Flight Details
On 17 August 1998, an ATR42-500 registered F-GPYI operated by Air Littoral was on a domestic public transport flight from Montpellier to Bastia. The aircraft was occupied by two flight crew, one cabin crew, and 35 passengers. The flight was cruising at Flight Level 180 between Saint-Tropez and the MERLU point.
Incident Description
During cruise, the crew noted the illumination of the "EMER BAT CHG FAULT" warning light accompanied by a single chime alarm. In accordance with the failure procedure, the crew isolated the emergency battery from the charging circuit and decided to continue the flight to Bastia. The landing was normal. After landing, the captain and a technician observed that the emergency battery was hot.
Investigation Findings
The incident was investigated by the French Bureau of Enquêtes-Accidents (BEA). The emergency battery was sent to the operator's maintenance facility for bench testing, which revealed that three of the twenty cells had negative voltage (polarity reversal). Subsequently, two examinations were conducted by the battery manufacturer SAFT. The first examination indicated that the battery's electrical characteristics were within specifications. The second examination showed that eleven cells had insufficient voltage at the end of charge and that their cellophane gas barrier was damaged. A review by the BEA covering from January to October 1998 showed an unusual frequency of "EMER BAT CHG FAULT" warnings in flight and unscheduled removals of emergency batteries on the same aircraft type.
Probable Cause
The probable cause of the incident was the increase in temperature of the emergency battery due to polarity reversal of some cells, as stated in the official report.