Simplified Technical English: what ASD-STE100 is and why accessibility people should care
A controlled language built for aircraft maintenance manuals turns out to be one of the best tools we have for cognitive accessibility and for writing agents. How it works, how it relates to the ISO plain language standard, where it fits, and where it does not.
Most writing advice is taste. Simplified Technical English is a rulebook. It tells you how long a sentence may be, which words you may use, which verb forms are allowed, and what a warning has to look like. It was written for people maintaining aircraft, many of them reading English as a second or third language, and it has been refined for more than forty years by an industry that cannot afford a misread instruction.
That makes it unusually useful outside aviation. A page written to its rules is easier to read for people with dyslexia, aphasia, low literacy, cognitive fatigue, or a screen reader running at speed. It translates better, by machine or by hand. And because it is a rulebook rather than a feeling, you can check a text against it, which is what the Simplified Technical English lab does.
What it is
ASD-STE100 is the specification for Simplified Technical English, usually shortened to STE. It is owned and maintained by ASD, the Aerospace, Security and Defence Industries Association of Europe, in Brussels. It began in the late 1970s as the AECMA Simplified English Guide, built with the American aerospace industry so that maintenance manuals for aircraft could be understood by readers with basic English. The guide was released in 1986, became an international specification in 2005, and was designated an international standard by ASD in 2025. It is not an ISO standard. The current edition is Issue 9, dated 15 January 2025. The official copy is free on request from the ASD-STE100 site, with one condition worth knowing: the dictionary text is copyrighted, so you follow the rules and use the words, but you do not republish the list. The lab on this site uses a short sample of common unapproved words for that reason, not the dictionary.
The specification has two parts.
The writing rules. Around fifty rules grouped into sections on words, noun clusters, verbs, sentences, procedures, descriptive writing, warnings and cautions, punctuation, and writing practices. Each rule is short, numbered, and comes with examples of text that follows it and text that does not.
The dictionary. A list of approved general words, roughly nine hundred of them, each with a single approved meaning and part of speech. "Close" is an approved verb meaning to move something so that it stops an opening; it is not an approved adjective meaning near. Alongside the approved words are many unapproved ones, each with the approved alternative to use instead: not "commence" but "start," not "utilize" but "use," not "prior to" but "before." Technical names and technical verbs specific to your product or field are allowed on top of the dictionary, so a procedure can still say "actuator" or "torque."
The combination is the point. The rules shape the sentences; the dictionary limits the words inside them; the result is text where each word means one thing and each sentence does one job.
The rules that do most of the work
You do not need the whole specification to get most of the benefit. These are the rules that change writing the most.
One instruction per sentence. "Release the pressure and install the locks before you begin" becomes two sentences, or a numbered list. A reader who is doing the task cannot hold three instructions in one breath.
Sentence length. No more than twenty words in a procedural sentence, no more than twenty-five in a descriptive one. Not as a style preference, as a limit.
Paragraph length. No more than six sentences in a paragraph, and the first sentence carries the topic.
Use the imperative for instructions. "Open the panel," not "the panel should be opened" or "the operator opens the panel."
Use the active voice. The passive hides who does what. "The valve must be closed by the technician" leaves a reader wondering whether that is a description or an instruction. "Close the valve" does not.
Use the present tense. Descriptive text stays in the present. Past and future tenses are used only when the time really is past or future.
Avoid gerunds and participles as nouns. "Testing of the unit is required" becomes "Test the unit." The -ing form is one of the biggest sources of ambiguity in English, because it can be a verb, a noun, or an adjective, and a reader has to work out which.
Break up noun clusters. "Fuel pump housing retaining bolt torque value" is seven nouns in a row. STE limits clusters to three and asks you to unpack the rest with prepositions: "the torque value for the bolts that hold the fuel pump housing."
Use articles. "Install bolt in hole" is telegram English. "Install the bolt in the hole" is readable English, and it is unambiguous about which bolt and which hole.
Warnings and cautions come first and stand alone. A warning tells the reader about a risk of injury; a caution tells them about a risk of damage. Each starts with a clear, simple command and is placed before the step it applies to, not after it, and not buried in the middle of a sentence.
Use vertical lists. When a sentence would contain a sequence, put the sequence in a list, one item per line, in the order the reader will do it.
One word, one meaning. This is the dictionary's rule. Each approved word has one approved meaning, and you use it only in that meaning. The reader never has to disambiguate.
Why this is an accessibility topic
Accessibility guidance mostly talks about structure: headings, labels, alt text, focus. The words themselves get a single criterion, and it is at the highest conformance level, so it is routinely skipped. Yet for a large group of people, the words are the barrier.
People with dyslexia lose their place in a long sentence. People with aphasia or a brain injury may process one clause at a time. People with intellectual disabilities, or anyone reading in a second language, are stopped by a word with two meanings. People using a screen reader at three hundred words a minute cannot rewind a thirty-word sentence to find the subject. People in a hurry, in pain, or in a crisis, which is everyone at some point, read the way all of those groups read.
STE addresses every one of those situations with the same rules, and it does so without dumbing anything down. The content of a maintenance manual is not simple. The language is. That distinction is the whole argument for plain language, and STE is the most rigorous version of it in daily use.
It also makes the other accessibility work easier. A page in STE has shorter headings, clearer link text, and alt text that is already a decision rather than a description, because the writer has been forced to say what a thing is for in one sentence.
Where it sits next to ISO 24495
STE is often confused with plain language. They are relatives, not the same thing, and since 2023 plain language has its own international standard.
ISO 24495 is the plain language standard, developed in ISO's language and terminology committee by around fifty experts from twenty-five countries working in nineteen languages. Part 1, published in June 2023, sets out governing principles and guidelines for any document aimed at readers, in any language. Part 2, from August 2025, applies them to legal communication. Part 3, from May 2026, covers writing about science for general audiences. A Part 4 on organizational certification is planned. Unlike STE, the ISO standard is sold rather than given away, and it is deliberately not a style guide: it sets no readability score and no word list.
Its four principles are a useful lens on everything else in this note.
- Relevant. Readers get what they need. Identify the readers and their purpose before writing.
- Findable. Readers can easily find what they need. Structure, headings, and sequence follow the reader's path, not the author's.
- Understandable. Readers can easily understand what they find. Familiar words, clear sentences, an appropriate tone, and supportive design.
- Usable. Readers can easily use the information. Test with real readers and keep improving.
Put side by side, the division of labor is clean.
| ISO 24495 | ASD-STE100 | |
|---|---|---|
| What it is | Principles and guidelines for any document | A controlled language: fixed rules plus an approved dictionary |
| Scope | Any language, any document, print or digital | Technical documentation, English only |
| Decides | What readers need and whether they can find and use it | How each sentence and word is built |
| Readability rules | None by design | Sentence length, voice, tense, word choice, one meaning per word |
| Current edition | Part 1:2023, Part 2:2025, Part 3:2026 | Issue 9, 15 January 2025 |
| Cost | Sold by ISO and national bodies | Free on request; dictionary copyrighted |
A sensible house rule for anyone writing for the public: cite ISO 24495 as the authority for whether content is understandable, write everything to its four principles, and apply the STE rules to the parts that are step-by-step instructions, where every sentence is an action. The accessibility guidelines' "understandable" criteria have no formal rule set behind them; between them, these two standards supply one.
Where it does not fit
STE was built for a narrow job and it shows.
It is not for marketing, essays, or anything that wants a voice. The rules strip tone on purpose. A product page written in STE reads like a manual, because that is what the rules produce.
It is not plain language for the general public. Plain language guidance for government and health writing shares most of STE's instincts but keeps a larger vocabulary and allows warmth. STE is stricter and colder, which is right for a torque procedure and wrong for a letter about someone's benefits.
The dictionary is a floor, not a ceiling. Technical names are allowed, and a text can be full STE and still be full of terms the reader has never met. STE makes the grammar around those terms safe; it does not explain them. That is the glossary's job.
It needs a reviewer who knows it. Most of the rules are checkable by eye or by software, but "one word, one meaning" needs someone who knows the dictionary. Tools exist, and the lab below approximates a few checks, but the specification itself warns that software cannot replace a trained writer.
Using it as a prompt for writing agents
A tip has been circulating among people who write with language models: tell the model to write in ASD-STE100, and add the four principles from the book On Writing Well, which are clarity, simplicity, brevity, and humanity. The claim is that the output gets markedly better.
The reference is real. The claim, as far as anyone has shown, is anecdote. Nobody has published a comparison of the same model, the same task, with and without the instruction, scored against the rules. That does not make it a bad tip. It makes it an untested one, and this site has a view on untested claims, which is that you measure them.
The lab makes that measurement possible. Take a passage. Ask a model to rewrite it plainly. Paste the result into the checker and count the flags. Then ask the same model to rewrite it "in ASD-STE100, with one instruction per sentence, no sentence over twenty words, the active voice, the imperative for instructions, and no gerunds," and paste that. If the second count is lower, the tip worked for that model on that text. If it is not, you have learned something the tip could not tell you.
A few things the lab's checks cannot see, which a human reviewer can: whether a technical name was invented, whether "one word, one meaning" was respected, and whether the rewrite kept the facts. Models are good at shortening sentences and less good at noticing that they shortened the meaning too.
How the lab works
The lab has three passages: a maintenance procedure, a software help topic, and a descriptive paragraph. Each was written by hand at four levels, from the original jargon to full STE, and a slider moves between them while a panel lists what changed at each step. The rule checks run on whichever level is showing, and on anything you paste into the box below it. They are regular expressions, not a dictionary, so they catch the obvious cases, miss the subtle ones, and occasionally flag a word that is fine. The page says so where it matters.
Sources
- ASD-STE100 official site, for the free copy and the terms of use: asd-ste100.org
- The International Plain Language Federation's pages on the ISO 24495 series, for the parts, dates, and adoption figures: iplfederation.org
- ISO catalog entries for 24495-1 (standard 78907) and 24495-2 (standard 85774)
Three sentences to remember
If you take nothing else from the specification: one instruction per sentence; no sentence over twenty words; use the word that means one thing. Most of the benefit, for most readers, is in those three.