What happened
The aircraft was conducting test flight maneuvers when issues with the modified fuel system arose. Ice accumulation blocked a fuel vent line, which led to a partial collapse of the main engine feed fuel cell. This structural failure produced an erroneous fuel quantity reading on the cockpit gauge. Additionally, the main tank overflow shutoff valve was leaking, resulting in fuel overflowing from the tank.
The fuel overflow caution light illuminated, triggering an automatic shutdown of the auxiliary tank fuel pump feed to the main system. Due to miscalibration within this system, the aircraft's logic overrode the pilot's attempts to restart the auxiliary fuel pumps. Instead of immediately returning to base, the pilot remained in the test area to troubleshoot the issue. The pilot eventually noticed that the main tank gauge continued to read 'full' despite the ongoing problems.
While en route to Boeing Field, fuel starvation occurred. The pilot opted to attempt a forced landing in a small athletic field located within a residential area rather than ditching in Puget Sound. The aircraft touched down in the intended landing area but bounced across an adjacent street. The arresting action of telephone wires on the vertical fin brought the aircraft to rest in a residential backyard.
The investigation
Examination of the event revealed that ice had blocked a fuel vent line, causing a partial collapse of the main fuel cell and resulting in inaccurate fuel quantity readings. The main tank overflow shutoff valve was found to be leaking, contributing to the fuel overflow. The auxiliary tank fuel pump feed system malfunctioned due to miscalibration, preventing the pilot from restarting the pumps during troubleshooting.
Findings
Contributing factors included the ice blockage of the fuel vent line and the leaking overflow shutoff valve. The erroneous fuel quantity reading misled the pilot regarding the actual fuel status. The automatic shutdown of the auxiliary pump feed, combined with the system's inability to be restarted due to miscalibration, exacerbated the situation. The decision to remain in the test area for troubleshooting delayed the return to base until fuel starvation was imminent.