Unhappily, the accident record bears out this articles main point: Combining STCs and performing untested modifications can have lethal effects.
August 7, 2010, Saltsburg, Pa., Beechcraft Model 58 Baron
The accident aircraft was modified by one STC that installed vortex generators (VGs) and by another for more powerful engines, different propellers, winglets and modified engine cowlings, which also increased the airplanes VMCA. However, the airplanes airspeed indicator remained marked to indicate a lower VMCA.
Radar data depicted the aircraft performing maneuvers resembling VMCA demonstrations, and slowing to about 71 knots groundspeed before it descended in a left spin and impacted a house. Subsequently, the FAA issued an airworthiness directive requiring an inspection for airplanes equipped with the engine-conversion STC to ensure modified airplanes have the correct airspeed indicator markings.
August 18, 2013, Cataldo, Idaho, KR-2 Experimental
A video found in the airplane captured the accident flight from taxi to impact. The video showed the pilot initiating an intentional left spin. During the first few rotations, the engine quit and the propeller stopped turning. After several additional rotations, the spin stabilized but the airplane continued its descent until impact. During the descent, the pilot was observed applying various control inputs without effect.
The airplane had been modified from the original design and was equipped with a heavier engine than recommended by the kit manufacturer. The kit manufacturer reported the modifications to the accident airplane resulted in unknown spin characteristics. Regardless, the accident airplane had not been test-flown for spins.
