An inertial reference system is a self-contained navigation unit containing laser ring gyroscopes and solid-state accelerometers. It continuously calculates the aircraft position, ground speed, and attitude by integrating accelerations from a known initial location. Crews must align the system on the ground before flight so it establishes a baseline reference to true north and gravity.
Heard on the job
Don't move the aircraft while the IRS is aligning or we'll have to restart the whole sequence.
Insider tip
A typical airliner has two or three independent IRS units to provide redundancy and allow the flight management computer to vote out a faulty sensor.