History of Flight
On July 15, 2020, at 1202 mountain daylight time, a Piper PA-30 (Twin Comanche) airplane, registration N8488Y, sustained substantial damage during an accident near Vaughn, New Mexico. The pilot and passenger were not injured. The flight was operated as a personal flight under Title 14 Code of Federal Regulations Part 91.
The pilot, who also held an airframe and powerplant mechanic certificate, reported that the airplane's fuel tanks were topped off to full capacity (90 gallons) the day before the accident. After an uneventful takeoff, the pilot proceeded to his destination. During climb, he positioned the fuel selectors to the auxiliary tanks for about 45 minutes and then switched them back to the main tank position. Approximately 3 hours and 15 minutes into the flight, at 8,500 feet mean sea level, the right engine surged twice and ceased producing power. The pilot reduced power on the left engine to counter a yawing motion. He briefly attempted switching to the auxiliary fuel tank, but the engine did not restart; he did not use the cross-feed selection.
Unable to maintain altitude and with no airports nearby, the pilot notified air traffic control of an off-airport emergency landing. The airplane touched down on desert terrain at about 6,300 feet msl. During the landing roll, the right wing struck a fence. After exiting the airplane, the pilot inspected the right wing tank and found it empty, which he attributed to a possible fuel system leak.
The pilot noted that the airplane's single-engine service ceiling at a gross weight of 3,600 lbs was 5,800 feet msl, and the single-engine absolute ceiling was 7,100 feet msl. He stated the fuel consumption was about 17 gallons per hour total. The left and right main fuel tanks each hold 30 gallons (27 usable), and the left and right auxiliary tanks each hold 15 gallons (all usable).
Aircraft Information
The Piper Twin Comanche Service Manual describes the fuel system as two independent units connected by a crossfeed for emergency use. Each engine draws fuel from its own set of cells through a screen, shut-off selector valve, electrically operated auxiliary pump, and engine-driven pump to the injector unit. Fuel valve controls are located in a panel between the pilot seats.
The Pilot's Operating Handbook states that for emergency single-engine operation, the crossfeed can increase range. To use fuel from the opposite side, the fuel selector for the inoperative engine is set to main or auxiliary, and the operating engine's selector is moved to cross-feed. For single-engine landing, fuel must be drawn from the main cell on the same side as the operating engine.
The handbook also notes the propellers are constant-speed, controllable, full-feathering units. Feathering is accomplished by moving the propeller controls fully aft through the low RPM detent; feathering takes about three seconds.
Examination and Testing
The airplane was equipped with a JPI EDM-760 Engine Monitor, which recorded exhaust gas temperature (EGT), cylinder head temperature (CHT), and battery voltage from engine start to the accident. Extracted data showed EGT and CHT values varied together during takeoff and climb. At 1154:16, about 3 hours 14 minutes after takeoff, EGT on the right engine momentarily dropped, then dropped again 36 seconds later and continued decreasing until the last recorded data at 1202:58. Battery voltage was notably higher just before the EGT drop.
The right wing was removed for recovery. The right fuel cap remained in the filler neck of the main fuel cell, which contained no liquid but trace sand/dust. Bladders were intact and snapped in place. The fuel screen to the right main fuel cell was clear, and the outlet was unobstructed when tested with light air pressure. The main cell air vent showed no blockages. The right auxiliary fuel cell contained an estimated 0.5–1 inch depth of fuel; the fuel line appeared to contain some fuel and was not blocked. The auxiliary cell air vent showed no blockages.
Both fuel selector handles were found in the OFF position. The selector bowls contained liquid consistent with 100LL AVGAS; water was not detected. Fuel lines were tight with no leakage. Examination of the right fuel selector revealed no anomalies.
The right propeller blades were unfeathered, with both blades bent aft slightly at the tips. No chordwise scoring or leading edge damage was observed.
The right engine, mount, propeller, and associated components had been removed as a unit. External examination showed no evidence of catastrophic failure. Cylinder rocker box covers revealed light oil film. Combustion chambers examined via borescope showed no mechanical damage or foreign object ingestion. Manual rotation of the crankshaft established thumb compression in all cylinders.
The engine was mounted on a test stand and started on the first attempt. It ran at various rpm with no defects noted. Upon shutdown, the propeller feathered without difficulty.