Accident Overview
On August 25, 2016, at approximately 1005 central daylight time, a Cessna U206G airplane, registration N891LL, was substantially damaged during a forced landing near Cave-In-Rock, Illinois. The pilot sustained minor injuries, and the camera operator received serious injuries. The airplane was registered to and operated by the State of Illinois under 14 Code of Federal Regulations Part 91 as a public use flight. The local aerial mapping flight departed from Abraham Lincoln Capital Airport (SPI), Springfield, Illinois at 0845 under day visual meteorological conditions without a flight plan.
Flight History
Prior to departure, the pilot confirmed a fuel load of 550 lbs (89 gallons) by dipping the fuel tanks. For the first 45 minutes, the fuel selector was in the “both” tanks position. After noticing a fuel imbalance, the pilot moved the selector to the “right” tank position, where it remained for about 30 minutes. While wings level at 3,500 feet above ground level, the pilot and photographer observed an abrupt loss of engine power. The pilot switched the fuel selector back to “both” and attempted to restart the engine three times without success. During restart attempts and the descent, the pilot did not feather the propeller, citing concern about the time required to unfeather it. The Pilot Operating Handbook's “Engine Restart During Flight” checklist directs the propeller control to be placed to feather. With the propeller not feathered, the pilot reported a high vertical descent rate.
Emergency Procedures
The pilot executed a forced landing, and the airplane impacted a grassy area at a high descent rate. Both wings and the engine firewall were damaged when the airplane hit a berm. Before exiting, the pilot turned the battery switch off and moved the fuel selector to the “off” position.
Post-Accident Examination
At the accident site, the airplane rested left wing low, with fuel leaking from the left-wing tank vent. During recovery, 20 gallons of fuel were drained from the right wing and 5 gallons from the left wing. The airplane was moved to the operator's hangar, where investigators reviewed records and examined the airframe and engine systems.
The engine was a Soloy Turbine PAC (Allison 250-C20S free turboshaft engine with Soloy reduction gearbox) installed in 1988 under supplemental type certificate SA2353NM. Examination revealed no evidence of fire, uncontained failure, or malfunction. Minor impact damage was present on the outer combustion chamber, but the engine was otherwise undamaged. Both oil and fuel filters had bypass indicators, neither of which indicated bypass. The N2 rotor rotated smoothly when turning the power output shaft, and rotational continuity from the power turbine to the propeller was confirmed. The N1 rotor rotated normally after energizing the starter-generator, and the N1 spun up normally. Borescope inspection of turbine blades showed no evidence of thermal erosion, foreign object damage, or abnormal combustion.
Engine and Fuel System Testing
The fuel supply line was disconnected from the fuel spray nozzle, revealing a few drops of fuel. When the starter-generator was engaged, no fuel flowed to the nozzle. The fuel line to the fuel-flow transducer was disconnected, and again no fuel flowed. However, when the fuel line to the check valve assembly was disconnected and the starter-generator engaged, after a brief air purge, fuel flowed freely. After reconnecting lines and purging air, fuel flowed freely to the nozzle, producing a normal spray pattern.
The check valve assembly prevents raw fuel buildup in the combustion case if the burner drain valve remains open when the engine is not operating, which could cause a hot start. It also prevents fuel nozzle drip at shutdown. Based on its design, an engine restart is not possible when air from the fuel system has reached the check valve, and there is no in-flight procedure to clear the air.
A vacuum test on the engine fuel system held 8 pounds per square inch vacuum for two minutes with the fuel line at the selector capped. Wing fuel vent systems were clear, and vent valves operated normally. Float switches and low-level lights functioned correctly. Vacuum tests from the engine pump filter bowl to the fuel valve showed no defects. Compressed air tests on wing fuel feed tubes found no air leaks. The fuel selector valve operated normally in all positions.
After examination, the engine was shipped to Rolls-Royce for a functional test run. It started normally and demonstrated normal acceleration capability. The fuel pump was disassembled with no evidence of cavitation or malfunction. The Shadin Fuel Flow Indicator, which did not interface with the airplane's fuel quantity system, retained last data after power cycles; at power-up it indicated 325 lbs fuel remaining and 225 lbs fuel used.