| 1 | U0 | Ultra Air S.A.S. | ICAO: ULS, Callsign: Air Ultra, | CO | AVCODES |
| 2 | U1 | Videcom International Limited | Computer Reservations Systems (CRS) | GB | AVCODES |
| 3 | U2 | UFS, Inc. | ICAO: UFS, Callsign: Feeder Flight, Founded in 1993. Ceased operations during 1999. | USA | AVCODES |
| 4 | U2 | Easyjet Airline Company Limited | ICAO: EZY, Callsign: Easy, IATA designator changed to U2. Designator U2* reinstated by IATA | GB | AVCODES |
| 5 | U2 | Easyjet Airline Company Limited | ICAO: EZY, Callsign: Easy, IATA designator changed from U2*. Designator U2* reinstated by IATA | GB | AVCODES |
| 6 | U2 | easyJet UK Limited | ICAO: EZY, Callsign: Easy, | GB | AVCODES |
| 7 | U3 | Avies SA | ICAO: AIA, Callsign: Avies, Founded and commenced operations in 1991. Ceased operations on 01 April 2016 after AOC withdrawn by Estonian CAA. | EST | AVCODES |
| 8 | U3 | Air Plus Argentina, S.A. | ICAO: LMP, Callsign: Air Flight, IATA Prefix: 017, Founded in 1999. Ceased operations during 2002. Name changed from Flight Bus S.A. | RA | AVCODES |
| 9 | U3 | Paramount Jet Ltd | | USA | AVCODES |
| 10 | U3 | Air Company "Sky Gates Airlines" LLC | ICAO: SAY, Callsign: Sky Path, IATA Prefix: 728, AOC issued on 14 September 2016 | RUS | AVCODES |
| 11 | U4 | Burjat Airlines | ICAO: BUT, Callsign: Buravia, | RUS | AVCODES |
| 12 | U4 | Buddha Air Pvt. Ltd. | ICAO: BHA, Callsign: Buddha Air, | NEP | AVCODES |
| 13 | U4 | Progress Multitrade Co. Ltd. | ICAO: PMT, Callsign: Multitrade, IATA Prefix: 253, Founded on 14 January 2003. AOC granted 14 October 2003. Ceased operations during 2008. | K | AVCODES |
| 14 | U4 | Bond Air Cargo, LLC | ICAO: BCU, Callsign: Bacair, IATA Prefix: 029, IATA Designator changed from B7*. | USA | AVCODES |
| 15 | U5 | International Business Air | ICAO: IBZ, Callsign: Interbiz, Designator U5 reinstated by IATA | S | AVCODES |
| 16 | U5 | Brendan Airways, LLC d/b/a USA 3000 | ICAO: GWY, Callsign: Getaway, Founded in 2001. Ceased operations on 30 January 2012. | USA | AVCODES |
| 17 | U5 | Karinou Airlines | ICAO: KRN, Designator U5* recalled by IATA | RCA | AVCODES |
| 18 | U5 | Aura Airlines S.L. | ICAO: GWR, Callsign: Lemon, Ceased operation on 27 October 2022. Name changed from Aura Airlines S.L. t/a Gowair Vaction Airlines. Designator G6 recalled by IATA. | E | AVCODES |
| 19 | U5 | United Nigeria Airlines Company Limited | ICAO: NUA, Callsign: United Nigeria, | WAN | AVCODES |
| 20 | U6 | Ural Airlines | ICAO: SVR, Callsign: Sverdlovsk Air, | RUS | AVCODES |
| 21 | U7 | USA Jet Airlines, Inc. | ICAO: JUS, Callsign: Jet USA, Designator U7 recalled by IATA | USA | AVCODES |
| 22 | U7 | Northern Dene Airways Ltd. | ICAO: NKA, Callsign: Norcanair, Designator U7* recalled by IATA. Name changed from Norcanair | CDN | AVCODES |
| 23 | U7 | Meridiana African Airlines (U) Limited d/b/a Air Uganda | ICAO: UGA, Callsign: Ugdana, IATA Prefix: 926, Founded in 2007. | EAU | AVCODES |
| 24 | U7 | Uniworld Commercial Group, S.A. (Uniworld Air Cargo) | ICAO: UCG, Callsign: Uniworld, IATA Prefix: 399, | PA | AVCODES |
| 25 | U8 | Armavia Aviacompany LLC | ICAO: RNV, Callsign: Armavia, IATA Prefix: 669, Ceased operations 01 April 2013. Founded in 1996 | AM | AVCODES |
| 26 | U8 | Austrian Airtransport (Osterreichische Flugbetriebsgesellschaft MbH.) | ICAO: AAT, Callsign: Austrian Charter, Designator U8 recalled by IATA | A | AVCODES |
| 27 | U8 | Tus Airways Ltd. | ICAO: CYF, Callsign: Tus Air, Established in June 2015. | CY | AVCODES |
| 28 | U9 | Tatarstan State Enterprise Aircompany | ICAO: KAZ, Callsign: Air Tatarstan, IATA Prefix: 966, Name changed to Tatarstan JSC Aircompany | RUS | AVCODES |
| 29 | U9 | OJSC Aircompany Tatarstan | ICAO: TAK, Callsign: Tatarstan, IATA Prefix: 966, AOC revoked by Russian Authorities. Name changed from Tatarstan State Enterprise Aircompany | RUS | AVCODES |
| 30 | U9 | Unilode Aviation Solutions Netherlands BV | Non-airline Unit Load Devices (ULD) companies | NL | AVCODES |
| 31 | UA | United Airlines, Inc. | ICAO: UAL, Callsign: United, IATA Prefix: 016, Name changed from United Air Lines, Inc. | USA | AVCODES |
| 32 | UA | Albatross Airlines | | AUS | AVCODES |
| 33 | UA | United Air Lines, Inc. | ICAO: UAL, Callsign: United, IATA Prefix: 016, Name changed to United Airlines, Inc. | USA | AVCODES |
| 34 | UA | United Airlines | | USA | TRAFFIC |
| 35 | UAA | UAS/AEF Leuchars | | GB | AVCODES |
| 36 | UAA | University Air Squadron | | | ICAO1 |
| 37 | UAAF | UNITED AIR LINES INC (AIR CODE UA) | | | SCAC4 |
| 38 | UAAI | UNITED ARAB AGENCIES INC | | | SCAC4 |
| 39 | UAAR | UNRAU, AARON | | | SCAC4 |
| 40 | UAAU | MOBIUS-GROUPPE EOOD | | BG | BIC |
| 41 | UAAX | American Acid Co. | 8/1920-7/1923 | | NAKINA |
| 42 | UAAX | AMERICAN ACID CO. | | | PWRC |
| 43 | UAB | United Arabian Airlines | Callsign: United Arabian, | SUD | AVCODES |
| 44 | UAB | United Arabian Airlines | | | ICAO1 |
| 45 | UABP | UAB Panevezio autobusu parkas | City: Panevezys | LT | TRAFFIC |
| 46 | UABS | UNITED ABACO SHIPPING | | | SCAC4 |
| 47 | UAC | United Air Charters | Callsign: Unitair, ICAO designator changed from UAC | ZW | AVCODES |
| 48 | UAC | United Air Charters | | | ICAO1 |
| 49 | UAC | UNITED AIRCRAFT CORP. | | | PWRC |
| 50 | UACB | UNIVERSAL AM-CAN LTD (BROKERAGE DIV | | | SCAC4 |
| 51 | UACL | UNIVERSAL AM-CAN, LTD | | | SCAC1 |
| 52 | UACL | UNIVERSAL AM-CAN LTD | | | SCAC4 |
| 53 | UACS | UNIVERSAL AM-CAN US | | | SCAC4 |
| 54 | UACU | HAPAG LLOYD A.G | | D | BIC |
| 55 | UACU | UNITED ARAB SHIPPERS | | | PWRC |
| 56 | UACU | United Arab Shipping | | | RSRV |
| 57 | UACU | UNITED ARAB SHIPPING CO SAG | | | SCAC4 |
| 58 | UACW | UNITED ARAB CHEMICAL CARRIERS LIMIT | | | SCAC4 |
| 59 | UACX | Universal Atlas Cement Co. | 7/1932; 1/1941-1/1952; eliminated 4/1953 | | NAKINA |
| 60 | UACX | Universal Atlas Cement Division of U.S. Steel Corp. | eliminated 7/1965 | | NAKINA |
| 61 | UACX | Universal Atlas Cement Co. | Jan-52 | | NRHS |
| 62 | UACX | Universal Atlas Cement Division of U.S. Steel Corp. | eliminated 7/65 | | NRHS |
| 63 | UACX | UNIVERSAL ATLAS CEMENT DIV., U.S. STEEL CORP. | | | PWRC |
| 64 | UAD | UAS/AEF Colerne | | GB | AVCODES |
| 65 | UAD | Univex Srl | | I | AVCODES |
| 66 | UAD | University Air Squadron | | | ICAO1 |
| 67 | UAE | Emirates | IATA: EK, Callsign: Emirates, IATA Prefix: 176, | UAE | AVCODES |
| 68 | UAE | Emirates Airline | IATA: EK | | ICAO1 |
| 69 | UAE | Emirates | | UAE | TRAFFIC |
| 70 | UAEA | United Aram Emirates Army | | UAE | TRAFFIC |
| 71 | UAEU | HAPAG LLOYD A.G | | D | BIC |
| 72 | UAEU | UNITED ARAB SHIPPING CO (S.A.G.) | | | PWRC |
| 73 | UAEU | United Arab Shipping | | | RSRV |
| 74 | UAEU | UNITED ARAB SHIPPING CO SAG | | | SCAC4 |
| 75 | UAF | United Arab Emirates Air Force | Callsign: Uniforce, | UAE | AVCODES |
| 76 | UAF | United Arab Emirates Air Force | | | ICAO1 |
| 77 | UAFU | UAFL UNITED AFRICA FEEDER LINE | | MUS | BIC |
| 78 | UAG | UAS/AEF Cambridge | | GB | AVCODES |
| 79 | UAG | AS Union Aviation | Callsign: Sakta, | LV | AVCODES |
| 80 | UAG | Afra Airlines | | | ICAO1 |
| 81 | UAH | UAS/AEF Newton | | GB | AVCODES |
| 82 | UAH | AEF Cranwell | | GB | AVCODES |
| 83 | UAH | University Air Squadron | | | ICAO1 |
| 84 | UAI | Union Africaine Des Transports | Callsign: Unair, | CI | AVCODES |
| 85 | UAI | Uni-Air International | Callsign: Uniair, | F | AVCODES |
| 86 | UAI | NIUAS/13 AEF | | GB | AVCODES |
| 87 | UAI | Union Africaine des Transports | | | ICAO1 |
| 88 | UAIN | UTILITY AVIATION, INC. | | | SCAC4 |
| 89 | UAIX | United Asphalts, Inc. | added 10/1992; 1/1994-7/2005; eliminated 1/2007 | | NAKINA |
| 90 | UAIX | United Asphalts, Inc. | added 10/92; 1/94-7/2000 | | NRHS |
| 91 | UAIX | UNITED ASPHALTS, INC. | | | PWRC |
| 92 | UAIX | United Asphalts | | | RSRV |
| 93 | UAIX | UNITED ASPHALTS, INC. | | | SCAC4 |
| 94 | UAJ | UAS/AEF Glasgow | | GB | AVCODES |
| 95 | UAJ | University Air Squadron | | | ICAO1 |
| 96 | UAK | Kiev Aviation Plant | Callsign: Aviation Plant, Ceased operations. | UA | AVCODES |
| 97 | UAK | Kiev Aviation Plant | | | ICAO1 |
| 98 | UAKU | ALARA LOGISTICS | | USA | BIC |
| 99 | UAL | United Airlines, Inc. | IATA: UA, Callsign: United, IATA Prefix: 016, Name changed from United Air Lines, Inc. | USA | AVCODES |
| 100 | UAL | United Air Lines, Inc. | IATA: UA, Callsign: United, IATA Prefix: 016, Name changed to United Airlines, Inc. | USA | AVCODES |
