ERA10FA404 · Beech 58

The pilot had previously owned the accident airplane about 22 years ago, and it was subsequently modified under a supplemental type certificate (STC) that installed vortex generators (VGs), which decreased the airplane’s air minimum control airspeed (Vmca) from 81 knots to 74 knots. Another subsequent STC modification, STC SA1762SO, installed more powerful engines, different propellers, winglets, and modified engine nose cowlings. STC SA1762SO took into account a change to only the original type design and increased the airplane’s Vmca to 87 knots; however, the airplane’s airspeed indicator remained marked to indicate a Vmca of 74 knots. A representative of the current holder of STC SA1762SO reported that, to his knowledge, no flight testing was performed on the accident airplane or any similar make and model airplane to determine the interrelationship between his company’s STC and the previous STC. Therefore, the actual performance data for the accident airplane, including the Vmca, were unknown. However, the Vmca for the accident airplane was likely higher than the 74-knot Vmca marked on the airspeed indicator.

The pilot purchased the airplane 4 days before the accident and performed three full-stop landings 2 days before the accident to get current. An individual familiar with the pilot believed that the pilot had not previously flown a reciprocating-engine-equipped airplane in about 3 years. Due to the pilot’s recent purchase, an insurance company broker “suggested” that the pilot obtain a multiengine instrument proficiency checkride; a Federal Aviation Administration (FAA) designated pilot examiner acting as a certified flight instructor (CFI) was on board for the accident flight. The CFI did not have an exemption from 14 Code of Federal Regulations 91.109(a) to give instruction in an aircraft equipped with a throw-over control yoke.

According to uncorrelated radar data, after departure, the flight proceeded north-northwest and climbed to 3,600 feet where two 360-degree nearly level turns to the left were made, followed by a 360-degree turn to the right. The airplane then proceeded north-northwest and climbed to 4,200 feet briefly with the ground speed decreasing to 127 knots, then it descended to 3,900 feet and remained at that altitude, at which heading changes occurred, and the ground speed decreased to about 71 knots. Witnesses reported seeing the airplane flying level before it descended in a left spin and impacted a house.

The only major components of the airplane that were not extensively heat damaged consisted of the outer section of the left wing and one cargo door, both of which were found in close proximity to the house. Both engines and their accessories and both propellers were extensively heat damaged. Although the right engine-driven fuel pump drive coupling was found fractured, this likely occurred during postaccident rotation of the crankshaft in order to facilitate removal of the propeller. The extent of the heat and impact damage to the airplane limited the airframe and engine testing that could be performed; however, there was no evidence of preimpact failure or malfunction on the observed components.

Based on the airplane’s decreasing airspeed and nearly level altitude, the pilot was likely performing either imminent stall or simulated loss of engine power airwork before the airplane aerodynamically stalled and then entered a spin. Because the airplane was equipped with only a throw-over control yoke, the CFI had limited ability to assist in the recovery of the airplane. Although it was not possible to determine which low-airspeed maneuver was being demonstrated, one scenario that is consistent with the radar data evidence (and is typically performed during multiengine checkrides) is the Vmca demonstration, which requires a power reduction on one engine(and is consistent with the witnesses' descriptions of "sputtering" engine sounds). If the pilot were performing a Vmca demonstration, it is possible that the airplane began to lose directional control earlier than expected because the actual Vmca of the airplane with multiple STC modifications was unknown, and the airspeed indicator was improperly marked.

Although the limitations and conditions section of STC SA1762SO stated that the installer must determine that the relationship between that STC modification and any other previously approved modifications “will not produce an adverse effect upon the airworthiness of that airplane,” the investigation found that the FAA does not provide any guidance to an installer to help determine the interrelationship between multiple STCs. As a result of this accident, on December 29, 2011, the FAA issued Airworthiness Directive (AD) 2011-27-04 that requires an inspection for airplanes equipped with STC SA1762SO and that specifies corrective action, if applicable, to ensure that the airplanes have the correct Vmca marking on the airspeed indicator, taking into consideration other STC modifications. AD 2011-27-04 is available from the FAA’s website at <http://www.faa.gov>.

Verbatim NTSB analysis and probable cause from the NTSB dataset