The Story: Of the estimated 12,600 B-17 Flying Fortresses built during World War II, only a few are still flying today, and in 2021, one of them was found sitting on the ramp with its left wing visibly pulled two inches away from the fuselage. The discovery happened during a routine pre-flight walk-around, but the true scope became clear when the wings came off for inspection: the left front spar lower fitting, a primary structural attachment tying the wing to the airframe, had completely separated at the wing-to-fuselage joint, while the matching joint on the right wing was already cracking.

This wasn’t the first time the FAA had found the same failure mode at exactly this joint. On April 26, 1994, an inspection of one B-17 revealed corrosion on approximately a third of the 64 bolts that attach the wing spar chords to their terminal fittings; two bolts were corroded down to nearly half their original cross-section, with one spar chord end broken into pieces. That finding led directly to AD 2001-22-06, which required inspection of the same wing terminal-to-spar chord joints across the board. By the time that rule was finalized, cracks had turned up in 5 of the 12 B-17s still flying or restorable at the time, nearly half the surviving fleet.

The mechanism, as the FAA later laid out, comes down to metallurgy: the spar chords are aluminum, the terminal fittings are steel, and dissimilar metals in direct contact tend to galvanically corrode. Moisture trapped inside the wing at the joint feeds that corrosion, and the pressure from the corrosion products building up between the terminal fitting and the spar chord is what drives the cracking investigators have found.

Owners pushed back hard on the 2001 rule at the time, pointing to meticulous maintenance, airplanes flown well under gross weight, and wartime-era technical orders that permitted continued flight with cracks within specified tolerances. The FAA’s answer was blunt: a technical standard written to get a bomber through one more combat mission isn’t a valid basis for judging long-term fatigue and corrosion risk. Two decades later, that logic holds even more: these airframes are now pushing eighty years old.

That 2001 inspection method had its own blind spot: it wasn’t reliably catching cracks hiding inside the steel fitting, deep inside the spar chord tube, which is exactly what led to the 2023 AD.

Field Notes:

• Applicability: Applies to all Boeing Model B-17E, F, and G airplanes certificated in any category, including those flying on experimental airworthiness certificates (which the FAA included because these aircraft routinely carry passengers on warbird experience flights).
• Credit for Prior Inspection: Operators who already inspected under AD 2001-22-06 within the last 10 years or 2,500 flight hours do not need to take immediate action. All other affected aircraft must be inspected prior to further flight.
• Required Action: Before further flight, operators must inspect all eight wing terminal-to-spar chord joints using one of two approved methods:
  • Option 1 (Full Teardown, Wings Off): Magnetic particle inspection of the steel terminal fittings plus eddy current inspection of the aluminum spar chords.
  • Option 2 (Less Invasive Option): Eddy current bolt-hole inspection of just the most inboard fastener on the left and right lower forward joints.
• Mandatory Repairs: Any cracking or corrosion found must be repaired under an FAA-approved method before further flight.
• Reporting Requirement: All inspection results, even “no findings,” must be reported to the FAA’s West Certification Branch within 10 days. The FAA is explicit that this is an interim action to gather fleet data before committing to a permanent structural fix.

Why It Matters: Start with the fleet: 18 B-17s on the U.S. registry today, and of those, only 3 are currently airworthy, with several more under long-term restoration and one additional example flying in the UK. Back in 2001, the FAA counted just 12 airplanes worldwide, 10 of them U.S.-registered, and even that smaller number has continued to shrink over the two decades since, despite sustained regulatory attention to exactly this structural weak point. There is no production line to fall back on; every one of these airframes is irreplaceable.

The economics underline that reality. The FAA estimated the 2001 inspection and repair work at roughly $90,000 per airplane, a serious burden for aircraft kept flying mostly by nonprofits, volunteers, and donor support. Keeping a Flying Fortress in the air is as much preservation as operation, and only the organizations that can absorb bills like this one keep their airplanes flying.

Then there’s the failure mode itself: not a crack caught during scheduled maintenance, but a primary wing attachment fitting that had already let go. Layer on the fact that the fix meant to catch this, a two-decade-old AD written after corroded bolts were found in spar chords, had its own blind spot, and you get the uncomfortable timeline connecting 1994, 2001, and 2023: an inspection is only as good as its ability to actually find what it’s looking for, and for a fleet this small, this historic, and this closely watched, getting the right inspection has been a two-decade-long moving target.

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