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Carolus39

B 747-400 Q.O.S can't maintain cruize alt

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Hello,I have this situation with the 744.It can not maintain the crz alt.When I enter FL 320 as crz alt, the aircraft will take me to FL 390.Even when I set the MCP ALT to 32000 it still goes higher.The situation is than, that it keeps climbing and I end with a flaps of 30 at FL 250, so that the aircraft stalls for it can not increase speed.In the Eicas there is still CLB even when I am descending or cruizing.It is always fl 39000 I noticed, is there some special reason for this???This is not nice flying, for I get into troubles with ATC.The aircraft also strays away from the track sometimes, but perhaps that is because I have the setting to have control over the steering of the aircraft even when the AP is on. Thanks for reading this.Carlos

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Since you are exceeding the MCP ALT it is almost certain that VNAV is not engaged. If you post a screenshot of the panel when it is misbehaving I bet we can spot the problem.

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Hello DanThis time I have enter VNAV at 500ft.It works fine this time.But look at the Leg page and the VNAV page.The CRZ alt setting is FL 320 and the aircraft wants to go to FL 390.I have add attachments to look at.ThanksCarlos

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Hello DanI have also start climbing to FL 390.I espected the CRZ on the eicas to change into CLB, but it remain CRZ.According to FMC the optimum is FL 376.So why FL 390 ?????On the EHSI it should also say VNAV ALT in stead of VNAV PATH, when at FL 390.VNAV PATH means it still has some more to go.Anyhow that is how I understood it.ThanksCarloshttp://forums.avsim.net/user_files/174443.jpghttp://forums.avsim.net/user_files/174444.txthttp://forums.avsim.net/user_files/174445.jpg

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"Since you are exceeding the MCP ALT it is almost certain that VNAV is not engaged. "The plane should not fly through the MCP altitude irrespective of what mode is engaged (V/S, VNAV, FLCH) (with any Boeing).(Edit) unless G/S is captured.Cheers.Q>

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"I have also start climbing to FL 390."Did you choose to climb to FL390, by entering this as a cruise altitude in the FMC?"I espected the CRZ on the eicas to change into CLB, but it remain CRZ."There is a mode called "cruise climb". Perhaps the airplane logic sees this as a submode of cruise and remains in CRZ."According to FMC the optimum is FL 376.So why FL 390 ?????"In VNAV, the airplane altitude is limited to MAX, not OPT. If you set the MCP and FMC Cruise altitude to FL390 and the VNAV Cruise page MAX altitude is at or below this, the aircraft will climb to that altitude. It is the pilot's choice to fly to MAX. Enter FL376 in the FMC if you wish to fly at the optimum altitude. The displayed MAX value should always be higher than the displayed OPT altitude."On the EHSI it should also say VNAV ALT in stead of VNAV PATH, when at FL 390.VNAV PATH means it still has some more to go.Anyhow that is how I understood it."No, actually, VNAV ALT means the aircraft has some more to go ;) VNAV PTH means that the MCP altitude matches the selected FMC cruise altitude.Hope this helps.Cheers.Q>(EDIT) Sorry, I have no explanation why the FMC Legs pages show FL390... unless you accidentally pushed the Altitude knob (this will change the FMC Cruise altitude to whatever is in your MCP altitude window.. and the airplane will start climbing to that altitude).

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Carlos,How long is your route? If it's decently long the plane is probably calculating automatic step climbs for fuel burn efficiency and thus the FL390 you're seeing... You can stop it from doing this by changing the STEP SIZE ICAO option on the PERF INIT page to 0...

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"How long is your route? If it's decently long the plane is probably calculating automatic step climbs for fuel burn efficiency and thus the FL390 you're seeing... You can stop it from doing this by changing the STEP SIZE ICAO option on the PERF INIT page to 0..."Good point, Ryan, but would an initial cruise altitude of FL320 generate a step climb to FL390? (with "ICAO" step size). Wouldn't the FMC generate step climbs to FL360 and then to FL400?Either way, Carlos is seeing some strange stuff looking at his initial entries on the ground.Cheers.Q>

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