Reading aircraft drawings

Repairs and alterations start with a drawing. Most use orthographic projection: the part seen straight on from the front, top and side, each view flat. An isometric view shows it at an angle so it looks solid; an exploded view pulls the parts apart along their lines of assembly to show how they go together; a sectional view shows the part as if cut through, with cross-hatching on the cut faces.

Lines

• Visible line: thick and solid, an edge you can see.
• Hidden line: short dashes, an edge behind the surface.
• Center line: long and short dashes, the middle of a hole or a symmetrical part.
• Phantom line: a long dash and two short ones, a part's other position (a door open) or a part that's next to this one.
• Dimension and extension lines: thin lines with arrows and the size.

The title block (usually bottom right) gives the drawing number, the part's name, the scale, the material, the standard tolerance and who drew and checked it. The revision block lists changes; always work from the latest revision. A parts list or bill of materials names every part on an assembly drawing.

A tolerance is how far a size may vary: 1.250 ± 0.005 means anything from 1.245 to 1.255 inches is acceptable.

On the aircraft, positions are given as stations, in inches from a reference: fuselage stations (FS) fore and aft from the datum, buttock lines (BL) left and right of the centerline, water lines (WL) up and down from a reference height, and wing stations (WS) out along the wing.