I recommend
the article in Air Clues 19 (see the sticky on this forum) written by Lt Col Nick English, AAC, regarding a loss of tail rotor drive during a combat sortie in Afghanistan. Of particular interest to pilots (of all sorts) is his description of their decision processes, simulator training and it influence on this incident, and in general how the crew reacted to an emergency that began as the pilot flying sensing a vibration in the aircraft and then went downhill from there. If we had a thread on this in Rotor Heads or Mil Aviation, my search terms are not finding it.