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Distance To Destination Calculations?

Posted: Thu Jul 15, 2010 4:03 am
by wardialer
Everytime I fly from Zurich to Los Angeles, the inflight Information on the AVOD displays the Distance to Destination of being 5931 miles.

However, when I add the all the waypoints like VORs, Intersections, etc from the actual flight plan (from Flightaware)......it shows more like 6300 miles on my GPS instead being 5931 miles....Why is this?

Does the onboard inflight information system calculates the distance computed as a STRAIGHT LINE DISTANCE not including the waypoints which tends to add more miles?

Thx.

RE: Distance To Destination Calculations?

Posted: Thu Jul 15, 2010 4:26 am
by hercppmx
ZRH-LAX on Great Circle Mapper shows a distance of 5938 miles, pretty close to what the AVOD shows. Since actually flight paths vary due to WX, and Traffic etc. I'd Assume the system has great circle distances between airports programed, as more of general information.

RE: Distance To Destination Calculations?

Posted: Thu Jul 15, 2010 4:27 am
by tdscanuck
Quoting wardialer (Thread starter):
Does the onboard inflight information system calculates the distance computed as a STRAIGHT LINE DISTANCE not including the waypoints which tends to add more miles?

I suspect so...in order to do the true route distance, the IFE would have to be much more tightly integrated with the FMC than is normally the case. Just having the origin and destination airports is much easier, and almost as good for a lot (most?) routes. It's probably doing great circle distance, not straight line distance, though.

Tom.

RE: Distance To Destination Calculations?

Posted: Thu Jul 15, 2010 4:35 am
by dxing
Quoting wardialer (Thread starter):
However, when I add the all the waypoints like VORs, Intersections, etc from the actual flight plan (from Flightaware)......it shows more like 6300 miles on my GPS instead being 5931 miles....Why is this?

Because the other 99% of the people on the airplane would be hard pressed to tell you when they are over Canada much less what waypoint or VOR they are coming up on. The IFE map is a matter of convenience and gee whiz more than a technically acurate display.

RE: Distance To Destination Calculations?

Posted: Thu Jul 15, 2010 4:46 am
by wardialer
Ohhhhh...My appologies.

I meant to say Great Circle....

And what about the FMC? Does that only compute the Great Circle distances between the two airports or does it calculate the total distance with all the waypoints added?

And of course, as I mentioned, with having the waypoints included in the flight plan, the total distance would a more compared to the Great Circle distance between those two airports.

RE: Distance To Destination Calculations?

Posted: Thu Jul 15, 2010 4:54 am
by tdscanuck
Quoting wardialer (Reply 4):
And what about the FMC? Does that only compute the Great Circle distances between the two airports or does it calculate the total distance with all the waypoints added?

The FMC does true distance, since it builds the flight as a series of legs between waypoints (with fuel burn, ETA, etc. for each leg). That will include everything in the flight plan, including climbs and descents, any programmed holds, winds, step climbs, SIDS/STARs if applicable, etc. The accuracy of the estimates is entirely a function of how much of the true flight profile the crew enters to the FMC.

Quoting wardialer (Reply 4):
And of course, as I mentioned, with having the waypoints included in the flight plan, the total distance would a more compared to the Great Circle distance between those two airports.

Absolutely. Other than some possible oddities in areas where the earth is a little out of whack of truly spherical, the great circle route will be shortest path and any deviations will lengthen the distance flown.

Tom.

RE: Distance To Destination Calculations?

Posted: Thu Jul 15, 2010 5:10 am
by wardialer
Ok, thanks.

So, the Airshow that gives the passengers the inflight Information, that system just calculates the distance between the two airports using the Great Circle.

Am I correct?

RE: Distance To Destination Calculations?

Posted: Thu Jul 15, 2010 6:25 am
by zeke
Quoting wardialer (Reply 4):
Does that only compute the Great Circle distances between the two airports or does it calculate the total distance with all the waypoints added?

Great circle tracks and distances between navaids/waypoints, as VORs transmissions etc take great circle paths.

Quoting tdscanuck (Reply 5):

The FMC does true distance, since it builds the flight as a series of legs between waypoints (with fuel burn, ETA, etc. for each leg).

Not sure if you really wanted to use "true" in that context, "true" in navigation terms is normally associated with rhumb lines, not great cicles. The tracks displayed on an FMC are normally magnetic, allowing crews to cross check them with published charts, which also show magnetic tracks.

RE: Distance To Destination Calculations?

Posted: Thu Jul 15, 2010 3:22 pm
by tdscanuck
Quoting zeke (Reply 7):
Not sure if you really wanted to use "true" in that context, "true" in navigation terms is normally associated with rhumb lines, not great cicles.

Good point...better word would be "route distance", the distance the aircraft must actually move to follow the planned route.

Tom.

RE: Distance To Destination Calculations?

Posted: Thu Jul 15, 2010 4:57 pm
by wardialer
Actually this comes to mind....I have downloaded a while back a manual of the AIRSHOW-400 series inflight system, and it shows you how the DIST TO DESTINATION and the TIME TO DESTINATION is entered via the AIRSHOW controller.....

Here is a quote from operating manual below. Hope this helps all of us

Airshow automatically
displays the destination
identifier from the flight
management system in most
aircraft. After a destination
identifier has been entered
(either from the flight
management system or
manually), the future track
line is displayed on maps

Set Destination. The system
typically receives destination
information from the flight
management system. However,
this option can be used to enter a
new destination identifier. Enter
each letter of the identifier until the
correct airport is displayed.

.




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