There should not be any ice at +9 degrees Celcius. They were flying quite low, so it should still be positive temperature in the altitude they were flying.
However would be another possible failure, yes. Just for my understanding, I’m puzzling, wouldn’t you see the intake icing on the manifold pressure gauge?
Forgetting alternate air is like forgetting carburetor heat
I don’t think so, because carb icing is pretty common and in some engine installations you can get it really easily. But fuel servo icing is much harder to get. I have seen just two instances in 22 years, with both happening when prop TKS was not used.
But as Udo says above, this is all very unlikely to have been fuel servo icing – unless it happened at a much higher altitude and the recovery (pull alt air immediately) was mismanaged. I met a Baron owner who got it at about 20k and the engines didn’t restart until 2k – above the Irish Sea! The DA42 pilot got something similar; a very late restart. If you pull alt air immediately then the stoppage is only tens of seconds (but feels way longer!).
Yes, intake icing would drop the MP. Fuel servo icing just ices up the four tiny pressure measuring tubes – see the above link.
@UdoR the reason I posted the FFAREX was to demonstrate that although there may well have been positive temperatures pilots were also experiencing.bands of fine rain where the temperatures were much lower than the ambient air and yet did not alter the visibility. In other words you were passing through them noticing little if anything. The problem was the REX got a bit sidetracked onto the fact that the PA18s problem manifested as carb icing.
We also must remember that the Twinkie probably came from a higher altitude and was possibly descending to IAP platform altitude so you could have had a cold aircraft entering an area of perhaps supercooled fine water droplets.
For fuel servo icing (see link above) you need an OAT of around -15C although this depends in the air intake duct geometry. The OAT and speed need to be roughly right to deliver around -5C at the fuel servo inlet.
I have never heard of invisible fine rain, let alone one which drops the OAT like above.
Carb icing can happen in positive temperatures (because of high depressurization leading to a ~20°C temp decrease), but other sort of icing does not happen so easily.
Also remember that they would have aimed for the initial fix at 3000ft, so about 5°C lower than LFPO (but still not the most likely airframe / intake / servo icing conditions). They may also have collected engine ice higher (not using alternate air in clouds) and not have time to make it melt (0°C should have been around 5k feet).
@Peter wrote “I have never heard of invisible fine rain, let alone one which drops the OAT like above.”
If you read the REX I posted, now you have. It is the reason I posted about it in this thread.
If you have ever flown through a fog bank you might well have felt a temperature drop. I have known the OAT when flying through such a bank to drop by 10°C and then pick up again immediately on leaving it. I know a fog bank is not the same as a bank of fine rain. One reduces visibility the other, in this case, and recorded by several other pilots over the last few weeks had no effect on visibility.
If you read the REX I posted, now you have
Doesn’t mean it is real 
Fog bank, yes, and similarly flying into IMC (stratus) tends to immediately drop the OAT by 3C or so.
Are you saying that the pilot of the PA18 and others who have been reporting the same, are lying?
In which case what makes euroga an accurate reflection of ga?
And with fog banks are concerned try flying over the Marais Poitevin, the coastal area just inland from La Rochelle or Biarritz or the foothills of the Massif Central. There you will see and feel much greater temperature differences than 3°C. I regularly see around 10° and even in a certified aircraft you can certainly feel the difference.
The differences in temperature between the base of the valleys and the plateaus they cut through. Eg.Seine, Rhone, Loire even with a height difference of only 100 or so feet can regularly register a 5°C temperature difference.
I don’t think so, because carb icing is pretty common and in some engine installations you can get it really easily.
Ok then forgetting alternate air in IMC at low temperatures is poor piloting skill.
Forgetting alternate air in IMC full stop, whatever the temperature, if your POH indicates it should be used when entering visible moisture.
In some types (at least in the PA46) it specifies it should then stay selected until the landing is completed. This is to avoid the situation where the air filter gets moisture which then freezes when climbing higher into colder air, even if you’re then VMC on top.
