Definition
A safety and risk-management strategy that uses multiple, independent layers of protection against a hazard, so that if one layer fails another is still in place to prevent an accident or undesirable outcome.
Plain English
Don't rely on just one safeguard. Stack several so that no single failure can cause an accident.
Context Anchor
Seen in aviation safety, maintenance, risk management, and accident-prevention discussions.
Derivation
From military strategy, where 'defense in depth' meant arranging multiple lines of defense one behind the other rather than relying on a single front line. The same idea is applied to aviation safety: multiple barriers between the pilot and an accident.
Why Pilots Care
It prevents accidents by ensuring no single human error, equipment fault, or procedural gap can cause harm on its own.
Analogy
It is like locking a door, setting an alarm, and having outside lights. Any one measure helps, but the protection is stronger because there is more than one layer.
Intuition Check
Do not read this as a military term about fighting off an attack. In aviation safety, it means layered safeguards that help prevent one problem from becoming an accident.
Example Sentence 1
Using a written checklist, a fuel reserve, and a weather briefing together is a good example of defenses-in-depth.
Example Sentence 2
During the safety review the team added another layer of defenses-in-depth by cross-checking the work order against the aircraft logbook.