DFW08TA092 · Robinson R44

During a routine night surveillance flight, the helicopter's pilot heard the single reciprocating engine "fluctuate" slightly; he noted that the engine's rpm gauge was above the "normal operating limits", and the gauge [needle] was "jumping up-and-down". In order to keep the engine within operating limits, the pilot disabled the engine governor and decreased the engine rpm. During the return to base, the pilot heard the engine rev up and the low rotor rpm alarm. During the precautionary landing in a parking lot, the helicopter's main rotor blade contacted a light pole. The helicopter subsequently made a hard landing, coming to rest on its side. The magneto, governor controller, clutch actuator, engine/rotor tachometer and drive-belts were removed for examination. The governor, clutch actuator, tachometer, and drive belts all passed either a visual or electronic test inspection. A visual inspection of the magneto revealed that the exterior of the magneto was absent any damage, but was lightly covered with an "oil like" liquid. The magneto was bench tested and produced regular sparks beginning at 196 rpm's. The magneto’s signals to the governor box were clear and equal, with only a very slight overlap between both signals. The magneto was disassembled; the light oil like substance was present on the inside of the magneto. The inspection also revealed "whiskers" of residue/corrosion on the magneto contacts; which were black instead of the normal silver color. During the test it was noted that the magneto should have produced a regular spark at 150 rpm, and that the residue/corrosion could send extra impulse signals to the governor box; producing an indication that the engine is revving. However, it was also noted that because there was only a slight overlap of the electrical signals, and since the magneto passed the other test items,a malfunction could not be duplicated.

Verbatim NTSB analysis and probable cause from the NTSB dataset