The third conditional is used to talk about past situations that did NOT happen and to imagine a different result in the past.
We use it to express regret, criticism, hypothetical outcomes, or analysis, especially in aviation when reviewing incidents or procedures.

Structure

If + Past Perfect, … would have + Past Participle

• “If the crew had noticed the leak…”

• “…the incident would have been avoided.”

Meaning

We use the third conditional to say:

• “This didn’t happen, but here is what would have happened if it did.”

• It is 100% unreal past — we are imagining an alternative history.

Example (aviation)

If the pilots had checked the weather earlier, the flight would have departed on time.
→ They didn’t check it early, so the flight was delayed.

Negative forms

• hadn’t + V3

• wouldn’t have + V3

Example:
If the engine hadn’t failed, we wouldn’t have diverted.