When I worked north and west out of Fairbanks, AK, our cutoff was -40. The nice thing about -40 is you don't have to convert or ask whether it's farenheight or Celsius. And that's the only good thing about -40.
I tried to avoid flights colder than -30 F, but sometimes stuff happens. Temps below -30 start to get dangerous. I had a cub on skis parked on Alexander Lake in Galena one winter after we'd finished moose surveys. Too dark to go home day before so parked overnight, planning to leave in AM. Got up and checked temp.....-38 in Galena...good enough.
Plane was plugged in overnight. The lake sits in a depression in the middle of town, and dumps into the Yukon at its south end. Pre flighted plane.....brrrr, really cold! Got covers off, engine nice and warm, loaded passenger, got aboard and fired up.
When you start a warm engine in very cold temps like this, the "engine warm up" is actually a gradual cool down. Let it run at 1000 rpm for a few minutes, taxi and go.
I taxied to the north end of the lake, turned around and noticed this cub had an OAT gauge in the left side window....which read -60F.
Now, I'd been on the road counting moose for ~ three weeks, and I was ready to go home. Weather was clear, and I knew that with the typical interior Alaska inversion, the temp at several hundred feet would be a lot warmer.
So, I pushed the throttle up....not to max, and took off. Not my best ever decision.

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That said, temps at 1000 feet were a balmy -25F. Unfortunately, strong easterly winds slowed us down to a crawl, and now I started thinking about gas. So, down into the inversion to minimize headwind.....

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Coming past Tanana, I asked my back seater if he wanted to stop there for a bathroom break. He said sure. I called Tanana FSS and asked for current weather. "Tanana is reporting sky clear, visibility 2 miles mist and haze, wind calm, temperature minus 58, say intentions". Before I could say anything, backseater said "I don't have to pee."
Onward to FAI, arriving in the dark at -35 F after 4.7 hours enroute.
Not one of my brightest piloting experiences.
Two points: "Get home itis" is very real, and rationalization can become king.
Secondly, in deep interior parts of the country, if it's -20 in some town, it may well be -30 or colder on the surface of a lake in the area. And, guess where you're going to land if anything goes wrong?
I was enrote from FAI to Chloya Lake in the Upper Yukon one winter. Exxon had a 250 man seismic camp on the ice there, and I was the environmental monitor. Fort Yukon was the nearest reporting station...about 25 miles from the lake, and FYU was reporting light wind and -19 F. FAI was -30, so I should have been suspicious.
Got over the lake and the helos were all parked. Hmmmm, should be working with good weather. Duh!
As the 185 descended below the trees, it literally felt like I was diving into water.....and the cabin was instantly very cold. I taxied back, plugged the plane in, threw the engine cover on, then walked over to the official NWS certified thermometer. -58 F.
Guess I'll spend the night.....
The surface of that lake is probably ~ 30 feet lower than FYU, and both thermometers were correct. Cold air settles.
Be careful out there.
MTV