AIM Text
- IRUs are self-contained systems comprised of gyros and accelerometers that provide aircraft attitude (pitch, roll, and heading), position, and velocity information in response to signals resulting from inertial effects on system components. Once aligned with a known position, IRUs continuously calculate position and velocity. IRU position accuracy decays with time. This degradation is known as “drift.”
- INSs combine the components of an IRU with an internal navigation computer. By programming a series of waypoints, these systems will navigate along a predetermined track.
- AHRSs are electronic devices that provide attitude information to aircraft systems such as weather radar and autopilot, but do not directly compute position information.
- Aircraft equipped with slaved compass systems may be susceptible to heading errors caused by exposure to magnetic field disturbances (flux fields) found in materials that are commonly located on the surface or buried under taxiways and ramps. These materials generate a magnetic flux field that can be sensed by the aircraft's compass system flux detector or “gate,” which can cause the aircraft's system to align with the material's magnetic field rather than the earth's natural magnetic field. The system's erroneous heading may not self-correct. Prior to take off pilots should be aware that a heading misalignment may have occurred during taxi. Pilots are encouraged to follow the manufacturer's or other appropriate procedures to correct possible heading misalignment before take off is commenced.
Source: FAA Aeronautical Information Manual · current edition · paragraph 1-1-15.
The short answer
This AIM section describes self-contained navigation sensors: Inertial Reference Units (IRUs) of gyros and accelerometers giving attitude, position, and velocity; Inertial Navigation Systems (INSs) adding a navigation computer to fly waypoints; and Attitude Heading Reference Systems (AHRSs) feeding attitude to systems like autopilot. As FAA advisory guidance, it also warns that slaved compass heading errors can occur from magnetic flux fields on ramps and taxiways.
Research Notes
Common Questions
What is IRU drift and why does it matter?
The AIM explains that once aligned with a known position, IRUs continuously calculate position and velocity, but IRU position accuracy decays with time — a degradation known as “drift.” On long-range flights, inertial systems generally need periodic updates from an external reference to maintain acceptable position accuracy.
How is an INS different from an IRU and an AHRS?
Per the AIM, an INS combines the components of an IRU with an internal navigation computer, so by programming a series of waypoints it navigates along a predetermined track. An AHRS, by contrast, provides attitude information to systems such as weather radar and autopilot but does not directly compute position information.
Can my heading be wrong after taxiing over the ramp?
Yes. The AIM advises that aircraft with slaved compass systems can suffer heading errors from magnetic flux fields in materials on or buried under taxiways and ramps; the compass flux detector may align with that field rather than the earth's, and the erroneous heading may not self-correct. Pilots should follow manufacturer procedures to correct a possible heading misalignment before takeoff.