and ATC availability.
That applies only in controlled airspace. Out of controlled airspace you don’t need a permission from ATC. Just tell them what you do.
This leaves the question about what to do with IFR shortcuts through uncontrolled airspace. Presumably the best way is to file an IFPS-acceptable flight plan and then (politely) insist on flying the shortcut through uncontrolled airspace.
Same thing: you don’t need permission to leave controlled airspace. But once out of controlled airspace you must ask a clearance if you want to enter controlled airspace again. The ATC must give the clearance unless they have too much traffic. Flying at low level IFR makes it usually easy to get a clearance.
If you want to cross a TMA the worst that happens usually is a small detour requested by ATC to maintain separation with an approaching airliner.
you don’t need permission to leave controlled airspace
I know you know this but you do need a clearance for any change of trajectory in CAS, so you do need a clearance for e.g. a descent out of CAS, etc.
The questions Bosco is asking in post #13 are ones I have referenced previously and came under some nasty attacks for it, including one French-resident poster apparently leaving the forum a week or two ago! It however seems obvious that the reason why this area does not get a lot of “exercise” is indeed because not many pilots are exercising it 
We have focussed on french system.
@wbardorf and @boscomantico, would you tell us how this request would be handled in other countries ? (UK, Germany, Italy for eg.)
In the UK you cannot do this at all. There are multiple facets to that issue, one of which is a watertight separation between “Eurocontrol IFR” (which is in CAS) and everything else. And once you drop out of CAS you cannot get back in. And converting a VFR flight to a Eurocontrol IFR flight (a “popup clearance” in the USA) is generally impossible – reportedly because UK ATC have not paid for the software module that is required to realtively efficiently generate the IFR flight plan fragment which is required for such a flight We have had many previous threads on this. However, IFR OCAS is freely available and is treated identically to VFR flights i.e. no implied CAS clearance, no need for radio contact, etc.
Peter wrote:
We have had many previous threads on this. However, IFR OCAS is freely available and is treated identically to VFR flights i.e. no implied CAS clearance, no need for radio contact, etc.
Yes many threads I should get back into.
IFR OCAS the way you describe remains astonishing to me, so I should really extend my flying trips :-) Do you at least file a FPL ?
