B777 Alternate Navigation Radio Page

[Haroon]

Hi

In FCOM on 11.50.6, it says:

If both FMCs fail, access is by the NAV RAD key on the left and right CDUs only.

• the left CDU tunes the left radios and center ILS.

On 11.50.7, it says:

Note: ILS course and frequency must be entered on the left, center, and right ALTN NAV RADIO page.

What does it mean?

Regards

[777boeings]

It means if you only enter the ILS freq on the left CDU, the ILS indications are only available on the left PFD. Same goes for the right.

[NSEU]

Seems contradictory to me.

"The Left CDU tunes.... the C ILS"

"ILS course and frequency must be entered on the Left, Center and Right Altn Nav Rad pages"

Are there only two CDUs on the 777 and are there Left & Centre ALTN NAV pages on the Left CDU?

Or is it the same as a 747-400? i.e. With both FMCs inop, the MMRs and VOR Receivers can be only be tuned by the onside/respective CDUs during Alternate Navigation. Left CDU for Left VOR/Left ILS, etc.

[777boeings]

In alternate Nav, the CDUs are no longer connected to the FMC. So each CDU is independent same as 744.
Three CDUs available.

[Haroon]

Thanks for the feedback

It means if you only enter the ILS freq on the left CDU, the ILS indications are only available on the left PFD. Same goes for the right.

thats understood, but:

If both FMCs fail, access is by the NAV RAD key on the left and right CDUs only.

This statement indicates that NAV RAD page cannot be accessed by the third (center) CDU. However the note on the next page says:

Note: ILS course and frequency must be entered on the left, center, and right ALTN NAV RADIO page.

So if only left and right CDU NAV RAD key is active then how do we access the center NAV RAD Page?

And why do we need to set ILS freq/course on the Center NAV RAD page if the left CDU tunes the left radios and center ILS?

Regards

[NSEU]

Looking at the 777 Maintenance Manual, it gets even more complicated:

There may be two Alternate functions.

In the ILS Section of the manual:

"Manual Tuning
You use the left or right CDUs to manually tune the receivers. The center CDU tunes the receivers if the left or Right CDUs fail."

"Alternate Tuning
If the AIMS FMCF [auto] tuning functions fail... You use the right CDU to tune the right receiver. You use the left CDU to tune the left and center receivers. There is no alternate tune function from the center CDU."

In the VOR section of the manual, the Center CDU is not mentioned as a backup if any of the CDUs fail. During Alternate Tuning, the Left CDU is used to tune the Left VOR receiver, the Right CDU is used to tune the Right VOR receiver.

Unfortunately, this still doesn't clarify the statements in your FCOM.

[Haroon]

Thanks for your time NSEU, but as you said, the issue is still there.

I wonder if Boeing can be contacted through some informal online setup instead of the formal company protocols.

[nitpicker330]

Simply try it in your next Sim.

Fail both FMC's and check it out. :ok:

[Haroon]

that will be the ultimate step :cool: