An illustration showing the stringers/longitudinal stiffeners which run the length of the SBD's fuselage.
An illustration showing the stringers/longitudinal stiffeners which run the length of the SBD's fuselage.

Overview:

Over the last few months (amongst his numerous other tasks) Pioneer Aero's Chief Engineer, Martin Hedley, has endeavored to sort through the stringer assemblies for the SBD’s upper fuselage. These stiffeners run the length of the fuselage, and are positioned radially around its circumference. As the image below endeavors to reveal, Position 0 (at the top) and Position 15 (at the bottom) are unique, whilst Positions 1 through 14 have left and righthand mirror image equivalents. Note that Position 6 for each side runs along the split line between the upper and lower fuselage halves. The beefier stringer assemblies are often referred to as longerons, while the others are sometimes referred to as stiffeners. Most of the stringer assemblies comprise several sections of extrusion, linked together by splice plates, with joiners, clips and other attachments also arrayed along their length. Martin Hedley used a similar regimen to determine each stringer’s potential future airworthiness.

The typical arrangement of longitudinal stiffener assemblies around the SBD's fuselage. Positions 3L (left) and 3R (right) are actually the longerons which form the cockpit canopy rails, for instance. (image captured from manufacturing drawing #5090219)
The typical arrangement of longitudinal stiffener assemblies around the SBD's fuselage. Positions 3L (left) and 3R (right) are actually the longerons which form the cockpit canopy rails, for instance. (image captured from manufacturing drawing #5090219)

Stringer Identification:

Most of the stringers in the SBD’s fuselage begin with part numbers sharing a common root (#5090219), with their dash numbers providing specific identities. Odd dash numbers, represent the stringers running along the righthand side of the fuselage, while even numbers refer to those on the left. Since the SBD’s fuselage is mostly symmetrical, a stringer running down one side of the fuselage and its equivalent from the opposite side are represented by adjacent dash numbers. For instance, stringer #5090219-2 and stringer #5090219-3 are essentially identical to one another, albeit mirror images, running along the same section of fuselage but on opposite sides of the airframe.

The upper fuselage stringers #5090219-39, -22, -23, and -38 after their initial inspection, but prior to the removal of smaller components. Stringer -23 suffered from excessive corrosion, requiring its replacement, while the other three are all refurbishable to airworthy standard. (image via Pioneer Aero Ltd.)
The upper fuselage stringers #5090219-39, -22, -23, and -38 after their initial inspection, but prior to the removal of smaller components. Stringer -23 suffered from excessive corrosion, requiring its replacement, while the other three are all refurbishable to airworthy standard. (image via Pioneer Aero Ltd.)

Refurbish or Replace?

To determine which stringer assemblies might be salvageable, Martin Hedley first performed a visual inspection. Any components with significant crash damage or corrosion were immediately set aside for replacement. Those which initially appeared reusable proceeded to the next step, which involved media-blasting suspect areas to remove dirt, old paint and surface corrosion. This sometimes revealed excessive corrosion previously hidden from view, which immediately rendered the part in need of replacement.

A selection of stringers from the lefthand side of the SBD’s fuselage. From top to bottom we see the original stringer #5090219-8 (already primed) which runs between Fuselage Frames #9 and #14 at Position 2. Then the original (but damaged) stringer -10 and its replacement which runs between Fuselage Frames #10 and #14 at Position 2. [Note stringers -8 and -10 sit back-to-back with respect to each other.] Then we can see the original stringer -12 and its replacement which runs between Fuselage Frames #7 and #13 at Position 3. The areas beside the masking tape reveal the only corrosion present on these stringers, but it was too extensive to permit its repair, unfortunately. (image via Pioneer Aero Ltd.)
A selection of stringers from the lefthand side of the SBD’s fuselage. From top to bottom we see the original stringer #5090219-8 (already primed) which runs between Fuselage Frames #9 and #14 at Position 2. Then the original (but damaged) stringer -10 and its replacement which runs between Fuselage Frames #10 and #14 at Position 2. [Note stringers -8 and -10 sit back-to-back with respect to each other.] Then we can see the original stringer -12 and its replacement which runs between Fuselage Frames #7 and #13 at Position 3. The areas beside the masking tape reveal the only corrosion present on these stringers, but it was too extensive to permit its repair, unfortunately. (image via Pioneer Aero Ltd.)

Next, all of the clips, joiners, and/or small structural items were removed from the stringer assemblies, allowing each part a thorough cleaning. Those parts which cleared this stage had any minor imperfections repaired, followed by a coat of primer paint. For those stringers which needed remanufacture, new sections of the appropriate extrusion were joggled and cut to length as required. The original and replacement stringers were then positioned on the fuselage frames.

A closeup showing the aft and of original stringer #5090219-10 (in green) and its newly manufactured replacement. This stringer runs between Fuselage Frames #10 and #14 at Position 2 on the SBD’s lefthand side. The area beside the masking tape shows the only corrosion on the original stringer, but it was too extensive for salvage, sadly. (image via Pioneer Aero Ltd.)

A closeup showing the aft and of original stringer #5090219-2 (in green) and its newly manufactured replacement. This stringer runs between Fuselage Frames #10 and #14 at Position 1 on the SBD’s lefthand side. The bead-blasted area beside the masking tape shows the only corrosion on the original stringer, but it was too extensive for salvage, sadly. (image via Pioneer Aero Ltd.)

The aft ends of upper fuselage stringers #5090219-24, and -25 after the initial inspection, but prior to the removal of the joiners which connected them to adjacent stringers. The joggled ends sit on the flanges of Fuselage Frames #7 L/R. The joiners pass through a hole in these frames and attach to stringers -26 and -27 (even-to-even and odd-to-odd). Both of these stringers and their respective joiners are refurbishable. (image via Pioneer Aero Ltd.)

The original stringer (part #5062879-18) which mounts above the baggage door, and its newly-manufactured replacement. The original piece suffered from significant corrosion in several places, rendering it beyond airworthy salvage. (image via Pioneer Aero Ltd.)

Stringer #5090219-43 side-by-side with its replacement. This was the only stringer in the fuselage’s upper aft end which buckled as a result of the crash. This damage, along with corrosion, combined to render the part beyond airworthy salvage. (image via Pioneer Aero Ltd.)

A close up showing the end of the stringer featured in the previous image, along with its newly fabricated replacement. This component’s part number is 5062879-18, but note that a truncated version of that number “2879-18“ has been pencilled in at some point during its original manufacture. While incomplete, this detail was sufficient to help factory workers keep track of individual parts. Pioneer’s engineers have found quite a few such handwritten part numbers across the airframe, while other parts sometimes have their identifiers printed in ink or even stamped into the metal itself. (image via Pioneer Aero Ltd.)

A righthand exterior view of the SBD’s rear fuselage area from Fuselage Frames #7 at right (hidden beneath skin) to #15. Frames #9 through #14 are all refurbished original components, while Frames #7 and #15 are replacements, as discussed in previous posts. Note the original stringers which still retain their dark green wartime paint - from top to bottom #5090219-13, -27, and -43, along with the upper stringer for the baggage door opening - all need replacing. They have been temporarily fitted here to assist with the attachment of the original skin. The non-airworthy stringers will then be replaced, one at a time, allowing the others to hold everything else in place properly during the process. (image via Pioneer Aero Ltd.)
A righthand exterior view of the SBD’s rear fuselage area from Fuselage Frames #7 at right (hidden beneath skin) to #15. Frames #9 through #14 are all refurbished original components, while Frames #7 and #15 are replacements, as discussed in previous posts. Note the original stringers which still retain their dark green wartime paint - from top to bottom #5090219-13, -27, and -43, along with the upper stringer for the baggage door opening - all need replacing. They have been temporarily fitted here to assist with the attachment of the original skin. The non-airworthy stringers will then be replaced, one at a time, allowing the others to hold everything else in place properly during the process. (image via Pioneer Aero Ltd.)

Rivet Hole Placement:

The original skin was clecoed to the original fuselage frames atop the stringers. This held the skin in place and positioned the replacement stringers precisely as needed,  allowing rivet holes to be drilled through them (using the old skins as a template for these holes). Below are images describing this process for the righthand side of the upper fuselage, followed by a gallery showing it for the lefthand side.

Righthand Side:

An exterior view of the righthand side of the SBD’s fuselage looking forwards. At the time this image was captured, all of the stringers had either been restored or replaced, the original skin being used to hold them correctly in position. Pioneer was able to reuse roughly 80% of the original stringer material in the upper half of the fuselage. (image via Pioneer Aero Ltd.)
An exterior view of the righthand side of the SBD’s fuselage looking forwards. At the time this image was captured, all of the stringers had either been restored or replaced, the original skin being used to hold them correctly in position. Pioneer was able to reuse roughly 80% of the original stringer material in the upper half of the fuselage. (image via Pioneer Aero Ltd.)

An interior view of the middle, righthand side of the fuselage between Fuselage Frames #5 (at left) and #6. Both of these frames are repaired, original components, as discussed in previous posts. The stringers seen here from top-to-bottom and left-to-right are numbered #5090219-23 (replaced) and -25 (original), then -39 (original) and -43 (replaced). (image via Pioneer Aero Ltd.)

An interior view of the righthand, rear fuselage showing Fuselage Frames #10 (at left) and #11. As is evident, these frames form the fore and aft edges of the baggage door opening. The unpainted stringers #5090219-13 (upper), -27, and -43, along with the upper stringer for the baggage door opening, are all remanufactured. Note the circular array of open rivet holes at image left; this is where the life raft stowage tube mounts to the righthand side of the fuselage. (image via Pioneer Aero Ltd.)

An interior view of the righthand, rear fuselage from Fuselage Frames #12 (at left) to #15. Referencing the manufacturing drawing depicted earlier in this article, the stringers in view range from Position 0 (image upper right) to Position 6. #5090219-13 (unpainted) is a replacement has been replaced. The two rows of presently unfilled holes either side of Position 0 are mark where the fin leading edge fairing will eventually be riveted to the fuselage skin. (image via Pioneer Aero Ltd.)

An interior view of the righthand side of the rear fuselage looking aft from Frame #7 (at left) in the gunner's cockpit to Frame #13. Fuselage Frames #9 to #13 are all restored original components, as discussed in previous posts, while Frame #8 is new-build and Frame #7 is awaiting replacement. All of the unpainted stringers are replacements; #5090219-13 (upper), -27, -43. The stringer running horizontally above the baggage door opening is also a replacement. (image via Pioneer Aero Ltd.)

An interior view of the righthand, rear fuselage from Fuselage Frames #13 (at left) to #15. Frames #13 and #14 are both refurbished original components, while Frame #15 is newly-built, as discussed in previous posts. Pairs of stringers from Position 0 (upper right) through Position 6 (lower skin edge) are in view, labeled (from top-to-bottom) as #5090219-5 and -7 (both original), the ends of -9 and -11 (both original), -15 and -17 (both original), -31 and -33 (both original) and -47 and -151 (both original). Three of the joiners between each pair of stringers have been replaced, however. (image via Pioneer Aero Ltd.)

An interior view of the upper fuselage looking aft from Fuselage Frame #11 (foreground) to #15. All stringers have either been restored or replaced. The hole at fuselage top with filleted corners is where navigation lights are mounted, while the more rectangular hole to its aft is the opening for loading the flare tubes. The row of unfilled holes, about half way up either side of the fuselage forward of Frame #11 marks where the aft end of the upper longerons will attach. (image via Pioneer Aero Ltd.)

Lefthand Side:

An exterior view of the lefthand side of the SBD’s upper rear fuselage showing the original skin clecoed in place against original frames and stringers. This helped align everything so that the replacement stringers could be positioned correctly. The skin was then used as a guide to drill rivet holes in the new material. This process ensures that everything is positioned correctly. (image via Pioneer Aero Ltd.)
An exterior view of the lefthand side of the SBD’s upper rear fuselage showing the original skin clecoed in place against original frames and stringers. This helped align everything so that the replacement stringers could be positioned correctly. The skin was then used as a guide to drill rivet holes in the new material. This process ensures that everything is positioned correctly. (image via Pioneer Aero Ltd.)

These are the ribs which form the aft edge of the ammunition door openings for the forward fixed machine guns on either side of the fuselage. While they hold the fuselage skin at the correct curvature, the front ends of four stringers also attach to them. The lower one pictured (with the bead-blasted upper end) is the righthand example, while the other is from the left side of the airframe. There is a slight crease in the latter rib (though not a crack) which resulted from the SBD’s wartime accident. If the team tried to straighten it in its current condition, it would simply have cracked. However, by heat treating the part to temporarily reduce its temper, it became soft enough (for a brief period) to rework without damaging it. After being gently bent back into position, the part soon age-hardened back to its preferred T3 condition. (image via Pioneer Aero Ltd.)

A closeup of the crease in the rib mentioned in the previous image showing where the tightest part of the fold was located. After annealing the part to soften it, the team was able to gently work it back into its original shape. While this may appear to be a crack (it isn’t), the disruption you are actually seeing is the granular structure of the outer layers of aluminum pulling apart. This damage was very shallow, however, and easily blended smooth to return the part to airworthy condition. (image via Pioneer Aero Ltd.)

A view of the interior, lefthand side of the fuselage in the pilots cockpit area looking aft towards the rollover structure. Fuselage Frames #3 (at right) through Frame #5 are visible. These two frames are fully refurbished (as discussed in previous posts, while Fuselage Frame #4 is a replacement. Two stringers are visible (#5090219-22 and -38), both being original. (image via Pioneer Aero Ltd.)

An image showing the forward lefthand side of the SBD’s fuselage with a range of components temporarily attached together in the upper fuselage jig. Running across the top we can see the replacement longeron which came with the project. Just visible to the left is a non-airworthy Fuselage Frame #1 from a donor aircraft which is being used for trial-fitting components together. The rib to its right is the original and recently straightened ammunition door edge support described in the previous two images. Then we can see the original, though non-airworthy Fuselage Frame #2, which is due to be replaced. After that comes the original, fully-refurbished Fuselage Frame #3, then the original Fuselage Frame #4 with the temporarily attached, forward section of rollover structure (which will be replaced). The upper-most stringer supports the end of the tray forward of the instrument panel which has yet to be inspected. The two rows of stringers consist of sections #5090219-18, -20, -34 and -36. Only -34 had to be replaced, with the other three are original parts now being airworthy. (image via Pioneer Aero Ltd.)

An interior view of the lefthand side of the rear fuselage area. Fuselage Frames #13 (at right) through #15 are visible. Fuselage Frames #13 and #14 are both original, while Frame #15 is a replacement, as discussed in previous posts. This image reveals Position 1 (upper in photo) to Position 6 (lower skin edge). The stringers, top to bottom, are numbered #5090219-4 and -6 (both original), the end of -8 (original), end of -10 (replaced), -14 and -16 (both original), -30 and -32 (both original) and -46 and -150 (both original). As is evident, each component has its own tag with its unique identify number stamped etched it, ensuring that each part ends up back in its correct location. (image via Pioneer Aero Ltd.)

An interior view of the lefthand side of the fuselage in the gunners cockpit area looking forward toward the roll over structure. Fuselage Frames #3 (at right) to through Frame #7 are visible. Fuselage Frames #5 and #6 are repaired originals, as discussed in previous posts, while Frame #7 is a replacement. Stringer #5090219-40 (lowest row, between Frames #6 and #7) is a replacement, while the other three (-22, -24 and -38) are all original. The dark green horizontal rib nestled between Frames #6 and #7 forms part of the turret support structure. It has been partially bead-blasted, but is expected to be returned to full airworthy condition sometime soon. (image via Pioneer Aero Ltd.)

An interior view of the lefthand side of the rear fuselage area looking aft. Fuselage Frames #10 (at right) through #14 are visible, all of them being original, as discussed in previous posts. Referring back to the manufacturer’s drawing presented earlier in this article, this image shows roughly 1/4 of the fuselage cross-section, from Position 0 (upper left) to Position 6 (lower skin edge). The stringers (top to bottom) are #5090219-124 (original), -2 (replaced), -8 (original), -10 (replaced), -12 (replaced), -28 (original) and -44 (original). (image via Pioneer Aero Ltd.)

An interior view of the lefthand side of the fuselage within the gunner’s cockpit area. Fuselage Frames #7 (at right) and #8 are both replacements. Stringer #5090219-12 at the top, then stringer -26 (aft of Frame #7) and stringer -40 (lowest row, forward of Frame #7) are replacements. Stringer -24 (middle row, forward of Frame #7) and stringer -42 (lowest row) are originals. The primer green, square sheetmetal component with a circular cutout at its center is the refurbished original structure for holding the flare pistol mount. (image via Pioneer Aero Ltd.)