DICKINSON, ND, USA
N36990
CESSNA 414A
THE CORPORATE TWIN-ENGINE AIRPLANE WAS TAXIING FOR DEPARTURE WHEN THE PILOT TURNED THE COCKPIT/CABIN HEATER ON. APPROXIMATELY FIVE MINUTES LATER AN EXPLOSION OCCURRED WHICH CAUSED TWO OF THE THREE BAGGAGE DOORS TO COMPLETELY SEPARATE FROM THE AIRFRAME. A THIRD DOOR REMAINED ATTACHED WITH BY A HINGE. DURING THE ON-SCENE INVESTIGATION EXPLOSION/FLAME DAMAGE TO THE BAGGAGE COMPARTMENT INTERIOR AND THE INTERIOR SIDES OF THE DOORS WAS NOT OBSERVED. INSPECTION OF THE HEATER UNIT REVEALED THE COMBUSTION AIR PRESSURE SWITCH WOULD NOT INTERRUPT ELECTRICAL POWER TO THE FUEL SOLENOID VALVE WHEN AIR PRESSURE WAS REMOVED FROM IT. THE SOLENOID ALLOWED FUEL TO ENTER THE HEATER UNTIL CONDITIONS ALLOWED THE EXPLOSION.
On August 30, 1993, at 0615 mountain standard time, a Cessna 414A, N36990, registered to Fisher Sand and Gravel Company of Dickinson, North Dakota, and piloted by a commercially certificated pilot, received minor damage when the nose baggage doors departed the airplane after a baggage compartment explosion while taxiing for departure at the Dickinson Municipal Airport, Dickinson, North Dakota. Visual meteorological conditions prevailed at the time of the incident. The pilot and four passengers reported no injuries. The flight was going to originate from Dickinson, North Dakota. According to the pilot-in-command of N36990, he had turned the cockpit/cabin heater on during taxi. Approximately five minutes after having turned the heater "ON", the pilot stated, an explosion caused two nose baggage doors (doors) to completely depart the airplane. The left and right side doors were blown 250 and 100 feet away from the airplane respectively. A third door, left forward, separated from its normal closed position but remained attached to the fuselage opening by one hinge. The on-scene investigation revealed no evidence of combustion within the nose baggage compartment, on the internal surfaces of the three doors, the compartment floor surface or structure, or external damage to the Janitrol/Southwind Heater (Model number 13E55-1) other than a ruptured air intake hose. The hose was ruptured between the airplane's nose mount and attach point to the heater unit. The heater blower unit cage fan had a black substance similar to soot on its vanes and shaft mount. The heater unit was examined during a tear down. Findings of the tear down were: The combustion blower motor would not operate when power was applied. The fan assembly could be turned by hand. The motor had two worn commutator brushes. The motor's armature was worn but not below manufacturer's specifications. The combustion air pressure switch (Part number CDAIR 4157), which controls the heater fuel soleniod valve, was found closed and would not open. The switch was removed and tested independently. During the test the switch would not interrupt electrical power when air pressure was removed from it. The switch had been installed 302 hours prior to the incident. It had been inspected 52 hours prior to the explosion. The switch did not have any FAA Parts Manufacturing Authority (PMA) or approved markings on its surfaces. The heater assembly was bench checked and operated within factory specifications. The unit's spark plug and fuel nozzle had deposits upon their surfaces which were similar to carbon. An automotive coil was found installed in the ignition unit of the heater. This part did not have any FAA/PMA or approved markings on its surface. No internal or external fuel leaks were found on this unit. The tear down report is appended to this report.
the total failure of the airplane's heater system combustion air pressure safety switch.
Source: NTSB Aviation Accident Database
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