What signal does a modal verb send?
A pilot reading an FAA airworthiness directive does not just read the words — they read the strength of the obligation each verb carries. Must, should and may sit on a sliding scale from legal requirement to optional suggestion, and confusing them can change a polite recommendation into a regulatory demand or vice versa.
Strong, weaker, and not-quite-obligation
Strongest — a legal or regulatory obligation:
- must / must not — the standard regulatory verb. Operators must identify and replace every affected clamp.
- are required to — formal and slightly more impersonal. Airlines are required to comply by 18 June.
- have to — same strength as must, but the obligation feels external rather than self-imposed. Technicians have to use the corrected procedure.
Weaker — recommendation, not requirement:
- should / should not — best practice. Operators should verify which version of the maintenance task they are using.
- is expected to — anticipated but not yet enforced. Airbus is expected to issue revised documentation before the deadline.
Possibility, not obligation at all:
- may need to / might need to — conditional necessity, depending on circumstances. Airlines that recently completed maintenance may need to assess past work.
- could — open possibility. The FAA could allow extensions, but it has not signalled any.
- do not have to — no obligation; the action is optional. Distinct from must not, which prohibits the action. Airlines do not have to ground aircraft pre-emptively, but they have to comply by 18 June.
A note on a common trap: must not prohibits an action outright, while do not have to means the action is not required. The two are not interchangeable.
Try these
Fill each blank with one item from the word bank. Every item is used exactly once. The text below is drawn from last week’s airworthiness directive story.
Word bank: must · must not · are required to · is expected to · should · may need to · could · have to
The FAA’s airworthiness directive on the A350 oxygen clamp takes effect on 18 June 2026. Operators (1)____ identify and replace every affected clamp before that date. Technicians (2)____ apply the original torque value to the new clamp, even if the existing documentation still lists it; the new component (3)____ be tightened to the revised specification. Airbus (4)____ issue corrected maintenance documentation before the compliance deadline. Until then, operators (5)____ verify which version of the maintenance task they are working from. Airlines that recently completed maintenance work (6)____ assess whether the older torque value was applied. The FAA (7)____ allow individual extensions to the deadline, but it has not signalled any so far. Operators who miss the deadline (8)____ ground affected aircraft until compliance is achieved.
Once you’re confident with present obligation, the matching past form opens up: Structure: ‘must have’ & ‘can’t have’ covers modals of past speculation — the verbs you’ll need for any post-incident report.