112. The following authorities shall have the power to convene a field general court-martial, namely:-(a) an officer empowered in this behalf by an order of the Government or of the Chief of Air Staff; (b) on active service, the officer commanding the forces in the field, or any officer empowered by him in this behalf; (c) an officer commanding any detached portion of the Air Force on active service, when, in his opinion, it is not practicable, with due regard to discipline and the exigencies of the service, to try the alleged offender by a general court-martial.