Is it obligatory to repeat the prayer (Isha or Fajr) that was performed on an airplane, without having searched for a designated prayer area within it?

If you knew or strongly believed that you prayed in the best possible manner, then there is nothing upon you, and your prayer is valid. However, if you were negligent in asking and could have performed the prayer correctly, then your prayer is invalid, and you must repeat it. If you doubt whether it was 'Isha or Fajr, then repeat both prayers.