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Descent FMA on Beta 13

Started by emerydc8, Wed, 14 Oct 2015 09:38

Kieber

Hi,

if I may interrupt for a moment.....the behavior of the FMS for descent which hardy here describes confirms exactly my state of knowledge and the LH OM.


The descent may be planned at economy Mach/CAS (based on Cost Index) or a manually entered Mach/CAS. VNAV will not command an economy target speed greater than 349 knots (VMO/MMO minus 16 knots) or a pilot entered speed greater than 354 knots (VMO/MMO minus 11 knots).

Speed Reversion:

If flight plan modifications or unknown winds occur when above the first speed constraint, VNAV varies speed to maintain the path up to the following limits:

With greater than 349 knots (VMO/MMO minus 16 knots), the scratchpad message DRAG REQUIRED displays. The airplane may accelerate up to 354 knots (VMO/MMO minus 11 knots) to maintain the path. If further correction is required, VNAV may allow the airplane to rise up to 150 feet above the path. If VNAV can no longer maintain the airplane within 150 feet of the path without further acceleration, speed reversion occurs, the pitch mode annunciation changes from VNAV PTH to VNAV SPD, VNAV resets the target speed to 349 knots (VMO/MMO minus 16 knots), and the scratchpad message DRAG REQUIRED displays again.


I don't know, if there are different custom features, sorry.....

HTH

Cheers
Walter

Hardy Heinlin

Hi Walter,

thanks for the confirmation :-)


Cheers,

|-|ardy

emerydc8

Hi Walter,

The flight plan modifications Boeing refers to are modifications that would still keep you within +/-150' of the path--not a modification that puts you 5,000 above the path. If you modify the flight plan on the legs page but you are still within the path limits then, yes, it will attempt to pitch to get you exactly back on path.

However, if a modification takes you outside the path, like in my situ, then you should see IDLE/HOLD||VNAV SPD and the airspeed it will maintain is the mach/speed on VNAV 3/3. It's not going to exceed the FMC speed with the window closed to try to get back on path when in VNAV SPD. In VNAV SPD, it has given up trying to maintain the path.

It will also do this when you fly past your ToD point and set a lower altitude in the MCP. When you press the altitude knob, you will see IDLE/HOLD||VNAV SPD but the aircraft will not dive and accelerate to 349 knots in an attempt to catch the path at this point. It will just do a normal descent and maintain the descent speed in VNAV 3/3. So, unless LH has modified their software, there is no path capture feature in VNAV SPD when you are above the path. If there was, then all the discussion in the Big Boeing Book about "dialing in some drag" (speed intervening to a higher speed and using speed brakes) would be moot. If the aircraft would do this on its own (increase to 349 knots), then why would you have to speed intervene at all?

Jon D.

Hardy Heinlin

Hi Jon,

whether speedbrakes are required or not, depends on the situation. If you are 5000 ft too high, you may capture the path in less than 3 minutes without speedbrakes. If that lies well before the next constraint, you keep it clean, if not you extend the brakes.


|-|ardy

emerydc8

Hi Hardy,

I agree that it is possible that you could capture the path if you are close enough without using speedbrakes. But the most common way to get back down to the path when you are descending in VNAV SPD would be to speed intervene and dial in a higher speed (the FMC is not going to do this automatically). You can usually get away without deploying the brakes if you have enough room and you started out with a low enough descent speed. When you get back on path you just push the speed button in, the speed window will close and you will go from VNAV SPD to VNAV PTH and resume an idle descent.

I just don't think the speed is going to increase to VMO-11 or even FMC target + 15 if you wind up above the path in VNAV SPD after a modification. Of course, it will if the modification is small enough that you can remain in VNAV PTH, but once you are in VNAV SPD it's going to descend at the speed in VNAV 3/3. Maybe LH has modified its software to do this, but this isn't my understanding of how the system works on our planes.

I am checking with some other pilots to verify this.

Hardy Heinlin

So do you agree that VNAV SPD is allowed to reach VMO-16 once it was on idle path with VNAV PTH, within 150 ft, and above the first speed limit altitude? If it never was under this condition before (e.g. late descent at CRZ, or route mod/exec during descent), VNAV SPD must hold command speed?

If that is true, the LH OM is not wrong, it is just incomplete because it doesn't explicitely mention these exceptions.

emerydc8

I agree that in VNAV PTH the speed can go up to VMO-11 to hold the path and if it can't hold it within 150' then speed reversion occurs, the command speed is reset to VMO-16 and the FMA will go to VNAV SPD.

But I don't think it will trigger a speed reversion condition if you do an altitude or speed modification and the new path is 5,000' below you. In this case, I think it will go to VNAV SPD and descend at the ECON speed on VNAV 3/3 (presuming ECON is being used). At that point, it is up to you to either speed intervene to a higher speed to get down or use speed brakes, or both.


Hardy Heinlin

Quote from: emerydc8 on Fri, 16 Oct 2015 23:00
I agree that in VNAV PTH the speed can go up to VMO-11 to hold the path and if it can't hold it within 150' then speed reversion occurs, the command speed is reset to VMO-16 and the FMA will go to VNAV SPD.
You mean the magenta bug will jump to VMO-16?

You know, my definition of command speed is the speed displayed on the FMC pages and marked by the speed bug. Target speed, on the other hand, e.g. command+15, has no indication.

emerydc8

Right. Sorry for not being clear on the nomenclature. The command speed is the magenta speed on the top of the speed tape. The target speed is what the FMC is targeting.

If you are descending above the path in VNAV SPD with the speed window closed, the command speed will be the same as the target speed.

Hardy Heinlin

By "right", you mean the magenta bug will jump to VMO-16? :-)

This would make sense. It would maintain the rule of VNAV SPD always aiming at the bug. To get VNAV SPD aiming at VMO-16 instead of the bug, the bug has to jump to VMO-16. If you don't want to go to the mountain, have the mountain come to you.

If this is true, the FMC will forget what your previous SEL SPD was. The only chance to get back to the original speed (when the path is recaptured) is to check the ECON speed of the current cost index. But then the FMC will also have forgotten whether the ECON mode was selected at all prior to the VMO-16 speed reset. It cannot just chose ECON on its own once it has changed to SEL SPD.

emerydc8

No. In a VNAV SPD descent with the window closed, there shouldn't be any difference between the magenta command speed and the speed the FMC is internally targeting.

Also, even in VNAV PTH, I am pretty sure that you will not see the command speed change as the aircraft increases or decreases speed to stay on path. It is hard to test this with an unforecast tailwind, but if you are descending in VNAV PTH with the speed window closed and you deploy the speedbrakes, the speed will be allowed to decay up to 15 knots below the ECON descent speed before power is added; but you will not see this speed decrease reflected in the magenta command speed. It's still going to match what is in 2L on VNAV 3/3.

I am also pretty sure that when Boeing is discussing the speed reversion scenario where it hits VMO-11, goes into VNAV SPD and resets the target speed to VMO-16, you are not going to see this reflected in the magenta command speed. This all goes on internally.

Boeing and Honeywell do an absolutely terrible job of describing the function of the FMS. Whoever wrote these manuals should have been fired. What's even worse is that decades later they haven't even acknowledged it nor have they made an attempt to clarify their initial piece of work.

Hardy Heinlin

Another possibility (instead of automatically setting SEL SPD to VMO-16):

When VNAV SPD engages at VMO-16, the MCP SPD window automatically opens and sets the current speed (VMO-16). That would be the most elegant solution.

emerydc8

That could very definitely be a possibility; but since I have never seen speed reversion in the real airplane (and probably most other pilots haven't either), I don't know. And Boeing sure doesn't feel it is necessary to let us in on it. I think you may have something with the speed window opening at VMO-16.

Hardy Heinlin

Quote from: emerydc8 on Sat, 17 Oct 2015 00:18
No. In a VNAV SPD descent with the window closed, there shouldn't be any difference between the magenta command speed and the speed the FMC is internally targeting.
I just realized you may have misunderstood my question.

"By "right", you mean the magenta bug will jump to VMO-16?"

If the bug (command speed) will jump to VMO-16, there will be no differences whatsoever because everything is reset to VMO-16. As you say, there shouldn't be any difference. And this reset won't break the rule then.

Hardy Heinlin

There is another case of VNAV SPD not aiming at command speed:

When below the path and above the first speed limit altitude, VNAV SPD maintains best hold speed or minimum maneuver speed, whichever is higher, plus 5.

That's in PSX and in LH.

Of course, you can test this only if you disconnect the A/T.

emerydc8

I think I was referring to two different situations. One is where speed reversion occurs because VNAV PTH couldn't maintian the path even at VMO-11, so it reverts to VNAV SPD and resets the target speed to VMO-16.

The other is where you either start down late or modify the route so that you are well above the path and descending in VNAV SPD. In this case, you will not see a magenta command speed more than what is in VNAV 3/3 at 2L and it will not dive to capture the path.

emerydc8

Good point. In that case, you will not see the best hold speed/min maneuver speed commanded in magenta.

Hardy Heinlin

Quote from: emerydc8 on Sat, 17 Oct 2015 00:45
I think I was referring to two different situations. One is where speed reversion occurs because VNAV PTH couldn't maintian the path even at VMO-11, so it reverts to VNAV SPD and resets the target speed to VMO-16.

The other is where you  ...

We have clarified this already :-)

It's now about situation #1. What does "reset to VMO-16" mean?

Will the command speed (speed bug and SEL SPD) jump to VMO-16?
Or will the MCP SPD window open and set VMO-16?
Or will nothing change?

emerydc8

That's the $64,000 question. I like your idea of the MCP speed window opening at VMO-16, but unless we get into a sim and try it, I don't how we get the answer. I will ask some people I know. It's just not something you would intentionally do in the real plane.

Hardy Heinlin

OK, so if everyone agrees, I'll modify my code and will never let VNAV SPD aim at a speed higher than FMC command speed. The only way to exceed it is to open the MCP window. And that will happen automatically when reaching VMO-16, when VNAV PTH changes to VNAV SPD.

Does anyone have any objections?