sbclark

NGX stops climbing prior to target altitude

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Lately, I have been experiencing an issue where I set a target altitude into the MCP of 9,000 feet, for example, and somewhere around 5,000 feet it stops climbing. Pressing VNAV numerous times to force the A/C to start climbing again fails. However, pressing LVL Chg  and setting V/S to whatever FPS setting, then the A/C will still climbing again.

Any idea what is causing this?

Bill Clark

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The problem is not one of climb performance, I think.  You didn't mention what final cruise altitude was, I can't assume you have set up the route with what you've shared.  Most importantly, you didn't share what the FMA was indicating.

Pressing VNAV multiple times is an indication that you are not familiar with this aircraft. Once VNAV is selected, it will follow the vertical path within the constraints of the MCP ALT. Basically, there are three pitch modes: VNAV, FLCH and VS and once one is selected the only way to deselect that mode is to either select another or turn FD off.

So...,, somewhere around 5000 when it stops climbing, what is the FMA indicating?  That is the thrust/roll/pitch annunciator above the attitude indication in the PFD.

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Thanks Dan,

I will not be able to fly again today, so I can't tell you what the FMA is until Sunday.

I've been flying the NGX for 3 years and I have only recently experienced this issue. I appears after leveling off, turning off and then turning on VNAV, and waiting for the climb to continue doesn't happen. The only thing that works is FLCH and VS.

I will have to fly again to collect the FMA information.

Bill Clark

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Hi Bill,

Looks like you have an altitude restriction in the FMC.

If you depart using VNAV any departure speed or altitude restriction in the FMC will be adhered to even if you set a higher altitude in the MCP window.

After setting a higher altitude in the MCP ALT window you need to push ALT INTV on the MCP Panel to delete the next altitude restriction (depending on your SID you might have to push ALT INTV a few times as some SID's have multiple altitude restrictions). If you don't do this you have to wait until you have passed the restriction otherwise the aircraft will not climb in VNAV.

Reverting to LVL CHG or V/S which is a basic climb mode ignores any restriction in the FMC so the aircraft will climb!

 

Good Luck!

 

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I went back and looked at the chart, and there is a 5K altitude restriction.............it was buried in the text details.........I missed it.

So problem solved.

Thanks for your help.

Bill Clark

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7 hours ago, sbclark said:

I went back and looked at the chart, and there is a 5K altitude restriction.............it was buried in the text details.........I missed it.

So problem solved.

Thanks for your help.

Bill Clark

Check out the LEGS page and you should see it.

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Yeah, it will fly according to any altitude and speed restrictions, descending it will do what is necessary to keep within the relevant profile.  That is why most NGX pilots prefer using V/S when descending, in order to maintain a constant descent.  Yesterday I used VNAV for my descent, and the V/S fluctuated as the A/P was trying to staying within the descent profile.  The FMC would indicate whether I was above or below the descent profile, and the A/P would make the necessary adjustments to get back on or stay within the profile.

I usually set my MCP altitude 500-1000ft above (or below, as required) the relevant assigned altitude (or altitude restriction) so that once the restriction is passed, the aircraft will continue descending/climbing, in an attempt to keep the descent/climb as continuous as possible.  However, I still think V/S is your best option in this regard anyway.

The above is just my experience and understanding, there will be more experienced NGX pilots that could probably give you better advice.

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Basically, in the real world, as part of departure brief, one of us reads the chart and the other confirms the LEGS page has all the legs and restrictions in it.

That way there are no surprises. where you have restrictions that are NOT BELOW or NOT ABOVE in them, it's far safer to select this altitude in the MCP and not any other as VNAV can drop out of be inadvertently deselected in the heat of multiple ATC instructions, etc. It's a major GOTCHA.

 

So preparation is the key. read your SID, check your LEGS pages and set the MCP to protect yourself from trouble.once it levels at an intermediate point, you would expect to still have VNAV PATH showing. if the restriction is passed without you resetting MCP, the change to VNAV ALT would alert you.

Happy flying! 

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