Date: 201212
Local time of day: 1801-2400
Location: ZZZ.ARTCC, US
Flight conditions: IMC
Light: Night
Aircraft: Dash 8-100
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Cruise
ATC / advisory: Center ZZZ
Aircraft component: Propeller Ice System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Training / Qualification
Anomaly: Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Flight Crew; Person Flight Attendant; Person Passenger
When detected: In-flight
Result: Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport; General Declared Emergency
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: While in cruise flight the First Officer; Flight Attendant; and numerous passengers observed bright sparks coming from the number two propeller aft of the spinner and forward of the engine nacelle. While in the cockpit all I could see was repeated bright flashes on the First Officer's glareshield. The First Officer turned to look out his window and described to me a 'ring of sparks' flying out of the propeller hub. I brought the power back to 50 percent torque to slow the aircraft while we troubleshooted. The power reduction did not get the sparking to stop and no abnormal indications were noted on the engine instrument panel. The Flight Attendant called to inform us that both passengers and she had noticed the sparks. After the call; the First Officer and I decided the best course of action was to land as soon as possible. I notified ARTCC of what we were seeing from the engine and requested immediate return. ATC assigned us lower altitude and a turn direct ZZZ. I declared the emergency and requested that ATC contact company operations as our SATCOM was inoperative. We were 40 miles from ZZZ when we turned the aircraft. I then called the Flight Attendant to tell her that we were returning but she did not need to prep the cabin for evacuation or brace the passengers. I asked that she just proceed as if this were a diversion with a normal landing. During our descent I noted that all deice was still on and AC A and B phases were showing normal rises in load with the associated green lights. I elected to keep the prop heat on as we were now IMC and I was not seeing the flashes on the First Officer glareshield. The First Officer contacted ZZZ to inform them of our situation. We then completed the normal checklists and landed using the ILS. I stopped the aircraft on the runway; and ARFF requested that I shut down the number 2 engine so personnel could get close and inspect. After inspection we were told no smoke or flame observed and we were clear to start the engine and taxi to the gate with ARFF trucks following behind. Once at the gate I spoke with row 1 and 2 passengers about what they saw. I was told sparks and bright light coming from the back of the propeller. I then made a PA thanking the passengers for remaining calm and apologizing for the inconvenience. I then met with fire and rescue personnel. One firefighter used a thermal imaging camera to note that the space between the number 2 prop and nacelle was noticeably hotter than on the number 1 side. No further action was taken by ARFF and I contacted Operations for Maintenance and Dispatch. Maintenance had told me that a combination of deice fluid and other contaminates could accumulate in that space to the point were friction would create some sparking. I would like to know for future reference if this is the case is it acceptable to continue the flight. I would also like to know if it was electrical arching is it acceptable to keep using the prop heat to get the plane on the ground as most of the north east was IMC with below freezing temps and snow showers.




Synopsis: DH8A passengers and crew observe sparks coming from the aft edge of the propeller hub during cruise flight. No other anomalies are noted but the decision is made to return to the departure airport. Electric propeller deice was in use and continued to function normally.