Iowa State University

Rockwell Shrike Commander 500

N81589 was following a Boeing 727 on ILS approach in IFR conditions. Approximately 2.5 miles from runway, N81589 suddenly deviated left nearly 90°. 39 seconds later, pilot reported he encountered severe turbulences, then said he was trying to climb. N81589 subsequently flew thru tree tops on rising terrain for approximately 2 city blocks, then hit power lines and a pole and crashed against a tree. No evidence of turbulences due to weather at/near approach altitude. Investigation showed 4.7 miles separation between aircraft when the 727 passed the point of deviation (pod). At pod, the 727 was on the ILS glide slope at 850 feet; N81589's altitude was not determined. Wind was from 120° at 3 to 6 knots, shifting more southerly withalt. Prevailing visibility was 1/2 mile; RVR for runway 12L was 6,000 feet; light rime icing was reported. Witnesses heard power interruption of one engine at pod; but there was no indication of power loss where aircraft hit trees. Radar data showed aircraft slowed to 80 knots at pod, accelerated to 100 knots during deviation, slowed to 70 knots before impact; vmca was 74 knots. Safety board believes the pilot most likely lost control due to slow speed and light airframe icing; possibly encountered wake turbulences. No reason found for loss of engine power. All seven occupants were killed.