When an EFC is set on the HOLD page

[Hardy Heinlin]

Good morning,

I'm modifying my EFC show/hide logic on the FMC HOLD page. Here's another question that refers to the ND too. Maybe someone knows the answer.

As we know, the ETA of the active hold fix (i.e. active waypoint) is indicated on:

FMC RTE DATA
FMC PROGRESS 1
FMC POS REPORT
FMC HOLD
ND top right corner
ND map waypoint in DATA mode

Let's say, we start the hold at 0000z and each hold takes 6 minutes. So the FMC and ND will indicate this ETA:

0006z ... and when the fix is passed they will indicate:
0012z ... and when the fix is passed they will indicate:
0018z ... and so on ...

Say, we are on the first round with ETA 0006z, and ATC says EFC is 0023z. We set 0023 on the HOLD page in the EFC field. So the next possible hold fix ETA after 0023z is 0024z. (We can't exit the hold without first passing the hold fix again; after the EFC we still have 1 minute to go to the fix in this example.)

So during the first round we may get two different ETA indications:

(A) 0006z
(B) 0024z

According to the manuals, the fix ETA on the HOLD page will display (B). I assume (B) will be displayed on all the above FMC pages. But what about the ND?

ND top right corner: (A) or (B) ?
ND map waypoint in DATA mode: (A) or (B) ?


Thank you for your attention :-)

|-|ardy

[United744]

Just purely for consistency it would be (B)? I can't see why it would show different data.

[Hardy Heinlin]

On normal, non-hold legs, the active distance and ETA in the top right ND corner refers to the direct distance while the FMC's DTG refers to the wpt sequencing point. On fly-by curves, that sequencing point lies abeam the actual waypoint as on a fly-by path that waypoint will never be reached, i.e. the direct distance shown in the ND's top right corner will never decrease to zero. On a large fly-by radius at a large course change, the remaining smallest direct distance can be several miles. But the FMC's along-track DTG will reach zero. That's the difference between ND and FMC. The question is: What's the difference when the active leg is a hold?

[Jeroen Hoppenbrouwers]

Trying to switch brain on.

The difference between FMC and ND can be explained if you imagine you switch the FMC guidance off. If the FMC does not sequence waypoints, you need to manually do stuff, such as the lead-in turn for a smooth fly-by. Under such circumstances the ND should not attempt to guide you -- that is OFF -- so it shows you actual distance to actual fix. I think this is reasonable.

What do you keep on your knee pad when flying a hold without FMC support? Fix, inbound track, wind corrections as experienced, ... If there is a time at all in your mind, it probably is the EFC time, not the next-fix-crossing time.

Would this reasoning make sense?

[Hardy Heinlin]

I guess we'll get the answer in a few days. Some kind gentlemen are testing it on the real deck ...

[Hardy Heinlin]

According to observations on the real 748 and 744, the hold ETA shown on the ND (in the top right corner and on the map under the waypoint symbol) refers to the current, active waypoint arrival, no matter how many rounds will be flown thereafter. So 2R on the HOLD page is the only place where the "time of exit" is displayed.


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[Hardy Heinlin]

We noticed that an EFC entry on the real 748 and 744 will increase the indicated HOLD AVAIL by a couple of minutes for every additional round that the EFC will cause. E.g. if the EFC adds two more 9-minute-rounds, the aircraft will be holding for additional 18 minutes, and the indicated HOLD AVAIL will increase by circa 3 minutes. The FMC simulation in PSX behaves the same.

This effect seems strange at first glance, as the reserve fuel and the distance to go after the hold won't change by an EFC entry. But it reveals that both the real FMC and the PSX FMC use a pretty similar, simplified formula for this.

The logic is this: The more holds you fly, the more fuel will be burnt in the hold, thus the lower the gross weight at the hold exit. Thus the lighter the aircraft on the remaining route, with lower fuel flow accordingly. Thus more fuel remaining at the destination. Thus more "hold avail". A perfect algorithm would take this interdependency into account, regardless of any EFC entries, and would probably assume maximum possible EFC and use multiple computation iterations to approach the respective maximum possible "hold avail". PSX doesn't use iterations here either. Nevertheless, in the real FMC, after an EFC entry, it takes 30 seconds until the HOLD AVAIL value appears. Pretty long time.

Honeywell customer service doesn't seem to answer the question why the HOLD AVAIL increases by circa 2 minutes for every additional round. I'd say the reason is simply the absense of iteration methods. Maybe they don't want to talk about it :-) -- Make, say, 20 iterations of a 30-second-computation and you'd have to wait 10 minutes!

(Before you try this in PSX, you need to wait for update 10.151 in which the HOLD AVAIL will no longer be blank after an EFC entry.)


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[Hardy Heinlin]

The modified hold stuff discussed above is now available in PSX 10.151:

https://aerowinx.com/board/index.php?topic=4191.0


Regards,

|-|ardy