Concerning that C150 that wouldn't recover. A Canadian instructor was killed under similar circumstances, teaching spins to a student in a C152. The result was an AD on the tail. The aircraft spun into a lake and the instructor was killed but the student survived. See below. I currently own and fly a Beech C23 (Sundowner 180) fitted with the aerobatic kit so it's cleared for spinning. It's got an uncoventional entry: at stall, full rudder into the direction of the spin and full and immediate opposite aileron. The aircraft then goes slightly inverted before spinning. Anything else degrades into a very rapid and steep spiral dive with very rapid and dangerous speed buildup. Very entertaining to say the least.
Here is the summary of the report on the C152 accident which can be read at:
http://www.tsb.gc.ca/en/reports/air/1998/a98q0114/a98q0114.
1. During a practice spin exercise, the rudder locked in a full left deflection, which could not be overcome by the crew.
2. The aircraft was released for flight with a rudder bar return spring missing, which, in combination with other factors, probably allowed the rudder to lock in a full left deflection.
3. Tests conducted on an aircraft similar to the accident aircraft showed that the design and condition of the stop bolt and rudder horn stop plate allowed the stop plate to over-travel the stop bolt and jam.
4. Because the direction of cable pull tends to close the mouth of the horn, increasing the jamming effect, applying right rudder force would only have tightened the jam rather than broken it.
Mike