But unfortunately the FAA
has opened a few years ago a door, which should have been never touched:
On the B737 New Generation the take-off is permitted with a certain amount of frost on the UPPER WING surface, in the so called cold fuel soaked zone.
Teaching winter operation & de-icing/anti-icing I often get the question, which other aircraft types are also good for this.
Saying only this one, often gets me head shaking as an answer.
THis shoudl have never happened, as it is also clear against the "Clean Aircraft Concept" as already published in the ICAO document nearly 20 years ago.