Flight Numbers Explained: How Airlines Decide Route Numbers

Sitting in a bright corner office of the ninth floor of Chicago's Willis Tower, Patrick Quayle, VP of international network at United Airlines, beams as I walk in to meet him. Earlier this month, United Airlines announced new service between Newark and Cape Town, South Africa, one of the first carriers in the United States to ever offer that route, and the airline's only service into sub-Saharan Africa.

We're here to talk not necessarily about why United chose that route, but how the route number was selected: fight 1122 from EWR-CPT and 1123 on the way back.

But let's back up for a minute: each flight operated in commercial airspace has a number associated with it, a label that easily tells planners (and knowledgeable passengers) where an airplane is headed, and between what two airports. In many cases, those flight numbers repeat daily; flight 100 on American Airlines always goes between JFK and London Heathrow seven days a week, while 101 runs the same route in reverse.

In some special cases, the routes have a special meaning. Boarding flight 1 to any destination is often a matter of prestige. Whether it's British Airways's flight BA1 from London to New York — once flown by the Concorde, now by the Airbus A318 in all-business configuration — or United's UA1 from San Francisco to Singapore, flight 1 is often a flight of distinction, operated with a flagship aircraft and special service.

But how do airlines decide what route gets labeled flight 1, or how any particular route gets numbered? The answer lies in a complex set of tiers combining traditional and modern rules, tweaked to each carrier specifically. And while no naming convention is identical between airlines, there are some parallel trends as well as some red-hot flight numbers that most airlines tend to avoid.

At United, that process starts and stops at Quayle's desk. Together with Ted Novkov, United's Director - Network Planning and International Scheduling, and a team of route network planners, Quayle helps pick flight numbers via a combination of manual and automatic tools to ensure that there are no repeat numbers and potential safety issues.

At American, there's a similar setup. I tracked down Brent Alex, American's Manager of Global Airport Access and Network Planning, to chat about the same topic and figure out what goes into naming AA1 versus AA100.

Overall Trends

There are definitely similarities in the ways that many carriers select route numbers. In general, lower flight numbers are assigned to international flights, though the merger of multiple carriers over the years has muddied many of the old delineations. At United, for example, flights 1 to 200 are assigned primarily to international routes from the Continental era, before the two carriers merged. 800-1000 are now United international routes. Google any flights in the low hundreds across American, Delta or United, though, and there's a high probability that that route will serve an international destination.

Where possible, adjacent numbers are often chosen for the same flight going in the opposite direction. DL6, for example, flies from Haneda, Japan to Los Angeles. Flight 7 is the same route in reverse.

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There's also a bit of a trend in the way that flights are numbered by the direction in which the aircraft operates, though many of those old rules have also been diluted by mergers and growth, especially on the domestic front.

"From a domestic system (point of view), that's true," says Alex at American. "Internationally, our flights generally are even going east, odd going west. And then odd going south, even going north."

Brackets for a carrier's mainline versus regional fleet are also typically used. At American, flights 1 to 2949 are all assigned to the mainline fleet while 2950 to 6099 are all reserved for swaths of regional partners such as Skywest or Envoy. Delta and United use a similar formula. This helps route planners and other staff quickly identify which carrier is operating a flight and often where it's headed.

How Flight Numbers Are (And Are Not) Selected

So what happens when an airline launches a new route and a new flight number is needed?

At legacy carriers, a combination of manual and automatic tools are used to figure out what flight numbers are open within a specific bracket. American, specifically, uses a tool called FNOM, or the Flight Number Optimization Model, to automatically search for route numbers, consider the overall schedule and suggest possible numbers. "We use the automatic tool mostly for the domestic system where we don't have as much of a preference on what the numbers are," says Alex at American. "In that automated tool there is a function where those international numbers will kind of be locked down so they can only be assigned to that route."

Lest one think that the tool simply picks the lowest available number and assigns it to the route accordingly, there's much more nuance to the process. "[United's tool] looks at things like consistency and what's being published. If a certain number is already published, let's keep it the same. It's very smart in the sense that it doesn't create unnecessary schedule changes, and at the same time, we have our international flight numbers that we restrict, saying 'Don't use those because we have them for international'," says Novkov at United.

That careful balance between manual and automatic selection also keeps the airlines out of hot water when considering certain flight numbers across the network. At American and United specifically, flight numbers associated with the 9/11 terrorist attacks have been permanently retired. Flights that have crashed often have their number retired: Swissair eliminated SR111 after the 1988 accident when an MD-11 plunged into the Atlantic; Air France doesn't fly AF447 anymore, which crashed in 2009; and you won't find MH370 and MH17 — two flights that gained worldwide recognition for the wrong reasons — in the Malaysia Airlines timetable.

Route numbers to and from China are also often carefully picked. "The number eight is lucky for China," says Quayle at United. "If you'll notice, we have flight 88 which is New York to Beijing. We have flight 8 which is San Francisco to Chengdu. Because eight is a lucky number in China, we fly eights to China. But four is an unlucky number, so we make a point not to do that. "

Flight 666 is also generally avoided by air carriers unless its part of a marketing campaign like Finnair's flight 666 to Helsinki (HEL) a few years back.

Airlines can also organize flight numbers in blocks that indicate at a glance where they are going. For example, with typical German precision, Lufthansa divides the world by flight number: LH400 to LH499 are flights to North America, South America gets flights in the 500s, Africa from 560 to 599, the Middle East and Central Asia is the 600s, and Asia-Pacific the 700s. Even numbers are outbound from the home country, and odd numbers fly back. (British does it the other way; odd flights are outbound, starting from that BA1 to New York.) The geography-based model is in use at other airlines too: On Alitalia, for example, if your flight is in the 600s you know you're going to the Americas, and in the 700s to Asia.

As it turns out, United flight 1122 from Newark to Cape Town was chosen because that was the first date — November 22nd — that Quayle visited South Africa when he was a youngster. Meaningful to him and a handful of people in his network, of course, but most people flying across the Atlantic will never know the significance of that route number. Many more stories of how a particular route got its number may never get told.

This story has been amended with Patrick Quayle's correct title and name spelling.