What happened
On the morning of September 7, 2002, a private hot air balloon flight in Hämeenlinna, Finland, resulted in a collision with high-voltage power lines. The aircraft, a Cameron N-65 with registration OH-APU, had been flying at approximately 1,200 feet for about ten minutes under favorable weather conditions.
As the pilot descended to an altitude between 300 and 500 feet to search for a landing site, the pilot flames on both burners unexpectedly extinguished. The pilot attempted to relight the flames using the burners' integrated piezo igniters as well as a handheld igniter, but all attempts failed. As the balloon lost heat and began to descend rapidly, it struck a 110 kV power line. The impact caused an electrical discharge to flow through the balloon, resulting in significant damage to the envelope and the basket's structural components. A small fire broke out in the basket, which the pilot extinguished using an onboard fire extinguisher. No injuries were reported.
The investigation
The investigation focused on the failure of the burner's pilot flames and the subsequent inability to reignite them. Investigators examined the condition of the burners and the maintenance history of the aircraft. The inspection revealed that the pilot flames were significantly smaller than normal. Specifically, the regulator piston for the left burner was completely seized, while the right piston was found to be very stiff. The presence of reddish-brown oil was found in the left regulator, and the investigators noted that the maintenance instructions for this specific burner model were not sufficiently detailed regarding specific service intervals or components.
Findings
- The primary cause of the accident was the insufficient size of the pilot flames, which made them too weak to remain lit in the wind encountered during descent.
- The left burner's regulator piston had seized due to a lack of regular lubrication.
- The burner maintenance instructions lacked specific requirements for lubricating the regulator pistons.
- The existing annual inspections failed to detect the diminishing size of the pilot flames because the burners had not been opened or serviced for lubrication during those checks.