Accident Overview
On July 3, 2017, at approximately 1815 central daylight time, a Cessna 208B airplane, registration N9714B, was substantially damaged during a forced landing near Alpine, Texas. The commercial pilot, the sole occupant, sustained minor injuries. The aircraft was operated by Martinaire Aviation LLC under 14 CFR Part 135 as a cargo flight. The flight departed from Alpine-Casparis Municipal Airport (E38) at about 1812, destined for Maverick County Memorial International Airport (5T9) in Eagle Pass, Texas. Day visual meteorological conditions prevailed for the instrument flight rules (IFR) flight.
Engine Failure and Forced Landing
While climbing through approximately 500 feet above ground level, the pilot heard a loud bang followed by a squealing noise and an immediate loss of engine power. The pilot released back pressure on the controls and pulled the propeller control to feather. During the forced landing, both wings were damaged due to impact with power poles, and the airplane came to rest in a field.
Engine Examination Findings
A preliminary engine examination at the salvage facility revealed distress to the compressor and power turbine blades. The left ignition lead was not connected with the ignition plug, and the pin on the left plug exhibited oxidation not evident on the right plug. The power control and reversing linkage was in place and secure.
Subsequently, the engine was removed and transported to Pratt & Whitney Canada (PWC) facilities for detailed examination. The power turbine blades were all present but fractured, with varying lengths of blade span remaining. The power turbine wheel was sent to the PWC laboratory, where no evidence of fatigue was found. The power turbine vane ring (PTVR) was battered in various locations, with 11 thumbnail-sized fractures at the leading and trailing edges, and one through hole about 0.10 inches in diameter near the trailing edge. The PTVR was not an original equipment manufacturer (OEM) part.
The compressor turbine (CT) shroud had four locations with deep impact marks consistent with a blade release, and remaining surfaces were heavily battered from contact with smaller fragments.
The compressor turbine vane ring (CTVR) had part number 3029051 with an unreadable serial number; an additional identification number, 13-769 0531R13, was vibro-peened on the part. Operator records were insufficient to ascertain the CTVR's origin. The CTVR did not conform to an OEM part; vane cooling air exit slots differed in shape and size. The investigation could not determine if cooling air performance was compromised. The CTVR vane sealing plates welding differed from an OEM part, and the axial locations of the vane trailing edge were staggered. A variation of axial stagger would induce flow variation to the CT blades, increasing vibrational amplitude, but the investigation could not determine if this would affect blade longevity. The CTVR could not be tested due to damage.
The CT blades were all present but fractured at varying heights. Material analysis confirmed operational overtemperature conditions. Significant creep in the blade micro-structure indicated an imminent cracking probability.
The most recent borescope inspection of the CT blades occurred on April 12, 2017, with no defects reported. However, a video recording of this inspection revealed one CT blade was cracked. The technician who conducted the inspection stated he was focused on tip clearance and did not notice the crack.
The operator was approved by the FAA for an 8000-hour time between engine overhaul. Raw trend monitoring data was manually collected by pilots during cruise and monitored by Camp EHM, who advised the operator on engine health. Camp EHM did not report any significant trend changes since the last inspection interval. Although trend monitoring has occasionally detected a CT rotor with significant creep, it cannot detect cracks in a single blade, and manual monitoring does not capture engine temperature exceedances.