Summary
We propose to adopt a new airworthiness directive (AD) for certain Model 750 airplanes. This proposed AD would require an inspection to determine the serial numbers of the auxiliary power unit (APU) generator and the left and right engine direct current (DC) generators, and related corrective actions if necessary. This proposed AD would also require revising the airplane flight manual. This proposed AD results from a report of a DC generator overvoltage event which caused smoke in the cockpit and damage to numerous avionics and electrical components. We are proposing this AD to detect and correct an overvoltage condition on the DC electrical busses caused by exciter stator winding failures, and subsequent failure of the generator control unit (GCU) overvoltage protection circuitry, which could result in damage to critical electrical and avionics components.