Glasflügel 304 eS FES lithium battery fire at touchdown; cause undetermined

During a normal touchdown at Parham, the forward FES lithium polymer battery of a Glasflügel 304 eS ignited from electrical arcing. Smoke entered the cockpit on rollout; the pilot completed the landing and exited safely. The dislodged battery cover let an external fire develop, extinguished by foam after CO2 failed. Substantial damage to the FES battery box and surrounding fuselage; pilot uninjured. The cause of the arcing could not be determined. This was the second of three known FES battery fires; three safety recommendations followed for mandatory fire warning systems on FES-equipped sailplanes.

1. Aerotow landing: After an aerotow launch from Parham, 38 min ridge soaring, a brief (4 min) FES motor use during a rain shower, and ~1 h 15 min further soaring, the pilot returned to Parham for a normal circuit and smooth touchdown on grass runway 04.
2. Battery ignites on touchdown: At the moment of touchdown, an electrical arcing event in the forward FES lithium polymer battery caused it to ignite. The battery compartment cover fractured upward from inside, becoming dislodged. AAIB could not determine the cause of the arcing despite comprehensive examination.
3. Smoke enters cockpit: During the landing roll the burning battery produced smoke and fumes that moved forward into the cockpit; the pilot noticed a burning smell and visible smoke. No FCU warnings were recalled.
4. Pilot evacuates glider: The pilot completed the landing roll then vacated the cockpit normally without injury upon recognizing the smoke and fire.
5. External fire develops: After the pilot exited, the missing battery compartment cover allowed flames to emerge from the forward battery compartment, causing extensive fire damage to the fuselage battery box and surrounding structure.
6. Firefighting with foam: The airfield fire crew first attempted a CO2 extinguisher without success, then successfully extinguished the fire by applying aqueous film-forming foam into the FES battery compartment.
7. Fire damage; safety recommendations: Pilot uninjured. Substantial fire damage to the forward FES battery, its compartment, and adjacent fuselage structure. The G-GSGS fire was the 2nd of 3 known FES battery fires at the time. A survey of in-service batteries found metallic debris in a significant proportion; vibration testing showed cell pouch fretting but not severe enough alone to cause arcing. AAIB Special Bulletin S3/2017 issued three Safety Recommendations to EASA requiring fire warning systems in FES-equipped powered sailplanes. Sailplane and FES manufacturers implemented additional mitigations.