<USE totalizer fuel

[Hardy Heinlin]

Good morning,

on the legacy FMC, the <USE USE> prompts only appear when totalizer fuel disagrees with calculated fuel.

Once a USE prompt is selected, the other display is blank and can no longer be reselected.

(A manual fuel entry on the PERF INIT page also blanks the totalizer fuel, but it will reappear when that manual entry is deleted with the DEL key. So that action isn't identical to pushing the USE prompt.)

Now on the NG FMC the <USE prompt for the totalizer is displayed even under normal conditions as long as the calculated fuel method is active (that's the default).

Question: Isn't it risky to enable the <USE prompt under normal conditions? When it's accidentally pushed, it can't be reset anymore, right?


Regards,

|-|ardy

[DougSnow]

Polar says the <USE prompt is displayed if there is a fuel disagree by more than 4000 kgs for 5 minutes, instead of being continuously displayed... maybe an operator option?

[Markus Vitzethum]

I checked this morning, too, and it's the same for e.g. Atlas or NCA (747-8F): 9000 lbs / 4080 kgs for 5 min.

Markus

[Hardy Heinlin]

It's not about the disagreement value of 9000 lbs or the 5 minute time in particular. That's clear. I'm referring to the disagreement condition in general which is no longer involved in the same way in the NG FMC. There's no doubt about that (real deck observations, and described in the FCOM too).

Be sure to check for "legacy FMC" and "NG FMC" within the same Atlas/Polar 744 FCOM. There are two chapters.

Page 11.42.25 - item 6 (Legacy FMC)
Page 11.47.24 - item 6 (NG FMC)

Another new feature is the <SEL> marker in line 6. It's on the CALC side at 6R when the USE is at 5L. When you see this <SEL> you know it's an NG FMC.


Regards,

|-|ardy

[andmiz]

It can't be changed back to CALC, once pressed?  Both TOT and CALC are always displayed, and the option always exists.  Maybe as you say, it's an operator option.

We use the lower of the two calculations in normal ops.

[Hardy Heinlin]

Yes, in normal operations, both TOT and CALC are displayed, and there is a single <SEL> and a single USE key.

CALC selection is the default, and so that single <SEL> is at CALC and that single USE is at TOT.

When the disagree alert is active, two USE keys are shown like in the legacy FMC.

We use the lower of the two calculations in normal ops.

When the TOT is the lower one, how do you select the TOT in normal ops? Do you see that USE key at the TOT side in normal ops?


|-|ardy

[Hardy Heinlin]

I see now that the NG FMC will not blank the "fuel used" indications when the "<USE" totalizer key has been pressed, and that it's possible to switch from "totalizer" to "calculated" and back at any time.

I guess it will blank them only when it's pressed under "fuel disagree" conditions (as described in the legacy FMC chapters).

And I now think there was a misunderstanding regarding the "fuel disagree" logic in the legacy FMC:

The old manuals illustrated these conditions with two "USE" prompts:

<USE   fuel qty   USE>

It made me think the real FMC displays this stuff exactly as illustrated. But the illustration probably didn't mean to say that both "USE" prompts appear at the same time; it just meant to show where they can appear. I guess in the legacy FMC too there can be just one "USE" prompt at a time, like in the NG FMC.

The only difference to the NG being the <SEL> markers in line 6. These markers are not mentioned in the old legacy chapters.


Regards,

|-|ardy

[Hardy Heinlin]

I don't trust the manuals in this part. Some effects are obviously different on the real FMC.

Here's another part that makes no sense to me:

USE TOTALIZER
Push -
• blanks CALCULATED display

USE CALCULATED
Push -
• blanks TOTALIZER display

This quote is in the legacy chapter as well as in the NG chapter. In the NG chapter it may be a copy & paste error carried over from the legacy chapter; the real NG clearly doesn't blank the non-selected side. Instead, it displays the <SEL> on the selected side.

And I can't believe why it should blank the non-selected side in the legacy FMC either. One method is always selected -- typically the CALC side. Why should this status blank the other side? OK, we could assume that the blanking only happens when a fuel disagree alert occurs. But then it's even more important to display both values, left and right. When I've got a fuel disagree alert I want to compare the TOTALIZER value with the CALC value. And when I have selected the other method, I still want to have the USE prompt ready for the non-selected side, and that only makes sense when there is a value displayed under that USE prompt. Before I press that USE prompt I want to see what value this USE prompt is referring to :-)


|-|ardy