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RPRIEVER

Aircraft doesnt descend as planned

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Greetings all, Yesterday I was flying KMCI-KORD. I was cruising at FL330, on the BENKY1 arrival. Per the chart, the BDF VOR says to "expect to cross at FL240", and BENKY says to "expect to cross at 11000". Approx 40 miles from BDF, the controller told me to descent via the arrival. I changed the BDF and BENKY legs from soft altitudes to hard altitudes (the fonts changed from small to big). Also changed the MCP altitude to 11000, and VNAV was engaged. However, I noticed that the T/D point was after BDF, and the plane didnt descend to meet the BDF crossing restriction (I had to use V/S). Unfortunately I didnt take a screen shots. Any ideas what might have happened?

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Sounds like something went wrong with the legs. You can press alt inv. Altitud intervention. That'll start the descent. But I'm guessing the Vnav didn't like getting a change to the altitude restrictions that close to ToD. Dis you get any FMC messages?

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Dix, I don't recall getting any FMC msgs. I was approx 40 miles from BDF, so if you use the 3-for-1 rule that meant I had to start my descent approx 27 miles away. I had a tailwind of around 40kts, so that wasnt a big deal there. What does ALT INV do exactly? When is it used, compared to V/S, LVL CHG, or the FMC "DesNow" function?

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I had a tailwind of around 40kts, so that wasnt a big deal there.
That tailwind will definitely screw up your path unless you entered into the FMS. Cheers,

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Dix, I don't recall getting any FMC msgs. I was approx 40 miles from BDF, so if you use the 3-for-1 rule that meant I had to start my descent approx 27 miles away. I had a tailwind of around 40kts, so that wasnt a big deal there. What does ALT INV do exactly? When is it used, compared to V/S, LVL CHG, or the FMC "DesNow" function?
ALT INV will "ignore" the altitude restriction as set in the FMC. It will obey the MCP altitude setting. Very useful in step climbs and descents.

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It's sort of like a VNAV altitude override.
Correct! Back in "The Queen" if ATC clears me for a higher/lower requested altitude, I adjust MCP and modify my FMC altitude, then press FLCH then VNAV accordingly. Now in the NGX, all I have to do is adjust my MCP then press ALT INTV...

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Correct! Back in "The Queen" if ATC clears me for a higher/lower requested altitude, I adjust MCP and modify my FMC altitude, then press FLCH then VNAV accordingly. Now in the NGX, all I have to do is adjust my MCP then press ALT INTV...
If I'm in ALT INTV mode, do I have to press VNAV to get back to that mode? For example, let's say I'm cruising at FL330. The chart requires me to cross XYZ at 14000. My T/D point is miles ahead. ATC tells me to descent and maintain FL240. I would change the MCP altitude to FL240 and press the ALT INTV button. Do I have to press the VNAV button again, or is VNAV still engaged?

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No. Just alt inv. Then when you want to let VNAV manage the altitudes again, just hit alt inv again

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No. Just alt inv. Then when you want to let VNAV manage the altitudes again, just hit alt inv again
Correct!

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Greetings all, Yesterday I was flying KMCI-KORD. I was cruising at FL330, on the BENKY1 arrival. Per the chart, the BDF VOR says to "expect to cross at FL240", and BENKY says to "expect to cross at 11000". Approx 40 miles from BDF, the controller told me to descent via the arrival. I changed the BDF and BENKY legs from soft altitudes to hard altitudes (the fonts changed from small to big). Also changed the MCP altitude to 11000, and VNAV was engaged. However, I noticed that the T/D point was after BDF, and the plane didnt descend to meet the BDF crossing restriction (I had to use V/S). Unfortunately I didnt take a screen shots. Any ideas what might have happened?
I just had this happen too on the BENKY1. So, I flew another flight to try to figure out what was going on. Turns out it's the 5000A restriction at BABUU that is causing problems. Delete the 5000A restriction and the BDF and BENKY restrictions work. Also noted that any final altitude restriction that is an "at or above" type will screw up earlier restrictions in the route. Seems like a bug in the FMC to me but I'll hit the manuals again to see if there's an expected behavior I'm missing.

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No. Just alt inv. Then when you want to let VNAV manage the altitudes again, just hit alt inv again
Is there any way to see in what mode you currently are? alt inv or vnav? Kind regards, Daniel

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ALT INTV is not really a mode. It tells VNAV one of several things: -if you are on departure and ATC tells you to level off, for example, at FL100, you put 10000 into the MCP ALT window. If you are climbing with VNAV, it will level off at FL100. When you want to continue your ascent, you put in the new altitude restriction into the MCP ALT window, and hit ALT INTV. VNAV wil continue the climb as normal. -If you are in cruise, you can use ALT INTV in a similar way to start you descent early. When you put in the MCP ALT window the altitude that you are cleared to descend to, and hit ALT INTV, VNAV will enter DES mode, and will descend at 1000ft/min until it intersects the original VNAV profile. After that it will descend as normal. If you started you descent after ToD, I'm not completely sure, but i think the FMS might ask you to add some drag to meet profile restrictions. Do note that if you want to descend fast, using ALT INTV with VNAV is not a god idea. LVL CHG is much better for that. -during approach, ALT INTV can be used just like during departure, only you use it to let VNAV descend to a new altitude. Also, as far as i can recall, VNAV, ALT INTV, and LVL CHG (or FL CHG in the 747) all work the same in the 737 as in the 747.

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