# 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 a flight 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.

When I first saw the parity rule, I assumed it would have numerous exceptions. It says “typically” and I thought that might mean the rule holds 80% of the time. But just based on my experience, it doesn’t seem to hold at all.

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 website, 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.

## 7 thoughts on “Airline flight number parity”

1. It goes further for interstates. Spurs get an odd prefix and bypasses get an even prefix.

2. Mark Sundstrom

US Highways (non-Interstate) also mostly follow the even-odd rules. Regarding airline flights, I recall that this used to be mostly true but is often no longer because there are more flights and because airlines reserve some blocks of codes for flights with partners, etc. I see situations where a flight out from a hub to another city and the returning flight have the same number. I have also read that the various arline computer systems have a hard coded 4 digit limit on flight numbers. It would be interesting to hear from industry insiders about this.

3. In case of Delta airlines, flying to Europe get odd numbers and going back always would be even. Flying to Asia the other way around. So they mostly hold that convention. Also Dal555 is Seattle to Phoenix so east/south gets odd, I guess

4. I don’t know how general this is and maybe you know it already, but in the street of Paris (and probably in other places,) when facing the direction of increasing numbers, the even numbers are always on the right and the odd numbers are always on the left, and never the opposite. It is useful because when you see a house number, not only it tells you the number you’re at, but in addition its parity tells you which direction you should go to reach the address you’re looking for. If you’re facing house n°7 and you’re looking for n°9, it’s on the right. A useful bit of information…

5. silverpie

More on highway numbers: for interstates, low numbers are in the south and west (also, exits are numbered in that direction). For US highways, it’s the other way around (101 is a special case; it’s officially a two-digit number with a first digit of 10).

Also, major interstates end in 0 or 5; for US highways, it’s 0 or 1.

6. I’m actually working in an airline as flight scheduller. Part of my job is assigning the Commercial flight numbers to new flights. The iata standars do not impose a rule for flight number and every company has its rules. In my company every route (flight departing from a origin to a destination and back) has a specific series. For example, 100s are assigned to flights from jfk to lax and back. The parity is related to the direction of the flight. In the example, 100 can be assigned to a flight from jfk to lax and 101 to the opposite direction lax to jfk.
In our case, odd or even is a question of where is the base of your aircraft. If your aircraft is based on jfk, the first flight jfk-lax will be 100 and the return flight wil be 101.
But as I mention before, there isn’t a standar for this but this rule is very convenient for safety reasons in case you use your comercial fligth number on the radio comumnication between pilots and control tower.

7. Kaleberg

There might be some confusion with the rule for flight levels: north-east-odd, south-west-even. If you are flying from LAX to JFK you might fly at 35,000 feet or 31,000 feet but not 34,000 feet.

Notice that Rick said COMMERCIAL flight numbers. There are also RADIO flight numbers, the ones used by air traffic control. These are usually the same, but the idea is that no control region should ever have to deal with two flights with the same number. Having a Delta 242 and a US Air 242 in the same airspace would be a recipe for confusion and possible catastrophe.

For “would be” you should read “has been”, and there were certainly a few “incidents”, close calls or worse, before this rule was put into place. Aviation code and practice, like legal code and practice, are full of scar tissue. (I had to read and analyze the gross weights and flight planning system for a major airline and some of the comments were scary.)