What happened
On March 5, 1998, a Finnair DC-9-82, registration OH-LMH, arrived at Arlanda Airport in Sweden. Upon docking at the passenger bridge, a mechanic noticed steam emitting from the battery vent pipes and determined that the aircraft batteries had overheated. The crew and passengers also noted a strange odor in the cabin.
As the mechanic reported smoke emanating from the fuselage, the captain was advised that the batteries were overheating. To ensure safety, the mechanic disconnected the aircraft's electrical power. Because the aircraft was without power, the captain could not use the standard PA system and had to enter the cabin personally to instruct the 101 passengers and 7 crew members to evacuate immediately. All passengers exited through the main door into the terminal building. There were no injuries during the evacuation.
The investigation
The investigation focused on the physical condition of the batteries and the performance of the charging equipment. Both nickel-cadmium batteries showed severe damage. The battery casings were swollen, and the nylon cell covers had melted due to temperatures exceeding 186°C. Electrolyte had evaporated and leaked through the vent pipes, leaving a white crusty residue.
Technical examination of the Sundstrand battery charger revealed that a thyristor (SCR 4) had failed in a conducting state. This caused the charger to provide a continuous, high-level current to the batteries regardless of their state of charge or the status of the temperature sensor. The investigation also noted that the aircraft lacked any onboard battery temperature monitoring or warning system, a deficiency common to the DC-9/MD-80 series due to older certification standards.
Findings
- The primary cause of the battery damage was overheating caused by a faulty thyristor in the battery charger, which resulted in an excessive charging current.
- The high current caused the electrolysis of the battery fluid, producing hydrogen and oxygen gases.
- The aircraft lacked a temperature monitoring or warning system for the nickel-cadmium batteries.
- The investigation noted that while the airline was transitioning to Acme Electric chargers (which fail safely), the existing Sundstrand chargers were prone to this specific type of failure.
Safety action
The investigation recommended that the airline complete its planned transition from Sundstrand chargers to Acme Electric chargers as quickly as possible, as the latter are known to cease charging upon failure, thereby preventing overheating.