IRS transfer switch on the 737 NG

[WhiteKnight]

My question is:
Loss of both Engine driven Generators, you are only on standby power and only the first officer`s primary attitude display is failed. The checklist says:

IRS Transfer switsch ..... Both on Left

What does this help?
As I understand, the left IRS is normally powered by the AC stby bus and the right IRS from the AC transfer bus 2. If AC power is lost, either or both systems automatically switch to backup DC power from the switched hot battery bus.The right IRS loses it DC power if AC power is not restored within 5 minutes.

So the left one should remain on DC power. So what does this checklist step actually do?

anyone any ideas?

[Port Strobe]

So the left one should remain on DC power. So what does this checklist step actually do?

Look up the description of the switch in the Vol 2, it should say;

IRS Transfer Switch
Should either IRS fail, the IRS transfer switch is used to switch all associated
systems to the functioning IRS.

So after the right IRS has failed, by placing the IRS transfer switch to "both on L" then everything that requires IRS data is fed from the operative side as opposed to those that normally receive data from the right IRS being starved of info from the failed IRS.

[WhiteKnight]

thank you for your response. So what instruments would that be? The FO`s Display units are blank with standby power only, and would not restore!!?

[Port Strobe]

I see what you're getting at now, sorry, missed the point initially. Just my guess here but perhaps it's a preparatory step for you getting a source of AC power back but only after more than 5 minutes when the right IRS has dropped out, say an APU start attempt is unsuccessful at high altitude so you try again lower down and AC power becomes available, then the FO's displays are powered once again but attitude information will be lost from it's normal source, using the IRS transfer switch guarantees usable information quickly. As for trying to start the APU at high altitude, it might be justified if you happen to be very close to an airport and you'd bet the mortgage you'll be on the ground within half an hour. Someone else may be able to shed some more intelligent light on the inclusion of the IRS transfer switch step in the NNC though?