Once had to carry out a single engine departure and transit on a twin engined Puma HC1 helicopter after one failed to start. It required a running takeoff to get airborne. Crew only and approx 20nm, but it was overwater apart from the last three nm or so. But that was under RAF regulations. It required the Air Officer Commanding’s authorisation but he willingly granted it because he had actually been on the aircraft when the second engine wouldn’t start. We flew back to base, fetched a spare aircraft and went back to recover him and his entourage.