Why am i experiencing extreme nerve pain in tooth with a crown when flying?

Depends: If the tooh was not root canaled, the nerve may be dying and the pressure changes will ellicit the pain. If the tooth did have a root canal and you are having pain, the occlusion (bite) on the tooth may be too high and needs adjustment. If the occlusion is too high, the periodontal ligament gets inflamed and can be very painful with pressure changes at altitude. Many pilots experience this.

Abscess: It is possible that the tooth has an abscess around the root end that is exacerbated while flying due to pressure differentials at flight altitude. See a dentist before the next flight!

Pulpitis: You almost certainly need a root canal treatment. The nerve is dead or dying and the pressure change as you change elevation is causing the pain.

Pulpitis: The tooth is likely experiencing mild pulpitis or inflamed pulp all the time. When the tooth is under the pressure change during flight, it irritates the already inflamed pulp to the point of being painful. The tooth will likely need a root canal to treat the acute pulpitis.