The FAA is proposing for the Cessna Citation X an Airworthiness Directive (AD) that would require an inspection to determine the serial numbers of the APU generator and the left and right engine direct-current generators. It would also require related corrective actions, if necessary, and a revision to the airplane flight manual. The proposed AD is based on a report of a DC generator over-voltage event that caused smoke in the cockpit and damage to numerous avionics and electrical components. The event occurred on the ground before an engine was started. Investigators determined the event to be the result of a short in the APU generator windings and subsequent damage to the over-voltage protection circuit in the generator control unit (GCU) due to prolonged holding of the generator reset switch in the cockpit. This condition, if not corrected, could result in an over-voltage condition on the DC electrical busses caused by exciter stator winding failures and subsequent failure of the GCU over-voltage protection circuitry associated with the engine and APU DC generators. A spokesman for Cessna told AIN the company issued a service letter in September 2009 and subsequently 162 generators were replaced at no cost to the customer. The FAA is accepting comments on the proposed AD until December 27.