B757;Stby bus fail;Ils tuning

[gandalfur]

During stby bus failure on the B757 (AC bus failed among others). All ILSīs become inop. (I haven't experienced or seen this personally). Why is this?
The C ILS is on the stb AC bus but but that does not explain the total failure. Is the ILS tuner maybe directly linked to the C ILS and not the others?

Regards,
G

[Piper19]

I've looked and looked into the maintenance manual (not a pilot - sorry); I reallly can't figure out why this would happen. All three systems (capt/FO/stby) are completely independent. Even the single tuning panel receives 3 seperate power sources internally. I also don't see how the 757 could have been certified autoland cat3B if you loose all ILS's when 1 of the busses fails.
Where did you see such a situation, is it described as such in the FCOM?

[NSEU]

Not familiar with the 757, but it may be like the 767. I seem to recall that the ILS panel displays fail when the C(?) ILS is depowered, but if you turn the knobs on the ILS control panel, it still tunes the other two receivers (you can check the frequency you have selected by looking at your EADI's).

Rgds
NSEU

[gandalfur]

Thanks for the replys.

This situation was seen in the simulator, I don't know the details excactly. It seems though that the ILS panel display fails, as you said NSEU, but really doesn'd explain why as piper19 pointed out.

[NSEU]

Another possibility could be the power source of LCD background lighting (without this, the displays are close to invisible). The control functions may have independent power sources, but not the lighting.

Anyway, if ever you are faced with this scenario again, rotate the knobs to see if the EHSI displayed frequencies change.

Cheers.
NSEU

[stilton]

Thats correct on the 757 NSEU.

[NSEU]

Thanks, Stilton )

Cheers