News:

Precision Simulator update 10.180 (14 October 2024) is now available.
Navburo update 13 (23 November 2022) is now available.
NG FMC and More is released.

Main Menu

CDU standby navigation

Started by Hardy Heinlin, Sun, 11 Feb 2018 14:23

Hardy Heinlin

Good evening,

as we all know, each CDU is hardwired with other specific devices; when the FMCs fail, the data from other devices is no longer shared within the network. So in a standby navigation scenario:

CDU L gets data from IRU L, ILS L, VOR L, ADF L.
CDU R gets data from IRU R, ILS R, VOR R, ADF R.
CDU C gets data from IRU C, ILS C.

Non-conditional route waypoints are copied from FMCs to CDUs whenever a modification is executed, and only if the respective FMC is operative, of course. A failed FMC cannot transfer any data.

As far as I recall, the links between FMCs and CDUs for standby memory actions are hardwired as well:

CDU L gets data from FMC L.
CDU R gets data from FMC R.
CDU C gets data from FMC L or R, whichever is the master FMC.

This means, when FMC R fails, CDU R gets no data from FMC R anymore. It gets no data from FMC L either, because CDU R is hardwired with FMC R only. Data synchronization across FMCs is only possible as long as both FMCs are operative. As there is no wire directly from FMC L to CDU R, CDU R cannot get data directly from FMC L. CDU R can only get FMC L data via FMC R. So if FMC R fails: CDU R gets no data.

Correct or wrong?


As we all know, cheers!

|-|ardy

Hardy Heinlin

#1
Test demo:

Load the situation file Basic 004 - Cleared for takeoff.situ

Adjust your layout so that you see CDU L and ND R (you can return to your old layout by pressing the "0" key on the numpad.)

Set the range of ND R to 80.

Set F/O's NAV SOURCE selector to CDU R (check that "CDU R" is displayed in green at the bottom of ND R).

On CDU L, put NOXAD over OLITO and execute.

Notice that the CDU-R-route on ND R draws a direct-to to NOXAD. (FMC L has sent this route modification to FMC R, and FMC R has sent this to CDU R and ND R.)

Nice.

Now go to Instructor > Situation > Malfunctions > Nav, and click "Activate" in the line "FMC failure - R".

Wait until the message RESYNCING OTHER FMC disappears (it tries to resync, but it will fail).

Clear all scratchpad messages.

On CDU L, put SINDA over NOXAD and execute.

Notice that the CDU-R-route on ND R remains unchanged. This is because CDU R gets no data from FMC R anymore. And it cannot get data directly from FMC L either as there is no direct wire from FMC L to CDU R.

mark744

Hardy

I think that if one FMC is operable, it would synchronise waypoints to CDU L R and C
so that they will be available for Standby Navigation if the other FMC fails too.

As we know, if both FMCs are failed, Standby Navigation is by using just CDUs &  requires lat/long waypoints to be entered into all 3 CDUs along with three ILS frequencies. but only if both fail

But I am only making assumptions about the synchronising.

BTW
The QRH for ONE failed FMC, makes no reference to route mods being different in each CDU, surely it would.

( the QRH for two failed FMCs goes into lots of detail  )


Hardy Heinlin

I just checked an engineering diagram and it looks like either FMC is directly connected to all three CDUs, while on the same diagram each IRU is explicitly connected to just one CDU. So you are probably right, Mark.

If I only could remember where I got that other information from. I'm still not 100% sure as these diagrams are simplified. But I tend to believe now that all three CDUs, indeed, can get data from one FMC directly, probably from the master FMC.


Regards,

|-|ardy

John H Watson

From my training notes:

"FMC Inputs - 1"

"The Left FMC receives data from:
Left and Right CDU." (Book 14 p194...) Centre CDU not mentioned (I assume no C CDU data is sent to the FMC)


"FMC Inputs -2"
"The Left FMC receives data from FMC Master Relay
Captain's and F/O's instrument source switches"
"FMC Master Relay" (A list of Master switched data is given on another page).

Regarding outputs from the FMC:

"In the normal configuration, the left and center CDU receive data from the L-FMC." The right CDU normally receives data from the Right FMC. Source selection is determined by the Captain's and F/O's ISSS position." (ref book 14 page 216).

I think the FMC Master Switch only affects those systems listed on page 208: 

FCC's, ILS receivers, ADF receivers, DME interrogators, VOR receivers, AFCS mode control panel and Engine EECs.



Jeroen Hoppenbrouwers

I had the pleasure of sticking my finger in FMC data busses a while ago, and although block diagrams may suggest "connections", the actual wiring typically is split in different busses for different purposes.

I am pretty sure that the connection from, say, FMC-Left to the three MCDUs only carries DISPLAY DATA.
There most likely is a DIFFERENT bus from FMC-Left to MCDU-Left that carries the flight plan data.

We need wiring diagrams, not block diagrams, to be sure.


Hoppie

John H Watson

Hoppie, Wiring Diagrams generally only show wiring... not the data going on them. Wiring Schematics on the other hand, do sometimes show data.

Hardy Heinlin

The normal stuff is clear to me. I'm just wondering whether those typical descriptions also refer to the data transfer into the CDU route memory. Maybe they just refer to CDU screen data and keypress signals.

We have:

• CDU keypress signals going to the FMCs --- NAV source selection dependent
• FMC text going to the CDU screens --------- NAV source selection dependent
• FMC route updates going to CDU memory -- Hardwired or NAV source selection dependent?


In case of a single FMC failure, with FMC L operative, I'm seeing two possibilities:

1. All CDU route memories get data from the operative FMC, independent of the NAV source selections.

Or:

2. CDU C memory gets data from the operative FMC; CDU L memory gets it if captain's NAV source is set to FMC L or CDU L; CDU R memory gets it if F/O's NAV source is set to FMC L or CDU R. (If the source selection is relevant, at least the selection of CDU C or of the inoperative FMC should disconnect the link.)


In other words: The NAV source selections are irrelevant and the memories of all CDUs get data in any case, -- or the NAV source selections are relevant and CDU L and R memories only get data when the related sources are selected.


|-|ardy

Jeroen Hoppenbrouwers

Quote from: John H Watson on Mon, 12 Feb 2018 01:23
Hoppie, Wiring Diagrams generally only show wiring... not the data going on them. Wiring Schematics on the other hand, do sometimes show data.
Correct -- I intended to say that if the wiring diagrams show multiple parallel 429 lines, i.e. more than one bus going from the same FMC to the same MCDU, it could reveal that some of these busses go to one, two, or three MCDUs.

What data goes over that then is phase two of the research project...


Hoppie

Hardy Heinlin

#9
Phase 2 has started:

From observations on the real deck (some years ago) we know this: When FMC R fails and F/O's NAV source is set to CDU R, CDU R is in standby navigation mode and its IRS LEGS page shows its CDU route memory (with all legs it previously got from the FMCs, except for conditional waypoints).

Now if these IRS LEGS are still being updated by FMC L, these updates should be visible on the screen of CDU R. I'm having difficulties to imagine that you can see any updates from the FMC on this screen when that CDU is in standby navigation mode.


Regards,

|-|ardy


P.S.: For a test I just changed my code so that the CDU R standby memory is still being updated by FMC L. Of course, standby route modifications in CDU R are not sent to FMC L. It only goes vice versa. So when I make a direct-to in this CDU route and execute, two seconds later this CDU route will be replaced by the FMC route. So the CDU route is shown, but I have no reasonable control over it. This makes no sense. I could allow this control only when the F/O's NAV source is set to CDU R. This way you could spend an hour making a fine new standby route in CDU R and be happy, and then lose all your work when you turn your NAV source selector away from CDU R for a second.

mark744

#10
In answer to you P.S.

Having F/Os Nav Source to CDU R, with a working FMC makes no sense as the aircraft is not forced into standby navigation.

so loosing that route that is input while in CDU R isn't unreasonable when re  selecting FMC L, which  is a superior mode, with full Autopilot function and normal operation under the control of FMC L, with data being able to be input from either CDU,all as normal (except two FMCs cant confer calculations)


Hardy Heinlin

Quote from: mark744 on Mon, 12 Feb 2018 14:45
there is no reason to set F/O's NAV source to CDU R if there is still an operating FMC

I agree. But if you do it anyway (accidentally or whatever), the system design should provide a clean solution for such a case, e.g. it could inhibit the access to the IRS LEGS page as long as the FMC is feeding the CDU's standby memory.

It does this for CDU C. You can only access the IRS LEGS page on CDU C in case of a dual FMC failure. For CDU L and R, however, the access is already enabled in case of a single FMC failure.

(Please note my "P.S." edit in my previous comment.)

Hardy Heinlin

Quote from: mark744 on Mon, 12 Feb 2018 14:45
Come to think of it, I believe the Source Select Switch just directs the source of the date that goes to be displayed on the ND (and PFD if it's a FMC) and doesn't control where the CDU gets it's information.

The NAV source selection also determines whether this CDU is controlling FMC L or R, and whether this CDU screen is getting text from FMC L or R. It's not just for the ND and PFD.

mark744

I see.
However

QUOTE
It does this for CDU C. You can only access the IRS LEGS page on CDU C in case of a dual FMC failure. For CDU L and R, however, the access is already enabled in case of a single FMC failure.
QUOTE

For one FMC failure, the correct Source Select Switch selection is to the operable FMC not to CDU

United744

As I understand it, a *single* FMC failure does not cause reversion to standby navigation. The CDU on the failed side connects to the remaining FMC, and can update/receive data from it as normal.

Whether NAV source selection can result in CDU standby navigation, I don't know (I would guess if FMC R fails, and the ND R source is selected to FMC R, that it will cause the CDU to go to standby navigation mode because its on-side FMC has failed).

If you had a failure of FMC R, and selected the ND L to FMC R, I would expect CDU L to go to standby nav, again because the FMC R has failed.

Hardy Heinlin

Quote from: mark744 on Mon, 12 Feb 2018 15:38
For one FMC failure, the correct Source Select Switch selection is to the operable FMC not to CDU

Yes. But why can the IRS LEGS page be accessed when the pilot incorrectly selects CDU R? Why does the system not protect itself against this incorrect selection? With the example of CDU C I just want to show that the system is able to inhibit the access if it just wants to.

United744

Quote from: Hardy Heinlin on Mon, 12 Feb 2018 15:52
Quote from: mark744 on Mon, 12 Feb 2018 15:38
For one FMC failure, the correct Source Select Switch selection is to the operable FMC not to CDU

Yes. But why can the IRS LEGS page be accessed when the pilot incorrectly selects CDU R? Why does the system not protect itself against this incorrect selection? With the example of CDU C I just want to show that the system is able to inhibit the access if it just wants to.

I would suggest because the selected FMC has failed, it reverts to standby nav (thus, selected side IRS nav data).

If it didn't revert to standby nav, that would be even stranger, as you would have selected the failed FMC, yet still be using the operable FMC.

mark744

also, there is little risk in being allowed to access the IRS Legs page it cant be followed by LNAVs or autopilot.

so its only a map for one pilot side, then if the NS Select is set to the operable FMC, the CDU gets updated with the FMC waypoints that were or can be used by the LNAV/ autopilot as normal

Hardy Heinlin

What I want to avoid is this: Say, the pilot incorrectly selects CDU R on the NAV and modifies the CDU R route for 10 minutes, and then notices the incorrect NAV selection and selects FMC R. -- 10 minutes of work in vain. If the access to the IRS LEGS had been inhibited from the start, the pilot wouldn't have wasted 10 minutes in useless work. The deleted route mods are useless anway. But the wasted time could have been avoided by not inviting the pilot to work on the IRS LEGS page.

In short: If I let CDU R get data from FMC L, I'd like to let the data flow to CDU R directly regardless of the NAV source selection. Any mods on the IRS LEGS will then be overwritten by FMC L after a few seconds already; so the pilot can instantly see that the work is useless.

mark744

I completely see your logic, but I don't think the aircraft should override the selection the pilot has made.
This is a Boeing, after all, not an Airbus  ;)

I dont think Boeing philosophy would prevent the pilot from doing something incorrect with no danger to the aircraft, just wasted time. Crew should follow the QRH and Boeing would expect this to be done.
(not Boeing philosophy for this age of aircraft anyway)

They dont prevent Alternate flaps if there is no fault, or alternate gear without a fault etc etc