Hi M2dude,
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
I'm struggling with the maths here - but if the chance of any RLG failing during the flight was say Z, and we consider the 3rd IRS equivalent to the SAARU
Then with 2 separate IRS units, the chance of the first RLG failing would be 6Z, and the chance of the second one failing in the remaining unit would be 3Z. So the chance of being down to just 1 IRS = 18 Z squared.
On the 777 unit, the chance of the first RLG failing = 6Z, and the chance of the second failing = 5Z. The chance of the second RLG being in the same axis = 1/3. So chance of total failure = 6Z * 5Z * 1/3 = 10 Z squared. Then you are down to the single SAARU. So the 777 arrangement appears to be 1.8 times more reliable than 2 separate IRS units.
Is that correct?