747-400 strange FMC message.

[sotiris 1]

Hi,While on CRZ at FL360 I got a message by FMC telling me to descent to FL240 without an obvious reason. First things first, I took a look at engine display and all four were working properly. Next I checked pressurization status and panel with no findings-all normal. Checked systems with EICAS, all normal too. At the time I had crossfeed valves open and closed them just in case they were involved in that strange situation. I levelled-off at FL240, saw that no other abnormality occurred and climbed again to FL360, because I had to correctly manage my fuel on board as I had to fly for some 6 hours more. The message was repeated afterwards every 10-15 mins and I elected not to descent at the third time it happened for no apparent reason (I already had wasted some tonnes of fuel with all that up-and-downs). I only pressed again the speed MCP button whenever it happened (A/T was not affected or disengaged by that). Finally, I felt that it could not go on any further, and forced to divert, having run the 1/3 of planned trip.Any ideas, anybody ?Regards,Sotiris.

[Ivan Kovacevic 0]

It might be that, if you were using Active Sky or something like that, the outside temperature spiked to a value that is considerably higher than the maximum cruise temperature for that flight level at that weight. It has happened to me a few times the TAT remains positive at FL300+ over Europe - it was caused by an ActiveSky error.

[sotiris 1]

Temperature change took place (from SAT -48 to SAT -45). It was not a significant change. ActiveSky was on.

[craig_read 43]

Hi,I don't really understand the message? Could you be more specific as to what you mean by an FMC message. What was actually presented to you on the screen, and also which screen? Did you check your CRZ page to determine the active optimum and maximum altitudes during your flight, if so what was the FMC giving you?If you were safely cruising at FL360, then obviously the max is above this, perhaps it is a programming error within the flight plan entries?Some screenshots of the message and more detail with regard to your plan would help make a diagnosis..ThanksCraig

[downscc 1,327]

If the message was "MAX CRUISE ALT FL240" then I bet that happened after alt-tabbing to another app, opening the msfs map, or some other action that took the focus away from the running simulation. If this is the case, then ignore the message and verify you are still in VNAV mode. You may have to press the VNAV button even tho it is lit to get speed back under FMS control. Just a guess.

[sotiris 1]

Dan,I believe that you would probably win, if you did bet on this. In fact, I used Alt & Tab and went to MSFS map several times before it happened for the first time. Actually, the VNAV button went out, but not right away after I got back from other FS tool-programs. It happened 10-15 mins later. It also appeared even when I used MCP speed button (speed button light out). That is a reasonable answer to the issue. I suspected that it would be a PC software issue and not a real 747-400 one. I am an Aegean Airlines staff and nobody among company mechanics could find a clue on that matter. I don't know whether it is quoted in PMDG Ops Manual. If it is not, I think it should.Craig,All these items you suggest to be searched were well checked before I send that e-mail. Of course the CRZ FL was the FMC proposed step-climb level. When FMC has an message for the crew, it is shown in amber on EICAS screen. Then, the crew check the CDU showing messages like the above quoted. Neither was any programming flight plan error.Many thanks to both of you.Sotiris.

[Guest lgvpilot1]

If the message was "MAX CRUISE ALT FL240" then I bet that happened after alt-tabbing to another app, opening the msfs map, or some other action that took the focus away from the running simulation. If this is the case, then ignore the message and verify you are still in VNAV mode. You may have to press the VNAV button even tho it is lit to get speed back under FMS control. Just a guess.

My money would be with Dan I have had this once or twice after ALT+TAB and personally I have found apart from the FMC message the autothrottle disconnects , but still armed I have found the solution is A.. dont ALT+TAB :( B..select alt hold on the MCP then select VNAV As I stated it works for me though option A seems to be the best :( Mark