What happened
On 20 April 2022, the BB78Z hot air balloon, registration VH-RJR, was preparing for its inaugural flight carrying 13 passengers from Royal Park, Victoria, to Moorabbin Airport. During the ascent over the Melbourne CBD, the pilot activated the burners, which raised the air temperature within the envelope. This temperature increase caused increased upward pressure on the vent panel, forcing the panel's edge through the aperture and creating gaps that allowed air to escape.
As air leaked from the envelope, the pilot struggled to maintain altitude and flight path control, necessitating much higher burner usage and rapid fuel consumption. While searching for a safe landing site at a low altitude, the balloon's basket struck two buildings. The pilot attempted to reach Elwood Beach, but the increased fuel burn meant altitude could no longer be sustained during the approach. Consequently, the pilot performed a forced landing in a suburban street. During this maneuver, the basket hit trees and the ground, resulting in 3 minor injuries to passengers.
The investigation
The investigation focused on a design change made to the deflation system prior to the manufacture of VH-RJR. At the operator's request, the manufacturer, Kubicek, implemented a larger deflation system to improve descent performance. This change was approved internally as a minor modification without the necessary tests or analysis to confirm its suitability for this specific balloon model.
The approval relied on two incorrect assumptions: that a design from a larger balloon would work on the smaller BB78Z and that a tethered production test would be sufficient for validation. Because the BB78Z has fewer gores than the larger model, the spacing between vertical load tapes at the vent edge was wider, allowing the vent panel to be pushed out of place by internal pressure. Furthermore, the manufacturer's production test recorded an internal temperature of 114 °C, but post-test calculations suggested the actual temperature was only approximately 89 °C. This lower temperature prevented the pressure-related leak from manifesting during the initial test.
Findings
- The modified deflation system featured wider spacing between vertical load tapes, which allowed the vent panel to shift and leak air.
- The manufacturer approved the design change without conducting required safety analyses or empirical testing.
- The manufacturer lacked a reliable process to verify the accuracy of temperature readings during production inflation tests.
- The failure to identify the design flaw was compounded by an inaccurate temperature reading during the manufacturer's production test.