747 FMC Failure
 

With the failure of the operating FMC LNAV and VNAV both drop out (struck through FMAs).
If the autopilot is in at the time of failure it remains engaged but in what modes? Is just an ATT mode? With LNAV and VNAV gone there will be no modes annunciated on the MCP. Obviously, selecting HDG and ALT HOLD puts one back "in charge" but I'm just curious to know what the autopilot is working on prior to reselecting modes.

CWS R/P I would imagine.

It will drop to CWS as previous poster said.

I'd be surprised to see CWS in a B744... ;)

Hmm - agreed, CWS would indeed be a big shock!

Autopilot remains engaged in attitude stabilising mode from my understanding.

Sorry guys I'm not a B747 driver. Im still on the B737.

LNAV and VNav
 

Autopilot remains engaged. Autothrottle disengages. FMA's will be _______ LNAV with an amber bar across it, and VNAV also with an amber bar across with a careted >autopilot EICAS message meaning the autopilot is in a degraded mode. Autopilot goes into HDG hold and VS but the FMA's do not change. But it's been 2 years since I've last flown the B744

According to the FCOM, AFDS failures affecting only active modes section:-

• the autopilot remains engaged in an attitude stabilizing mode
• an amber line is drawn through the mode annunciation
• the EICAS caution message AUTOPILOT displays

So with dual FMC loss I thought autopilot reverts to ATT mode (albeit not annunciated on FMA). I know on initial engagement autopilot/flight director logic with no prior selected modes is into HDG hold with < 5 deg bank or ATT if >30 deg but always thought the above solely applied when both FMCs fail? Good question posed.

The sim instructor likes to fail both FMC's just before a turn in the airway, so fly the aircraft using heading select and try not to get too distracted as PF.

Autothrottles are gone too, so beware if it happens near a level off during descent.

IRS and GPS navigation is still there, it is your database that is lost. So if already in oceanic airspace, why not continue instead of turning around(entering oceanic may be a different matter). You already have your flightplan in the FMC which is still there but creating a new flightplan is time consuming doing it the old fashioned way with lat/long. But maybe a route copy is good for destination in case you go around and want to have your approach waypoints still available.

I have to admit, when practicing it in the sim, I remember that some waypoints seemed to not be connected anymore. Anyone know more about this?

Only Nav/Rad, Prog, and Legs pages are available but no fuel prediction.

Anyways, aside from following the QRH for stuff like manual pressurization, go to the just go to the performance inflight and get you planned Vref. Then on a piece of paper, write up a landing data card like in the old days with min maneuver speeds being Vref 30+80, 60, 40, 20, and 10 for flaps up, 1, 5, 10, and 20 and then of course mark down Vref and it is all easily referenced plus go-around N1.

Vectors is best but in the sim you may have to manually tune VORs for arrival and manually tune and identify the ILS. And of course no VNAV performance.

Probably some other stuff as well.

Am I wrong in saying, it is time for you to work for a living? You still have GPS data which is a hell of a lot better than the old AC Delco Carousel in the 741 that was usually off by 30 NM after an 8 hour flight, Uhh VOR is still active, ILS etc. Ok your RNP is crap, you just burned some extra fuel and you may be restricted from allowing your aircraft to land it's self. Am I out of line here?

No not out of line just not what the guy was asking. He wanted a technical answer to the exact state the autopilot (if engaged) falls into during dual FMC failure, prior of course to reselecting HDG Sel what have you.

Thanks for the replies folks, question answered. Our FCOM is a little "thin" on detail sometimes.

Grounded27, what Cliff said. A technical query to further my understanding of the aircraft systems, that's all. Don't worry, I can cope without autopilots and autoland; the first 20 years of flying career were spent landing large turboprops on patches of dirt in the dark.

JammedStab...

Alternate (CDU) navigation can only understand basic point-to-point navigation. If the original FMC route contained SIDs/STARs, the CDU's cannot handle these, so these legs are removed and route discontinuities occur.

If the FMCs are not permanently lost, the full route can sometimes be restored (but will require re-executing).

And remember: you can still do a Cat3B auto-land with a dual FMC failure.
But you do have to use manual thrust, I think?

Depends on your OpSpecs. We can't do CatIII without autothrottles.

Not even IIIA?

We only need autothrottle for IIIB.