The pilot was flying under designated powers of auth. In other words, a supervisor was not available to deploy on the "det" and the captain was given temporary powers of auth by a senior supervisor at Benson.
I was lucky enough to have a number of experienced SH navs on the Sqn when I was a junior shag - therefore although I would go on such mini-dets as captain, I was left in no doubt by the Sqn staff who was in charge of the det and who would authorise every sortie. This was an effective check and balance allowing me to accrue captaincy experience with a supervisory hand on my collar. Shame that the SH nav is almost extinct.