What happened
On August 17, 1998, an ATR42-500, registration F-GPYI, operated by Air Littoral, was performing a scheduled domestic flight from Montpellier to Bastia. While cruising at flight level 180 between Saint-Tropez and the MERLU waypoint, the flight crew observed the "EMER BAT CHG FAULT" warning light accompanied by a single chime alarm.
Following established emergency procedures, the crew isolated the emergency battery from the charging circuit. The flight continued to its destination without further incident. However, upon arrival on the ground, the flight commander and a technician discovered that the emergency battery was extremely hot to the touch.
The investigation
The investigation focused on the maintenance history of the emergency battery and the procedures used by the operator's maintenance facility, Air Littoral Industrie (ALI). Laboratory testing of the battery revealed that three of its twenty cells had experienced polarity reversal. Further examination by the manufacturer, SAFT, determined that eleven cells had insufficient voltage at the end of the charging cycle, resulting in damaged cellophane gas barriers.
Investigators also identified a significant trend of similar incidents within the fleet. Between January and October 1998, twenty-four unscheduled battery removals were recorded across ten different batteries. The investigation found that the maintenance "cycling" process performed by ALI did not comply with the manufacturer's Component Maintenance Manual (CMM). Specifically, the recharge process lasted only one hour at nominal capacity, whereas the CMM required a five-hour process (one hour at nominal capacity followed by four hours at 1.5 A).
Findings
- The primary cause of the incident was improperly performed maintenance operations, which resulted in the installation of a defective emergency battery.
- The abbreviated charging procedure left the battery in a constantly undercharged state.
- The shortened testing duration prevented the detection of faulty cells or damaged gas barriers.
- The resulting imbalance in cell voltage and charge led to excessive temperature increases during flight, triggering the thermal runaway protection system.
- Difficulty in tracking the individual history and specific locations of replaced cells within the battery made it challenging to assess the overall health of the components.