What happened
On April 20, 2015, during a routine equipment upgrade at Innsbruck Airport (LOWI), the owner of a Schempp Hirth Ventus cM motor glider discovered significant damage to an ACK E-04 emergency locator transmitter (ELT) while removing the unit from the aircraft. Upon removal, it was observed that the housing of the ELT battery pack had partially melted.
The ELT had been installed in the aircraft's landing gear compartment on December 29, 2011. While the battery's expiration date was still valid at the time of the incident, the aircraft owner could not provide records for the most recent annual inspection of the device. The damage was confined to the ELT battery pack itself; the aircraft's structure and the mounting plate showed no signs of heat damage or impact.
The investigation
The Austrian Federal Safety Investigation Board (SUB) conducted a technical examination of the damaged battery pack, comparing it against an identical, undamaged battery pack from the same production period.
Investigators found clear evidence of internal heat development on the rear of the battery housing, which had deformed the plastic in a pattern matching the internal contact springs. Upon opening the housing, investigators found corrosion on the individual cell terminals, likely caused by sulfur dioxide leakage from a damaged cell reacting with moisture.
Further examination of the electrical components revealed that the insulation material on the thermal switch connection had melted. The investigation also noted that the thermal switch, designed to interrupt the circuit at 93°C, had been activated, as evidenced by a negative continuity test.
Findings
- The primary cause of the malfunction was a short circuit between the thermal switch connection and the outer casing (negative pole) of one of the individual cells.
- This short circuit led to intense localized heating, which melted the insulation between the positive pole cap and the outer casing.
- The resulting secondary short circuit between the positive and negative poles caused enough current to melt the internal connection element of the cell.
- Corrosion on the cell terminals was present, likely due to the chemical byproduct of a compromised cell.
- The investigation ruled out improper assembly, as the findings were consistent with the structural design of the connection points.