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Video of VNAV Weirdness


Frafty

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Hey all, filmed a video (nvm the landing...) to help demonstrate the bus chasing the magenta dot. Some other things you'll notice:

https://www.useloom.com/share/f1d0f6f8c030489aaab9d02e01e5aa3e

 

  • There's a descent rate of around 2200 fpm. That's a little steep, no?
  • As the plane descends at that FPM, the speed naturally climbs. 
  • As the speed climbs, it asks for more drag, so I deploy the speedbrake.
  • Magenta marker gets away as the a/c pitches up to slow down.
  • We get to around 10K and the a/c pitches up to slow the speed down (dangerously close to 10K and the speed isn't near acceptable levels yet)
  • Then the aircraft can't maintain the speed restriction because it resumes that same 2200 fpm descent under 10K

 

Is everything seen normal behavior? It seems the FPM is way too steep, but I'm also used to flying Boeings more where they manage the speed descent profile pretty well (especially when approaching the 10K restriction. The descent profile remains right on par even when the 737 slows down for the restriction). 

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Does this happen on every flight?

You would probably need to post the flightplan in order to troubleshoot this effectively.

 

This could be a results of poor flightplanning... 

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23 minutes ago, Anders Bermann said:

 

This could be a results of poor flightplanning... 

ESGG NEGIL M852 DETMO DETMO2F RWY34 ESNN, with real world pilot restriction FL70 at Detmo

Its another bus compare them with same flightplan load etc TOD 20nm before, very close to TOD on NGX and real world NGX

 

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I too experienced a couple of weird decents.

At one example (Approach into Warsaw Chopin) the descent rate from the autopilot exceed 10'000 feet per minute! This only happened once however.

 

Thought apart from that I had at least three other examples where the autopilot created a decent rate of about 5'000 - 6'000 fpm. 

I might be mistaken, but for me it too seems that the calculated TOD is a bit too close to the destination airport, thus causing such excessive descents.

 

If you need the actual STAR that I've been using for recreation purposes just let me know.

 

Regards,

Jan

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I descended at the TOD designation the FMC gave. The first few thousand feet in the descent are usually fine. It's not until it comes up to a restriction (be it the 10K restriction, or another somewhere in the flight plan) where it starts to struggle. I even "managed" the descent to give it time to prepare, but still ran into those issues. 

 

Trying to maintain the speed and at the rate of descent, in the past 6-7 flights done with it, it's left me way too high to intercept anything when the STAR dumps me out at the IAF. 

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Cool. For your reference, here's another example of odd VNAV behavior. 

 

The restriction says be between FL280 and FL170 by the time I get to ALAGU. No way that's gonna happen. Not sure why the magenta dot isn't actually moving either. 

 

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 And then after I pass it, the chase begins. 

 

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One thing I just noticed, which could be the problem in this specific case: despite the restriction being clearly visible in the flight plan on the display, it's not in the FMC (see AGLAGU at the top). 

 

 

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Sorry for all the posts: just wanna give you as much info as possible. I think I might've uncovered another reason for the odd descent behavior. 

The a/c is trying to descend and unnecessarily low speeds above 10K. There's no speed restrictions in the FMC, yet for some reason, it's trying to slow to speeds you would expect to see below 10K, and we're well above that here in the screeshot. Most a/c (unless there's a restriction in place) don't slow to speeds below 250 until after crossing 10K. I'd expect to see the speed in the 270kt range around this point in the screenshot. 

 

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It almost looks like the MCDU is trying to reach the constraints of the way point AFTER the next one.......in one of the examples above it only starts to increase the 1000fpm decent AFTER arriving at the constrained WP 

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