EASA ATPL — Instrumentation (022) Exam Prep

Correct answer: C. Airspeed will decrease

With a blocked pitot tube (and ram air pressure trapped inside), the ASI effectively becomes an altimeter. During descent, static pressure increases. Since the trapped pitot pressure remains constant and the ASI displays the difference between pitot and static pressure, the indicated airspeed will decrease as the aircraft descends (because the static pressure increases, reducing the pressure differential).

Correct answer: B. It computes the flight plan, performance data, and provides guidance commands to the autopilot and flight director

The FMGC is the core of the Flight Management System. It manages the active flight plan, computes lateral and vertical navigation (LNAV/VNAV), calculates performance parameters (V-speeds, fuel predictions, optimum altitude), and provides guidance commands to the autopilot and flight director. In Airbus aircraft, there are two FMGCs for redundancy.

Correct answer: C. The Sagnac effect — interference of counter-rotating laser beams

Ring laser gyroscopes use the Sagnac effect. Two laser beams travel in opposite directions around a closed triangular or square path. When the gyro rotates, the beam travelling in the direction of rotation has a slightly longer path, creating a frequency difference between the two beams. This frequency difference is directly proportional to the rate of rotation and is measured by detecting the interference pattern.

Correct answer: A. The compass will show a turn towards the north (acceleration error)

Acceleration error affects the magnetic compass on easterly and westerly headings. In the Northern Hemisphere, the compass CG is displaced towards the equator-seeking (south) end due to the magnetic dip. During acceleration, inertia causes the compass card to tilt, and the south-seeking end dips, causing an apparent turn towards north. The mnemonic is ANDS: Accelerate North, Decelerate South (in the Northern Hemisphere).

Correct answer: B. Frequency-modulated continuous wave (FMCW) at 4.3 GHz

Radio altimeters use FMCW (Frequency Modulated Continuous Wave) technology operating in the 4.2–4.4 GHz band. The transmitter sends a continuously frequency-modulated signal towards the ground. The received reflected signal has a frequency offset proportional to the time delay (and thus the height above terrain). The beat frequency between transmitted and received signals gives a direct measurement of AGL height.

Correct answer: B. Compressibility and density (pressure altitude and temperature)

Converting CAS to TAS requires two corrections: compressibility correction (significant above approximately 200 kt and/or 10,000 ft, converting CAS to Equivalent Airspeed/EAS) and density correction (using actual air density derived from pressure altitude and total air temperature to convert EAS to TAS). Position error correction is already accounted for in the CAS to IAS conversion.