What was found
APPROXIMATELY 35 MILES FROM STL, A BRIGHT BLUE GLOW WAS NOTICED COMING FROM THE LEFT ENGINE NACELLE JUST BEHIND THE PROPESSER SPINNER. THE BLUE GLOW WOULD OCCASIONALLY FLASH BRIGHT RED AND ORANGE. ALL SYSTEMS AND ENGINE INDICATIONS WERE NORMAL. A PRECAUTIONARY LEFT ENGINE SHUT DOWN WAS PERFORMED. THE AIRCRAFT LANDED WITHOUT INCIDENT. MAINTENANCE INSPECTED THE ENGINE AND FOUND THAT THE LEFT PROPELLER BRUSH BLOCK HAD SHORTED OUT. THE BRUSHES WERE REPLACED IAW THE AMM CHP 30-60-05 AND THE AIRCRAFT RETURNED TO SERVICE.