What happened
On the morning of 6 July 2000, an Aerostar S81A hot air balloon, registration ZK-SKY, was conducting a scenic flight near Methven with 13 passengers and two crew members. The flight began under favorable weather conditions, with the balloon climbing to approximately 5,300 feet.
As the crew approached a chosen paddock for landing, the pilot-in-command initiated a go-around to avoid a potentially hard landing. During this maneuver, the handling pilot utilized the burners to climb above the earth wire of a major power line. However, the balloon subsequently began to descend. The envelope made contact with the earth wire of the Benmore-Hayden power line, causing the balloon to become momentarily suspended.
Unable to climb clear of the wire despite continued heating, the pilot-in-command made the decision to vent the balloon and descend between the live power conductors. The balloon landed upright without any injuries to the passengers or crew and without sustaining any damage to the aircraft.
The investigation
TAIC investigators examined the flight path, the aircraft's weight, and the environmental conditions at the time of the incident. The investigation looked into the possibility of electrical fields from the high-voltage DC lines exerting a physical force on the balloon. Scientific testing conducted by the University of Canterbury found no measurable force generated by the electrical field on the balloon fabric.
Investigators also reviewed the experience levels of the crew, noting that while the pilot-in-command was highly experienced, the handling pilot had very limited experience with this specific aircraft type. Additionally, the investigation found that the balloon had been loaded to approximately 10% above its maximum specified weight, though this was not considered a primary factor in the descent.
Findings
- The primary cause of the incident was a misjudgement by the handling pilot regarding the balloon's response to burner inputs during the go-around.
- The pilot-in-command failed to sufficiently monitor the handling pilot's actions to ensure a safe margin of height was maintained above the power line.
- A low-level temperature inversion likely affected the balloon's buoyancy, complicating the climb.
- The power line, located just beyond the landing paddock, acted as a significant obstruction during the missed approach.
- The decision to descend between live conductors, while successful, presented unquantifiable risks.
Safety action
Following the incident, the operator implemented several safety measures, including:
- Instructions to all pilots to avoid landing within 300 meters of the power line and to avoid low-level flight over the lines in light winds.
- A new requirement for office staff to verify passenger weights during booking to ensure the aircraft's maximum all-up weight is never exceeded.