Read the written dimension first, then use its arrowheads and extension lines to identify exactly which two points it controls.
An aircraft drawing may be enlarged, reduced or viewed on a resized screen. The number on the drawing carries the intended value; a ruler placed over the displayed image does not.
What does a dimension line show?
A dimension line is a thin line that normally ends in arrowheads. The number placed with it states the distance, angle, radius or diameter being specified.
The FAA’s Aviation Maintenance Technician Handbook explains that dimension lines show the distance between two points. The arrowheads indicate the points or extension lines that bound the measurement.
What are extension lines?
Extension lines project outward from the object so the dimension can sit clear of the visible outline. They identify the precise surfaces, edges or centre references being measured.
Follow each extension line back towards the object. A small gap may separate it from the outline, so do not assume the line must physically touch the feature to control it.
Check the units before reading the number
Find the drawing’s stated units in the title block, general notes or dimension itself. A bare number is not safely interpreted until you know whether the drawing uses inches, millimetres, degrees or another unit.
Do not mix values from separate drawings without confirming that their units and configurations agree. The general-arrangement drawing guide explains why the title, revision and configuration come before the geometry.
Scale does not replace a written dimension
A scale such as 1:10 describes the relationship between the drawing and the object when reproduced correctly. Digital display, copying and printing can change that physical relationship.
Use the stated dimension for the controlled value. Scale can help you understand proportion, but it should not override a written measurement. The three-view aircraft drawing guide shows how to cross-check the same feature across side, plan and front views.
Diameter, radius and angle dimensions
- Diameter: the Ø symbol identifies the full width of a circular feature through its centre.
- Radius: an R identifies the distance from the centre of an arc to its edge.
- Angle: a degree symbol or angular dimension controls the relationship between two directions or surfaces.
Confirm whether the dimension applies to a visible feature, a hidden feature or a sectioned interior. The guides to hidden lines and section views help separate those cases.
What does a tolerance add?
A tolerance defines the permitted variation around a nominal value. It may appear beside the dimension, in the title block or in a general note that applies to a class of dimensions.
Never assume that a rounded display number means the tolerance is equally broad. Read the complete dimension and the drawing’s tolerance convention together.
A practical reading sequence
- Confirm the title, revision, configuration and units.
- Find the written dimension.
- Follow its arrowheads to the extension lines or controlled points.
- Identify whether the value is linear, angular, a radius or a diameter.
- Check the applicable tolerance and notes.
- Cross-check the feature in another view or section.
- Use the written value ahead of measurements taken from the screen.
Why line hierarchy matters
Dimension and extension lines should remain legible without overpowering the object outline. The article on line weight in technical aviation artwork explains why visible edges, annotations and reference marks use different visual strength.
From technical dimensions to display artwork
Display art can use selected dimensions to add context, but those marks should support the aircraft rather than crowd it. Decorative artwork is not controlled engineering or maintenance data.
For current display work that combines precise aircraft geometry with restrained technical detail, explore the FGR4 Vector Series.
The short answer
Find the number, follow the dimension line and arrowheads, then trace the extension lines back to the controlled features. Confirm the units and tolerance, and trust the stated value ahead of a ruler placed on a resized image.
Sources
Source checked 5 November 2026. Hero is an original FGR4 conceptual diagram and is not approved maintenance data.