How does FMC calculate T/D

[Qbix]

Hi,
could anyone tell me if FMC takes the actual wind at the cruise alt to calculate T/D or it takes the wind from PERF page (LEG alternatively)? Let's leave FORCAST page and other issues alone.

Any refernece appreciated!
Thx.

[BOAC]

Perhaps useful to define 'FMC'? I believe the 737 uses 2/3 INS derived cruise wind plus any forecast stuff (and still screws it up...:))

[EMIT]

FMC's use current conditions for some ways out in front, then start averaging between actual conditions and forecast conditions as put in by you until a certain range, beyond which they will use exclusively your forecast. Actual changeover ranges may depend on aircraft type (slow, short range, or fast long range aircraft) and on FMC manufacturer.

For descents, there will be an algorithm, based on the "standard phenomenon" that wind speed increases with altitude (conversely, decreases during descent), which may, for instance, make a straight interpolation between a value of 0 kts 2.000 ft below sea level and xx kts at a level for which a known wind exists (either measured or forecast).

For wind direction, again straight interpolation, meaning: if between two levels, for which a wind value is known (actual or forecast), there is a difference of say 30 degrees in wind direction, then for a point for which the level will be halfway between those two levels, the wind direction change will be 15 degrees.

If you want to see all this magic in action, just take a look at your legs page (route data, or next page, whatever applies to your aircraft type). Usually, manually input values are in large font, values calculated through interpolation or propagation from other waypoints is usually in small font.

[Qbix]

:ugh: of course I forgot to add that I meant B737, 10.7 update!

[Bullethead]

G'day Guys,

Also, can somebody tell me why the feckin FMCs, or should I say feckin FMC software designers, used T/D and T/C for TOD and TOC which were the traditional abbreviations for Top Of Descent and Top Of Climb????? It's still three characters either way.

Just one of my pet peeves. :confused:

Regards,
BH.

[tcas1]

Wild guess but maybe the reason they use T/C and T/D is that there is probably an NDB in the world somewhere named TOD and or TOC.

[Bullethead]

Hmmm,

I'll have a look in the FMC database next time I go flying and report back.

T/C and T/D are FMC generated symbols presented on the nav display and you wouldn't want to be able to line select them or TOD or TOC.

Prolly more a case of being designed by non-pilots.


Regards,
BH. :ok:

[twistedenginestarter]

Just like any specialist area, the insiders like to change the jargon at frequent intervals. Once other people know what you're talking about, the air of mystery falls away.