What happened
On March 19, 2001, a Raytheon/Beechcraft B 300 (Super King Air 350) was conducting a training flight from Berlin-Tempelhof to Berlin-Schönefeld. The flight was intended to serve as a proficiency check for a second pilot to extend his type rating on the aircraft. The flight began under visual meteorological conditions (VMC) above a solid cloud layer.
During the flight, the crew performed several clean-stall maneuvers. While the pilot in command was demonstrating a third stall, the aircraft's airspeed dropped to 93 knots. As the pilot applied power to recover, the aircraft experienced an abrupt left wing drop. The maneuver caused the aircraft to descend into the cloud layer, resulting in uncontrolled flight in instrument meteorological conditions (IMC). The crew, experiencing spatial disorientation, managed to stop the descent and rotation through intuitive control inputs. Once the aircraft emerged back into VMC, the crew noted that the flight instruments had displayed erroneous readings due to the extreme flight attitudes and accelerations experienced during the excursion.
Following the incident, the crew conducted a radar-assisted approach to Schönefeld. Upon landing, the crew discovered structural deformations to the airframe.
The investigation
The BFU investigation utilized radar tracks, radio communications, and the Flight Data Recorder (FDR). While the Cockpit Voice Recorder (CVR) was initially available, the relevant portions of the continuous loop recording were lost because the aircraft's electrical system was re-energized after the aircraft was moved to a hangar.
The FDR data revealed that during the three stall exercises, the airspeed was reduced to 97, 95, and 93 knots, respectively. The investigation established that between 17:45 and the recovery of stable flight, the aircraft was subjected to extreme loads. The FDR recorded two positive load factors of 4.8 and 5.0. For a Beechcraft B 300 in the commuter category, the certified positive limit load factor is 3.1; the recorded loads exceeded this limit by approximately 7.5%.
Findings
- The pilot in command may not have initiated recovery measures immediately after the stall warning horn sounded.
- By failing to reduce back pressure on the elevator, the airspeed continued to decay below the minimum control speed, VMCA, which is 94 knots for this aircraft.
- The primary cause of the excursion was asymmetric thrust resulting from a delay in power delivery from one of the engines when both levers were advanced simultaneously.
- The aircraft's descent into the clouds was unavoidable because the altitude remaining above the cloud layer was insufficient to recover the controlled flight attitude while still in VMC.