Accident Overview
A single-engine airplane conducting a Part 135 mail cargo flight in dark night instrument meteorological conditions impacted terrain following an in-flight breakup. Several minutes before the impact, the pilot informed air traffic control that the vacuum pump had been lost.
Wreckage Examination
An on-site examination of the wreckage revealed that the vacuum pump drive was separated from the engine. The flex coupling displayed a torsion-type separation with wear consistent with engine operation after the separation.
Vacuum Pump Disassembly
Disassembly of the model 211CC vacuum pump found that five of the six vanes were intact, while vane #4 was broken. Four pieces were reassembled to recreate approximately 80 percent of vane #4. Three pieces were located in the mounting flange, and one piece was in the back flange. Examination of vane slot #4 revealed a groove approximately 0.450 by 0.120 by 0.020 inches deep on both sides of the slot. These grooves corresponded in dimension and position to the missing portion of the vane. Three carbon pieces were found; two exhibited extensive wear and were smooth and round like a "BB." The rear fracture surface of vane #4 displayed a smooth, polished spherical wear mark matching the carbon pieces. The pump housing had severe wear with washboard marks around its entire circumference. At the bottom of the bore, a burr was found on the edge of one of the back flange discharge ports, and corresponding rotational marks were found on the pump's rotor.
Service Life and Overhaul
According to Parker Hannifin, the rule of thumb for vane wear versus service life is 0.025 inches of wear per 100 hours of operation. Using Parker's original new vane length of 0.845 inches, the estimated operating time of the accident pump's vanes was approximately 1,380 hours. Parker stated that the vacuum pump had been overhauled. Parker's service letters require replacement of model 211CC vacuum pumps after 500 hours of operation or 6 years from the date of manufacture, whichever occurs first.
Gyro Examinations
Examination of the directional gyro revealed an intact housing and a freely moving gyro, with no rotational marks found on the gyro or inside the housing. The turn coordinator gyro showed no rotational scoring when removed from the housing; a small portion of the housing was fractured and missing. The attitude indicator gyro was free to move within its intact housing, and no rotational scoring was noted when the gyro was removed. The filament of the vacuum enunciator bulb (PL33[0]) exhibited stretching.
Conclusion
The investigation focused on the vacuum pump failure and its components, but no probable cause was explicitly stated in the source.