Narrative:

After established at cruise altitude of FL360 with direct routing we experienced a very loud electrical arcing sound emanating from the avionics bay under the flight deck. Shortly afterward blue-white smoke with a very strong ozone smell began seeping from the floor panels around the captain's yoke and rudder pedals. Captain immediately turned on the third pack (no 2) to increase airflow into the cockpit area. This additional airflow effectively suppressed any further infiltration of smoke into the cockpit. The electrical arcing in the avionics bay could also be felt through the heels resting on the floor panels. An emergency was declared with ARTCC with an immediate turn and descent for ZZZ which was about 30 NM to the southeast of our position. Weather was cavok and the airport was in sight at FL360. Consideration was given to donning O2 masks; but considering that cockpit smoke was effectively suppressed by third pack airflow and in order to facilitate the numerous radio calls that had to be rapidly made a decision was made to defer O2 mask usage unless situation further deteriorated. Captain was greatly concerned that arcing might ignite other flammable materials in avionics bay which could have created a fire situation that would rapidly spiral out of control. If this should occur; then one the first indications would be a change in the smell emanating from avionics bay. Donning O2 masks would have negated this sensory awareness! ARTCC handed us off to approach at around FL250. Both controllers gave us excellent assistance with free rein to maneuver at will to expedite landing and recovery. First officer did an outstanding job of hand flying the aircraft to an expedited visual landing. Airspeed was close to the Q limit during the entire descent with momentary overspeeds. We crossed the outer marker about 8 minutes after leaving FL360. Both crew members were highly motivated and in complete synch with each other to get the aircraft on deck as fast as safely possible. Emergency personnel were waiting on the aircraft after landing and were prompt in their check of the exterior areas around the forward avionics bay. We taxied to assigned parking after clearance from emergency personnel. Maintenance found the number 3 avionics exhaust fan motor had electrically shorted and burned up even though it was not in any situation where it would have normally been on. The electrical self-destruction of the fan unit was highly unusual; also unusual was that so much electrical arcing damage had occurred before the circuit breaker for the unit popped and severed the current flow to the fan motor! If the arcing had ignited other flammable materials; the fire situation could have dangerously escalated with no feasible means of suppression. Number 3 avionics bay exhaust fan motor internally shorted and self-destructed in a commanded off state; without adequate circuit breaker protection of the high amp circuit to prevent dangerous electrical arcing and burning in the avionics bay. More reliable system design and machinery. As an aside; it is almost inevitable that using 30 to 40 year old aircraft will lead to greater system malfunctions and breakdowns of an urgent and destructive nature. Do not retrofit older airplanes with systems that those aircraft were never designed by the manufacturer to have. It can have very negative unintended consequences.

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Original NASA ASRS Text

Title: DC-10 Captain at FL360 reports very loud electrical arcing sound emanating from the avionics bay under the flight deck; followed shortly by blue-white smoke with a very strong ozone smell. Flight diverts to the nearest suitable airport and lands safely. Post flight reveals that the Number Three Avionics Bay Exhaust Fan Motor internally shorted and self-destructed with the fan not operating.

Narrative: After established at cruise altitude of FL360 with direct routing we experienced a very loud electrical arcing sound emanating from the avionics bay under the flight deck. Shortly afterward blue-white smoke with a very strong ozone smell began seeping from the floor panels around the Captain's yoke and rudder pedals. Captain immediately turned on the third pack (No 2) to increase airflow into the cockpit area. This additional airflow effectively suppressed any further infiltration of smoke into the cockpit. The electrical arcing in the avionics bay could also be felt through the heels resting on the floor panels. An emergency was declared with ARTCC with an immediate turn and descent for ZZZ which was about 30 NM to the southeast of our position. Weather was CAVOK and the airport was in sight at FL360. Consideration was given to donning O2 masks; but considering that cockpit smoke was effectively suppressed by third pack airflow and in order to facilitate the numerous radio calls that had to be rapidly made a decision was made to defer O2 mask usage unless situation further deteriorated. Captain was greatly concerned that arcing might ignite other flammable materials in avionics bay which could have created a fire situation that would rapidly spiral out of control. If this should occur; then one the first indications would be a change in the smell emanating from avionics bay. Donning O2 masks would have negated this sensory awareness! ARTCC handed us off to Approach at around FL250. Both controllers gave us excellent assistance with free rein to maneuver at will to expedite landing and recovery. First Officer did an outstanding job of hand flying the aircraft to an expedited visual landing. Airspeed was close to the Q limit during the entire descent with momentary overspeeds. We crossed the outer marker about 8 minutes after leaving FL360. Both crew members were highly motivated and in complete synch with each other to get the aircraft on deck as fast as safely possible. Emergency personnel were waiting on the aircraft after landing and were prompt in their check of the exterior areas around the forward avionics bay. We taxied to assigned parking after clearance from emergency personnel. Maintenance found the Number 3 Avionics Exhaust Fan Motor had electrically shorted and burned up even though it was not in any situation where it would have normally been on. The electrical self-destruction of the fan unit was highly unusual; also unusual was that so much electrical arcing damage had occurred before the circuit breaker for the unit popped and severed the current flow to the fan motor! If the arcing had ignited other flammable materials; the fire situation could have dangerously escalated with no feasible means of suppression. Number 3 Avionics Bay Exhaust Fan Motor internally shorted and self-destructed in a commanded off state; without adequate circuit breaker protection of the high amp circuit to prevent dangerous electrical arcing and burning in the avionics bay. More reliable system design and machinery. As an aside; it is almost inevitable that using 30 to 40 year old aircraft will lead to greater system malfunctions and breakdowns of an urgent and destructive nature. Do not retrofit older airplanes with systems that those aircraft were never designed by the manufacturer to have. It can have very negative unintended consequences.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.