In a deep stall, the turbulent airflow off the wing completely blankets the horizontal stabiliser, making it totally ineffective. That's one reason why development commercial aircraft carry a tail-mounted drag chute - deploying it in a deep stall pitches the aircraft nose down and puts the tailplane in clear air, I think.. .. .Yes, that was Mike Lithgow in that prototype BAC-111, along with, I believe, BAC's chief aerodynamicist and a significant proportion of the aerodynamics department as that flight was specifically investigating the phenomenon. It was not a forced landing, they never made it out of the stall and all on board were lost. I was in the second year of my aero engineering degree course at the time and I seem to remember quite a lot of discussion of it in the following weeks.. .. .Any high tailed aircraft could be susceptible to it but, nowadays, the implications are understood.. .The Gloster Javelin was a martyr to it (big delta wing and a T-tail right behind it), quite a few were lost to deep stalls. I think Hunter pilots were told not to bounce Javelins because several were lost trying to manoeuvre away from their attackers.