An integrated standby instrument system (ISIS) is an electronic aircraft instrument intended to serve as backup in case of failures in a glass cockpit instrument system, and thus is designed to operate as reliably and independently as possible from the aircraft's main instrument system, with provisions for backup power, and embedded sensors when possible.
An ISIS is designed to combine the functions of separate equivalent mechanical instruments that had previously been included as backup in such cockpits, including altimeter, airspeed indicator, and attitude indicator. More sophisticated integrated standby systems can include additional functions.
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Integrated standby instrument system Wikipedia(Text) CC BY-SA