Whilst recorded data may help post display analysis by a FDD, real time analysis and FDD intervention has many considerations regarding its viability. If a pilot approaches the start of a manoeuvre and is off parameters (speed, height or positioning) he will fly an alternative manoeuvre. If the cloudbase changes during a display, he will change from a full to a rolling or flat display. Those are aspects of display flying which are demonstrated by a competent, professional display pilot. There may be several options and a FDD cannot know which one a pilot has chosen. If a rigid fixed sequence is mandated it will put pressure on a pilot to enter a manoeuvre from marginal conditions because he is not allowed any options and this can actually compromise safety. As an example, if I am flying an aircraft where I use 350 KAS as a minimum entry speed to enter a loop and 300 KIAS to enter a 1/2 Cuban and I reach the planned entry point for a loop at 320 KIAS I may elect to fly a 1/2 Cuban followed by a positioning turn for the subsequent manoeuvre. How is a FDD to know that? Does he call a stop? Intervention may cause distraction, confusion (if the reason is not clear to the pilot) and can actually be detrimental to safety. I once watched a MiG-29 practise at Farnborough where the pilot was consistently infringing the crowd line. The FDD told the Kommisar who was supervising to tell the pilot to land which he did immediately - 2/3 down the runway and ran off the end! I have been the leader of a FCC when an item infringed a separation minima due to misjudgement. I was told to tell him to stop and clear by someone who often fulfilled my role but had no authority at that display. I refused to call stop because at that point of his display (a decelerating transition towards the crowd) he would then have had to clear over the crowd creating a greater safety hazard. The pilot corrected for the next manoeuvre and I was happy for him to continue, and the person who interjected subsequently apologised. Display flying and supervision is not black and white; it requires judgement based on experience and knowledge.