Flight number

In the aviation industry, a flight number or flight designator is a code for an airline service consisting of two-character airline designator and a 1 to 4 digit number. For example, QF9 is a Qantas Airways service from Perth, Australia to London Heathrow. A service is called "direct" if it is covered by a single flight number, regardless of the number of stops or equipment changes. For example, QF1 flies from Sydney to Singapore to London on Qantas Airways. A given flight segment may have multiple flight numbers on different airlines under a code-sharing agreement. Strictly speaking, the flight number is just the numerical part, but it is commonly used for the entire flight designator.

The flight designator of the operating carrier of a commercial flight is used as a call sign. This is distinct from the aircraft's registration number, which identifies a specific airplane.

Conventions
A number of conventions have been developed for defining flight numbers, although these vary widely from airline to airline, and are increasingly being modified. 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, for airlines in Mainland China, one-digit and two-digit numbers are only reserved for administrative charters. Furthermore, 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.

Codeshare
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.

List of flight number 1 by airline
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.

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 number changes
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.
 * 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.
 * 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.
 * 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.

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 number conservation
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).

Flight designator
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.

Spacecraft
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).