B737NG - IRS FAULT
 

Hello everyone,

With one IRS FAULT, the checklist in the INFLIGHT section ask you if restoring partial capability by selecting attitude mode on the failed IRS is desired or not.

Considering that this action is not reversible (if you select ATT on the other IRS, you basically lost also the other IRS) would it not be better to skip this procedure (i.e. it is NOT desired to select ATT mode on the remaining IRS) and follow the remaining part of the checklist where you just need to select the IRS TRANSFER SWITCH both oh left or right?

In the event that you loose both IRS, you should still have the independent standby instruments available, right?

During a LOFT exercise in the sim during the climb on a short sector from london to amsterdam I elected to transfer the IRS both on left (the good one!). We continued the flight and landed safely at destination without problem. However I was told after during debriefing that restoring partial capability on the other failed IRS would have been the right thing to do. What I wanted to avoid was to get distracted with this task, fly the aircraft and brief for a procedural approach at destination. Maybe on a long sector during cruise I would have done differently but not in this specific case. Just wondering what you think about it?

Maybe I misunderstand the situation you are describing, but if you are handling an IRS FAULT light you might have a failure of that IRS in NAV or ATT. If only the NAV mode has failed you lose groundspeed and present position data from that IRS, in that case switching that IRS to ATT turns off the FAULT light and leaves the other IRS intact and supplying the FMC with position information. You lose nothing by following the procedure, just switching to the other leaves you with only one normal attitude source when the other one might still be working.

Your action during the LOFT is understandable, however in a simulator you are supposed to follow the QRH.