Date: 201507
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Light: Daylight
Aircraft: Citation Excel (C560XL)
Operator: Fractional
Operating under: Part 91
Flight plan: IFR
Mission: Ferry / Re-Positioning
Flight phase: Taxi
ATC / advisory: Ground ZZZ
Aircraft component: AC Generator/Alternator
Component problem: Malfunctioning
Reporter organization: Fractional
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion
Anomaly: Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Air Traffic Control; Person Flight Crew
When detected: Taxi
Result: Aircraft Aircraft Damaged; General Flight Cancelled / Delayed
Contributing factors: Aircraft; MEL
Primary problem: MEL

Narrative: Picked up the aircraft with the AC [Generator] bearing R [Right] MELed. I reviewed the MEL requirements and found that we could continue for 20 flight hours. I briefed the First Officer and we continued our planned schedule. After landing in Airport ZZZ we deferred the Right Windshield (WS) Heat due to an inflight failure on the previous leg after a Right WS fault light illuminated. Maintenance Control scheduled a Ferry flight to Airport ZZZ1 for maintenance. During taxi-out the Ground Controller reported smoke coming from our Right engine. We noticed a high pitched whining sound similar to bearing failure type of noise from the right side as well. We shut down the Right engine and requested CFR to assess the Right engine smoke issue. I determined that we would not continue to taxi at this point and shut down the other engine. The CFR personnel suggested utilizing Halon to cool the right AC Generator which we had determined was the source of the smoke. They gained access to the generator by lowering the Right engine nacelle. There was significant burnt paint and charring around the generator cooling port on the nacelle. After 20-30 minutes; CFR said we could reposition the aircraft to the FBO ramp which we did via a tug. I contacted an Assistant Chief Pilot (ACP); Maintenance Control; and Crew services to advise. We were shut down and sent to the hotel. If this is a reasonably expected outcome from the AC [Generator] bearing warning light; then it should not be an MEL item; as the light indicates an eventual bearing failure. If we had gone airborne there is no doubt that it would have been an Inflight Fire/Engine Shutdown event.[Recommend] the AC bearing light should not be on the MEL. In the future if I am faced with this MEL item; I would refuse to fly with it based on this experience.

Callback: Reporter stated their Cessna 560XLs have Pratt-Whitney PW-545 engines. The AC Generator is mounted on the forward right-hand; outboard side on the Right engine; just aft of the Fan case. The total amount of time flown with the AC Bearing deferral was about three hours of a 20-Flight hour MEL deferral. Reporter had carried MELs before with the AC Bearing deferred; but was always concerned since the AC Bearing Annunciator light meant that Primary bearing failure was imminent.Reporter stated they picked up the aircraft with the Right engine AC Bearing already deferred and wrote up the Right Windshield heat due to the illuminated fault light on arrival. The Right Engine AC Generator does provide electrical power to the Right windshield. Reporter is not sure if the window heat fault could have been a RED flag indication of the AC Generator about to completely fail.Reporter stated that given the extent of the burnt; damaged area around the AC Generator and cooling port on the nacelle; the length of time; and the amount of Halon poured into the nacelle by the Crash/Fire/Rescue (CFR) personnel; it is highly doubtful the engine fire bottles would have been capable of extinguishing that type of engine electrical fire; especially in flight. They do fly over open water. All the more reason not to allow further MELs of AC Generator Bearings. Reporter has Boeing aircraft experience and has flown Citations for 20 plus years.



Synopsis: Captain reports that deferral of the Engine AC Generator Bearing Annunciator light with a 20-flight hour continued operation should not be allowed in the MEL for Cessna CE-560XLs. Primary Bearing failure of the Right Engine AC Generator after only three hours operation since deferral caused an electrical fire that burnt and scorched the nacelle; AC Generator; and wires during taxi-out. Pilot suspects the engine fire bottles would not have been sufficient to extinguish that type of fire in flight.