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Schaschlik

VNAV Descend Problem

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Hello everyone,

 

I recently bought the 737 and did Tutorial1 and some practice flights beside it and everything works like a charm except for one problem I can't get my head around or find an answer in either the manuals or the Internet.

 

Here is the situation: I'm flying into OMDB and I've reached T/D so I dial in my glideslope capture altitude as I don't fly with ATC. There is an altitude restriction of 13000B ahead and the 737 sinks just and the right rate to hit that Waypoint at excact 13000. After that however, it continues to sink way too steep and I reach the capture altitude way ahead of the planned waypoint.

In the CDU Legs page it shows the correct altitude profile which it should fly but somehow it is disregarded. I'm descending in VNAV Path but it looks more like I would descend using manual V/S.

Other approaches worked fine for me, the descent path always led me right onto the right altitude at the right point.

 

Any idea?

Thanks for your time :)

 

Yannic

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What is your target air speed at the intercept waypoint? Make sure you follow your VNAV profile with the proper flap settings. The aircraft must be properly configured for low speed flight as indicated on the PFD. The flap setting required at or below each indicated speed must be set or the FMC will ignore the input.

 

You should also enter a zero altitude on the MCP. The aircraft will respect at, above or below altitudes entered and accepted by the FMC and indicated on the CDU Legs page. If properly configured, the NG will capture the glide path at the best altitude given speed and CDU constraints. Also, you might check your NGX setup screens and see if G/S capture is allowed before the LOC is active.

 

Hope this helps,

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Hello everyone,

 

I recently bought the 737 and did Tutorial1 and some practice flights beside it and everything works like a charm except for one problem I can't get my head around or find an answer in either the manuals or the Internet.

 

Here is the situation: I'm flying into OMDB and I've reached T/D so I dial in my glideslope capture altitude as I don't fly with ATC. There is an altitude restriction of 13000B ahead and the 737 sinks just and the right rate to hit that Waypoint at excact 13000. After that however, it continues to sink way too steep and I reach the capture altitude way ahead of the planned waypoint.

In the CDU Legs page it shows the correct altitude profile which it should fly but somehow it is disregarded. I'm descending in VNAV Path but it looks more like I would descend using manual V/S.

Other approaches worked fine for me, the descent path always led me right onto the right altitude at the right point.

 

Any idea?

Thanks for your time :)

 

Yannic

You're certain you were in VNAV PATH and not VNAV SPD?

 

Were you on the path or below it?

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Thanks to both of you for your answers.

 

@Rick, it was around 150k, flaps came down after the DECEL point right before VOR/LOC, I don't think that was the problem.

 

@Spin, yes I'm absolutely sure I was in VPATH as I double checked for that. I was way below the path.

 

 

I did the same approach again today and maybe I should have added that I saved the flight right after T/D, on my third attempt it worked fine and even though I'm pretty sure I did everything like on the descents before I wouldn't bet money on it.

Might it be that while loading the flight something messed up within the systems?

 

Thanks again for your answers and your patience with me :)

 

Yannic

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Yeah, I'd try saving the flight before TOD. Keep in mind that if you are using an external weather source that it may have an impact on VNAV performance if winds change.

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It sounds like it was is SPD. It really doesn't get below path like that if you're in PATH. Take some pics next time and we'll see what we can figure out.

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