DC-3/Dakota/Skytrain
Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Flight Crew Last 90 Days 90; Flight Crew Total 2800; Flight Crew Type 2000
Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor; Inflight Event / Encounter Other / Unknown
Flight Crew Overcame Equipment Problem; General Maintenance Action
Aircraft
The [DC-3] aircraft was being used as a training platform for a Second in Command (SIC) Check. I was the PIC. We had just completed a left turn to establish a direct track to the airport and initiated a descent when both crew members smelled smoke. The smoke became rapidly worse. On a scale of 1 to 10 (with 10 being the worst) I would estimate that the smoke accumulation in the cockpit was never worse than 4. Upon turning around in my seat to look back down the companion way and into the cabin to see if I could determine the source of the smoke I saw smoke emanating from under the cabin floor. I briefed the SIC in training to remain at the controls and to keep up the present heading (and descent) direct towards airport ZZZ. I left the flight deck with one of three fire extinguishers we had onboard.I looked for several seconds for the source of the smoke; but could see no fire. I elected to discharge the contents of the fire extinguisher under the floor. The smoke immediately started to disperse and I immediately returned to the flight deck and took control of the ship. Flight Engineer made a radio transmission on the ZZZ CTAF frequency and informed all ZZZ traffic [of our situation] and were returning to ZZZ and wanted priority on Runway XX (conditions favored Runway XX and we were established approximately straight in prior to the occurrence.) We heard one response from an aircraft in the pattern at ZZZ; they broadcast that they would leave the pattern to the west as a result of our situation. ZZZ has recently had a control tower built. The Tower is not operational; but I asked 'ZZZ Tower' if they had heard our distress call. I received no response. We continued our approach at best possible forward speed. Approximately 30 seconds after my call I heard a transmission (I assume from the Tower staff who are known to be monitoring activity prior to the official opening of the Tower) that 'there was no visible smoke outside the aircraft'. The smoke within the aircraft did not return.I made a normal landing and immediately vacated the runway to the right. I shut the aircraft down on [the] taxiway and both crew members (only people on board) vacated the aircraft with an additional fire extinguisher. No sign of fire; smoke or damage was visible outside the aircraft in any location. Approximate five minutes after shutdown; the ZZZ volunteer Fire Department arrived and proceeded towards us. The Fire Fighters inspected the aircraft and then accompanied the aircraft (now under tow by hangar staff) to our hangar. The Fire Fighters made a thorough inspection of the aircraft; completed a brief report on site and then departed.Upon deeper inspection; it was determined that the right-hand fuel boost pump motor had become excessively hot during operation. The right-hand boost pump Circuit Breaker (CB) was found to have tripped. When removed from the aircraft; the fuel boost pump assembly smelled particularly strongly of burned electrical insulation. The pump itself turned freely; but the motor was seized. It was determined that the fuel boost pump motor was the source of the smoke. There was no fire; and no damaged was observed to any aspect of the aircraft structure or systems other than the boost pump assembly itself. The rest of the aircraft is currently undergoing a condition inspection. It is my position that this event could best be described as an occurrence only.
Reporter stated the Douglas DC-3 aircraft uses a Direct Current (D.C.) Motor driving a vane type fuel pump. Douglas uses the same vane type design for the engine driven fuel pump. The D.C. fuel boost pump however; has a small gear reduction driven by the D.C. motor. Both pumps generally run forever; but the D.C. fuel pump gear reduction is sealed after assembly and occasionally fails; seizes up; jamming the D.C. motor and causing smoke. The D.C. fuel pumps are mounted below the cabin floor just outboard of the fuselage near the forward spar. He suspects the smoke emanating from the D.C. fuel boost pump stopped after the Circuit Breaker (CB) tripped. He also has fifteen plus years of experience working with aircraft heavy sheet metal.
Captain reports he and the First Officer (SIC) smelled smoke emanating from under the cabin floor in a DC-3 aircraft during a training flight. Captain discharged a portable fire extinguisher dispersing the smoke. Maintenance noted the right-hand (RH) electric D.C. fuel boost pump internal reduction gearing had failed; seizing the motor and tripping the Circuit Breaker (CB).