Whenever we did airtests on the VC10 (four engines tail mounted) we had an extra cockpit instrument added which displayed angle of attack. There was a limit beyond which we were warned not to go, lest a deep stall set in. There was apparently no recovery possible from this deep stall except by utilising the tail parachute they fitted only to the original test aircraft on which they defined the safe operational envelope.
Hope this helps. BRGDS