EASA ATPL Package (former JAA) Instruments/ Electronics

INS\/IRS

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Sample Question: INS\/IRS

Question 6141
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With reference to inertial navigation systems, a TAS input is:

A
required to provide a W/V read out.
B
required for the AFCS to compute a heading according to track.
C
required for Polar navigation.
D
not required.

Understanding INS/IRS for ATPL and Advanced Pilot Training

Inertial Navigation Systems (INS) and Inertial Reference Systems (IRS) are core aircraft systems in the ATPL syllabus, providing self-contained attitude, heading, and navigation data without external aids. A classic INS uses a gimballed stable platform, delivering north/south and east/west accelerations along with attitude and true heading. Modern strapdown IRS mount accelerometers and laser ring or MEMS gyros directly on the aircraft axes; high-rate computations transform body-axis measurements into the earth frame for attitude and navigation. After alignment, the system outputs true heading, attitude, and linear accelerations that are integrated to velocity and position. With zero acceleration, velocity remains constant—an important principle behind inertial mechanization and performance checks.

Alignment is critical. A strapdown system aligns by sensing local gravity to establish the vertical and by measuring Earth’s rotation to find true north, positioning the reference trihedron before entering NAV mode. Accurate initial position entry on the CDU and a stationary aircraft are essential; alignment errors propagate into heading and position. Gyro drift—particularly in azimuth—induces position error roughly proportional to elapsed time, so pilots monitor drift rates and cross-check with GNSS and radio updates where allowed by procedures and aviation regulations. In triple-fit IRS installations, the three indicated positions on the CDU may differ slightly because each unit computes position from its own sensors; the FMS may “mix” or select sources to enhance integrity.

INS/IRS data feeds multiple aircraft systems. The autopilot uses IRS attitude; the PFD’s sideslip/skid indication is typically derived from inertial lateral acceleration and yaw rate; and the FMS depends on IRS position and velocity when GNSS is unavailable. Wind computation requires combining groundspeed/track from INS/IRS with TAS and pressure data from the Air Data Computer (ADC). If TAS input is lost due to an ADC failure, wind direction and speed are no longer available, even though inertial data remain. Conversely, if an IRS can no longer compute latitude/longitude, the FMS’s navigation solution is degraded until another source restores position. Good procedures include careful preflight alignment, validation of initial position, cross-checks against radio or GNSS, and routine monitoring of drift and residuals.

What the INS/IRS Question Bank Covers

  • Platform vs. strapdown architectures, sensor alignment with aircraft axes, and transformation to earth-referenced outputs.
  • Alignment theory and procedures: gravity/earth rotation sensing, true heading determination, and impact of initial position errors.
  • System outputs and uses: attitude, true heading, north/east accelerations, integration to velocity/position, and autopilot/PFD interfaces.
  • Error sources and performance: gyro drift, time-proportional position error, and cross-checking with GNSS/radio aids.
  • System integration: FMS dependency on IRS position, triple-mix behavior, and ADC/TAS inputs for wind calculation.
  • Operational considerations and ATPL-level procedures within aviation regulations for dispatch, monitoring, and fault management.