GPS incorrect

Pro Member First Officer
RichT First Officer

Can someone please explain the following to me.

I take off, climb to cruising height and set auto-throttle. I look at the GPS and it tells me my ETA is for example 16:00. Several hours later I return to check everything is OK, and the ETA had changed to 20.35.

Why, if the speed is constant, does the ETA change by over 4 hours?

It is also the same with the estimated time en-route. Sometimes this will show for example 10 hours, only for me to return several hours later again and it says 9.5 hours.

4 Responses

Pro Member First Officer
Bob (Traches) First Officer

How long of a flight are we talking here? The easiest explanation is winds-- the GPS doesn't know what the winds are and therefore can't account for them. Having a 50 knot headwind can make a huge difference in your ETA...

Pro Member First Officer
RichT First Officer

This is a flight from Tokyo to London Heathrow.

Departed at 12:55 local time (04:55 GMT) - flight is approx 12 hours long so should arrive in London at 16:55. I actually arrived at 00.15, a flight of nearly 20 hours!!! No head wind is that strong.

It seems whenever flying from east to west it just cannot cope with the time.

Pro Member First Officer
Pete (pdegraff) First Officer

Hit Shift+Z once to see the actual wind displayed in the upper left of the screen. It shows the direction (magnetic heading) from which the wind is blowing and the velocity of the wind in Knots ...
as for instance "273 Mag 47" (wind from the west (273 degrees magnetic) at 47 knots).
Maybe the wind IS that strong, especially if you're flying against a jetstream.
In just ordinary flying (I fly the small stuff) I've seen a 60 knot headwind while flying east to west (in the northern hemisphere) - IF I'm above ... say .. 12,000 feet.

Pro Member First Officer
RichT First Officer

Thanks for the Shift+Z tip, I never knew about this!


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