What happened
On June 2, 2005, at approximately 18:20 local time, preparations were underway for a promotional flight in Frankfurt-Sindlingen, Germany. The aircraft, a newly manufactured Lindstrand LBL HS 110 with British registration, was being inflated using a burner and a cold-air blower to maintain internal pressure.
As the Rotax engine was started and the airship was prepared for takeoff, a loud cracking sound was heard. The pilot experienced a violent jolt in the gondola and noted a significant drop in internal envelope pressure. The inflation process resulted in a massive tear in the envelope near the left tail fin, spanning several fabric panels both horizontally and vertically. The internal ventilation hose also ruptured. The incident caused heavy damage to the aircraft, but there were no injuries to the pilot.
The investigation
The BFU examined the damaged envelope and compared it to a previous version of the same model. The investigation focused on the structural integrity of the new envelope, the inflation system, and the cockpit instrumentation. Investigators analyzed the stitching, the placement of load tapes, and the design of the ventilation system. The investigation also looked into the functionality of the pressure and temperature monitoring equipment during the pre-flight phase.
Findings
Significant manufacturing defects and design deviations were identified as the primary causes of the failure. The investigation established that the new envelope featured a different ventilation design compared to its predecessor; instead of a large funnel-shaped opening, air was introduced through a much smaller 40 cm slit. This restricted fresh air intake while the blower continued to operate at full capacity, leading to excessive internal pressure within the envelope and tail system.
Furthermore, the structural strength of the envelope was compromised by several factors:
- The stitching of the fabric panels was substandard, with several longitudinal seams failing to align precisely.
- The load tapes, which are intended to provide structural reinforcement, were interrupted at the locations of the eyelets.
- The positioning of the eyelets for the left rudder control cable was poorly chosen, creating high stress concentrations.
- The pressure relief valves were more tightly secured than on previous models, preventing the necessary release of excess air.
Additionally, the pilot was unable to effectively monitor critical operating limits because the envelope temperature sensor was non-functional and the pressure gauge used Inch units without color-coded operating ranges, making it difficult to identify the maximum allowable pressure of 20 mm/Ws.