What keeps an airplane warm in the cold?

Angels.

Or maybe a system powered by the primary jet unit that utilizes a pack valve flow control and shutoff valve (FCSOV) that controls the flow of bleed air into the pack, along with a primary heat exchanger that cools the engine/APU bleed air, combined wth a compressor and reheater mounted on the same shaft used to raise the temperature of the air before it reaches the turbine inlet to vaporize any remaining water droplets for turbine protection…

Because Jesus loves us and wants us to be warm.

Compressor bleed air. https://en.wikipedia.org/wiki/Bleed_air

On the ground (hangar), or while in the air?

@rebbel In the air.

Hey mods, good job on taking the criticism and greenlighting @JLoon original post.

“Just be nice, and so will I.”
—JLoon (profile)