What happened
On July 14, 2007, at approximately 20:25, a Sky Balloons Sky 120-24 hot air balloon departed from an ultralight airfield for a non-commercial flight. The aircraft was carrying four people, including the pilot, who had chartered the balloon from an aviation company. After more than 40 minutes of flight, the pilot accidentally activated the rapid deflation system (SDS).
This activation caused an opening at the top of the balloon envelope that could not be closed. As the air escaped, the lower opening of the envelope closed, preventing the pilot from effectively managing the descent. Despite continuous and massive use of the burners, the aircraft lost altitude steadily. During the descent, the pilot attempted to heat the air through the envelope material itself to counteract the loss of lift.
The descent could not be arrested, and the balloon collided with the roof of a house in a densely populated area of Rheda-Wiedenbrück. The basket slid down the slope of the roof and came to rest on a small rooftop terrace. The envelope, which had caught fire due to the intense burner use, draped over the house and an adjacent building. Three passengers sustained light burn injuries from falling pieces of burning envelope material. The accident also caused significant damage to the aircraft, a nearby house, and a parked vehicle.
The investigation
The BFU investigation focused on the mechanical operation of the rapid deflation system and the pilot's preparation. Investigators examined the aircraft's flight manual, which contained contradictory instructions regarding the operation of the SDS. The investigation also reviewed the pilot's experience and the weight and balance calculations performed prior to takeoff.
Technical examination of the Sky 120-24 revealed that several load tapes and their connections to the suspension lines were destroyed, and some lines had become detached from the envelope. Additionally, two gas cylinders were found on board that were not listed in the aircraft's official documentation, and the velcro straps on the envelope were worn.
Findings
- The primary cause of the accident was the accidental activation of the rapid deflation system.
- The pilot was inexperienced, having held a license for only a few months and having performed only five total flights, with this being the first flight on this specific aircraft model.
- The pilot had not received a proper briefing on the handling of the SDS upon taking over the aircraft.
- A critical safety feature, a safety lock/pin mentioned in the manual, could not be located on the aircraft, which contributed to the accidental activation.
- The flight manual provided confusing and contradictory information regarding the operation of the SDS.
- The pre-flight weight and balance calculation was incorrect, as the weight of the gas cylinders was significantly underestimated (209 kg instead of the documented 76 kg), although the aircraft remained within operational weight limits.