When you open a set of aircraft technical drawings, you expect to see front views, top views, and side views arranged in the familiar pattern of orthographic projection. But aircraft are not made entirely of flat, square surfaces aligned perfectly with those viewing planes. Wings have dihedral angles, fuselage frames meet at compound angles, and bracket flanges can be tilted in ways that make standard views show only a foreshortened, distorted image. To solve this problem, drafters add auxiliary views — special projections drawn from a vantage point that looks directly at an inclined or oblique surface, revealing its true shape, true size, and true angular relationships. For the Aviation Maintenance Technician (AMT), reading these views accurately is an essential skill, because incorrect interpretation can lead to fabricating a part with the wrong hole spacing, wrong angle, or wrong contour.
This article explains what auxiliary views are, how they are constructed, why they appear in aircraft drawings, and exactly what information they convey. It also covers the testable rules and the traps that catch technicians who skim over these projections.
The Foundation: Orthographic Projection and Its Limitation
Standard aircraft drawings use orthographic projection, in which the object is viewed from infinitely far away so that projection lines are parallel rather than converging. This produces three principal views — front (frontal plane), top (horizontal plane), and right side (profile plane) — arranged at right angles to each other. Any surface that is parallel to one of these three planes appears in its true size and shape in that view. The problem arises with inclined surfaces, which are tilted relative to one principal plane but parallel to one axis, and oblique surfaces, which are tilted in two or more directions simultaneously.
An inclined surface, when viewed from the front or top, appears foreshortened — compressed along the direction of tilt. Its angles and distances are distorted. You can see that the surface exists, but you cannot measure it accurately from that view. An oblique surface is even more troubling: it appears foreshortened in all three principal views. In both cases, an auxiliary view is the solution.
How Auxiliary Views Are Constructed
An auxiliary view is created by projecting the inclined or oblique surface onto an auxiliary plane — an imaginary plane that is perpendicular to the line of sight aimed directly at the surface in question, and perpendicular to one of the principal planes. Because the auxiliary plane is parallel to the inclined surface, the projection onto it captures true shape and true size.
In practice on a drawing, the drafter establishes a reference line (also called a fold line or hinge line) between the principal view and the auxiliary view. This reference line represents the edge where the auxiliary plane meets the principal plane. Projection lines are drawn perpendicular to this reference line, and dimensions are transferred from the adjacent principal view to position features accurately. The result is a view that, when you unfold the imaginary planes flat, shows the surface exactly as it would look if you held the part up and looked straight at that face.
Types of Auxiliary Views
There are two broad categories worth knowing for the AMT knowledge test:
- Primary (single) auxiliary view: Projected from one of the three principal planes. It gives the true shape of an inclined surface that is perpendicular to that plane and tilted relative to the other two. This is the most common auxiliary view in aircraft drawings.
- Secondary auxiliary view: Projected from a primary auxiliary view to reveal the true shape of a fully oblique surface. Secondary auxiliaries are rare but appear on complex structural drawings, such as compound-angle fuselage bulkheads.
In both cases the construction logic is the same — look perpendicular to the surface, project onto a parallel plane, and transfer depths from the appropriate adjacent view.
What Auxiliary Views Show in Aircraft Drawings
In the context of aircraft technical drawings, auxiliary views are used for several recurring situations:
- True hole patterns: A rivet pattern on an angled rib flange will appear as an ellipse in the standard views because the holes are drilled perpendicular to the inclined surface. The auxiliary view shows circular holes at their actual diameter and true center-to-center spacing — the critical information needed for layout and drilling.
- True lengths of edges and members: A spar web that runs at a dihedral angle will appear shortened in the front view. The auxiliary view gives the actual developed length needed for cutting stock.
- True angles: Bend lines, flange angles, and joggle angles that are inclined to the principal planes can only be accurately measured in an auxiliary view.
- True contours and radii: Curved surfaces that are inclined, such as a fillet at an angle, require auxiliary views so the technician can select the correct radius gauge or form block.
How to Identify an Auxiliary View on a Drawing
Auxiliary views are positioned adjacent to the principal view from which they are projected, and they are connected to it by thin projection lines drawn perpendicular to the reference line. On many aircraft drawings, an arrow or label may indicate the direction of sight. On older aircraft drawings — many of which you will encounter on legacy aircraft still in service — the auxiliary view may simply appear as an extra view rotated or positioned off the main drawing area, sometimes labeled VIEW A-A or identified by cutting-plane arrows marked with letters.
A key identification rule: the reference line is parallel to the edge view of the inclined surface in the principal view. If you see a slanted edge in the front view and then a separate view whose reference line matches that slope, you are looking at an auxiliary view of that surface. Projection lines crossing the reference line at right angles confirm the relationship.
Why Auxiliary Views Matter for Aircraft Maintenance
Aircraft manufacturing depends on extremely tight tolerances. A misread auxiliary view can produce parts with incorrect hole locations, wrong flange lengths, or improper bend angles. In structural repairs, fabricating a doubler or gusset from principal views alone — when an auxiliary view is provided — would introduce dimensional errors. Major repairs and alterations must be accomplished in accordance with approved data (14 CFR 43.13); when a drawing furnished as approved data includes an auxiliary view, that view must be read and applied correctly to keep the work within the tolerances the data specifies.
Beyond fabrication, auxiliary views also aid in inspection. Inspecting a rivet row on an inclined surface requires knowing the true edge distance and spacing, which only the auxiliary view provides. Using the foreshortened distances from a principal view would lead to incorrect accept/reject decisions.
Key Numbers and Rules
- An auxiliary view is projected perpendicular to the inclined surface — meaning the line of sight is 90° to the surface being depicted.
- The reference line in an auxiliary view represents the intersection of the auxiliary plane with the adjacent principal plane, and is always parallel to the true edge of the inclined surface in that adjacent view.
- In a primary auxiliary view, one set of dimensions (those parallel to the reference line) comes from the related principal view; the depth dimension is transferred from the principal view adjacent to the one containing the reference line.
- Secondary auxiliary views require projecting from a primary auxiliary — the drafter establishes a second reference line perpendicular to the line of sight of the oblique surface.
- Partial auxiliary views are common in aircraft drawings — only the inclined surface is shown, not the entire object. Features not on the inclined surface are deliberately omitted to avoid confusion.
- Hidden lines are often omitted from auxiliary views unless they are essential to clarify a feature; this is standard practice to reduce clutter.
Common Test Traps
- Confusing a section view with an auxiliary view: A section view shows an internal cut through the object; an auxiliary view shows an external surface from a special angle. Both may appear off to the side of the main views, but section views are generated by cutting-plane lines, while auxiliary views are generated by projection lines from an inclined surface.
- Assuming dimensions in principal views are accurate for inclined surfaces: The entire point of the auxiliary view is that the principal views are not accurate for that surface. Never measure hole spacing or flange length from the foreshortened image in the front or top view when an auxiliary view is provided.
- Misidentifying the reference line: The reference line is not an outline of the part — it is the fold line between planes. Mistaking it for a part edge leads to incorrect dimension transfer.
- Ignoring partial auxiliary views: Because only the inclined surface is shown, some students assume the view is incomplete or a detail view of a different part. Read the projection lines and the reference line carefully to confirm what feature the partial auxiliary is depicting.
- Mixing up primary and secondary auxiliaries: A secondary auxiliary is projected from the primary, not from the original principal views. Transferring dimensions directly from the principal views to a secondary auxiliary introduces errors; intermediate values from the primary auxiliary must be used.
Mastering auxiliary views takes practice with actual drawings, but the underlying logic is straightforward: wherever a surface is tilted, aim your line of sight straight at it, project onto a parallel plane, and read true dimensions from the result. That discipline — looking at the right view for the right measurement — is at the heart of precision aircraft maintenance work.