The Inertial Reference System (IRS) is a fully independent navigation system onboard many aircraft. On the A320 the IRS rotary switches are sequenced 1, 3, 2. This is because in an aircraft the left system is usually considered primary (captain side) and the right system is secondary (First Officer side). So, although this may seem unintuitive, placing a third (backup) system in the middle makes sense to pilots. Unfortunately, the Position Monitor mage on the MCDU uses the 1, 2, 3 sequence (which probably looked perfectly normal for the engineers developing the MCDU software)
Switching off an IRS during an abnormal situation is non-reversible and thus a critical (and therefore ‘monitored’) action. But if the middle position (i.e. Nr 2) seems to be inaccurate on the display, it is quite tempting (logical) to switch off the middle IRS (i.e. Nr 3) possibly aggravating the situation!
