History of Flight
On December 27, 2020, at 1812 Pacific standard time, a Beech 35 airplane, registration N5474D, sustained substantial damage during an accident near Santa Ynez, California. The pilot and two passengers sustained minor injuries. The flight was operated as a personal flight under Title 14 Code of Federal Regulations Part 91.
The pilot reported departing with approximately 32 gallons of usable fuel. While en route to Lompoc, California, he altered course slightly to avoid inclement weather. The airplane passed 1 mile north of the Santa Ynez airport. When approximately 17 miles from Lompoc, the pilot switched the fuel selector from the right main tank, which was indicating near empty, to the left main tank, which indicated slightly less than half full. As he manipulated the selector handle, engine rpm decreased. Concerned that he could not engage the handle into the detent, the pilot returned the selector to the right tank.
The pilot elected to perform a 180-degree turn and land at Santa Ynez because it was closer. He attempted to troubleshoot the reduced rpm but was unable to restore engine power. The airplane could not maintain altitude, and the pilot executed a forced landing short of runway 08. During the landing, he attempted to avoid a tree but collided with a fence.
Airplane Information
A postaccident examination revealed that the airplane's fuel system had been modified under supplemental type certificate (STC) SA2660SW, converting the engine to an IO-407-N fuel-injected model. The fuel system consisted of four bladder-type cells. The main left and right tanks were located in the inboard wing adjacent to the fuselage, each with a total capacity of 20 gallons (17 gallons usable). The auxiliary tanks were 10-gallon fuel cells in the mid-wing section, outboard of the wheel-wells, each connected to its respective main tank.
The fuel selector had positions for left, right, auxiliary, and off, located to the left of the left-forward seat. Both auxiliary cells were connected to a common port in the fuel selector valve, so both fed simultaneously when set to auxiliary.
Individual electric sending units were installed in all tanks, transmitting fuel quantity to a single cockpit gauge. The pilot could switch the auxiliary fuel gauge selector switch on the left subpanel to "AUX" or "MAIN" and the "FUEL GAUGE" switch to the desired right or left position to view respective tank levels.
From the fuel selector, fuel flowed to an electric-driven fuel pump, then through the firewall to the engine-driven fuel pump. It then continued to the servo, then to a manifold mounted on top of the engine, which divided fuel to each cylinder through GAMI injectors. The fuel return line was located at the engine-driven fuel pump and routed only to the left tank; at normal operating power, about 8–10 gallons per hour would return to the left tank.
Tests and Research
Investigators established continuity in the fuel system. They plumbed a water supply to each fuel supply line at the wing roots and disconnected the line at the fuel pump with a bucket placed underneath. With the electric boost pump activated and a position selected on the fuel selector, fluid flow was observed from the selected tank inlet through the system into the bucket. All selector positions corresponded to the desired tank(s), and the detents were felt to be strong. Disassembly of the selector revealed no anomalies. No debris was flushed from the system. The filter was clean with no debris. The fuel pump was intact; carbon vanes moved freely, and light scoring was observed on cavity walls. The fuel servo had both mixture and throttle cables attached; no anomalies were found. The fuel manifold had a pliable diaphragm that was intact.
Fuel caps remained secure on the tanks. The right and left main-tank fuel bladders had wrinkles in the bottom and were collapsed inward, not taut against the wing structure, consistent with attachments not being secured. The aft section of the left main bladder was bulged upward in the area of the fuel-sender float.
Investigators filled the left-wing bladder with water and simulated the wing dihedral. With the quantity sensor wired to the cockpit gauge, eight gallons of fuel were added. The fuel gauge showed about half-full with eight gallons inside. The sending unit float cleared the bulged portion by approximately 3 inches.
External examination of the Continental Motors IO-470-N-3E1 engine revealed no evidence of catastrophic failure. Cylinder rocker box covers were removed, showing a light oil film on rocker arms and valve assemblies. Upper spark plugs were all similar with light gray coloration consistent with normal operation, except the No. 3 cylinder had a whiter, rust color. Combustion chambers examined through upper spark plug holes with a borescope were mechanically undamaged, with no evidence of foreign object ingestion. Valves and seats appeared undamaged; exhaust valves had red coloration. Manual rotation of the crankshaft via propeller produced spark at each lead. Thumb compression was established in all cylinders, but the No. 3 cylinder initially lacked compression until the valve was staked several times. The upper rocker arm toe was not aligned with the valve end on several cylinders.
The JPI was sent to the Safety Board recorders laboratory but did not contain any recoverable non-volatile memory.