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A theory of presentation and its implications for the design of online technical documentation
©1997 Detlev Fischer, Coventry University, VIDe (Visual and Information Design) centre

Appendix III-Engineering Technical Graphics

This appendix describes the document type and the design of Enginering Technical Graphics (ETGs). ETGs became the starting point for the design of the cinegram (cf. App. I–Cinegram). They differ in many respects from the generic technical documentation (cf. App. II–ATA specified documents). ETGs are invaluable tools for visualising systems in their functional context (cf. figure III.1)

Simplified oil system ETG (click to enlarge)

Figure III.1. The Trent 700 simplified oil system diagram (original size: A3) (click to enlarge)

The surface of ETGs is covered with optimised views of functional components which themselves consists of a large number of parts. All components are interconnected in some way - mechanically, through the network of pipes, or through wiring. An average full ETG has the height of an A4 page and is about 10 feet long.

ETGs also cover time and state changes such as the direction of flow, which is shown through solid black arrows placed within pipes and components. State changes are shown through several images of the same component in different states.

ETGs are cheap and reliable, and do not require electricity. They are usually out of date, so that the user needs to know which parts of the ETG have been superseded by modifications. Many engineers have laminated copies of ETGs rolled up in their desk drawer; others put them up at the wall. ETGs are easily portable so they can be taken to meetings, customer visits, or to the shop floor.

Often, engineers annotate ETGs, for example, to update them or to add particular IPC references or part numbers.

ETGs are frequently used for scheduled checking procedures in which engineers trace fluid flows in order to identify possible locations of blockages or leaks. The use of ETGs is mostly combined with the use of other resources, such as faxed reports detailing the problem, or concurrently articulated to-do lists. ETGs trigger navigations to other resources, e.g., in order to check modification standards of a certain parts.

Design context

ETGs are designed in the Department of Visual Communication (now part of Technical Publications) at Rolls Royce plc in Derby. Their design follows an implicit and shared understanding of ‘how it is done’. The document type itself is traced back to a Mr. Dunwell who for the first time put all components belonging to one system onto one page. The protocols guiding  design decisions have emerged over years of practice in designing other ETGs.

Importantly, the design of a new ETG is usually is the work of one person who is given responsibility for all design decisions. This designer is occupied for many months with this project and can carefully plan the layout, size and type of elements, ensuring that everything will fit together. This results in very homogenous designs which clearly bear the style of their maker. In spite of different styles and approaches, there is a sense of a shared quality assessment apparent in designers' talk about each other's work.

Design process

First, the designer collects whatever material about the various system components is available, for example, schematics and illustrations. The domain boundary is a particular subsystem of the engine ‘as understood’ from prior practice and existing examples. Then, the first sketch of the ETG negotiates a number of provisional but important decisions:

Once the initial sketch is completed, work turns to the component views which differ from engineering schematics in that they often include relevant details in their combination of different section views into one artificial unified cross section view. The outer shape and appearance of components is often neglected for functional clarity. A pressure relief valve, for example, might be repositioned and turned through 90° so that its function is clearly displayed. The segmentation  necessitates remedial artefacts: the space between components which are set apart for clarity is bridged by connections with angular bends and dotted outlines while normal pipes are shown with rounded bends and solid outline. Where pipes cross, drop shadows indicate depth. Jumps in scale between components are indicated in a curved cut of the connecting pipe affording an rough estimation of the scale difference.

While the ETG is slowly built up, the decisions concerning components which are still subject to modifications are deferred. Often, it may be one lacking detail which creates a bottleneck. Many system components are manufactured by third-party suppliers who may be reluctant to provide precise documentation since they wish to protect their design knowledge.

At the draft stage of ETGs, people in various departments are asked what annotations they would like to be included: for example, oil jet flow speeds, pressure thresholds at which relief valves open, or short explanatory paragraphs.

Changes to the engine affect the system topology until very late in the design process. This necessitates changes to the layout of the ETG which can jeopardise the achieved optimisation of component and pipe track placements on the narrow real estate strip. It is clear that digital editing of graphics would make such changes less painful; currently, parts of the drawing are still scraped off with a knife.

Last update: 08 November 2007 | Impressum—Imprint