The much anticipated, overly speculated upon, FAA Airworthiness Directive (AD) for the B-17 wing attach structure was published on Wednesday, May 17, 2023, to become effective on June 1. The published AD will effectively ground the three U.S. based B-17s until required inspections and, possibly, repairs are completed. It will most likely ground the UK-based B-17 also in the near future given the severity of the structural issue and the normal close cooperation between the FAA and the UK Civil Aviation Authority.

To immediately quell the rampant speculation about on the web, this new AD does not:

• Permanently ground all B-17s
• Require the spars to be replaced (yet)
• Affect the carry-through wing structure in the fuselage

The New AD Answers Some Questions

The AD does answer most of the questions that have arisen in the past two years and, more specifically, the past four weeks, about the wing spars and attach points. As a recap, the EAA B-17G (44-85740, N5017N), otherwise known as Aluminum Overcast, was grounded by the EAA in April 2021 when a pre-flight inspection revealed an anomaly in the left wing attach structure at the fuselage. And last month, on April 15, 2023, the Yankee Air Museum voluntarily grounded its own B-17G (44-85829, N3193G), otherwise known as Yankee Lady, based on the expectation of an Emergency Airworthiness Directive (EAD) being issued.

B-17 Wing Structure

Also as a recap, the primary structure of the B-17 inner wing panel consists of two truss-type wing spars, forward and rear. Each truss-type spar is built up from two spar chords, upper and lower, that are assembled with tubular members forming “N” type trusses. The upper and lower spars, both front and rear, are themselves attached to the fuselage center section with steel terminal fittings that are bolted into the inner end of the spar chords. Eight close-tolerance bolts fix the terminal fitting into the spar chord. Then, the terminal fittings attach to matching wing attach fittings on the fuselage center section that sandwich the wing fittings. The two parts are secured with special tapered pins. Thus, there are four main attach points that hold each wing to the fuselage, along with two other specialized secondary attach points located between the upper and lower spars. The secondary attach points are where the sheer terminal is located and are held with a single bolt.

This illustration perfectly shows the problem area—the left forward, lower wing attach structure.

The Left Wing of Aluminum Overcast Shifted Two Inches

Over the past two years, the EAA has related in several information releases that the problem found in April 2021 that grounded Aluminum Overcast was found in the left rear shear attachment point. Based on the just-released AD, that was not, in fact, the case.

The AD provides information that the problem found was in the left forward terminal fittings where bolted into the wing spar chord. Apparently, that joint separated and allowed the wing to shift two inches at that attach point, no doubt revealed with a bulge or separation with the fillet that covers the joint on the lower wing just outboard of the bomb bay. That the wing itself could shift two inches is pretty significant in that there remained two other points of attachment for the forward wing spar, the upper terminal fitting and secondary single-bolt shear attachment. In any event, the fact that the wing shifted two inches at that attach points indicates a significant structural failure; fortunately, a well-done pre-flight inspection revealed the problem and the proactive EAA immediately grounded the airplane where it was, at Punta Gorda, Florida.

Further inspection of Aluminum Overcast by the EAA revealed that the identical lower forward terminal fitting to spar chord on the right wing had suffered cracking. Since April 2021, Aluminum Overcast had its wings removed for inspection and repair, suffered some tornado damage, and was ultimately trucked overland to Oshkosh, Wisconsin, where it currently sits in an EAA hangar being repaired.

What The New AD Requires

So, what of this new published AD? Well, it really is addressing the same terminal fitting to spar chord issue addressed by the 2001 FAA AD. But, the FAA clearly states that this new AD can be considered interim; problems found in inspecting airworthy B-17s will determine if a further AD requiring inspections or repairs will need to be issued.

Essentially, this new AD requires one of two actions:

• Remove both wings of the B-17 and separate the 8 terminal fittings-to-spar chords joints (four on each wing, upper and lower, forward and aft) and perform a magnetic particle inspection of the steel terminal fittings and an eddy current inspection of the 8 inboard holes in the end of the spar chords; or
• On the left and right lower forward terminal fitting-to-spar chord joint assemblies, remove the most inboard bolt and do an eddy current bolt hole inspection on both the steel terminal fitting and the aluminum spar chord.

The second option above seems to be the easiest to complete and directly addresses the problem found with Aluminum Overcast. However, the three inboard bolts (out of eight) on the terminal fitting to spar chord are difficult to remove due to other structure near the bolts with access difficult. For this reason, the 2001 AD allowed these three inboard bolts to remain in place and be inspected internally with a borescope for cracking and corrosion. Evidently, that was not sufficient to reveal problems as this is exactly where the April 2023 problem was found on Aluminum Overcast. So, now, operators will be required to more closely inspect this most inboard bolt hole by removing the bolt.

Even if the inspection process was relatively easy to comply with, the most important part of this new AD is the requirement that any cracking found will require a repair. Based on the history of the problems found with the joint, it can be presumed that cracking and/or corrosion will be found. The repair component then becomes the big question. Repairs methods will have to be specifically approved by the FAA.

What if Corrosion or Cracking is Found?

With this critical part of the wing attach structure, the question becomes how can it be repaired to adequately insure the airworthiness of the airplane. Replacing the steel terminal fitting is doable with new parts built to specifications. However, repairing corrosion or cracking in the bolt holes on the aluminum spar chords is more problematic. Replacing the entire 25-foot section of the spar tube is just about a non-starter due to 1) there are no replacement spar chord tubes available; 2) the design of the spar tube with an internal taper is difficult and expensive to extrude; and 3) the wing would need to be disassembled to rebuild either the aft or forward spar assemblies.

Can a new inboard portion of the spar chord tube be fabricated and spliced into the outer section of the spar chord tube? Can a doubler or other fabrication be engineered to repair the cracked or corroded holes? What are the other options? All good questions but, at my pay grade, I have no good idea about the solution. Reportedly, the EAA is busy working a repair to Aluminum Overcast and they hope to have it flying again in 2024 so, possibly, the EAA has proposed a repair and the FAA has accepted it. Or, maybe not. Time will tell.

The New AD is Probably an Interim AD

The new AD has another caveat: if one of the inspection options in the 2001 AD has been complied with within the past 10 years or 2,500 flight hours, the AD is not applicable. That inspection option pretty much duplicates the first option of this new AD: removal of the wings, separating the terminal fitting-to-spar chord joints, and doing a thorough inspection. However, to my knowledge, none of the airworthy or close-to-airworthy B-17s had this major inspection (requiring removal of the wings) in the past 10 years. Those aircraft under active restoration to flight status probably can meet this requirement.

A major part of this new AD is the reporting requirement. All inspection results, both good and bad, must be reported to the FAA. The FAA wants to know what is going on in these joints and they it will analyze inspection results and possibly issue another AD on this wing structure.

Effect Of the New AD

So, after June 1, this AD will ground the airworthy B-17s in the US. This currently consists of three airplanes: the CAF’s Sentimental Journey, the Erickson’s Ye Olde Pub, and the YAF’s Yankee Lady. These three will remain grounded until the AD is complied with.

There are three other B-17s that this AD will immediately affect as they are near airworthy: Collings Foundation’s N207EV, Tallichet’s Movie Memphis Belle, and the Mid-America Museum’s old Thunderbird. The latter is in a deep rebuild at Madras, Oregon, with the wings off, so it will be relatively easy to comply with the inspection requirements of the AD. The other two may require some disassembly. There are a number of other B-17s in very-long-term restoration to flying status. Most of these are completely disassembled so those aircraft can incorporate the requirements of the AD as they come together. The others: this might be a show stopper for barely-funded efforts to get their B-17s airworthy again.

An early account of addressing this B-17 structure is presented on the GossHawk Unlimited website about work they did to a B-17 many years ago. It has some illuminating photos such as the one below:

So, as a summary, the FAA is now carrying four ADs on the B-17, according to the FAA AD website:

• AD 77-17-11 (Effective January 23, 1979): Wing attach-Inspection and repair of the aft lower spar caps on both wings. Requires periodic inspections.
• AD 73-20-02 (Effective January 26, 1979): Amends/expands the prior AD with one that includes the forward, lower spar chord for inspection of cracks and requires repairs. Requires periodic inspections.
• AD 2001-22-06 (Effective November 30, 2001): Requires inspection of the spar chords, terminal fittings, and bolt holes. Requires periodic inspections.
• AD 2034-10-04 (Effective June 1, 2023): Requires inspection of all terminal fitting-to-spar chord bolts and holes, or removal and inspection of innermost bolt and associated components. No requirement in this AD for periodic inspections; however, there is reporting requirements for all inspection results.

As can be seen, these terminal fitting and spar chord joints have been a continual problem on the B-17 for the past forty-plus years.