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Anyone who is a fan of warbirds or has been following aviation news probably at this point has heard about EAA’s B17 “Aluminum Overcast”. While on the road in spring of 2021, the aircraft was grounded following evidence of movement on a wing fairing. While investigating the issue, a crack was found in the left wing’s sheer web. Because of this event, the remaining 8 B17 operators grounded their aircraft in preparation for the AD inspection that would be issued by the FAA.
Because of the unknown of what the inspection would entail or if the aircraft would be allowed to ferry back home if it was on the road when the inspection came, Yankee Air Museum opted to ground its B-17 for the 2023 season so it would not get stuck anywhere. In May 2023, the FAA released the AD outlining the inspection procedures. I recently had the opportunity to work with the Yankee Air Museum as the inspection procedure became live. In today’s video we take an up close look at the inspection procedure that took place on Yankee’s B17 named “Yankee Lady”
To take a look at the procedure, we dive into the wing root of the B-17, where the wing is connected to the fuselage. Although each wing has a forward and aft web spar, each having 2 primary mounts on it, the inspection is only concerned with the lower forward mounting areas on each wing. The affected area for the inspection is the inboard most bolt hole on the wing terminal which is where Aluminum Overcast was discovered to have had a crack. The terminal, in simple design terms, is more or less a fork with 2 tabs which 3 pins run through connecting the wing to the fuselage. The terminal is made of steel and slides into an outer aluminum piece which then mounts to the aluminum sheer web wing structure. Although it is held by multiple bolts, only the inboard most bolt hole is concerned because if a crack forms there, the end of the terminal could completely shear off, separating the wing from the fuselage. In theory, if there was an issue with any of the other holes, the terminal would still have bolts downstream of the defect hole holding it in place.
In order to perform the inspection, the bolt filling the hole in question must be removed. To gain access to remove the bolt, a section of the wings rib structure is cut out. This will later be replaced with a splice which can be easily removed and re installed in anticipation that this becomes a repetitive inspection. Once the bolt is removed a type of non destructive testing called eddy current is used to inspect the hole for cracks. A special spinning bolt hole probe is used in this procedure. This method of testing uses an electromagnetic field created by the coil that is induced into the material it is being tested. Any defects, whether on the surface or subsurface will change the impedance showing a change in phase and amplitude on the machines readout. This inspection procedure is widely used in the aerospace industry and notably used to inspect the fan blade roots on the CFM56 engine following an uncontained engine failure due to a crack on a Southwest Airlines Boeing 737 in 2018.
Thankfully for warbird and B17 lovers, both of Yankee Lady’s wing terminals and surrounding structure were found defect free, giving the aircraft a clean bill of health after the inspection. With a little bit of reassembly and all the paperwork, the aircraft will be back in the air soon.
The Yankee Air Museum is located in Michigan at Willow Run airport with various building locations on the airfield. Notably the museum takes up a portion of the historic Willow Run bomber plant which used to produce the B-24 in World War II as well as the building and hangar that used to house the Michigan Institute of Aviation and Technology before it relocated to a larger building. In addition to the B17 “Yankee Lady”, the museums flying fleet which is housed in the Roush Aeronautics hangar consists of a North American B25 named “Rosie’s Reply”, a Douglas C47 named “Hairless Joe”, a 4-AT-B Ford Tri-Motor and a UH-1 Huey helicopter as well as many non flying static displays.
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