ICAO Level 5 and 6 Structure: How To Form Complex Irregular Plurals?

ICAO Level 5 and 6 Structure: How To Form Complex Irregular Plurals?

“Aircraft” is the best example of an irregular plural in aviation English and yes, it is the same in the singular (“an aircraft”) and plural (“the aircraft”). No other correct options available. It’s one of the top mistakes non-native English speakers and one of the main reasons one cannot get a high score for the ‘Structure’ section. Today I’d like to take a closer look at the more advanced structures of irregular plural that derive from Latin and Greek.

‘-us’ ending becomes ‘-i’

When a singular word has ‘-us’ ending it will become ‘i’ in the plural:
cactus (singular) -> cacti (plural) or cactuses (plural)
In this case both “cacti” and “cactuses” are correct; however, “cactuses” is just the English plural with “-es” at the end. No worries, both are acceptable.

“Cacti and succulents have adapted to grow where there is not much fresh water, in places like deserts.” (BBC)

stimulus – stimuli
radius – radii
fungus – ‘fungi’
alumnus – only ‘alumni’

Cactus, cactuses and cacti as irregular plural

‘-um’ ending becomes ‘-a’

This is the main rule but in some cases just also the English plural can be found with ‘-um’ ending + ‘-s’.
bacterium – bacteria

“Bacteria are given two names—their genus and species—which are often descriptive of where the bacteria are found or what they look like.” (BBC)
datum – data
millennium – millennia
curriculum – ‘curricula’ but also ‘curriculums’
planetarium – ‘planetaria’ but also ‘planetariums’ – the second form is more common than the first.

“Children and adults alike visit observatories and planetaria, download images originating from spacecraft in orbit around planets, asteroids and comets, and grab the chance to look through a telescope without hesitation.” (CNN)
stadium – ‘stadia’ but also ‘stadiums’ – the same as ‘planetarium’
auditorium – ‘auditoria’ but also ‘auditoriums’

‘-ix’ ending becomes ‘-ces’

If a Latin or Greek word has an ‘-ix’ ending it becomes ‘-ces’ in plural according to the main rule.
cicatrix – cicatrices
appendix – appendices
aviatrix – aviatrices
matrix – matrices; matrixes – also acceptable

Amelia Earhart the legendary aviatrix

Amelia Earhart the legendary aviatrix

‘-is’ ending becomes ‘es’

The ‘is’ ending in most cases is transformed into ‘es’ apart from the uncountable words.

crisis – crises (because ‘crisises’ would be too difficult to pronounce)
“Talk story about how U.N. people manage during crises such as the recent mid-east hostilities – when they eat, sleep etc.” (New Yorker)
axis – axes
oasis – oases
basis – bases

How to check a proper plural form?

If you’re not sure what a plural form of a certain word is, here’s a short instruction that will help you become more confident in the complexity of irregular plural in English grammar.

Firstly, go into the Cambridge Online Dictionary website – click the link here.

Next, (1) type in a search box a word you want to check and (2) hit ‘Enter’ or click a magnifying glass.

Cambridge Online Dictionary - a search box

Finally look at the explanations next to a word:

Definitions and plurals in Cambridge Online Dictionary

It’s really not that difficult as it seems unless you’ve got some valuable learning resources. I hope this article is helpful for you 🙂

Emilia Barska
About me

General English teacher and Aviation English specialist. Devoting her free time to sharing her expert knowledge how to pass an ICAO exam with flying colors. She enjoys reading crime stories, listening to heavy metal music and sipping a vanilla latte.


What are participle adjectives?
ICAO Level 5 Structure: Participle Adjectives
May 31, 2018
ICAO structure - plans and intentions
ICAO Level 4 Structure: Plans and Intentions
December 11, 2017