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Author Topic: NG FMC: Alternate airports  (Read 1521 times)

Hardy Heinlin

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NG FMC: Alternate airports
« on: Sat, 29 Jun 2019 18:06 »
Good evening,

this is a giant forum post, but a tiny little problem for mankind.

As we know, when a pair of values can be entered with a slash in between, we have three possibilities:

• Enter one value without a slash
• Enter one value with a trailing or leading slash
• Enter two values with a slash in between

When we enter one value without a slash over a pair of already entered values, the other value of that pair may or may not be deleted.

Examples:

WIND/SLOPE
H10/U0.5 is displayed
Enter "H11" and you will get "H11/U0.5" (inner value remains)

FLAP/ACCEL HT
10/1000
Enter "20" and you will get "20/1000" (inner value remains)

Speed/altitude constraint
180/4000
Enter "5000" and you will get __/5000 (inner value deleted)

Fix info BRG/DIS
090/222
Enter 080 and you will get 080/__ (inner value deleted)


On the 777, Pegasus, 748 FMC etc. it is possible to enter a pair of airports that are supposed to be inhibited when the FMC scans the nearest (ETA related) four alternate airports. E.g. VHHH/VMMC in 5R on the ALTN page. When I overwrite 5R with "WSSS", will it show "VHHH/WSSS" or just "----/WSSS"?

Does anybody know? :-)


Cheers,

|-|ardy


One more:
If I enter "VHHH/" (with trailing slash) over "----/----" in 5R,
will I get "VHHH/----" or "----/VHHH" or "INVALID ENTRY"?


« Last edit: Sat, 29 Jun 2019 18:21 by Hardy Heinlin »

Hardy Heinlin

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Re: NG FMC: Alternate airports
« Reply #1 on: Sun, 30 Jun 2019 16:32 »
Another question, slightly more important than the previous one, but not essential either, probably.

For each of the four alternate airports displayed on the ALTN pages, the FMC allows the crew to enter one special ALT/OAT data record and one special WIND data record. This makes the ETA prediction more precise.

Does airline training actually include the handling of these FMC features? Or is this part considered a toy? Has anyone ever entered special ALT/OAT or WIND data in these dashed fields?

If so, where do you get the information from? E.g. if you have an alternate 500 nm left abeam of the route, and another one 600 nm to the right. I guess you want to know the average OAT and wind at the halfway point to the respective airport. These points lie outside the normal route, obviously.


Regards,

|-|ardy

Mariano

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Re: NG FMC: Alternate airports
« Reply #2 on: Sun, 30 Jun 2019 16:49 »
Hardy,

At my carrier, those pages are a toy.

I use them in every ETOPS sector. I enter FL100 winds and OAT to evaluate fuel remaining at the alternates in case of all-engine depressurization. Then, I play with the speed line to see how much more fuel company speed uses over LRC (company speed being optimized for largest ETOPS Area of Operation (low fuel efficiency) and mostly for planning).

We are not provided with a wind matrix in the documents (which I wish we did), but I do download the latest winds into my Jeppesen Flight Deck app, then select FL100 winds (graphical) and enter an average between ETP and each alternate, like you said. For temperature, I mostly guess by looking at the difference from ISA at current level and applying that difference to FL100 ISA temperature. I update the temperature value in the ALTN pages between each waypoint passage and the ten-minute post position plot.

It’s mostly a way to keep busy and force myself to stay situationally-aware.

Best regards,

Mariano

Hardy Heinlin

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Re: NG FMC: Alternate airports
« Reply #3 on: Sun, 30 Jun 2019 18:15 »
Interesting. Thanks!

While we are at cosmetics: Do you know if one and the same airport can be entered in multiple lines? E.g. enter VHHH in all four lines and enter four different wind/OAT data.

In my current model I trigger the INVALID ENTRY message when entering an airport that has already been entered in any of the four lines.


Cheers,

|-|ardy


P.S.: Do you also know what company speed thrust limits are optionally possible? I guess it can be CRZ, CLB, or CON -- never GA. You certainly can't plan to cruise with the GA limit. I think the default is CLB.
« Last edit: Sun, 30 Jun 2019 19:27 by Hardy Heinlin »

Hardy Heinlin

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Re: NG FMC: Alternate airports
« Reply #4 on: Sun, 30 Jun 2019 21:48 »
A very complex toy.

I'm wondering whether you automatically get a <SEL> when you select an XXXX ALTN page with a right LSK on the index page.

<SEL> airports won't get overwritten when the FMC scans the nearest airports every minute or so.

Imagine an XXXX ALTN page is displayed and XXXX has no <SEL> on the index page, and you're just about to enter data on the XXXX ALTN page right in the moment when the FMC scanner changes this airport from XXXX to YYYY.

I assume the scanner is disabled as long as an XXXX ALTN page is displayed -- or when the related XXXX is <SEL>.

That would also mean that the <SEL> marker jumps to the next line on the index page when you hit the NEXT PAGE key on an XXXX ALTN page.


:-)

Britjet

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Re: NG FMC: Alternate airports
« Reply #5 on: Mon, 1 Jul 2019 09:39 »
Hardy,

As you say, the 777 has this ALTN system. It is reached directly by a key in the CDU through the ALTN and FMC COMM pages.
ALTN airport selection can be AUTO (small font, which is uplinked when requested, or manually (large font).
Once the ALTN fields are populated by either method any remaining data (such as METARS) can be requested from the ALTN page directly.
The ALTN en-route winds can be requested specifically and uplinked via the FMC COMM page.
I never bothered with it very much. As I recall you could overwrite en-route winds with your own.
The 777 FCOM should give lots of hints if you have access to one?

Peter.

Hardy Heinlin

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Re: NG FMC: Alternate airports
« Reply #6 on: Mon, 1 Jul 2019 11:00 »
Thanks, Peter,

I have various official manuals about the ALTN system, and my model of this system is nearly finished now, including the features you just mentioned.

If you say "ALTN en-route winds" -- do you mean the wind data on the four XXXX ALTN pages? Or the usual RTE DATA in the legs when the route to the selected alternate is executed? As far as I understand it, there is no uplink function for the wind data displayed on the four XXXX ALTN pages.

There's just an uplink function for a list of 4 airport codes and 2 inhibited airport codes to be displayed on
the ALTN 1/2 index page, and there's another uplink function for a list of up to 20 airport codes to be displayed on the ALTN LIST 2/2 page. And there's a request function for ALTN WXR which can only be printed. I see no uplink feature for those 4 optional wind records.


Regards,

|-|ardy

Britjet

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Re: NG FMC: Alternate airports
« Reply #7 on: Mon, 1 Jul 2019 12:01 »
Hi Hardy,

Sorry - I think I got carried away there - reading through the material again I’m sure you are right (as I said - I never bothered with it much, mainly because - as I learnt the hard way - if you press ‘Divert Now” your whole world changes, and the dispatchers’ screens light up!).

I think any winds have to be input manually having selected the ALTN of choice, after which a separate page opens up.

Peter.

Hardy Heinlin

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Re: NG FMC: Alternate airports
« Reply #8 on: Mon, 1 Jul 2019 12:47 »
Ah, that's a relief :-)

But here's the next question ... for anyone who has played with it:

When you enter wind or OAT data in an XXXX ALTN page, will this XXXX airport automatically get the "manually entered" status and appear in big font on the list of four? Caution: I do not mean the <SEL>. I mean the big font status that is set when an ICAO code is manually entered. (The <SEL> can also be moved to a small font airport.)

If it remains in small font and the next FMC scan will replace that airport by another one, that wind/OAT data will certainly be autodeleted. The entered wind/OAT data would remain if that airport is displayed in big font, as the  scanner won't overwrite manually entered airports.

The pilot manuals describe just 3% of this complex system.


Cheers,

|-|ardy
« Last edit: Mon, 1 Jul 2019 12:58 by Hardy Heinlin »

Britjet

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Re: NG FMC: Alternate airports
« Reply #9 on: Mon, 1 Jul 2019 13:01 »
I don’t know, I’m afraid. I would have thought that it would change to large font..
Peter

Hardy Heinlin

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Re: NG FMC: Alternate airports
« Reply #10 on: Mon, 1 Jul 2019 13:18 »
I think so too. It will get chaotic otherwise.

But this is just for wind/OAT entries as there can be four different entries. Not so for manual ALT and SPD entries as those are always equally valid for all four airports; there's always one ALT and one SPD for all four airports -- so that part can't get chaotic :-)


|-|

Mariano

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Re: NG FMC: Alternate airports
« Reply #11 on: Mon, 1 Jul 2019 17:33 »
Hardy,

I’m making notes of all these items and will test them on Monday, July 8th.

I love the ALTN pages (yes, I know, I need to get out more 😂) so I’m very excited to hear it is nearly close to completion.

Best regards,

Mariano

andmiz

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Re: NG FMC: Alternate airports
« Reply #12 on: Wed, 3 Jul 2019 06:10 »
Hi Hardy.

I’ll email you some videos.

Hardy Heinlin

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Re: NG FMC: Alternate airports
« Reply #13 on: Wed, 3 Jul 2019 13:17 »
Thank you! :-)

I see, entering wind/OAT data won't turn the related ICAO code to big font.

I also see that the ETA/fuel font gets big in the line where the <SEL> is, even if the ICAO is in small font.

Good stuff.


Regards,

|-|ardy


And the first INHIB entry (without a trailing slash) into "----/----" goes to the left side. Nice :-)
« Last edit: Wed, 3 Jul 2019 19:22 by Hardy Heinlin »

Hardy Heinlin

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Re: NG FMC: Alternate airports
« Reply #14 on: Wed, 3 Jul 2019 21:08 »
Good evening,

most airlines probably don't use the ALTN REQUEST uplink function for the "list of four", but maybe someone can answer my question anyway:

The manuals say that each of the four uplinked airports gets a priority number. Bulfer says this number is displayed above the identifier. Understood.

But will these priority numbers disappear when the "ACCEPT>" key is pushed? (Probably not.)


Regards,

|-|ardy



And one more:

We know, when an uplink is pending on the PERF INIT page, TAKEOFF REF page, ALTN LIST page etc., the 5th CDU text line reads:

<REJECT          ACCEPT>

During this time the "ACCEPT>" word hides the data that is normally shown in 5R. But it doesn't matter because, for example, on the PERF INIT page it hides the STEP SIZE data which is not part of the uplink data to be previewed anyway. But on the ALTN page it hides the ALTN INHIBIT airport pair in 5R, and this very data may be part of the ALTN data uplink (up to 4 airports and up to 2 inhibits). I'm wondering if this page provides any ACCEPT> key in 5R at all?

6L has the WXR REQUEST (or INDEX in some versions). Maybe <ACCEPT is in 6L when an uplink is pending ...
« Last edit: Thu, 4 Jul 2019 02:14 by Hardy Heinlin »

Hardy Heinlin

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Re: NG FMC: Alternate airports
« Reply #15 on: Sat, 6 Jul 2019 06:57 »
Good morning,

this stuff is getting academic :-)


As we know, there are 3 selectable routing options:

(A) Direct to
(B) Along current route (with offset option), then direct to
(C) Along current route to a selected route waypoint ("overhead"), then direct to

Understood. No problem.


Now, to acquire four alternates (small font), the FMC goes two steps:

Step 1:
The FMC scans the database (or the uplinked list of 20) for the nearest four alternates.
"Nearest" means: It needs a reference point.
I'm wondering: Is this reference point always the aircraft position? Or does it depend on the selections (A) (B) (C)? When (B) or (C) is selected, the reference might be the respective exit point on the route; that could be thousands of miles away from the current aircraft position, and the four "nearest" alternates would then be thousands of miles away, accordingly.

Step 2:
The FMC has got four alternates from step 1. The FMC calculates their distances according to the selections (A) (B) (C) and calculates their ETAs based on distances/groundspeeds, then sorts the four by ETA for the page text.


Step 2 is clear.

Step 1 is not :-)

In other words, has anyone any clue whether the (B) (C) exit points (when selected) are used in step 1 as well?


Regards,

|-|ardy

jcomm

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Re: NG FMC: Alternate airports
« Reply #16 on: Sat, 6 Jul 2019 08:30 »
Alternates as in "EMERGENCY" would make sense only in terms of DIRECT TO, I guess..., so B) and C) wouldn't even be considered if active I guess...

Or.. is this ALTERNATES stuff also available for other purposes ?

Sorry for the so linear / unacked analysis... :-)

EDIT: Edited after morning coffee to strikethrough the useless question :/  And of course it makes all sense for B) and C) scenarios to take place.
« Last edit: Sat, 6 Jul 2019 10:31 by jcomm »

Hardy Heinlin

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Re: NG FMC: Alternate airports
« Reply #17 on: Sat, 6 Jul 2019 09:11 »
Now, after breakfast and some good piece of bread, I think my common sense can now answer my questions:

I say step 1 uses the same (A) (B) (C) selection -- just as as step 2 does.

It makes no sense to display the nearest four airports relative to the aircraft position when (B) or (C) is selected.

Three options for three different kinds of scenario:

(A) for emergencies (as you say, jcomm)

(B) in case the destination is suddenly closed

(C) for emergencies near no-go areas, e.g. if a direct-to is impossible from the current position


This is certainly the reason the four alternates on the ND are only displayed in PLAN mode. They may be somewhere along the route as per the (B) or (C) selection, and thus you can only see them on the ND by stepping through the LEGS page in PLAN mode.


|-|

Hardy Heinlin

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Re: NG FMC: Alternate airports
« Reply #18 on: Sun, 7 Jul 2019 06:23 »
Regarding the ALTN 1/2 page and the uplink feature:

Bulfer p. 69 says:

"The source of alternates can be from the nav database, a manual entry,
from the ALTN LIST page, or
uplinked directly to this page."


Any opinions on the word "directly"?

Does "directly" mean there is no <REJECT ACCEPT> involved?

Well, the uplink package is rather small anyway. If the data turns out to be inacceptable afterwards, just delete it to return to the small font suggestions. Easy.

Also the uplink package of a pair of inhibited airports is ridiculously small. If the pair is entered directly by the uplink and you're not happy with it, just delete it.

I think there's no <REJECT ACCEPT> involved here. Perhaps that's why it's not documented.

Does anyone disagree?


|-|ardy


I've learned that operators of the 748 and 744 NG FMC don't take these ALTN pages too serious as there are still some known bugs in the real FMC software. -- I find it impressive that these features are actually not new; they are over 20 years old, they are part of the 777 and Pegasus FMC. The above Bulfer quote is from 1999.

I think the <REJECT ACCEPT> principle was introduced in later years, for the new TAKEOFF REF and PERF INIT etc.
« Last edit: Sun, 7 Jul 2019 06:45 by Hardy Heinlin »

Hardy Heinlin

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Re: NG FMC: Alternate airports
« Reply #19 on: Sun, 7 Jul 2019 20:41 »
Just got the answer from another CBT. For the four alternates the <REJECT ACCEPT> appears in the 6th line. So no data is hidden. Only the alternate list of 20 is uplinked directly without <REJECT ACCEPT>.