My son was flying from a Rochester NY to Washington Dulles on flight 4578. They were to land, pickup passengers, then continue to Raleigh. The flight from Rochester was delayed so late arriving in DC. While in the air, the same flight number left DC to Raleigh - obviously different plane. How does that happen?

  • I've written about a similar situation regarding United 237 Delay 1 of the flights and you get 2 flights with the same number in the air at the same time.
    – Karlson
    Commented Jan 9, 2015 at 22:17
  • I think the question @Karlson answers might be marked as a duplicate, even though it does not directly address the legality point.
    – Vince
    Commented Jan 10, 2015 at 2:47
  • @Vince I don't see any reason to address legality of this...
    – Karlson
    Commented Jan 10, 2015 at 3:21
  • @Karlson I suppose you don't because flight codes most likely do not have much to do with legality. Which shows that the OP is confused with what a flight code stands for, overall.
    – Vince
    Commented Jan 10, 2015 at 3:27
  • Related: How are duplicate call signs dealt with? on aviation.se.
    – reirab
    Commented Aug 7, 2015 at 14:20

3 Answers 3


Yes, and No.

Flight numbers do frequently get used for multiple flights on the same day. For example today UA712 is used for both a Chicago to San Diego flight and a subsequent San Diego to San Francisco flight. In this case, both of these flights used the same aircraft (an Airbus A320, United "ship" number 4616).

On Saturday, UA338 will fly from Salt Lake City to Denver, and then Denver to Dallas Fort Worth - but in this case each of those legs will be flown by a separate aircraft.

Even in cases where the same aircraft is scheduled to fly both legs, delays and changes can mean that different aircraft end up being used for the two flights.

So that certainly means that two aircraft with the same flight numbers can be in the air at the same time. In fact, it's even possible that both flights will be very close to each other (a delayed flight arriving, whilst the on-time one after it with the same flight number is departing from the same airport).

To avoid confusion in situations like this, the flight number that the aircraft uses for the purpose of flight planning, air traffic control, etc will be changed in cases like this - frequently by the addition of a letter after the number.

So if flight UA111 is delayed, then the next segment with the same flight number may be referred to as UA111T, or UA111 "tango" or similar. As a traveler on that flight you won't see this - it'll still be referred to as UA111 because from a traveler perspective there's no confusion - there's still only one UA111 departing so there's no opportunity to (for example) catch the wrong flight.

One exception to this is if a flight is delayed until the next day. In this case there might be two flights with the same flight number departing the same airport on the same day. To avoid confusion here, airlines will either change the flight number completely (one of the XX123's becomes XX234), or they will add a letter similar to above (XX123 becomes XX123B). In this case, boarding passes and airport displays would all show the modified flight number so passengers would know which of the two flights they were on.


It's fairly common for a single flight number to cover multiple flight segments (legs): for example, Qantas flight 1 (QF1) is both Sydney to Dubai and Dubai to London.

In the US, it's apparently common to use the same flight number on separate services flown by separate planes, leading to situations like yours. I don't think this serves any practical purpose except allowing airlines to dupe people into buying "direct" flights that aren't.

In most of the world, the same physical plane flies both legs. However, if the originally planned plane is delayed/cancelled for the first leg, and there's lots of passengers waiting for the connection, and the airline happens to have a plane available at the connection point, it's rare but possible for the second leg to be flown by a different physical plane.

Updated to account for American exceptionalism.

  • 4
    In fact it's extremely common for the subsequent flights with the same flight number to be flown by a different aircraft. Happens all day every day in the US.
    – Doc
    Commented Jan 9, 2015 at 7:42
  • QF1 now flies via Dubai.
    – Hugh
    Commented Jan 9, 2015 at 11:31
  • 1
    Well, there are some practical purposes. Preserving flight numbers is one of them. As far as "American exceptionalism" is concerned, you will find that the U.S. is actually quite exceptional in regards to the number of scheduled airline flights, so preserving flight numbers is actually a valid concern with the larger U.S. airlines. Delta, for instance, operates an average of over 5,400 flights per day with nearly a thousand from Atlanta alone. Once codeshares are accounted for, Delta would literally be running out of flight numbers if they didn't do this.
    – reirab
    Commented Aug 7, 2015 at 14:33

You ask if it's legal. Yes, otherwise as for the reasons explained in the other answers, airlines could not operate delayed or multi leg flights using the number,

To expand on the other answers you need to remember that each plane has its own identifiers, so when an airline books a slot with air traffic control it will be saying,for example, Plane GB-AAA , flying from LHR (London heathrow) to BRS (bristol) operating as Flight Ba1111. So then if for some reason there were delays the airline could then book with air traffic control a second flight Plane GB-BBB flying from LHR (London heathrow) to BRS (bristol) operating as Flight Ba1111. which could be in the air at the same time,

Flight numbers identify the route and scheduled timings for the airline, what air traffic control get is a plane identifier, which has to be unique.

  • 3
    Actually what ATC uses is the flight's callsign. In general aviation the callsign is usually the aircraft registration, but scheduled airline flights usually use the flight number as their callsign (with the two-letter IATA prefix replaced with a three-letter ICAO operator code, e.g., BA becomes BAW, pronounced "Speedbird"). Of course, when there are several instances of the same flight number in the same airspace at the same time, one or both of them needs to use a different callsign. Commented Jan 10, 2015 at 15:45
  • 1
    When talking to ATC, your callsign will not be "GB-AAA operating as BA1111." That would be too long and would waste frequency time (which is important in busy airspace.) In the U.S., your callsign will normally be your airline callsign plus your flight number (i.e. "Speedbird 1-1-1-1" for BA1111, as Henning pointed out.) In Europe, sometimes the aircraft registration number is used instead of flight number for airlines, just like it is in GA in the U.S.
    – reirab
    Commented Aug 7, 2015 at 14:18

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