Flight number

A number of conventions have been developed for defining flight numbers, although these vary widely from airline to airline, and are increasingly being modified.^[3] Eastbound and northbound flights are traditionally assigned even numbers, while westbound and southbound flights have odd numbers. Other airlines will use an odd number for an outbound flight and use the next even number for the reverse inbound flight. For destinations served by multiple flights per day, numbers tend to increase during the day. Hence, a flight from point A to point B might be flight 401 and the return flight from B to A would be 402, while the next pair of flights on the same route would usually be assigned codes 403 and 404.

Flight numbers of less than three digits are often assigned to long-haul or otherwise premium flights. For example, flight number 1 is often used for an airline's "flagship" service (see below for a 'List of flight number 1 by airlines'). However Cathay Pacific assigns flight numbers which are less than 100 for cargo flights.

Four-digit numbers in the range 3000 to 5999 typically represent regional affiliate flights, while numbers larger than 6000 are generally codeshare numbers for flights operated by different airlines or even railways.

Likewise, flight numbers larger than 9000 usually refer to ferry flights; these carry no passengers and are used to relocate aircraft to or from a maintenance base, or from one air travel market to another in order to start new commercial flights. Flight numbers starting with 8 are often used for charter flights, but it always depends on the commercial carrier's choice.

The People's Republic of China uses a completely different system for assigning flight segments than most countries; prior to 1988 reformation, there was only one major airline in mainland China, CAAC, which initially used “the first digit of the flight number represents the base airport (1 North China, 2 Northwest China, 3 South China, 4 Southwest China, 5 East China, and 6 Northeast China) for domestic flights, and the end of the number has an odd digit for departures and an even digit for return trips”. In the 1980s, a second digit joined, indicating the destination of flights (many domestic flights of Air China and China Eastern from their base still follow this rule); one-digit and two-digit flight numbers are usually reserved for executive charters or special mission flights (e.g., the flight carrying the black boxes and American investigators of the China Eastern Airlines Flight 5735 in 2022 used flight numbers CA79 and CA80). With the privatization and booming of China's civil aviation, in 2004 the CAAC issued a system for allocating flight numbers across the country, with Air China allocated 1XXX, 4XXX, and 9XX, China Eastern 2XXX and 5XXX, China Southern 3XXX and 6XXX, Hainan Airlines 7XXX, Sichuan and Xiamen 8XXX, and stipulating that Chinese airlines should not use the same flight numbers each other. However, this rule is no longer strictly enforced, as the allocated numbers have become shortage of use.
As a result, there are three special cases in China:
1. Flight number 9XXX may be a regular flight, while smaller number like 500 or 2000 might be used on ferry flights or codeshare ones.
2. Flagship flights do not use one-digit or two-digit flight numbers, and may even be four-digit flight numbers.
3. Flight numbers in the event of an aviation accident will not be retired.

In a codeshare, airlines share their aircraft with others, resulting in the flight having more than one flight number on the same sector, and either the same or different flight numbers on joined sectors.

As an example, QF8412 flies from Dubai to Sydney, but it is codeshared with and operated as EK412. Another example is QF3920, which is a flight from Lima, Peru to Santiago, Chile that is also codeshared with LATAM 523.

Most flights are non-stop from A to B, and few are from A to B then to C (both A-B and B-C have flight number 1). Aircraft type may change due to operation need or unforseen circumstance.

A notable former flight number 1 was British Airways flight BA1, operated by the Concorde between London Heathrow and New York's John F. Kennedy airport. After the retirement of Concorde in 2003 the flight number was retired with it, however in 2009 it was given to the all business class A318 flight between London City Airport and New York JFK via Shannon in Ireland. This route ceased operation in 2020 due to the COVID-19 pandemic, and British Airways has since announced it will not be restarting the service.

Flight numbers are often taken out of use after a crash or a serious incident. Examples include:

• Following the disappearance of Malaysia Airlines Flight 370, the airline changed the flight number for subsequent flights following the same route to MH 318. ^[80]
• American Airlines Flight 77, which regularly flew from Dulles International Airport in Washington, DC, to Los Angeles International Airport, was changed to Flight 149 after it crashed into the Pentagon during the September 11 attacks.^{[81] [82]}
• After the crash of Air France Flight 447 over the Atlantic Ocean, a regular scheduled flight from Rio de Janeiro to Paris, was changed to Air France Flight 445. ^[83]
• Following the crash of Asiana Airlines Flight 214 in July 2013, Asiana change the numbered flight pairs from Asiana Airlines 213/214 to 211/212.^[84]

Other considerations may lead an airline not to change a flight number; there are at least four instances of the same flight numbers that have suffered two serious accidents:

• Flight 315 of Aeroflot (in 1959 and 1960)
• Flight 253 of Linea Aeropostal Venezolana (both in 1956, the first in June, and the second in November)
• Flight 869 of United Arab Airlines (the first in 1962 and the second in 1963)
• Flight 800 of TWA (the first in 1964 and the second in 1996)
• Flight 383 of American Airlines (the first in 1965 and the second in 2016).
• And the instance of the aforementioned "flagship" American Airlines Flight 1, it retains its designation despite three major accidents in 1962, 1941, and 1936. ]
• Flight 251 of Petropavlovsk-Kamchatsky Air (the first in 2012 and second in 2021)
• Flight 5 of Northwest Airlines (the first in 1941 and the second in 1990).
• Flight 63 of American Airlines (the first in July 1943 and the second in October 1943, along with an attempted bombing in 2001 and a near mid-air collision in 2006).

Airline mega mergers, in markets such as the United States, have made it necessary to break conventional flight numbering schemes. Organizations such as IATA, ICAO, ARC, as well as CRS systems and the FAA's ATC systems limit flight numbers to four digits (0001 to 9999). The pool of available flight numbers has been outstripped by demand for them by emergent mega-carriers. As such, some carriers use the same flight number for back-and-forth flights (e.g., DCA-PBI-DCA), or in other cases carriers have assigned a single flight number to a multi-leg flight (e.g., ICT-DAL-HOU-MDW-OMA-DEN-ABQ-LAS-BDL).^[85]

Although 'flight number' is the term used colloquially, the official term as defined in the Standard Schedules Information Manual (SSIM) published annually by the International Air Transport Association (IATA) Schedules Information Standards Committee (SISC), is flight designator. Officially the term 'flight number' refers to the numeric part (up to four digits) of a flight code. For example, in the flight codes QF103 and AF296Q, "103" and "296" are flight numbers. Even within the airline and airport industry, it is common to use the colloquial term rather than the official term.

Flight numbers are also sometimes used for spacecraft, though a flight number for an expendable rocket (say, Ariane 5 Flight 501) might more reasonably be called the serial number of the vehicle used, since an expendable rocket can only be launched once. Space Shuttle missions used numbers with the STS prefix, for example, STS-93. SpaceX uses sequential numbers for flights of reused boosters. As an example, Crew-2 used booster B1061.2 (the second flight of booster B1061).

• Airline call sign
• Codeshare agreement
• Change of gauge (aviation)