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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 II-ATA-specified documents

This appendix outlines the architecture imposed on technical manuals by the ATA 100 [1]  specification, and analyses some of the related boundary and granularity problems which effect manual navigation. The generic ATA-specified technical manuals have a wide scope: they cover all instances of the-engine and are distributed to all customers. Their design is a highly formalised process since documentation must comply with the industry-wide ATA standard and be updated regularly. Different versions exist for different mod standards used by individual airlines.

In many cases generic documentation is incorporated into and adapted to airframe maker's documentation, which is again transformed into airlines' technical documentation.

The structure of the generic aircraft documentation mirrors the structure of the aircraft: both are coded in the same way. The structure follows industry-wide set of standards set by the ATA (Air Transport Association) and described in the ATA100 and ATA2100 specifications. This means that there is a common referencing standard for all commercial aircraft.

At Rolls Royce, the ATA-specified technical manuals are permanently stored in a publicly accessible place for everyone's use. Each engine type is documented by a large set of manuals covering various aspects of the engine. For a typical engine, the most important manuals are the Maintenance Manual which describes all aspects relevant for normal ‘on-wing’ service (5 volumes); the Engine Manual which covers all procedures carried out during ‘off-wing’ engine overhaul (7 volumes); the Illustrated Parts Catalogue (IPC) which shows exploded view type illustrations and lists of all engine parts down to nuts, bolts, clippings and O-rings (3[4] volumes). Other manual types such as Time Limits Manual, Tools and Equipment Manual, and Standard Practices manual also conform to the ATA numbering system.

The ATA numbering system is organised as a hierarchical number space covering the entire aircraft. The basic six digit number format provides a unique address for each component. The number is grouped into three elements for hardware identification, and up to four elements for maintenance task identification (cf. figure II.1).

[schematic view of ATA reference number and meaning of number elements]

Figure II.1. ATA numbering system. The first three elements uniquely identify hardware components. The remaining elements (up to four, which means 18 digits altogether) refer to maintenance tasks.

The first three paired elements correspond to ATA chapter, section and subject respectively. On the top level are ATA chapters, which deal with the different aircraft assemblies and systems such as chapter 79-Oil system which is the subject of the Cinegram prototype (cf. appendix I–Cinegram). The second number element refers to sections within chapters, e.g., 79-21 stands for Oil cooling. The third number element refers to subjects within sections, i.e. to separate components; e.g., 79 -21-51 stands for one of the coolers, the Air-oil heat exchanger.


Despite its obvious advantages generic coding creates problems for presentation. A fundamental problem is that the functional system context is spread out according to the spatially oriented classification. For example, most parts of the oil system such as accessories and pipes are covered in chapter 79-Oil, a few require chapter 73-Fuel, and the lubrication and cooling of bearings is covered in 72-Engine (cf. figure II.2).

[Diagrammatic view of ATA allocation of oil system components]

Figure II.2. The diagram shows the ATA allocation of the components which are part of the oil system. The bearing chambers and gearboxes, among the most important in analysing oil system-related problems, are covered in a different chapter.

A further problem is that organisational boundaries introduce breaks in the ATA numbering logic. Only the first three digits are defined by the ATA—the rest is up to the airframe manufacturer (cf. figure II.1). The two largest manufacturers, Boeing and Airbus, have adopted different  strategies. Boeing has a serial concept which is basically in line with Rolls Royce's strategy; Airbus however allocates numbers for components according to an arbitrary principle where, for example, electrically controlled components with moving parts are allocated numbers starting with 51. The result is that the same component can have different ATA numbers in different manuals. The Air-oil heat exchanger, for example, is allocated ‘79-21-51’ in the Maintenance manual (which complies to Airbus' numbering system), and ‘79-21-11’ in Rolls Royce's Engine manual used for engine overhaul.

Even worse, there are two differently numbered IPCs (Illustrated Parts Catalogues) for the same engine. The Engine IPC has Rolls Royce's numbering, while the Powerplant IPC (‘powerplant’ is airframe makers' preferred term for the engine) has Airbus' numbering. The result of this lack of standard is confusing: each time someone refers to the IPC, the question arises: which IPC do you mean?´ [2]

A look at the list of contents of chapter 79 (cf. figure II.3) shows a number of structural inconsistencies reflected in the organisation, layout, and naming of sections and subjects within the chapter hierarchy. This inconsistency is put into relief as soon as one attempts to present the hierarchy as a tree diagram (figure II.4). The first puzzling observation is to find two near-identical section headings ‘Distribution 79-20-00’ and ‘Oil distribution 79-22-00’. What is the difference? A look at the next page reveals that here, section 79-20-00 is called ‘Oil distribution’. The omission of ‘Oil’ in the section name in the list of contents is revealed as an artefact providing surface differentiation, necessitated through the inconsistencies of the hierarchy.

The chapter breakdown in the tree diagram (figure II.4) reveals the existence of an unacknowledged subsystem level expressed in a change of the second ATA number element (which stands for section). In terms of subsystem function, it is necessary to differentiate oil cooling as a sub-section of oil distribution [3]

[Manual contents page showing structural inconsistencies of ATA numbering]

Figure II.3. The first page of the ‘List of contents’ of the Trent 700 Maintenance Manual.

However, in the list of contents, the different section number only appears on the subjects level. Here, the second number element is ‘21’ in both ‘Fuel-oil heat exchanger’ and ‘Air-oil heat exchanger’, presumably standing for ‘Oil cooling’. There is however no place allocated for a description of the Oil cooling subsystem (which by inference should have the ATA number ‘79-21-00’). Instead, a description of the system can be found, more or less by accident, under ‘Oil distribution 79-22-00’, hidden inside a long text covering all components in the distribution chain.

diagram of the hierarchy of ATA
chapter 79

Figure II.4. The tree diagram shows the hierarchy of ATA chapter 79, Oil system. The breakdown within the hierarchy is inconsistent; in some branches, elements appear to be missing (here indicated through boxes with hatched frames). The manual introduction only assumes three ATA levels: chapters, sections, and subjects. De facto however, there is a further subdivision, expressed in the change of the second element from ‘79-20-00’ (here shown on the sections level)to the omitted ‘79-21-00’ (Oil cooling) and ‘79-22-00’ (again, oil distribution!) on an unacknowledged sub-sections level.

The actual system description it presented inside the component description of the Air-oil heat exchanger—one of the cooling system components. With good luck, an oil cooling subsystem diagram contextualising the description can be found 19 pages away. It is, for want of a proper place in the chapter hierarchy, appended at the end of a number of component illustrations lumped together under ‘Oil distribution 79-22-00’ [4].

Problems with references

More oddities appear when one tries to guess what content the items listed in the ‘List of contents’ actually refer to. Overall, the availability of the topics ‘Description’, ‘Fault isolation’, and ‘Removal/Installation’ on the different levels of hierarchy does not seem to follow any clear principle. Both ‘Distribution’ and ‘Oil distribution’, for example, offer the topic ‘Fault isolation’. When following the page references, one finds two near-identical tasks, one in each section. Both tasks relate to similar fault indications: ‘TM A’ under ‘Distribution’, and ‘TM’ under ‘Oil distribution’ (both procedures refer to the Air-oil heat exchanger torque motor (TM)). The Air-oil heat exchanger subsection, however, merely contains ‘Removal/Installation’ procedures. There is no hint that faults are covered both under section ‘Distribution’ and ‘Oil distribution’. Also, these two sections do not cross-reference each other.

Component coverage is equally obscure. Under ‘Oil storage’, for example, the Oil tank is not separately listed. Instead, the oil tank cap, although clearly a part of the oil tank, is listed on the component level. Should the restriction to three levels have motivated the omission of the tank? Its inclusion would have made the inconsistency of having component and component part on the same level of hierarchy too obvious.

Another example are the figure reference numbers. When following up a figure reference, one often finds the respective figure separated by many pages from the reference in the text. Figure numbers often compete with page numbers: Fig. 604 can be found on page 603 of some topic. Moving from the figure to the corresponding text is even more difficult. Five-element task references are listed in figure captions such as

Fig. 603/TASK 79-00-00-991-003.

To find the task, one is forced to scan all the pages of the topic for the task code since neither the function element (991) nor the sequential number (003) correspond to any numerical sequence of task codes within the body of text.

ATA layout

The problems caused through the ATA architecture are compounded by a layout using monospace font, frequent capitalisation and poor visual structuring which flattens the document hierarchy. There is no cueing visualising the hierarchy of section, sub-section, and subject; the list of contents, for example, makes no use of indentation. In task descriptions, task headings are rendered in the same font, size, and text style as the rest of the text.

Most pages look very similar, with little visual landmarks for orientation. Once in the middle of a task description, there is no indication of the task to which the text belongs. To find out, one has to browse backwards until one discovers the rather undistinguished beginning of the task description.
Because of the similar look, it is easy to loose sight of the current task. Only the ATA reference in the lower right corner is heavily emphasised; however, the need to remember its second two number elements and the page number (which in itself falls into a semantic first digit and sequential pair of digits) makes memorising page numbers rather difficult. Task differentiation is at its worst when two nearly identical tasks are presented with minimal lexical and layout differentiation. Compare the following task titles in a Fault isolation section in figure II.5:

TASK 79-22-00-810-805 The Valve of the Air-Oil Heat Exchanger Does not Move to the Correct Position 792151 AIR OIL HEAT EXCH (Ei-4073KS) LVDT B
TASK 79-22-00-810-806 The Valve of the Heat Exchanger-Air-Oil Does not Move Smoothly to the Correct Position 792151 AIR OIL HEAT EXCH (Ei-4073KS)

Figure II.5. Nearly identical task titles in the Fault isolation section of the Maintenance Manual.

Apart from the odd, probably accidental, difference in naming the component, there is enough similarity in the symptom description to potentially cause a mix-up of the two tasks. The little difference there is is not emphasised; no formatting separates semantically distinct parts of the symptom description.

Illustrations are placed nowhere near the descriptions that refer to them. They are difficult to find in ways other than random page-flipping: the overview diagram and the component drawings are not included in the list of contents.

The main overview diagram is poorly designed and full of inconsistencies. It is a simple double-page block diagram, split by the yawning gap between opposite pages in lever arch files (cf. figure II.6).

Main overview diagram (click to enlarge)

Figure II.6. Overview diagram in the 79-Oil system chapter of the Trent 700 Maintenance manual>

Block layout does not appear to be governed by any obvious rule. Block sizes seem motivated neither by semantic differences nor by the amount of text they hold. However, for some reason, just three component types are symbolically coded. Two of the symbols, that for Magnetic chip detectors and that for Oil strainers, are also listed in a legend at the bottom of the page. A third symbol for pressure transmitters only appears in the diagram, not in the legend.

The component line drawings include small inset pictures, which show component and pipe locations on a very small 3D side view of the engine which does not show any pipes. Larger pictures showing component and pipe location can be found in the IPC, which means having to use an additional unit of resource.

Footnotes to Appendix II-ATA-specified documents

[1] For a list of ATA specifications and publications see http://www.air-transport.org/

[2] Although airlines usually work to the documentation (and numbering strategy) supplied by the airframe manufacturer, some prefer Rolls Royce's documentation.

[3] There must be doubts as to the wisdom of making oil cooling a sub-section of oil distribution. Although the cooling components are part of the distribution chain in terms of the flow topology, moving the oil and cooling the oil are distinctly different functions. In other cases, such as indicating, the ATA structure does differentiate by function regardless of place within the topology.

[4] It is only fair to relay the comment of a Rolls Royce service engineer about this point: ‘It is policy to put illustrations at the end of a given section and to refer to them from the TASK by fig. number. It is not really a problem for me.’ (Laffan 13/11/95)

Last update: 08 November 2007 | Impressum—Imprint