Mid Pacific Air

NAMC YS-11

An IFR flight was terminated with a visual approach. Conditions were conducive to airframe icing. The aircraft was being positioned empty, with a cg at 22.1% mac. On short final, at approximately 400 feet agl, 35° of landing flap was selected. The aircraft was observed to pitch downward to an unusual attitude and to enter a steep descent. A partial recovery was observed before the aircraft impacted a dirt hill 500 feet short of runway 28. Examination of the airframe after the accident revealed 1/2 to 3/4 inch of rime ice adhering to the leading edge of the horizontal stabilizer. No ice was found on any other portion of the airframe. Evidence in the cockpit indicated that engine, pitot, and windshield anti-ice systems were on, but wing/empennage deice was off. No evidence of a powerplant or systems malfunction was found. Both pilots were killed.

NAMC YS-11

The flight crew failed to adhere to appropriate procedures and directives when they failed to select the hp cock levers to the hswl (lock out) position while performing an approach to landing stall during a training/test flight. When stall recovery was initiated, both propellers 'hung up' when the high (cruise pitch) stops of each propeller failed to withdraw. As the power levers were advanced, turbine gas temperatures (tgt's) exceeded limitations; the left propeller auto-feathered, the right propeller was later feathered by the captain. Restart procedures were attempted without success, and a forced landing in a plowed cornfield ensued. Examination of the engines revealed that the turbines had been 'subjected to severe (and destructive) thermal degradation during operation' as a result of the propellers being constrained during low speed operations. Testing of the relays revealed that the high stop withdrawal relay for the right propeller functioned intermittently. All three crew members escaped uninjured.