This thread has naturally produced a lot of comments, some of them very good and helpful, but others have shown a limited understanding of low level aerobatics.
To help the general reader or would be pilot think a bit about what is going on when they next go to an airshow, I would like to make some comments (not specific to this accident) which apply to all aerobatic displays not just fast jets.
There are two categories of manoeuvres, those that include the possibility of something called a ‘gate’ and those that do not.
A ‘gate’ in this context is a point in the manoeuvre where it is possible to positively determine that all is well to continue and complete a downward manoeuvre without hitting the ground.
A nice easy example of such a ‘gate’ are the conditions that exist where an aircraft is upside down at the top of a loop and before the pilot carries on to flying the second half of the circle by what is called ‘pulling through’. Common sense says a minimum height will be required at the top of the loop and this will vary with the aircraft type. However the speed at this point also matters because too little and the aircraft will produce less lift on the way down, thus increasing the radius of the pull through. Perhaps surprisingly too much speed can also increase the radius of the pull through because of the extra centrifugal force involved at the higher speed. Thus the gate a pilot will look for at the top of the loop is a minimum of H feet in height and a speed between X and Y knots in the event he is outside one of these parameters then he must abandon the manoeuvre by rolling erect and carrying on as appropriate afterwards.
So a safe loop or pull through is easy eh? Well yes and no. There are many possible reasons why an aircraft is outside the gate but the pilot does not realise it. It could be he plain forgot to check, it could be the altimeter setting was wrong (remember the Thunderbirds F-16?), it could be he was distracted by some R/T that was going on at the time, or a number of other possibilities.
Please note that one thing that does not determine the safety of a loop is the height at which you start it. You could be an inch above the ground when you pull up but providing you fly up to your minimum height at the top, with your speed in the bracket, it just does not matter.
Now for the other category of manoeuvres – those that can have no gate. For me these are inherently trickier than those with a gate. Take the simple wingover as an example. This is initiated by pulling up into a climb, then pulling round in a tight turn to face the opposite direction to the original climb. This turn is overbanked so that the nose goes down during the turn and the aircraft is in a dive when rolling out on the reciprocal direction to the original pull-up. Hardly an aerobatic manoeuvre some might say – just a bit of overbanking. So easy and safe despite no gate at the top? My answer is again yes and no. Done a bit more enthusiastically (because one wants to put on a good show) the pull-up can be a bit steeper, the turn a bit harder, the overbanking a bit more and the next thing you know you are facing steeply downhill at too slow a speed and too low a height to pull out before hitting the ground. Of course the pilot avoids this by using skill, currency and being in practice. The trouble is such factors can vary from display to display unlike the loop case where checking the simple gate conditions does not require the same skill, currency or practice.
Planning, practising and executing an airshow routine involves much more than I have touched on here. Much more. Most of which is outside a simple post such as this and I would not want anybody to think otherwise.