What happened
A single-engine aircraft operating as a Part 13 and 5 mail cargo flight was flying in dark night instrument meteorological conditions when it impacted terrain following an in-flight breakup. Several minutes prior to the impact, the pilot notified air traffic control that the aircraft had lost its vacuum pump.
The investigation
An examination of the wreckage showed that the vacuum pump drive had separated from the engine. The flex coupling exhibited a torsion-type separation, with wear patterns suggesting the engine continued to operate after the separation occurred.
Upon disassembly of the model 211CC vacuum pump, investigators found that one of the six vanes (vane #4) was broken. Pieces of the broken vane were located within the mounting and back flanges. Grooves were discovered on both sides of the vane slot, matching the dimensions of the missing portion of the vane. Additionally, three carbon pieces with extensive wear were found; two of these pieces were smooth and rounded. The pump housing showed severe wear characterized by washboard marks around its entire circumference. A burr was also identified on the edge of a back flange discharge port.
Based on manufacturer service life guidelines for vane wear, the estimated operation time for the vanes in the accident pump was approximately 1,380 hours. However, manufacturer service letters require the replacement of model 211CC vacuum pumps after 500 hours of operation or six years from the date of manufacture.
Investigators also examined the aircraft's gyros. The directional gyro housing was intact and the gyro moved freely without rotational marks. The attitude indicator gyro was also intact with no rotational scoring noted. While the turn coordinator gyro showed no rotational scoring, a small portion of its housing was found to be fractured and missing.