Vibrating sticks

I've been to the National Museum of Flight a few times, and its always good value.  On this visit the BAC 1-11 was open - the only plane I've seen with steps at the rear instead of the side.  

This plane's unusual tail also extended to putting engines on its fins.  Apparently this could cause "deep stall" - an unrecoverable state that caused the plane to crash.  As a result of such a crash in 1963 the control sticks were fitted with a vibrator to warn the pilot when they were getting close to a deep stall.