Definition
In aviation risk assessment, a likelihood category indicating that an event is very unlikely to occur during the normal operation or expected lifetime of a system, activity, or training program, though it remains theoretically possible.
Plain English
Something that almost certainly will not happen, but isn't completely impossible.
Context Anchor
Used when estimating the chance of a problem during flight planning, training, or an instructor’s risk discussion.
Derivation
From Latin 'improbabilis' — 'im-' meaning 'not' and 'probabilis' meaning 'likely.' So literally 'not likely.' Knowing this helps because in risk matrices, 'improbable' is a precise tier on a scale, not a casual 'probably won't happen.'
Why Pilots Care
Misjudging an improbable event as impossible can lead to complacency, while treating it as probable can cause unnecessary operational restrictions.
Intuition Check
Improbable does not mean impossible. It means the event is unlikely enough that you do not expect it, but you still leave room for it in your decision.
Example Sentence 1
The instructor rated a complete dual engine failure on the training flight as improbable but catastrophic, so a contingency plan was still briefed.
Example Sentence 2
Although an improbable fuel contamination event remained on the matrix, the pilot elected to proceed after verifying recent fuel quality checks.