I have experienced first hand in a 269a at Houston Airport, the aircraft was bucking like a bronco within 2 seconds...2 seconds later the blades were through cockpit ( convertible !!) and chopped top off console and chopped tail off, then flipped upsde down and finally chopped through fuel tank and exploded, nothing left except charred remains on floor....me!! If you do not recognise the slow bowncy vibration building up it is too late to lift off as there will already be serious damage to the head etc much better to chop throttle...or better still land on grass!!!! All in all not a good machine for anyone to groung run unless they are competent pilot. Very sad.