A question of a very basic type: How are the frequent-flyer miles between cities really computed?

For example, on a recent flight, I was credited with 3255 miles. Yet the 777 in-flight monitor listed the distance between the two cities at take-off as 3265 miles.

A difference of 10 miles.

I do know that there are lanes that aircraft must observe, analogous to 'skyways'. But if airline X is offering you k miles between cities x and y, is this the average of the possible routes, or merely the number of miles of the minimum-distance route?

Does someone know? I know a difference of merely 10 miles is not much, but I feel that if this is happening to all travelling passengers worldwide, the airlines are saving many a reward trip.

Thank you.

This is timely, as the Air France version of the SkyTeam million miles promotion asks contestants to calculate mileage earned on a CDG-SEL-LAX-MEX-CDG trip.

I have already discovered that this Webflyer's mileage calculator produces a different answer from that using the individual airlines' figures!

My understanding is that distances are computed using great circle routes -- the minimum possible mileage. It's impossible for a plane to fly that route, but that's the shortest possible. The difference between the posting and the in-flight computer is probably due to the use of slightly different reference points and has nothing to do with actual mileage flown, which would reflect MUCH more than a 10-mile difference.

Bruce

A good question......but then maybe the smaller mile number is the correct mile amount. I believe the actual printed distance is the great circle distance AS MEASURED AT SEA LEVEL. Of course we all know the aircraft usually (and most importantly) flies somewhat above this height.....and screws us outta even more nice miles, but I nicely go along with this reasoning considering the alternative.

Here's a confusing example of calculating mileage.

I flew JFK to HKG via Vancouver on CX earlier this year. The flight had only one number, and I was awarded something like 8,000 miles on AA. Had I actually stopped and changed planes/flight numbers in Vancouver, then the sum of the point to points would have been closer to 8,700 (according to Expedia).

Just yesterday, I heard that CO was planning to introduce the first New York to HKG nonstop via the polar route beginning in March 2001. According to the press release, the flight miles only total 7,300 in this case. So, I wonder how many miles CO will actually grant on this routing.

According to the great cricle route calculator I use for this purpose (which is normally spot on), EWR-HKG is 8071 miles. I will be interested where CO plan to shave 771 off the shortest possible distance , but it suggests that CX were about right.

dholloway - what route were you on? (I guess LHR-BOS, which comes out as 3264 miles?)

A very good guess indeed. LHR<>BOS. You are correct.

Ack, so you were cheated! Is this grounds to boycott AA?

The CO press release claims the flight is 7,337 *nautical* miles. The "mile" we're used to, and the one that FF programs use, is the statute mile (5280 feet). 1 nautical mile = 1.15 statute mile. 7337 nmi = 8443 mi. Looks like there's actually a 372 mile bonus relative to james' circle route figure.

The difference between measuring the great circle route at sea level vs. at 6 miles (~32,000 feet) above the ground is small. To be exact, it's the ratio of 6 miles to the earth's radius, or 6/3963 = 0.15%.

I have consistently received 3265 miles for BOS-LHR on AA, about a dozen times over the past few years. If that was your routing, check the statement again to make sure the first "5" wasn't really a "6" and then contact AA. They goofed.

The only way to get a shorter distance than the great circle route is to tunnel through the earth. For example, the great circle route halfway around the equator is roughly 12500 miles, but the distance through the center of the earth is only 8000 miles. Of course, tunneling airplanes do not exist at present, and the great heat just a few miles down would melt one anyway!

Regarding the small extra distance involved in flying above the earth's surface, I am reminded of a problem I got in grade school: If the earth's equator is 25000 miles around, how long is a rope suspended one foot above the equator? The answer is 25000 miles plus 6.28 feet. Not much longer, in other words. A plane flying 6 miles above the equator would fly 25037.7 miles.

Bruce

I'm afraid it wasn't AA, but BA.

The interesting point is that I've always received 3255 miles LHR<>BOS each direction. I looked at past statements and this is indeed the same credited mileage over and over again.

This fact is the one that prompted me to post in the first place.

If AA consistently award 3265 miles, and BA 3255, whose miles are more 'correct'? I'm positive the awarded mileage for this particular itinerary is not the only one that differs between airlines.

The original question thus remains: when is a mile truly a mile?

BSchoebel is correct in stating that the airlines calculate mileage using the great circle route method.

Here is a website that calculated distance this way as well as generating some really cool maps:
http://www.chicago.com/airliners/gc.html

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That's the one I use too Mileage Addict. For Delta it is almost always aboslutely right. (I was surprised that it was 1 mile out for the AA BOS-LHR flight discussed above).

BA are cheating you - if the distance is 3265 miles, they can't do it in 3255. Maybe it is some sort of snob tax, imposed by BA It wouldn't put it past them to reduce the mileage they give you simply because you have benefitted so much already from being within their aura.

UA uses 3265 for LHR <> BOS too. BA is ripping you off of 10 miles each way.