Sample Question: Gyroscopes
The main reason for usually mounting the detector unit of a remote indicating compass in the wingtip of an aeroplane is to:
Understanding Aircraft Gyroscopes for Directional and Compass Systems
Gyroscopes underpin several critical aircraft systems taught in ATPL theory, notably the directional gyro (DG), attitude indicator, and the slaved or gyromagnetic compass. Their performance rests on two principles: rigidity in space and precession. Design efficiency is maximized when rotor mass is concentrated at the periphery and spun at high RPM, increasing angular momentum and stability. A typical directional gyro used for heading reference is a two degrees-of-freedom horizontal-axis gyro (the rotor spin axis does not count toward the DOF). Because the gyro’s sensitive axis must remain level, gimballing errors can arise in banked attitudes due to cross-coupled precession and misleveling, especially near gimbal limits.
The gyromagnetic (remote indicating) compass is a slaved system that combines a stable gyro with magnetic sensing to reduce pilot workload and drift. A flux valve (detector unit) senses the direction of the Earth’s magnetic field; it is commonly mounted in a wingtip to minimize deviation from aircraft magnetism and electrical systems. The flux valve signal is sent to an error detector, which compares the gyro’s heading with the magnetic reference. Any difference is amplified by a precession amplifier and applied to torquers to precess the gyro back onto magnetic heading. During synchronization or system checks, an annunciator alerts the pilot/maintenance that the magnetic and gyro elements are being aligned. Standard procedures in line with ATPL and common aviation regulations emphasize aligning or slaving the system in straight-and-level, unaccelerated flight and verifying correct operation before use as a primary heading reference.
Heading instruments are subject to predictable errors. A free or unslaved DG experiences apparent wander due to Earth rotation, also known as Earth-rate drift, given by approximately 15°/hour × sin(latitude): zero at the equator, maximum 15°/h at the poles, to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Representative ATPL examples include about 13°/h to the right at 60°N and 10.5°/h to the right at 45°N. A second component, transport wander, results from the aircraft moving over the Earth’s surface and depends on groundspeed, true track, and latitude. If a DG is pre-set to cancel Earth-rate drift at one latitude (e.g., 30°S) and then used at another (e.g., 60°N), the observed drift will combine the new Earth-rate component with any transport wander. Good practice is to use slaved systems when available and, for free DGs, to periodically realign with the magnetic compass and apply drift-rate corrections per the AFM/POH and operator procedures.
What this Gyroscopes Question Bank Covers
- Gyro design and performance: rotor mass distribution, high rotational speed, rigidity, and precession fundamentals.
- Directional gyro (DG): 2-DOF horizontal-axis configuration, bank-induced gimballing errors, Earth-rate drift, transport wander, and pilot procedures for setting and realigning heading.
- Gyromagnetic compass (remote indicating compass): flux valve sensing, error detector, precession amplifier, torquers, slaving logic, annunciator use, and wingtip detector placement to reduce deviation.
- Calculations and rules of thumb: Earth-rate drift versus latitude (0°/h at equator; up to 15°/h at poles), typical values at mid/high latitudes, and operational implications for ATPL-level exams.
- Procedural emphasis aligned with aviation regulations: straight-and-level synchronization, periodic checks, and integration within aircraft systems for reliable heading information.