What happened
An explosion occurred in the forward galley involving the aircraft's water heater. The event resulted in the destruction of the unit.
The investigation
Mechanical examination of the equipment identified two primary failures. First, the contacts of the water heater relay—which provides electrical power to three heating elements when energized—were found to be rough, ragged, discolored, and exhibited excessive erosion. The manufacturer noted that once a relay exceeds its service life, surface wear and erosion can cause the contacts to stick in the closed position.
Additionally, investigators tested the pressure relief valve, which is designed to open at 65 PSI. The valve failed to crack even when subjected to 300 PSI. Upon disassembling the valve, it was discovered that an O-ring had migrated from its intended location into the lower valve body. The O-ring became lodged between the head of the poppet and the inside diameter of the valve, creating a complete flow blockage that prevented the relief valve from opening.