FMGC doesn't hold speed

[Guest jan_fandango]

Hello,I have a problem with my 737NG. The FMGC system doesn't manages the speed during the flight. All settings are made and the FMGC says, that it would hold a speed from 320kt during the CRZ-flight. But, when I'm in CRZ, it doesn't hold this speed. VNAV und LNAV are activated. When I look at my PFD, it shows the speed 4857kts in magenta .... but on the FMGC are 320kts set. What should I do?Thanks for help.P.S: I have installed the newest version of 737NG.

[downscc 1,327]

This forum asks that you sign your posts...A 320KIAS cruise seems high, what cruise altitude did you set? Was it higher than the max cruise? Speed indication of 4857kts reads like a typo... if your problem is at cruise only then I presume it maintained target climb speed.. what was that?

[Guest jan_fandango]

A cruise altitude of FL260 was set and the FMS was set in ECON-Mode. The problem isn't only in CRZ, it sometimes is the same during climb. It then doesn't hold the speed during climb, too. All in ECON-Mode, so that the FMS chooses the speed. When I set the speeds manual in the FMS, it is the same problem :-(

[Mats_J 2]

Jan,What mode is your auto thottle in? Check top left of the primary flight dispay.Cheers,

[Guest jan_fandango]

Hi, it is in N1-Mode. On the PFD, there are displayed N1 | LNAV | VNAV SPDJan-Christoph Misch

[downscc 1,327]

What you're looking for is FMC SPD instead of N1 in the left-most position. You don't have speed control in N1 mode. At cruise, with N1 displayed, a way to get back to FMC SPD is to select SPEED (display indicates MCP SPD) then select VNAV. It should go to FMC SPD unless something else is missing. I notice you said you were in VNAV SPD, so I think I am missing something.I assume autothrottle is engaged, TOGO takeoff went normal, etc...

[Guest jan_fandango]

Hmmm, when I select SPEED an then select VNAV, FMC SPD | LNAV | VNAV PTH is displayed for a few seconds only on the PFD. The target speed is, like above, 4857kts (displayed in magenta) but in the FMS, there are 320kts. Then, after a few seconds, there is ARM | LNAV | VNAV PTH. The system doesn't hold the speed at all the time :-(Jan-Christoph Misch

[downscc 1,327]

This one is beyond my expertise. A target of 4857kts is obviously wrong. The ARM indication is normal when indicated airspeed is above target airspeed, and normally changes to FMC SPD when the aircraft deaccelerates into the target. I'm also uncomfortable with the FMC calculated cruise target of 320kts... a typical target I observe in the 737 at FL260 is closer to 0.71M/277kts.Hopefully someone with more experience will join into this thread. My gut says that I'm still missing the key to this puzzle, let's look at the completed preflight information: What is the economic factor, climb target speed above transition and speed target (kts & Mach) after preflight is complete but before takeoff? Is autothrottle switch armed normally, acceleration after flap retraction normal and maintains 250kts before transition? Do you get any FMC messages after takeoff?

[Guest jan_fandango]

Great, I've found the problem!I must set the CRZ-altitude only on the CRZ-page of the FMC and not, like i did before, on some other pages, too. Now, all is working as

[Guest dt1951]

You can also set the cruise altitude on the performance page as you are setting up your flight in the FMC.

[Guest jan_fandango]

Yes, but bevor I must set my altitude 2 times before it was on all pages. Now, I only must set it 1 time and it is on all pages AND all systems are working as they should :-)Jan-Christoph Misch