One not often discovered reason for abandonning 40deg flap settings...
On early model 40 flap versions..try a very low speed approach with full flap. say at about 45kts ias. if you are too high on approach and apply full slide-slip (max rudder) the diverted turbulent airflow from the 40 deg flap over the tailplane will cause an instant and complete tail-plane stall with sudden violent nose down pitching moment.
very alarming and clearly fatal at less than 50ft agl.
