Rebuilding Spar #4:

In a similar fashion to Spar #5, which we covered HERE, Pioneer’s Craig Cunha and a colleague carefully removed Spar #4 from the wing center section structure. They then disassembled it into its component parts, assessing each for its potential airworthiness. Bead-blasting each part revealed a good number which could be restored to flightworthy condition, while those too corroded for reuse still served as excellent patterns for manufacturing their replacements.

2024 05 03 Wing Center Section Spar #4 Restoration
Spar #4 after disassembly: the lower spar cap (clecoed to the web) has already been bead-blasted and is on its way to becoming airworthy, while the upper spar cap (being held) is about to receive the same treatment. The telltale white patch of corrosion next to the lightening hole is one of the reasons why the web required replacing. (image via Pioneer Aero Ltd.)


While both spar caps proved to be restorable, several areas of significant corrosion on the web meant that Pioneer’s engineers had to refabricate the part, using the original as a guide. The pulley brackets riveted to the spar have also been disassembled, bead blasted and inspected. The upper sections for each of these components were corroded beyond use, so replacements were made.

2024 05 03 Wing Center Section Spar #4 Restoration
The original web for Spar #4 (right) placed next to its replacement (left) part way through its manufacture. The original component served as a template, with each hole transferred directly from the old to the new. The whitish and light gray areas in the original web (visible in the foreground) indicate significant underlying corrosion damage, which is why the part needed replacing. The blue-tinted hue in the replacement material is due to a thin layer of plastic which protects the metal underneath from incidental marring; it will remain on the part until it is ready for painting. (image via Pioneer Aero Ltd.)


The lower sections of fuselage Frame #6, which rivet on to Spar #4, were removed, bead-blasted and checked over. Unfortunately, the left hand frame was corroded beyond limits, as per the SBD’s Structural Repair Manual specifications. To manufacture its replacement, Craig Cunha created a forming block which fit within the original frame, using the block to shape a duplicate frame section.


With each part either refurbished or replaced, it was time to prepare them for primer painting and reassembly. The small pulley bracket was also reassembled, with its small tabs riveted to the spar web.

2024 05 03 Wing Center Section Spar #4 Restoration
The aft face of Wing Center Section Spar #4 going back together. The components have been painted with primer and clecoed together, ready for riveting at a later date. (image via Pioneer Aero Ltd.)