Squared up the frame with: - Big square, 18x24 inch - Skin. It is perfectly square Then clampled in: - Square - Skin - Hinge - Rear end of ribs Checked alignment of primary datum at: -Rear end of skin with rear spar - Hinge with end of rear spar - 75 inches of rear end of spar with skin and hinge - Spar drill lines with pilot holes in skin Once squareness and alignment was checked, pilot drilled: - Rear spar and hinge using skin pilot holes - Ribs