What happened
On November 15, 2021, a Beech E-90, registration N290KA, was performing a business flight from Oakland County International Airport to Boyne City Municipal Airport. While established on the RNAV GPS Runway 27 final approach, the aircraft's groundspeed gradually decreased from 129 knots to 88 knots over the course of one minute. The last ADS-B data recorded the aircraft at 88 knots, approximately 3.2 nautical miles from the runway and 600 feet above ground level. Shortly thereafter, the airplane impacted the ground in a nose-down attitude in a forested area. The pilot and a student pilot passenger sustained 2 fatal injuries.
At the time of the accident, the area was experiencing lake effect weather, including an AIRMET for moderate icing. While one witness near the flight path reported no precipitation, witnesses near the impact site observed very heavy sleet and low visibility. The aircraft' $\text{power levers}$, $\text{speed/propeller levers}$, and $\text{fuel cutoff levers}$ were found in the full forward position, and the landing gear was down.
The investigation
Engine examinations at a recovery location showed that the impeller vanes of both engines were bent in a direction opposite to rotation, which is consistent with the ingestion of debris during operation. The third-stage turbine shrouds and nozzle vanes showed evidence of normal internal airflow and combustion.
Investigators examined the deicing system, including the bleed air regulator valve, de-ice distributor valve, and a section of the left wing de-ice manifold. No mechanical failures or malfunctions were found that would have contributed to the accident.
Previous reports from other pilots regarding this specific aircraft noted a tendency for rapid deceleration when propeller levers were moved to the high RPM position. Additionally, the airplane's flight manual cautioned that stalling airspeeds increase when ice accumulates on the wings and advised maintaining at least 140 knots during icing conditions.
Findings
- The pilot failed to maintain sufficient airspeed during the final approach.
- The aircraft exceeded its critical angle of attack while operating in icing conditions.
- The pilot's lack of recent experience in this specific turboprop aircraft likely contributed to poor airspeed control.
- The workload associated with inflight deicing tasks may have also impacted the pilot's ability to manage airspeed.
- The encounter with heavy lake effect sleet likely contributed to the aerodynamic stall.