ASD-STE100 · Specification for Simplified Technical English

One word. One meaning. One part of speech.

ASD-STE100 is a controlled language for technical documentation, born in civil-aerospace maintenance in the 1980s (as AECMA Simplified English) and maintained today by ASD, the Aerospace, Security and Defence Industries Association of Europe. It restricts vocabulary, grammar and sentence structure so that maintenance manuals are unambiguous, easier to read for non-native speakers, cheaper to translate, and safer to follow.

2 partswriting rules + dictionary

53writing rules in 9 sections

≈900approved words, one meaning each

20 / 25max words: procedural / descriptive sentence

The two halves of the spec

Every STE author works with both halves at once: the grammar constraints of Part 1 and the controlled vocabulary of Part 2. This page demonstrates the spirit of the most frequently used rules — it is a teaching toy, not the licensed specification.

Part 1 — Writing rules

How to build sentences and procedures: approved verb forms, sentence-length limits, one action per step, how to attach warnings and cautions, how to use articles, punctuation and noun phrases.

Part 2 — Dictionary

A controlled vocabulary. Each approved word has one approved meaning and one approved part of speech. Everything else gets rewritten into an approved alternative — or into a strictly governed technical noun or name.

Who uses it

Airframers, engine makers, airlines and MRO shops — and, increasingly, industries beyond aerospace: machinery, energy, medical devices, automotive. Any domain where a misread sentence costs money, hardware, or fingers.

Before / After

Flip between typical "engineering English" and its STE rewrite. Hover a highlight on the red side to see what is wrong with it.

Procedure Preparation before an inspection

Prior to commencing the inspection procedure, the operator should ensure that the aircraft electrical power has been deactivated, and it is also necessary to verify that the parking brake is engaged in order to prevent any possibility of unintended movement occurring.

Rule violations

41 words — over the 20-word limit for procedural sentences
Non-approved words: prior to, ensure, verify, in order to
Gerunds: commencing, occurring
Present perfect + passive voice
'should' instead of imperative steps
Several topics in one sentence

Do these steps before you start the inspection: 1. Set the aircraft electrical power to OFF. 2. Make sure that the parking brake is set.

Why this is STE

Imperative steps, one action per step
Every sentence under 20 words
Approved words only
Simple present + imperative only
Steps in the order you do them

S1 · 8 wordsS2 · 7 wordsS3 · 8 words

Descriptive text Why only approved lubricants

The utilization of lubricants which have not been approved could possibly result in deterioration of the seals located within the hydraulic reservoir, thus necessitating premature replacement thereof.

Rule violations

Non-approved words: utilization, thereof
Present perfect passive clause
Gerund: necessitating
Vague 'possibly' instead of the actual effect
Long, tangled nominal style

Use only approved lubricants. A lubricant that is not approved can cause damage to the seals in the hydraulic reservoir. If there is damage to a seal, replace the seal immediately.

Why this is STE

Simple present for facts
One topic per sentence
Cause and effect stated plainly
Approved vocabulary throughout

S1 · 4 wordsS2 · 15 wordsS3 · 11 words

Safety instruction Pressurized hydraulic reservoir

Warning: Care should be taken when opening the pressurized hydraulic reservoir as failure to do so may result in serious injury to personnel occurring.

Rule violations

Vague hazard ('care should be taken') — forbidden in warnings
Gerunds: opening, occurring
Hedged consequence with 'may'
Hazard, condition and consequence buried in one sentence

WARNING: Make sure that the reservoir is not pressurized before you open it. Pressurized hydraulic fluid can cause injury to you.

Why this is STE

Warning states the condition and the consequence
No vague phrasing
Short sentences, simple verb forms
'WARNING' = risk of injury; 'CAUTION' = risk of equipment damage

S1 · 12 wordsS2 · 8 words

Rules explorer

The most-cited rules, grouped the way the specification groups them. Rule numbers differ between issues, so these are shown by name.

Use only approved words, with their approved meanings

✗Utilize the special tool for this task.

✓Use the special tool for this task.

One approved meaning per word

✗Drain about 2 liters of fluid. ('about' is approved only as 'concerned with')

✓Drain approximately 2 liters of fluid.

Do not use -ing forms as nouns (gerunds)

✗Before installing the unit, do a check of the seal.

✓Before you install the unit, do a check of the seal.

Noun clusters: maximum three nouns in a row

✗the torque link pin cover

✓the cover of the torque link pin

Keep approved technical names intact

✓'aileron', 'circuit breaker', 'No. 3 fuel tank' stay exactly as defined — technical names are exempt from the dictionary when they are on your controlled list.

Use articles correctly in descriptive text

✗Valve is installed in reservoir.

✓The valve is installed in the reservoir.

Simple verb forms only

✗After the test has been completed, close the valve.

✓After the test is completed, close the valve.

No passive voice in procedures

✗The bolt must be tightened to 45 Nm.

✓Tighten the bolt to 45 Nm.

Approved verb, approved meaning

✗Inspect the surface for corrosion.

✓Examine the surface for corrosion. ('inspect' is not approved as a verb; 'examine' is)

Keep the verb close to its subject

✗The valve, if the pressure decreases during the test, opens automatically.

✓If the pressure decreases during the test, the valve opens automatically.

One action per step

✗Loosen the two bolts, remove the cover and do a check of the seal.

✓1. Loosen the two bolts. 2. Remove the cover. 3. Do a check of the seal.

Write steps in the order you do them

✓The reader performs the steps top to bottom — never make them hunt backwards for a condition that applied earlier.

Imperative mood, no actor

✗You must now set the power switch to OFF.

✓Set the power switch to OFF.

20 words max in procedures, 25 in descriptive text

✓A hyphenated word counts as one word; so do numbers and part numbers.

One topic per sentence

✗The pump is in the aft bay and the filter is adjacent to it and both were replaced last week.

✓The pump is in the aft bay. The filter is adjacent to the pump. The two parts were replaced.

Only simple conditions

✗If, when you have completed the check, the pressure is less than 3 bar, add fluid.

✓Do the check. If the pressure is less than 3 bar, add fluid.

Simple present for facts, definitions and descriptions

✗The valve will be closing when the temperature was increasing.

✓The valve closes when the temperature increases.

In descriptive text, use the passive only when the agent is unknown

✓The valve is installed in the reservoir. (who installed it is not known)

Keep paragraphs short

✓Paragraphs stay small and single-topic; a reader in a hangar must find the relevant fact in seconds.

WARNING = risk of injury or death. CAUTION = risk of damage to equipment.

✓Pick the word by the kind of risk, so the reader knows what is coming.

State the hazard and the consequence, not vague advice

✗Warning: be careful with the reservoir.

✓WARNING: Make sure that the reservoir is not pressurized before you open it. Pressurized hydraulic fluid can cause injury to you.

Common practice, not an STE rule: put the warning before the step it protects

✓A warning that arrives after the dangerous step is decoration, not safety.

Dictionary sampler

A few of the best-known substitutions. The real Part 2 dictionary holds roughly 900 approved entries, each locked to one meaning and one part of speech.

Not approved	Write instead	Note
utilize	use	The classic. Also 'employ' → 'use'.
prior to	before	Two-word latinate connectors are out.
in order to	to	Purpose needs only 'to'.
in the event of	if	Conditions are always 'if'.
due to the fact that	because	No padding phrases.
commence	start	Also 'terminate' → 'stop'.
obtain	get
assist	help
require, need	necessary	'need' is not approved either: 'clean filters are necessary'.
enough	sufficient
about (a quantity)	approximately	'about' is approved only as 'concerned with'.
additional	more
ensure	make sure
verify	make sure	'check' is approved only as a noun: 'do a check'.
inspect	examine	'inspection' is approved as a noun.
switch off	set to OFF	'deactivate' (approved from Issue 9) is for a system or function, not for power.

Live sentence checker

Paste text and watch it get measured against the headline rules: sentence length, non-approved words from the sampler above, gerunds, present perfect, and passive voice in procedures.

⚠ Toy checker — ~16 dictionary entries and simple heuristics, not the licensed specification.

Your text

Procedural (max 20 words) Descriptive (max 25 words)

Sentence boundaries are detected at . ! and ? — hyphenated words and numbers count as single words.

Findings

Is it STE?

Five sentences. Judge each one, get the rule explained.