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SuperCell

777 Pitch down and dive

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I was just wondering if there are any ways to fix these issues for me. My affinity mask is 5... and load is I think only 100% on my first core when landing. The freezing happens in both the NGX and 777 but only in highly dense areas. Virtual Memory was at ~2.9GB, and it's version 1.10.6320

 

Pitch: The airspeed increases and it starts gaining altitude whilst pitching down to keep on path. Is it just me being a noob?

 

Freeze: 


Nick Mathews

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Can't see the first video, it is private  :( .


James Goggi

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I confess, that is a very unusual looking combination of pitch/power/airspeed!
 

I notice that there is a lot of turbulence going on, so I would suggest this is weather related: I can only assume that you were in a mammoth updraft. If you watch the instruments, at the start of the video the pitch & power looks normal. Then the VS starts increasing, so the A/P pitches down (correctly) to maintain altitude. At the same time the airspeed starts falling off rapidly -- which given the 10 degrees nose-down and 70% N1 is unusual!

 

As the aircraft comes out of the updraft the vertical speed and airspeed trends reverse and the aircraft begins to recover back to the VNAV path.

 

Not really sure what else the autopilot could have done to be honest: the aircraft started climbing, so it pitched down to correct.


Simon Kelsey

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Is it something similar to this?

 


James Goggi

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Not sure how the two look similar at all, if I'm honest. In the first video, the aircraft is quite happily maintaining 9000ft and 235kts, with the power around 75% N1. This looks normal.

 

The aircraft then starts climbing after the start of the video, so the autopilot pitches the nose down to compensate. The speed and thrust remains fairly constant.

 

From around 0.17, the aircraft's rate of climb increases despite the nose down inputs, so the autopilot applies further nose down. The speed also begins to drop, so the autothrust increases power to around 90% N1.

 

This situation continues: aircraft climbing, speed dropping. The autopilot applies thrust and more nose down to counteract. What else would you do?

 

At around 0.38 the V/S finally starts decreasing and the speed trend begins to reverse and start to increase. At this point the aircraft is now around 10 deg nose down and above the VNAV path. So the autopilot continues to hold the nose down to recapture the path and bring the speed back up. It then overshoots slightly, reduces the rate of decent and modulates the thrust to maintain 240kts.

 

Again, I'm not sure what you're expect the autopilot to do in this situation. It is pitching the nose down because the aircraft is gaining altitude due to some sort of outside influence -- i.e. the weather. If the aircraft starts unexpectedly gaining altitude and losing airspeed, how would you fix that as the pilot? Column forward and thrust levers forward. That's what the A/P is doing in your video. What behaviour were you expecting?

 

The early descent video that James links to is a completely different situation: smooth air, cruising level, an early descent commanded and the aircraft responds by pitching down in to the overspeed clacker. Clearly, there's something wrong there.


Simon Kelsey

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Pitch: The airspeed increases and it starts gaining altitude whilst pitching down to keep on path. Is it just me being a noob?

I don't think having Flap 5 selected is helping with the negative pitch angle in the first video. I see later on flaps are retracted. If flap had been extended and speed reduced it might have gone better. Unlike the 737NG, the 777 will not schedule speed reduction with flap selection, you have to intervene youself. Also you are in turbulence and FSX is notorious for not responding well to AP or A/T speed errors in those conditions.


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Well it happens the moment after I put down flaps... so I was thinking it might have been caused by that. 


Nick Mathews

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The A/P starts diving in order to regain the lost speed, but it would just need 2-3000 ft/min *and* until the speed is again the scheduled one, then the airplane should increase pitch and resume normal rate of descent. What the PMDG 777 does in a buggy way is diving up to 10000 ft/min (without a reason) and then, of course, it has difficulties in recovering, thus reaching overspeed. I already submitted a ticket about that, but it looked like they were reluctant to aknowledge the bug... 


James Goggi

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I already submitted a ticket about that, but it looked like they were reluctant to aknowledge the bug...

 

I am not speaking for PMDG, but it you have a bug that is reproducible then in all likelihood it  was put into the bug tracking system. 

 

I've seen the 777 pitch commands respond slowly in VNAV to changing descent profiles and weather, but assume that the slow pitch changes are designed into the control laws of the system to reduce acceleration loads (G loads).  There are times when you have to fly the airplane by taking over from VNAV and using MCP SPD and V/S, in my opinion it seems realistic but I'm not T7 rated.

 

Are you T7 rated?


Dan Downs KCRP

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Well it happens the moment after I put down flaps... so I was thinking it might have been caused by that. 

But a change in pitch and lift due to flap extension is to be expected. You need to reduce speed at the same time.

 

The A/P starts diving in order to regain the lost speed, but it would just need 2-3000 ft/min *and* until the speed is again the scheduled one, then the airplane should increase pitch and resume normal rate of descent. What the PMDG 777 does in a buggy way is diving up to 10000 ft/min (without a reason) and then, of course, it has difficulties in recovering, thus reaching overspeed. I already submitted a ticket about that, but it looked like they were reluctant to aknowledge the bug... 

If the speed doesn't respond to pitch then the AP will pitch down more to try and control it. The problem could well be the way turbulence is done in FSX and how it affects dynamics, not the 777 itself.

 

What you say makes no sense. If it doesn't recover the lost airspeed at 2-3000 ft/min down and increases to 10,000 ft/min how does it end up overspeeding? It must have passed through the target speed long before that and so started pitching up again.

 

If it was a design bug then we should all suffer from it. I haven't seen the 777 misbehaving like this myself. Maybe PMDG can't reproduce your problem and therefore haven't placed it on the list to be fixed.


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Well it happens the moment after I put down flaps... so I was thinking it might have been caused by that. 

But a change in pitch and lift due to flap extension is to be expected. You need to reduce speed at the same time.

 

Well obviously, yeah. There is no way this is normal behavior, I was just trying to resolve the issue. It would balloon up, not down though. Everyone who I've talked to never said they had the problem. I'm using ASN SP2b or whatever the latest is called if that helps..


Nick Mathews

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Oh, I misread your post.. forget my balloon up comment.


Nick Mathews

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Nick,

 

I am convinced that your issue lies with Activesky. Check your settings, try reducing the maximum updraft/downdraft and maximum cloud/wind turbulence a little and see if you still get the problem.

 

As I say, the autopilot has not commanded a climb: quite the opposite, it is pitching down to counter an uncommanded climb that can only have come from a weather event. It continues to pitch down until the aircraft finally begins to descend, overshoots the VNAV path slightly and then pitches back up to stabilise the descent and regain the path.

 

Extending flaps will cause a balloon, but not to this extent: there are external influences at work here.


Simon Kelsey

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