Airline flight number parity

I read in Wikipedia this morning that there’s a pattern to the parity of flight numbers.

Among airline flight numbers, even numbers typically identify eastbound or northbound flights, and odd numbers typically identify westbound or southbound flights.

I never noticed this. I could see how it might be a useful convention. It would mean that your outbound and in return flight numbers would have opposite parity. That might help keep them straight.

I have coming up, so I wanted to see if the rule holds for my flights. But I’m traveling northwest. The rule couldn’t apply since the north component would say the number should be even, and the west component would say the number should be odd.

Next I looked back on some past trips. When I flew from Houston to Boston a few weeks ago, headed north and east, my flight number was odd, contradicting the rule. But on the way back I flew west from Boston to Chicago, and south from Chicago to Houston. Both of these flight numbers were also odd, in accordance with the rule.

Looking back over all the flights I could see in my account, there was no pattern of parity and flight direction. Most of my flights have been on United because Houston is a United hub. The few flights I could find that weren’t United didn’t seem to follow a parity rule either.

The source that the Wikipedia article cites is a book about Southwest Airlines. Maybe Southwest Airlines follows this rule. I checked a few flights on their web site, and it’s not clear that they follow the rule either. Maybe Southwest used to follow the parity rule.

Incidentally, there is a parity rule for numbering US Interstate highways analogous to the supposed rule for flight numbers. North-south highways are generally odd numbered, and east-west highways are generally even numbered. There are ambiguities and exceptions, but the rule generally holds. It holds more often than the flight rule.

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