Aircraft Accident Involving Gyro System Failure and Structural Wing Failure (N55MM)

Weather and Flight Progression The pilot received a full weather briefing before the IFR flight. Convective thunderstorm activity was forecast for the route, with numerous thunderstorms in the area at departure and a tornado watch in effect at the time of the accident. After leveling at FL210, the pilot reported that conditions were 'getting rough.' Shortly thereafter, he reported problems with the aircraft gyro system, then said all gyros were 'out.' He elected to descend in an attempt to locate VFR weather. As the aircraft descended, radio and radar contact were lost. ## Wreckage and Mechanical Findings Wreckage was found distributed over an area about 4-1/2 miles in length. An exam of the wreckage revealed evidence that the wings had failed in an upward direction. During an exam of the vacuum powered attitude gyro, no rotational scratching was found on the gyro or in the gyro housing. An exam of the left vacuum pump revealed that the plastic drive coupling had signs of heat stress and melting, that the internal veins were broken and cracked, and that there was a deposit of carbon material inside the housing. The right vacuum pump had evidence of rotational scratching. The gyros should have operated with 1 vacuum pump in operation.