---
type: Concept
title: "ASD-STE100 Simplified Technical English"
description: The controlled-language standard for technical documentation — 53 writing rules in 9 sections plus a ~900-word controlled dictionary, owned by ASD. What it is, what it constrains, what it deliberately does not cover, and why its scope boundary matters to anyone adopting it as a house standard.
tags: [writing, controlled-language, technical-writing, standards, ste, documentation]
timestamp: 2026-07-30
---

# ASD-STE100 Simplified Technical English

**Situating context:** Extracted from the free official Issue 9 specification (2025-01-15) and the STEMG's own FAQ during the [ste-house-standard-spike](/planning/prd/ste-house-standard-spike.md) discovery spike, which asked whether STE should become the standard for PMOS output. This concept holds the durable half — what the standard *is*. The PMOS-specific adoption question, its scope boundary, and the enforcement design live in the spike brief, not here.

## What it is

**ASD-STE100 Simplified Technical English (STE)** is a *controlled natural language*: a restricted subset of English defined by an explicit rule set and an explicit approved vocabulary. It exists to make technical documentation unambiguous on first read — originally for aircraft maintenance manuals read by mechanics whose first language is not English.

It has exactly two parts:

| Part | Content | Size (Issue 9) |
|---|---|---|
| **Part 1 — Writing rules** | 53 numbered rules in 9 sections, plus 8 general recommendations (GR-1…GR-8) | 53 rules / 9 sections |
| **Part 2 — Dictionary** | Approved words, each with **one meaning and one part of speech**, plus unapproved words with their approved replacements | ≈900 approved words |

The dictionary is the expensive half and the reason STE is a *controlled* language rather than a style guide: `test` is approved as a noun only, `oil` as a noun only, and a writer who wants the verb must use the approved one. Words outside the dictionary are legal only as **technical nouns** or **technical verbs** — domain terms admitted under rules 1.5–1.13, which is how STE stays usable in a field it has never seen.

## The nine sections

Rule statements below are **paraphrased**, with rule numbers as pointers into the specification. The document is ASD copyright and is not reproduced here (see *Licensing* below).

1. **Words (1.1–1.14).** Use only approved words, each as its listed part of speech and listed meaning, in its approved verb/adjective forms (1.1–1.4). Domain terms enter as technical nouns (1.5–1.11) or technical verbs (1.12–1.13): keep them short, never use a technical noun as a verb or a technical verb as a noun, never two names for one thing, no slang/regional/jargon terms. American English spelling unless a directive says otherwise (1.14).
2. **Multi-word nouns (2.1–2.2).** At most three words in a noun cluster; a longer term is written in full once and then shortened or hyphenated.
3. **Verbs (3.1–3.7).** Only the dictionary's verb forms (3.1), and only infinitive, imperative, simple present, simple past, simple future, and past participle-as-adjective (3.2–3.3). No auxiliary stacks / complex constructions (3.4). `-ing` only inside a technical noun (3.5). **Active voice** — passive only in descriptive text when the agent is genuinely unknown (3.6). Describe an action with an approved verb, not a nominalization (3.7).
4. **Sentences (4.1–4.5).** Short and clear (4.1). Never drop words or use contractions to shorten (4.2). Vertical lists for complex material (4.3). Explicit connecting words between related sentences (4.4). Keep articles and demonstratives in front of nouns (4.5).
5. **Procedural writing (5.1–5.5).** **Maximum 20 words per sentence** (5.1). One instruction per sentence unless the actions are simultaneous (5.2). Imperative form (5.3). A precondition is stated first, then comma, then the command (5.4). Notes inform; they never instruct (5.5).
6. **Descriptive writing (6.1–6.6).** Information gradually (6.1); key words/phrases carry the structure (6.2); **maximum 25 words per sentence** (6.3); paragraphs group related information (6.4), **one topic each** (6.5), **at most six sentences each** (6.6). The imperative is not permitted here — description is not instruction.
7. **Safety instructions (7.1–7.3).** Name the risk level with the applicable word ("warning", "caution") (7.1); open with a clear command or condition (7.2); explain the risk or the consequence (7.3).
8. **Punctuation and word count (8.1–8.7).** All standard punctuation **except the semicolon** (8.1); hyphens join directly-related words (8.2); parentheses for references, identifiers, work steps, abbreviations (8.3). Then the counting rules that make the length limits decidable: in a vertical list a colon ends a sentence (8.4); a parenthetical counts as one word (8.5); numbers, numbers-with-units, abbreviations, alphanumeric identifiers and proper nouns count as one word each (8.6); a hyphenated word counts as one (8.7).
9. **Writing practices (9.1–9.4) + GR-1…GR-8.** Restructure the sentence when word-for-word replacement fails (9.1); use approved words correctly (9.2); no invented phrasal verbs (9.3); consistent terminology and style (9.4). The general recommendations cover the conjunction "that", the preposition "with", pronoun use and antecedents (GR-3/GR-4), false friends, Latin abbreviations, and — new in Issue 9 — **inclusive language** (GR-7) and possessive form (GR-8).

## What STE deliberately does not do

The specification's own FAQ is unusually explicit about its boundaries, and this is the part most adopters miss:

- **It does not regulate formatting** — typeface, numbering, lettering belong to a publications specification or style guide.
- **It does not regulate abbreviations or units of measurement** — projects decide.
- **It cannot be used alone.** In the FAQ's words: *"No. It is intended to be used with other applicable specifications for technical publications, style guides, and official directives. A high standard of professionalism is necessary to use STE correctly."*
- **It is not general-purpose English.** STE targets maintenance and technical documentation. The STEMG's position is that STE's *principles* — short sentences, one topic per sentence, active voice — transfer to other writing; the full rule set and dictionary do not.
- **No tool can certify compliance.** Checkers flag violations; they cannot convert non-STE text into STE, and the writer remains accountable.

## The one-meaning trade

STE's power and its cost are the same mechanism. Because every approved word has one meaning, ambiguity collapses — and so does nuance. The dictionary's recurring-errors list replaces `should` and `may` with `MUST`, and `shall` with `MUST`; `CAN`, `CANNOT`, `WILL` and `MUST` are approved, while `may`, `might`, `should` and `would` are not. That is correct for a maintenance manual, where a hedged instruction is a defect. It is destructive for text whose payload *is* graded confidence — a recommendation, an estimate, a risk statement — because STE's replacement converts a recommendation into an obligation and a possibility into a certainty. Adopting STE for such text does not make it clearer; it makes it more certain than the author was.

This is the boundary any adoption must draw explicitly. It is also why STE splits **procedural** (section 5) from **descriptive** (section 6) writing and gives them different rules: it already recognizes that one register's rule is another register's error. Argumentative text — a decision record, a strategy memo, an evaluation verdict — is a third register the standard never addressed.

## History, status, licensing

- Originated 1986 as the *AECMA Simplified English Guide*, requested by the European airline industry for aircraft maintenance documentation; the name honors the ATA100 publications specification.
- Maintained by the **Simplified Technical English Maintenance Group (STEMG)**, a working group of **ASD** (Aerospace, Security and Defence Industries Association of Europe), formed 1983.
- **Issue 9, 2025-01-15** is current. Free of charge to anyone on request since Issue 6 (2013).
- **Copyright is fully owned by ASD.** The document states that no reproduction or publication in whole or in part may be made without ASD's written authority; irrevocable free reproduction rights are granted only to named categories (ASD/AIA/AIAC/ICCAIA members and their customers, member-state Ministries of Defense, and similar). **Practical consequence:** an adopter outside those categories may apply and cite STE freely, but must not vendor the rule text or the dictionary into its own repository — reference it by rule number and link to the source.
- Adoption has spread well beyond aerospace (machinery, automotive, electronics, medical, IT, and language services). Most non-aerospace organizations adopt the writing rules and skip the controlled dictionary, which is a partial adoption, not compliance.

## Known limitations

- **Correct application is hard.** It demands strong English *and* domain knowledge; research on STE adoption consistently finds writers apply it incorrectly without training.
- **It is often misused as a substitute for a style guide** rather than a layer on top of one — the failure mode the FAQ answer above exists to prevent.
- **A clean check is not a correct document.** STE-compliant prose that is factually wrong reads *more* authoritative than the sloppy version — a controlled language raises the confidence a reader places in the text without touching its truth.

## Related

- [output-eval](/okf/core/concepts/output-eval.md) — the three-tier gate any writing standard is graded through; a clean lint is a Quality-tier signal, never an Acceptance-tier one.
- [okr-anti-optimism](/okf/core/concepts/okr-anti-optimism.md) — the reporting stance that STE's modal replacements would flatten; the reason the register boundary above is load-bearing in PMOS.
- Sources: [asd-ste100.org](https://www.asd-ste100.org/) (specification + FAQ, free on request), [ASD](https://www.asd-europe.org/standards-specifications/simplified-technical-english/).