IRS

An inertial reference system is a self-contained navigation instrument that tracks an aircraft's position, velocity, and orientation in three-dimensional space without needing external signals. It does this by measuring acceleration in all directions using accelerometers and rotation using gyroscopes, then continuously calculating where the aircraft is and how fast it is moving. Once initialized on the ground with a known position, an IRS works independently and reliably for the duration of a flight.

The system contains a stable platform or a strap-down configuration of sensors that detect every change in motion. Modern commercial aircraft typically carry at least two independent IRS units, and sometimes three. Each unit continuously solves the equations of motion, integrating acceleration data over time to produce velocity and position. The accuracy degrades gradually, this drift is why long-range flights over the ocean require periodic updates from radio navigation aids or GPS.

Role in flight operations

The IRS provides heading, attitude, and position information to the autopilot, flight management system, and flight deck displays from takeoff through landing. Unlike VOR or NDB navigation, which depend on ground stations, the IRS is independent and unaffected by radio interference or coverage gaps. It remains the primary means of dead reckoning and is critical over remote areas, at high altitude, and during oceanic crossings where other navaids are unavailable.

Alignment takes 5 to 15 minutes while the aircraft is stationary and level on the ground. During this period the system establishes a reference frame and nulls out sensor errors. Any movement before alignment is complete compromises accuracy. In the air, the IRS performance degrades at a rate typically around 1 to 2 nautical miles per hour of flight due to sensor drift and integration errors, which is why updates from GPS or ground-based navaids are essential for precision approaches and long flights.

Technicians troubleshoot IRS issues through built-in test equipment and by comparing outputs between multiple units. Disagreement between two or three independent systems flags a problem. Common failures include accelerometer drift, gyroscope bias, and computational errors. An IRS that fails alignment or shows excessive drift must be replaced; these units are not field-repairable and are sent back to the manufacturer for overhaul.