In-flight upset - 154 km west of Learmonth, WA, 7 October 2008,

Inertial reference partThe IR part of the ADIRU provided information about the aircraft’s position,orientation, and velocity with respect to the Earth. It obtained its data from a set ofinertial instruments within each ADIRU, which continually measured accelerationin all three axes (pitch, roll and yaw) as well as rotational movement.The IR part constantly updated the aircraft’s three-dimensional position andorientation based on the movement it sensed from a known starting position andorientation. The process of determining this starting position was known as ‘inertialalignment’, which occurred at the beginning of each flight when the aircraft was onthe ground and stationary. Subsequent inertial measurements changed the calculatedposition, orientation and velocity by a very small amount for each measurementcycle. As the IR parameters were dependent on previous values, an introduced errorwould affect subsequent values. The IR parameters were also highlyinterdependent, and an error in one parameter would affect other parameters.Each ADIRU received GPS data from one of two multi-mode receivers in order toaugment the inertial reference computations.ADIRS control panelThe ADIRS control panel provided local fault indications for each part of eachADIRU (Figure 9). If there was a fault with the IR part of an ADIRU, an amber‘FAULT’ light illuminated. The relevant part of the ADIRU could be deactivatedby pressing the OFF pushbutton below the fault light. The ADR part of the ADIRUoperated in the same manner.Figure 9: ADIRS control panelThe panel also had an IR mode rotary selector for each ADIRU that allowed theflight crew to select one of three modes:• OFF; the ADIRU was not energised and the IR and ADR parts were notavailable- 16 -