CEN24LA146 · Schweizer Aircraft Corporation G-164B

The pilot was conducting an agricultural flight in the turbo-propeller equipped airplane when the engine lost partial power. The pilot was unable to maintain altitude and performed a forced landing in a nearby crawfish pond. The airplane came to rest inverted in the pond. The fuselage, upper wings, and empennage were substantially damaged during the forced landing.

A postaccident engine examination revealed no evidence of preimpact engine failure that would have precluded normal operation. There were multiple indications that the engine was producing power at impact, including rotational rubbing marks in the compressor, compressor turbine (CT), and power turbine (PT); however, the engine’s power output at impact could not be determined.

One of the two fuel control unit (FCU) air inlet adapter fittings contained debris that covered the air passage. A blockage or leak to the air inlet adapter fitting that reduces pressure to the FCU would result in a reduction in engine power, but a materials laboratory analysis of the debris found on the fitting indicated it was a combination of environmental debris and ester-based compound (oil) that likely accumulated following the accident while the engine was partially submerged in the pond. Additionally, the air inlet metering orifices in the FCU air inlet adapter were both confirmed to be free of obstruction when examined. Had debris buildup been present during the accident flight, compressor discharge air would have dislodged the blockage. Accordingly, the accumulation of debris found in the air inlet adapter fitting occurred postaccident and did not contribute to the reported loss of engine power.

The FCU-to-fuel pump coupling adapter remained intact. According to the engine manufacturer, the coupling will often experience a torsional failure when there is a sudden FCU flyweight stoppage at high power due to impact. However, it could not be determined if the coupling remained intact due to the accident sequence (forced landing, rollover in a pond) and/or reduced engine power at impact.

The PT6A-27 power section installed on the engine was not an approved configuration. The PT6A-27 and PT6A-15AG power sections are identical both mechanically and thermodynamically, but the PT6A-15AG power section has additional corrosion protection and hardware changes for agricultural operations. Although the PT6A-27 power section was not an approved configuration, it did not contribute to the reported loss of engine power.

The PT containment ring was missing three bolts, and the installed bolts were not safety wired in accordance with the engine maintenance manual. The missing bolts most likely backed out of the containment ring and migrated through the holes in the combustion chamber inner liner during engine operation. These three bolts showed varying degrees of oxidation and hardness changes that confirmed they were exposed to the gas temperatures of an operating engine. However, there was no evidence of upstream impact damage to indicate these loose bolts were ingested through the compressor. As such, the improperly installed PT containment ring bolts did not contribute to the reported loss of engine power.

The engine examination revealed damage to the 3rd stage CT rotor blades and CT vanes that was consistent with debris ingestion. It is unknown if the damage occurred during the accident sequence or if it was already present. If the damage was preexisting, it would have resulted in a slight decrease in engine performance; however, none of the findings of the engine examination could be definitively identified as the initiating event for the sudden partial loss of engine power described by the pilot.

Verbatim NTSB analysis and probable cause from the NTSB dataset