Pitot static frozen
Twice I have experienced this event.
Flying F/L 370 with sat -45degreesC when temp increases to 0 degrees c.
Boeing have a QRH checklist for this.
No auto throttle, no EPR.
No TAT. Set power and fly attitude. Usually max contin. N1 to hold altitude.
Not sure why, but lasts 10 minutes than back to -45 c.
Perhaps hot air from volcanic activity. Who knows.
This is taught only by experienced pilots.