Modern instrument flight relies on integrated flight support systems that provide pilots with precise attitude, heading, and air data information when visual references are limited or unavailable. Attitude and Heading Reference Systems (AHRS) and Air Data Computers (ADC) replace traditional gyroscopic instruments and pitot-static devices, combining multiple sensors and processing units to deliver accurate, reliable information essential for safe IFR operations.
Attitude and Heading Reference System (AHRS)
As aircraft displays have transitioned to new technology, the sensors that feed them have also undergone significant change. Traditional gyroscopic flight instruments have been replaced by Attitude and Heading Reference Systems (AHRS) improving reliability and thereby reducing cost and maintenance.
The function of an AHRS is similar to traditional gyroscopic instruments: it determines aircraft attitude (pitch and roll), heading, and rate of turn by combining data from gyroscopes, accelerometers, and magnetometers.
The genesis of this system was initiated by the development of the Ring Laser Gyroscope (RLG) developed by Kearfott located in Little Falls, New Jersey. [Figure 1] Their development of the Ring Laser Gyroscope in the 1960s/1970s was in support of Department of Defense (DOD) programs to include cruise missile technology. With the precision of these gyroscopes, it became readily apparent that they could be leveraged for multiple tasks and functions. Gyroscopic miniaturization has become so common that solid-state gyroscopes are found in products from robotics to toys.
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| Figure 1. The Kearfott Attitude Heading Reference System (AHRS) on the left incorporates a Monolithic Ring Laser Gyro (MRLG) (center), which is housed in an Inertial Sensor Assembly (ISA) on the right |
Because the AHRS system replaces separate gyroscopes, such as those associated with an attitude indicator, magnetic heading indicator and turn indicator these individual systems are no longer needed. As with many systems today, AHRS itself had matured with time. Early AHRS systems used expensive inertial sensors and flux valves. However, today the AHRS for aviation and general aviation in particular are small solid-state systems integrating a variety of technology such as low cost inertial sensors, rate gyros, and magnetometers, and have capability for satellite signal reception.
Air Data Computer (ADC)
An Air Data Computer (ADC) [Figure 2] is an aircraft computer that receives and processes pitot pressure, static pressure, and temperature to calculate very precise altitude, IAS, TAS, and air temperature. The ADC outputs this information in a digital format that can be used by a variety of aircraft systems including an EFIS. Modern ADCs are small solid-state units. Increasingly, aircraft systems such as autopilots, pressurization, and FMS utilize ADC information for normal operations.
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| Figure 2. Air data computer (Collins) |