CEN21LA237 · Grumman G164 B

The pilot was departing on an agricultural flight when shortly after liftoff the airplane had a sudden engine power fluctuation from high-power to low-power and back to high-power. The pilot stated that he was “thrown forward” into his safety restraints and then “thrown back” into his seat. The forward/backward motion only occurred once and was “pretty quick and hard.” The pilot stated that after the loss of engine power, the propeller speed remained at 100% but the airplane’s climb performance was significantly diminished.

The pilot pitched the airplane for best glide airspeed and entered a slight right turn from the runway heading to avoid trees. Believing the engine was still running, the pilot did not want to jettison the airplane’s load unnecessarily into the river near the airport. The pilot reported that the engine lost all power shortly after he entered the right turn. He then jettisoned the airplane’s load in an attempt to maintain a climb, but the airplane descended and impacted a levee, where it nosed over into the river. The airplane sustained substantial damage to the vertical stabilizer, rudder, upper wings, and the fuselage.

Postaccident examination of the engine and propeller revealed no evidence of internal failure or other anomalies that would have precluded normal operation. The rotational rub/scoring observed in the engine, metal spray on the turbine stator vanes, and earthen debris throughout the engine core are all consistent with the engine producing power at impact. The engine’s torsion shaft aft spline was fractured consistent with sudden stoppage damage sustained when the propeller impacted the ground. The propeller exhibited impact-related damage that was consistent with high impact forces with the propeller rotating in the normal blade angle range of operation at moderate engine power.

During takeoff and initial climb, the engine speed is controlled by the propeller governor and fuel flow is determined by the fuel control unit (FCU) Power Lever Schedule. Although postaccident testing of the FCU revealed anomalies with the Standard Day Acceleration and Deceleration Schedules, the FCU did not contribute to the loss of engine power because the Power Lever Schedule controls fuel flow to the engine during takeoff/initial climb. Additionally, according to the pilot, the propeller speed remained at 100% after the initial power fluctuation and is consistent with the engine operating with the condition lever set for takeoff.

Postaccident examination revealed that the airframe control linkage to the engine propeller pitch control (PPC) was disconnected from the splined end of the shouldered shaft of the PPC, and the retaining bolt that normally secured the airframe linkage to the splined end was loose and was not secured by safety wire. Additionally, although the PPC shouldered shaft had an internally threaded hole for a secondary retention feature, there was no evidence that the secondary retention feature was installed at the time of the accident.

There are no outside forces acting on the PPC other than the linkage connection to the cockpit power lever. The PPC and FCU main metering valve are connected through the concentric shaft and are rigged to move together. A disengagement of the airframe PPC linkage will result in the pilot’s inability to change fuel flow and/or propeller pitch (blade angle) during flight and landing. If the airframe control linkage disconnected from the PPC shouldered shaft during initial climb at takeoff power, vibration could potentially rotate the PPC cam and reduce engine power as if the cockpit power lever had been pulled back by the pilot.

The pilot’s description of being thrown-forward and then backward would have required the PPC cam to rotate from high-power to low-power and back to high-power before settling at a low power setting that would not sustain a climb. However, it is unlikely the propeller blade angle pitch change would be fast enough to cause the pilot to believe the engine had surged. As such, the pilot’s description of being thrown-forward and then backward is not consistent with a disconnected PPC airframe linkage.

Although the airframe control linkage to the PPC shouldered spline was found disconnected after the accident with a loose retention bolt that was not secured with safety wire, the investigation was unable to conclusively determine if the control linkage disconnected from the PPC while inflight or during impact. Additionally, the investigation did not reveal any evidence of a preimpact failure or other anomalies that would have prevented normal engine operation.

Verbatim NTSB analysis and probable cause from the NTSB dataset