Definition
A self-contained navigation device that uses ring laser gyroscopes and accelerometers to continuously sense the aircraft's motion and rotation in three dimensions. From an initial known position, it calculates and outputs aircraft position, attitude, heading, ground speed, and acceleration without relying on any external signal such as GPS, radio, or ground-based navaid.
Plain English
A box on the aircraft that knows where it is, which way it's pointing, and how fast it's moving — purely by sensing its own movement from a starting point. It needs no outside signal to work.
Context Anchor
Seen in modern aircraft navigation, flight instrument, and maintenance discussions, especially when checking systems that feed attitude, heading, and navigation information to the flight deck.
Derivation
Inertial comes from the Latin iners, meaning inactive or motionless — the same root as inertia, the tendency of a mass to resist change in motion. The unit measures the aircraft's resistance and reaction to motion (its inertia) and uses those measurements to track where it has gone.
Why Pilots Care
Supplies continuous attitude, heading, and position data when GPS or radio navigation is unavailable or unreliable.
Analogy
It is similar in basic idea to the motion sensors in a smartphone, but built to aircraft standards and used to support flight instruments and navigation instead of screen rotation or step counting.
Intuition Check
Do not think of an IRU as a radio receiver or GPS box. It primarily senses the aircraft’s own motion from inside the aircraft and calculates reference information from that motion.
Example Sentence 1
After parking at the gate, the technician powered down the IRUs as part of the shutdown procedure.
Example Sentence 2
Maintenance replaced the faulty IRU after it failed to provide accurate heading information.