One rather interesting point here is that while we consider that the Honeywell ADIRU (B777) is the equivilant of 3 DADCs, srictly speaking, the INERTIAL equivilent of this unit is that of only TWO IRUs. We require a RLG and accelerometer for each of the X, Y & Z axis, however the ADIRU houses 'only' SIX RLGs and accelerometers respectively. (Technically of course, we should have nine of each). However, these are extremely reliable individual components, far more reliable and accurate than those found in the Delco C1VA and Litton INSs of yesteryear. (I remember that the C1VAC used to cough on a very regular basis, gyro failures was one of the main culprits here). There is more redundancy however as far as the rest of the modules are concerned; 3 Power Supply Modules, 4 Processor modules and a total of 6 Databus Modules. (There are three 629 bus outputs, each with a dual Databus Module). The gyros/accelerometers are housed in fault containment areas, and the Honeywell blurb is that you can fail any TWO components. Unless I am totally missing something here, if you lost two RLGs or Accelerometers on the SAME axis, you would be in the poo. (Of course we still have the SAARU, and having said all this, I do not personally recall the complete failure of an ADIRU in flight, at least with my airline).
