Incident Overview
An aircraft conducting test flight maneuvers experienced problems with a modified fuel system. Ice blocking a fuel vent line caused a partial collapse of the main (engine feed) fuel cell, resulting in an erroneous fuel quantity reading. Additionally, the main tank overflow shut off valve was leaking, prompting tank overflow. The fuel overflow caution light illuminated, and the auxiliary tank fuel pump feed to the main tank automatically shut down. Due to miscalibration, this system overrode the pilot's attempts to restart the auxiliary fuel pumps.
Pilot Actions
Instead of immediately returning to base, the pilot remained in the test area troubleshooting the issues. The pilot eventually noted that the main tank fuel gauge continued to read 'full.' While en route to Boeing Field, fuel starvation occurred. The pilot opted to attempt a forced landing in a small athletic field in a residential area rather than ditching in Puget Sound.
Landing Sequence
The aircraft touched down in the intended landing area but then bounced across an adjacent street. The arresting action of telephone wires on the vertical fin brought the aircraft to rest in a residential backyard. All three occupants were injured, one seriously.
Probable Cause (Official Findings)
- Occurrence #1: Airframe/component/system failure/malfunction during maneuvering. Findings: fuel system vent blocked (total); fuel system tank distorted; engine instruments fuel quantity gauge false indication; fuel system fuel shutoff leak.
- Occurrence #2: Airframe/component/system failure/malfunction during maneuvering. Findings: fuel system pump switched off; remedial action not possible.
- Occurrence #3: Loss of engine power (total) - nonmechanical during cruise. Findings: precautionary landing delayed by pilot in command; fuel supply misjudged by pilot in command; fluid fuel starvation (cause).
- Occurrence #4: Forced landing during descent-emergency.
- Occurrence #5: In-flight collision with terrain/water during landing. Findings: terrain condition none suitable.