wardialer
Topic Author
Posts: 1142
Joined: Fri Sep 14, 2001 1:08 pm

Flight Info Question

Sat Feb 12, 2011 7:11 pm

Hi Guys!

I just wish to know on how does this Flight Information (from the below image link) know the Time to Destination if the Ground Speed is 0 mph?
Doesnt the flight computer start calculating the Time to Destination when the Ground Speed is active?

Please shed some light on this....Thx.

http://www.airliners.net/uf/view.fil...?id=126150&filename=phpEeiWR7.jpeg
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Flight Info Question

Sat Feb 12, 2011 7:32 pm

Quoting wardialer (Thread starter):
I just wish to know on how does this Flight Information (from the below image link) know the Time to Destination if the Ground Speed is 0 mph?
Doesnt the flight computer start calculating the Time to Destination when the Ground Speed is active?

When was this photo taken? If it was taken on the ground, the FMC is just spitting out the expected flight time based on the input flight plan.

Tom.
 
wardialer
Topic Author
Posts: 1142
Joined: Fri Sep 14, 2001 1:08 pm

RE: Flight Info Question

Sat Feb 12, 2011 7:56 pm

Its not my picture, but I believe its on the ground at the parking gate.

But how does the FMC know the predicted Ground Speed?

And since the Ground Speeds are not fixed through the whole entire flight due to winds aloft, and the flow of the jetstream, I still do not understand on how it will predict the exact Ground Speed in order to calculate the Time to Destination.

[Edited 2011-02-12 12:00:01]
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Flight Info Question

Sat Feb 12, 2011 8:14 pm

Quoting wardialer (Reply 2):
But how does the FMC know the predicted Ground Speed?

It doesn't. That screen is showing actual ground speed (provided by the avionics) and *planned* time to destination (calculated by the FMC). Once you're airborne, it will show remaining time to destination.

Quoting wardialer (Reply 2):
And since the Ground Speeds are not fixed through the whole entire flight due to winds aloft, and the flow of the jetstream, I still do not understand on how it will predict the exact Ground Speed in order to calculate the Time to Destination.

The FMC gets a flight plan, a cruise speed, and winds aloft (and cost index and a bunch of other stuff). It calculates time to destination from current position based on that information...that's what's showing on the screen. The FMC is not predicting ground speed, that's an input based on the current instantaneous state. The FMC will update time to destination as the flight progresses but, on the ground, it's not paying attention to current speed.

Tom.

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