In the storied history of Beechcraft King Air aircraft, few chapters are as intriguing as the saga of the wing attachment modifications, driven by a blend of engineering challenges, safety concerns, and the responses to those issues. Tom Clements of King Air Academy sheds light on this fascinating history, detailing how a serious safety concern was addressed and evolved over time.
In the early 1970s, the King Air 200 series faced a significant safety issue when two wing attachment bolts failed. One such incident involved an early model 200 based in Indiana. During flight, the crew heard a loud crack and observed the left wing slightly moving. They declared an emergency, managed to descend safely, and executed a successful landing. Post-landing inspections revealed that the lower forward wing bolt—critical for wing load-bearing—had fractured. Remarkably, the other three bolts (upper on the main spar and both upper and lower on the rear spar) managed to keep the wing intact.
This design difference—where the 200 model's bolt was rotated into a stationary nut versus the earlier models where the nut was rotated onto a stationary bolt—led to further investigations. Inspections revealed that the failures were due to "Environmental Corrosion" and "Hydrogen Embrittlement," highlighting the need for improved maintenance and inspection protocols.
David Saunders played a pivotal role in addressing these safety concerns. Through a vigorous letter campaign and direct outreach, he effectively highlighted the risks associated with wing bolts, creating substantial concern among King Air owners. Saunders’ efforts resulted in a significant retrofit: the installation of the "Saunders Spar Strap."
The Saunders Spar Strap involved connecting the right and left outboard wing sections with a steel strap routed through the wheel wells, designed to distribute the wing bending loads more evenly. A weight of 1,000 pounds of sandbags was placed on each wingtip to ensure that the strap would always bear some of the wing’s load. This modification was not just about enhancing safety but also about restoring confidence among operators.
Despite the successful implementation of the Saunders Spar Strap, Beechcraft had to respond to mounting concerns. At the 1975 NBAA convention, Beech’s then-president, Linden Blue, publicly addressed the lingering discomfort among King Air owners. He promised to develop a solution that would reinforce the wing structure, which led to the creation of the "Beechcraft Center Section Bridge."
Unlike the steel Saunders Strap, Beech’s Center Section Bridge was made of aluminum and was designed to provide equivalent strength. The aluminum design posed its own challenges, including the need for a thicker material to match the strength of steel and potential issues with "Galvanic Reaction" between steel and aluminum. The Center Section Bridge, which protruded significantly and was covered with an aerodynamic fairing, became a visible sign of Beechcraft's commitment to safety. Despite its efficacy, the Bridge resulted in a slight loss of cruise speed and came with a substantial price tag of approximately $50,000, reflecting the significant cost of development and production.
The King Air wing attachment story didn’t end there. Beechcraft introduced a new lower forward wing attach fitting design with models starting from BB-1158, BB-1167, and BB-1193 around 1984-1985. This new design used a bolt going through “knuckles” to transfer force from tension to shear, which improved overall safety and reliability.
The Saunders Spar Strap was installed via a Supplemental Type Certificate (STC), while Beechcraft's Center Section Bridge was an approved factory kit, avoiding the need for an STC. Over time, the urgency surrounding wing attachment modifications diminished, and the availability of the Center Section Bridge was phased out as newer designs took precedence.
Tom Clements’ account of the King Air wing attachment saga provides invaluable insight into a critical period in aviation safety history. It illustrates how safety concerns and engineering challenges were met with innovation and determination. For enthusiasts and operators of King Air aircraft, understanding this history not only honors past advancements but also underscores the ongoing commitment to aviation safety.
This chapter in King Air history remains a testament to the rigorous efforts of both individuals like David Saunders and manufacturers like Beechcraft, whose actions helped shape the safety standards we rely on today.
Cover Image: Beechcraft
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For more insightful information from Tom Clements, check out The King Air Book - Volumes I & II available for purchase here: https://kingairbook.com/the-king-air-book...