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Old 2nd Sep 2000, 06:05
  #14 (permalink)  
Posts: n/a

Ceppo, you're on the right track.

Your estimate of 75 degrees => (60 * 75) nm => 4500 nm would be valid at the equator, but New York and London aren't at the equator. New York is at about 40 degrees latitude, and London around 50, making 45 N a useful average.

Very useful, in fact, as 45 is halfway between the equator and the pole. So is the distance half that at the equator? No. As Autothrottle ahas already pointed out, the arc distance of a great circle varies with the cosine of the latitude. The cosine of 45 is about 0.7 -- you remember this from from calculating crosswind components, or or banked stall speeds, right? So based on our assuptions, the great circle distance from New York to London is approximately 0.7 * 4500 nm = 3150 nm.

It's always smart to double check your estimates using a completely different method. How about multiplying the speed of the aircraft you aspire to fly by its flight time from London to New York.

Plugging the actual latitudes and longitudes of New York and London into a great circle calculator that assumes a spherical Earth, gave me a distance of 5571 km, which is 3008 nm, so we're within 5 percent, sounds like an okay estimate.

Of course, no interviewer will use this question now, because they all read PPRuNe, didn't you know?