What happened
On October 7, 1972, a Semco-Balloon hot air balloon, registration HB-BOT, departed from the Willisau sports field for a scheduled training flight. The crew, consisting of an instructor and a student, intended to fly for approximately 45 minutes with a planned intermediate landing near Oberberg. After 30 minutes of flight, the pilot attempted to land but was unable to arrest the balloon's descent in time. At the critical moment, the active gas cylinder was nearly empty, causing a drop in burner performance and necessitating the opening of a new cylinder.
During this transition, the balloon descended into a wooded area at an altitude of roughly 10 meters. A tree branch punctured the envelope, creating a 2-3 meter tear that led to a rapid loss of hot air. To lighten the load, the passenger descended via a tether, followed by the student. As the balloon sank to an altitude of about 4 to 5 meters, the instructor attempted to offload the instrument box and gas cylinders to prevent a full crash. While attempting to lower the cylinders, the instructor opened the quick-release valve on a nearly full propane cylinder without first closing the main valve or extinguishing the pilot light.
The investigation
The investigation examined the flight sequence, the mechanical state of the HB-BOT, and the actions taken by the crew during the emergency. The inquiry confirmed that the instructor held valid licenses for hot air ballooning and instruction, and the student held a valid learner's permit. The aircraft, a 1970 model manufactured by Semco-Balloon, was found to be airworthy and free of pre-existing defects. The investigation also reviewed the weather conditions, which were characterized by light winds and clear visibility at the time of the incident.
Findings
- The burner output was insufficient for the weight of the three occupants, which hindered the pilot's ability to control the descent during landing.
- The pilot failed to follow established checklists, specifically by failing to close the cylinder shut-off valve and extinguish the pilot flame before releasing the quick-release valve.
- The escaping gas from the pressurized cylinder ignited upon contact with the active pilot flame, causing a fire.
- The heat from the fire caused a partially filled cylinder to rupture and a full cylinder to vent through its pressure relief valve, leading to the total destruction of the balloon.
Safety action
- The instructor sustained severe injuries during the descent and impact, while the balloon was destroyed.