During IRU/IRS alignment after power-up, what is its primary purpose?

• A. It updates weather data and traffic information.
• B. It aligns sensors to establish accurate attitude and heading references (often using GPS alignment data if available), which is essential for navigation accuracy.
• C. It tests engine parameters for checklists.
• D. It powers down redundant units to save energy.

Answer: (B)

The main point of IRU/IRS alignment after power-up is to establish a reliable reference for the aircraft’s orientation. The inertial sensors—gyros and accelerometers—need to be oriented properly with respect to a known frame so the system can correctly determine pitch, roll, and heading. If GPS data is available, it can be used to speed up the process and improve the accuracy of the initial heading by providing an external reference vector. Getting this alignment right is crucial because all navigation calculations depend on a trustworthy attitude and heading reference from the start; otherwise the aircraft’s navigation solution can drift or be incorrect.

Weather data updates, engine parameter checks, and reducing redundant units are separate functions and not related to establishing the initial attitude/heading reference, so they don’t reflect the primary purpose of this alignment process.