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badderjet

MCP Speed Bug - V2+20 caused...

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Gents, although I try to read this board on a regular basis I might miss a thing or two, so sorry should this have happened. Anyways, this just occured to me and because it was new to me I thought, oh please not another one. Without further ado, here is what happened: Preflight setup was performed normally. After manual departure with LNAV armed, I selected BUG UP speed at acceleration altitude which was set to 800' AFE (and I did so by visually placing the magenta bug over the UP speed, as probably everyone else would do). Now watch closely what the MCP thinks of this speed, which is also mirrored on the PFD readout: mcpspeed.jpg Now dialing the MCP speed up and down would simply retain the error - which turned out to be exactly 20 knots. So I suppose what happens is that at liftoff the actual BUG jumps to V2+20, but this seems to yield an internal discrepancy between the values, as the PFD readout and MCP speed window show the wrong number (or the bug is wrong, whatever). Reproducing this bug five minutes later in another plane with a different config was a piece of cake: mcpspeed2.jpg Now I wanted to see what speed the A/T would "think" was selected, so I'm still into that and will perform tests in a minute, but I already wanted to let you know about this. In fact, I just tried LVL CHG and that immediately cured the issue by resetting the bug, so I suppose the bug is wrong, not the numbers. Any thoughts? sig.gif

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As I understand it, with the TO/GA pitch mode the magenta speed bug should show whatever you have set on the MCP. The pitch bar will command MCP speed + 20 knots, but the bug should stay at MCP speed. That is the behavior I've seen, but maybe it depends on display options?

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In the pics you are still in takeoff mode !!
Yes, of course I am. After all this is right after takeoff. I don't need to go into any other pitch mode by that time, so it's irrelevant, because we ARE in TOGA and NOT in any other pitch mode. Hence the issue remains. Acceleration was at 800' AFE, climb thrust at 3000' which is perfectly common practise depending on the interpretation of the NADP in use. Bug and readouts are way off, that's undeniable. Changing the MCP speed should reflect the value in the bug, everything else would be a major design flaw IMHO. Thanks for your input however. Any other thoughts? sig.gif

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Yes, of course I am. After all this is right after takeoff. I don't need to go into any other pitch mode by that time, so it's irrelevant, because we ARE in TOGA and NOT in any other pitch mode. Hence the issue remains. Acceleration was at 800' AFE, climb thrust at 3000' which is perfectly common practise depending on the interpretation of the NADP in use. Bug and readouts are way off, that's undeniable. Changing the MCP speed should reflect the value in the bug, everything else would be a major design flaw IMHO. Thanks for your input however. Any other thoughts? sig.gif
It is not the other way around?At 800 climb trust (for noise abaitment procedures) and at 3000 Acceleration?

Matias Sorcinelli
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It is not the other way around?At 800 climb trust (for noise abaitment procedures) and at 3000 Acceleration?
No, it depends on the NADP.
Normally you would change to climb thrust before starting to accelerate. Altitudes depending on what kind of noise abatement procedure you were following.
Hm, as I said I think this would depend on the interpretation of the NADP. From a NADP 2 description:
On reaching an altitude equivalent to at least 800 feet AGL, decrease aircraft body anglewhilst maintaining a positive rate of climb, accelerate towards Flaps Up speed and reducethrust with the initiation of the first flaps/slats retraction or reduce thrust after flaps/slatsretraction.
So it seems to be valid procedure, and to back that up, in fact I used the procedure posted in one of Circletoland's popular
, and he drives the real deal so he certainly knows what he's doing. He set the values to 800/3000 on TAKEOFF page 2/2. It still has nothing to do with the issue of the values being off. sig.gif

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It still has nothing to do with the issue of the values being off.
As long as you are in TO/GA pitch mode, the pitch bar will command MCP speed + 20 knots. It seems likely that this is relevant to what you see, would you agree? The question is, how should the magenta bug behave. In the panel version that I have been using, it stays at MCP speed. Maybe it behaves different with a different display type, maybe what you see is a bug.

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As long as you are in TO/GA pitch mode, the pitch bar will command MCP speed + 20 knots. It seems likely that this is relevant to what you see, would you agree?
Yes, I totally would, no question.
The question is, how should the magenta bug behave. In the panel version that I have been using, it stays at MCP speed. Maybe it behaves different with a different display type, maybe what you see is a bug.
This is left to be determined. On the one hand, I somehow like the bug to jump to +20, because I'd obviously glance at the speed tape and it makes sense to have the bug at the target speed you actually want to fly. Also the FD commanding +20, even more important.However, this obviously leads to a MCP speed dilemma, so to speak. Thinking.gifsig.gif

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I still cannot see a problem ?
If you look at the speed tapes in the first post, you can see that the magenta speed target on the first one is 184 knots, but the magenta bug points to 204 knots. Similar in the next picture. So there is a 20 knot discrepancy. On all my takeoffs, the magenta speed target and the magenta bug have been in agreement while in TO/GA pitch mode. But like I said, it may be a matter of different display options.

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That's not the point, I am not holding the plane from anything. If I choose so I can stay in TOGA as long as I want up to 3000, according to NADP. The point is, look at the bug, and look at the MCP and PFD readouts. I can change the speed on the MCP, and the bug will move appropriately, but the reflected speed will be 20 knots off. That's the problem. Regardless of TOGA or any other pitch mode, this is improper behavior in ANY mode. If I choose the configuration from above (800/3000), I WILL be in TOGA up to 3000. Which exactly triggers said issue. So I ALSO will select bug up at 800, so I will accelerate at takeoff thrust while retracting flaps between 800 and 3000, so there is a phase where I will be in TOGA and accelerating during flap retraction. sig.gif

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I would be curious to know if you see the same issue with the 737-800WL with the PMDG livery. I tried an 800' accel / 3,000' thrust reduction takeoff, but bug and speed target were in perfect agreement. What livery were you using?

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Perhaps a bug on 2D panel. I think these are 2D panel screenshots right?


Mauricio Brentano

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I would be curious to know if you see the same issue with the 737-800WL with the PMDG livery. I tried an 800' accel / 3,000' thrust reduction takeoff, but bug and speed target were in perfect agreement. What livery were you using?
I will try it out. I have been using custom liveries. But after all, they only differ in their config files, no? At least both of them, quite differently configured, exhibited the described behavior. BTW If you say it works for you, what did the bug do? Did it go to MCP speed at some point?
Perhaps a bug on 2D panel. I think these are 2D panel screenshots right?
I will check that as well, but I highly doubt that. The gauges are shared between 2D and VC. sig.gif

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To kill to birds with one stone, here a shot of the PMDG livery 800WL in the VC: mcpspeed3.jpg Again, at 800 I select bug up (on speed tape), which is 214 at the current weight. MCP and readout reflect 194. Tom, at what point during departure did you try to manipulate the MCP and you say it changed the bug appropriately? sig.gif

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Did it go to MCP speed at some point?
Hand-flown takeoff, V2 = 146 knots set on MCP. Magenta bug and speed target on PFD both at 146. Following the pitch bar gives a speed of 166 knots, as expected in TO/GA. At acceleration altitude (800') speed on MCP set to 211 knots. MCP speed, PFD speed target and magenta bug all at 211. Stil in TO/GA, so following the pitch bars gives 231 knots. Once I press LVL CHG, MCP speed changes to the actual speed of the airplane, and the PFD speed target and magenta bug update accordingly. Behavior seems to be the same in VC and 2D.

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