WPR24LA146 · Robert Sterling 4P

The float-equipped, experimental amateur-built airplane had just come out of its condition inspection and the accident flight was the first time it had flown in about 6 years. The pilot stated that the engine performed normally during multiple post maintenance run-ups, and with no anomalies observed he departed with the intention of remaining within the traffic pattern. However, shortly after takeoff the engine lost power and the pilot performed a forced landing into a flat open area, where the airplane landed hard on its floats and tumbled over.

The airplane was originally an FAA type-certificated Piper PA-22 Tri-Pacer but had been reclassified to the experimental amateur-built category after undergoing extensive modifications, including a larger fuel-injected engine and an extension of the fuselage and wings.

Postaccident examination did not reveal any failures in the airframe or engine that would have precluded normal operation, and fuel was present in the fuel tanks and lines leading into the fuel injection system. Review of data recorded by the airplane’s engine monitor revealed that shortly after takeoff, a reduction in fuel flow was followed immediately by an increase in engine exhaust gas temperatures. Such an increase is consistent with a lean fuel-to-air mixture, often experienced during the initial phases of a fuel exhaustion event. The manifold pressure remained unchanged, indicating the pilot kept the throttle fully open. A short time later, with no change in manifold pressure, the engine began to lose power. The loss of power was partially interrupted as fuel flow resumed, but then all power was again lost as fuel flow dropped.

The manufacturer of the engine’s fuel injection servo gave specific instructions for the design of fuel supply system that were intended to prevent vapor lock. The instructions recommended that if a fuel strainer must be used, it should not be installed on the engine side of the firewall, and if it must be installed there, it should be configured to receive blast air. Additionally, a purge valve should be installed as close to the fuel flow divider as possible, preferably directly next to it.

However, the examination of the airframe revealed that the fuel strainer was mounted on the firewall right next to a hot exhaust pipe, and although a purge valve was installed, it was installed within the cabin, where its lines were routed through the engine compartment.

Due to the multiple engine starts performed before takeoff, the engine would have been hot and retaining residual heat, and as such the fuel system likely exhibited vapor lock. The installation of a purge valve indicated the airplane likely had a history of vapor lock. Such a vapor lock would have presented itself as a fuel starvation event as observed in the engine monitor data.

Verbatim NTSB analysis and probable cause from the NTSB dataset