Incident Overview
A certified flight instructor (CFI) was conducting seaplane training with a student who held a pilot certificate but was not seaplane rated. Witnesses reported that as the aircraft departed Lake Sammamish, the engine produced unusual sounds before becoming silent. One witness described seeing the airplane spinning in a flat attitude.
Investigation Findings
An investigation revealed no pre-impact part failure. However, during a test, the spark plugs would not fire continuously at the minimum allowable operational pressure. The aircraft had accumulated 255 flight hours since its last inspection.
Modification Details
The aircraft was equipped with an extended propeller hub in accordance with Supplemental Type Certificate (STC) #SA4NE. This modification extended the propeller rearward by 4 inches.
No further details regarding injuries, aircraft registration, or time of day were provided in the source material.