I am aware that flight into known icing is prohibited if your aircraft is not suitable to fly in icing conditions (unless e.g.: FIKI,...). Given the following example I am wondering if it would be legal to takeoff in Luxembourg in these conditions if your cruising level is well above the cloud tops: Cloud Layer with Cloud tops +/- FL050 and w/ Freezing Level FL040+ and light icing to be shown on a icing forecast / report. What indications are best to use to predict icing? Would this report indicate flight into known icing (I guess Yes, but I am not sure) thus making a takeoff "illegal"?, although you would only briefly pass the narrow layer of icing? The safest answer is obviously to not takeoff(although I do believe it would present no hazard in this situation), but I am simply questioning this from a legal point of view? Thanks!
This is a wonderful question! The regulations are vague around it. According to your situation presented above, it is certainly illegal. AIM 7-1-22 defines Known Icing Conditions as "Atmospheric conditions in which the formation of ice is observed or detected in flight."
I urge you to check out this letter which is a response to AOPA from the FAA regarding known icing conditions: https://www.govinfo.gov/content/pkg/FR-2007-04-03/pdf/07-1620.pdf
Here is a quote from that letter: "The NTSB has held that known icing conditions exist when a pilot knows or reasonably should know of weather reports in which icing conditions are reported or forecast."
The FAA argues that if a pilot willingly flies into known icing conditions, they could take legal action as per 14 CFR §91.13 (no reckless or careless behavior).
Just talked with some experienced pilots and they all concluded that this is a case into know icing as you expect icing to occur at temperatures below 0 degrees celsius thus climbing through a layer (visible moisture) known to be above freezing level is definitely "flight into known icing", especially from a legal point of view, no matter if you only pass it during a few seconds.