Crossbow Technology is the first company to receive FAA TSO approval for a low-cost attitude heading reference system (AHRS) that can replace all of the spinning rotor gyros in a general aviation airplane. AHRS has been the standard in turbine airplanes for many years, but the price has been too high to make sense in piston-powered airplanes until now. Several companies are developing solid-state AHRS gyro replacements for the piston fleet, but Crossbow is the first to receive TSO approval for a stand-alone unit.
The Crossbow AHRS500GA measures aircraft attitude, stabilized magnetic heading and rate of turn, tasks that require three different gyroscopes in a typical airplane. The entire unit measures about five by four inches and is just less than five inches high. The 500GA weighs just a couple of pounds and can be mounted anywhere in the airplane. The unit uses only four watts of power and, because it has no moving parts, should have an operating life of many thousands of hours.
The AHRS500GA uses micro-electronic mechanical sensors (MEMS) to replace the spinning rotor gyros that are the norm in general aviation airplanes. The MEMS gyros and companion accelerometers keep track of rate of change in aircraft attitude and heading and thus compute actual attitude and heading. In contrast, a spinning rotor gyro remains stable in space while its gimbals allow the airplane to move around the gyroscope.
The very best AHRS use laser beams traveling around a mirrored ring. Very small movements cause the light beam to change frequency and that change can be measured with great precision. A laser gyro "drifts" less than a degree per hour. All larger jets, including business jets, use laser gyros, but the price is well north of $100,000 each.
MEMS gyros are used mostly in the automotive industry to measure body tilt and skid to operate handling and traction control systems. A MEMS gyro costs very little but isn't very good over the long term. Raw output from a MEMS gyro drifts many degrees per minute, so if you tried to use the device in an airplane the gyro would show you inverted, or through the first roll after just a few minutes of level flight.
