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Each pilot on a Boeing 737 NG has a five-position switch labeled MAIN PANEL DUs that selects what appears on the two main display unit panels in front of her. Here's a picture of the left pilot's switch:

Boeing 737 NG MAIN PANEL DUs and LOWER DU selection knobs

Most of this seems pretty obvious to me: NORM gives you the PFD (Pilot Flight Display with attitude, altitude, speed, etc. indicators) on the outboard DU and the navigational display on the inboard DU; and the three INBD settings change the inboard display to show the engine primary display (normally on the centre console upper panel), PFD, or multi-function display (normally on the centre console lower panel).

What does the OUTBD PFD setting do? In NORM mode the PFD is already on the outboard DU, so it sounds as if this wouldn't change anything.

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It blanks the inboard display & forces the PFD to its normal place. It would take a combination of failures for this to be needed, but if the system sensed the wrong display as having failed & was trying to put the PFD on the inboard (actually failed) screen, then this would get it back to the (actually working) outboard screen. A really, really unlikely case.

In 20 years, I don't recall ever needing to use these switches for an actual failure in the aircraft. Maybe once or twice in the simulator.

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  • $\begingroup$ Ah! I had no idea that automatic systems might switch the displays around themselves. $\endgroup$
    – cjs
    May 19 at 4:40
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    $\begingroup$ @cjs They do, although not every failure is something they would necessarily detect. Something like a failed power source might be easier to detect than some types of damage to the display screen, for instance. The system will try to take care of you, but if it's not actually being helpful, these switches allow the pilot to override it. $\endgroup$
    – Ralph J
    May 19 at 8:35

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