What is the pilots legally accepted definition of known icing. Eg vis moisture and below zero or when you see ice forming on the airframe or gramet or Jepp icing forecast (all different)
I suggest a search for
or better still
“known icing” AND “faa definition”
and you get e.g. this.
The “FIKI” (flight in known icing) is a purely US topic, which maps onto varying-over-time FAA positions/definitions/statements and potentially specific US weather services (e.g. icing sigmets). It isn’t directly applicable in Europe, in the sense of breaching an aviation regulation (i.e. creating a criminal offence). EASA type certificates etc instead specify that flight is permitted in (specific severity categories of) icing conditions.
AIUI, the FAA implementation of FIKI attempts, or attempted in the past, to create a criminal offence (render a proposed flight illegal even before departure) if the wx forecast contains certain elements. Europe (specifically the various national CAAs which have approved various TCs and STCs, before EASA took over the whole thing) has not had this AFAIK. Well, historically, Europe never had wx forecasting services which were much good for GA flight
On top of that you have different requirements between the two systems in terms of what equipment is required for flight in icing. The FAA requires a heated stall warner, two alternators, etc, whereas over here none of those are required. You can argue about that of course e.g. a heated stall warner is rather desirable…
We have all experienced this kind of problem in IFR (not to mention similar cases in VFR ON TOP!).
According to the end of the story, before MTL, when you observe that the Alps are covered, and even more so with the LFLP ATIS (04 / M02, OVC 4500), given the reliefs in the corner of LFHN and LFLP , you understand that an APP RNAV to LFLP is not possible (probable icing).
I guess the LFLB TAF (at the start) and the METAR were not encouraging either … and I’m not talking about approaching LSGG to continue VFR to LFLI …
So on the IFR plan it could be considered to return to the 1st field having an IFR approach compatible with the aircraft equipment and having a control or an AFIS (or at worst a favorable STAP?) , LFLY for example, and either wait for better days or try to get closer as possible VFR to land not too far.
The observation of a layer BKN to see the ground made you choose to go under the layer (it would be more secure, and comfortable for the passenger, the layer is SCT but you did not have a great choice!) and cancel the IFR to return to this VFR progression that I just mentioned that allowed you to approach the VOR LTP with the moving map.
From then on, with the time stopped, the progression towards the low mountain especially 45 mn of the sunset was compromised as you say very well and LFHS with the highway was almost the ideal solution!
When flying in IFR with IMC conditions, especially at destination and in winter, the often “administrative” obligation of an ADEG (FPL alternate aerodrome), for a private pilot, must be perceived as real (a bit like when we say that a “landing is a missed go-around”!).
I will say in conclusion and my poor IFR experience (on my Bonanza F33A, obviously not de-iced), knowing the IFR flights from the Paris region passing Geneve to make the LFLP Approach before continuing in VFR to the Megeve altiport (from Spring to Autumn), I will have done the same thing keeping in mind that turning back must always be possible as long as it is likely that the weather conditions that you went through a while ago still have to be correct.
Finally, an instructive flight bringing a good experience on both Meteorology (TAF and real observation) and on human factors (how far to not go towards its destination)!