Three IRUs needed?
An observation from “xetroV”, in message #32 on 1Apr, regarding pilots’ analysis of a faulty IRU [when designers provided ONLY two IRUs for those mishap pilots to compare]:
-- [from 2nd paragraph] “... All these observations seem to suggest that the left IRU data was faulty, yet the crew decided to switch the right IRU to ATT. As far as I can see, the report does not attempt to find an explanation for this choice ...”
-- from 4th paragraph] “... after the crew had switched the IRS-R to ATT, the PIC said ‘put it back on nav again, put it back on nav again.’ Once an IRS is switched to ATT, it is impossible to re-align it during flight, so putting it back in NAV is useless....”
Those are good points.
Question inferred from xetroV’s observations:
Wouldn’t a THIRD IRU have aided these mishap pilots?
The 3rd IRU could have aided their fault analysis, and [with proper design switching] the PITCH and ROLL signals from the 3rd IRU could have been selected, or de-selcted, to either PFD. This design, with the 3rd IRU, would be analogous to the better AUX GYRO switching we used before abandoning the VGs (later B737 designs use only 2 IRUs).