Final answer:
The main concern when a snow-covered aircraft is warmed in a hangar and then taken back out into sub-freezing temperatures is condensation, which can refreeze into ice or frost on the aircraft.
Step-by-step explanation:
When a snow-covered aircraft is put into a heated hangar and then taken back into sub-freezing temperatures, the main concern will be condensation. As the aircraft warms up in the hangar, snow and ice melt and the aircraft's surface also becomes warmer. When the aircraft is then taken out into the cold, the warmer surface can cause moisture from the air to condense on it. This can quickly refreeze into ice or frost, presenting a significant risk to the safety and performance of the aircraft during subsequent flights.