The aircraft has 3 ILS receivers, but only ILS L and R in the CDUs. How does the ILS C is put into action?
$\begingroup$ What is CDUS?.. $\endgroup$– DeltaLima ♦May 23, 2019 at 20:24
$\begingroup$ @DeltaLima Plural of CDU, the user interface to the FMC. $\endgroup$– user71659May 23, 2019 at 20:28
$\begingroup$ Isn’t it only used to feed the center autopilot in automatic approaches with all three autopilots engaged? $\endgroup$– Cpt ReynoldsMay 23, 2019 at 20:34
$\begingroup$ I know there is a way to at least show the readings off of it (needles). Not sure about tuning it $\endgroup$– PIXPMay 23, 2019 at 21:01
$\begingroup$ When @Terry flys the -400 he can tune the ILS's to different frequencies, and fly them all, at the same time... $\endgroup$– DaveMay 23, 2019 at 21:58
It depends on which page you are viewing. There is the NAV RADIO page, and the ALTN NAV RADIO page.
In standard operation (i.e., nothing has failed, more on that in a bit), there are no separate L and R ILS entry lines on the NAV RADIO page (unlike for VOR and ADF).
A single entry, on either CDU, tunes all 3 receivers to that frequency. If FMC 1 and 2 failed, the alternate nav radio page (ALTN NAV RADIO) can be used. And this is where the third CDU comes in.
If both FMCs fail, ILS receivers can be tuned on the CDU ALTN NAV RADIO page. The left ILS receiver is tuned with the left CDU, the center receiver is tuned with the center CDU, and the right receiver is tuned with the right CDU.
Source: Boeing 747-400 FCOM 11.20.6 (emphasis mine)
Here's the location of the center CDU:
Note: Normal operation is based on auto tuning, say unlike a 737NG where the frequency is manually selected left and right. With the approach selected, and within range, the plane auto tunes the frequency. The crew can then confirm the identifier on their displays.