| 801 | UIT | Universitetet I Tromso | Callsign: Artic, ICAO Callsign changed from Spirit. | N | AVCODES |
| 802 | UITA | UNITED INTERMODAL TRANSPORT INC | | | SCAC4 |
| 803 | UITK | UNIFIED INTERMODAL TRANSPORT INC | | | SCAC4 |
| 804 | UITT | U.S. TRANS TECH, INC | | | SCAC1 |
| 805 | UITT | US TRANS TECH | | | SCAC4 |
| 806 | UIVL | UMUGISHA INVESTMENT LLC | | | SCAC4 |
| 807 | UIVQ | UNIVERSITY MOVING & STORAGE | | | SCAC4 |
| 808 | UIWX | Union Iron Works, subsidiary of Riley Stoker Corp. | added 1/1968; eliminated 4/1970 | | NAKINA |
| 809 | UIWX | Riley Stoker Corp. | added 4/1970; 4/1970-1/1994; eliminated 7/1998 | | NAKINA |
| 810 | UIWX | Union Iron Works, subsidiary of Riley Stoker Corp. | added 1/68; eliminated 4/70 | | NRHS |
| 811 | UIWX | Riley Stoker Corp. | added 4/70; 4/70-1/94; eliminated 7/98 | | NRHS |
| 812 | UIWX | RILEY STOKER CORP. [UNION IRON WORKS] | | | PWRC |
| 813 | UIWX | Riley Stoker Corporation | | | RSRV |
| 814 | UIZ | ULEN INDUSTRIES | | | PWRC |
| 815 | UJ | Montair Aviation, Inc. | Designator UJ reinstated by IATA. Designator UJ recalled by IATA | CDN | AVCODES |
| 816 | UJ | State Airline Ukraine National Airlines | IATA designator changed to UR | UA | AVCODES |
| 817 | UJ | USA Jet Airlines, Inc. | ICAO: JUS, Callsign: Jet USA, Designator U7 recalled by IATA | USA | AVCODES |
| 818 | UJ | Uganda Air Limited | Callsign: Ugandajet, | EAU | AVCODES |
| 819 | UJ | Air Lanka Limited | ICAO: ALK, Callsign: Air Lanka, Commenced operations on 01 September 1979. IATA designator changed to UL | CL | AVCODES |
| 820 | UJ | Sun Freight Logistics Company Limited | ICAO: SFG, | T | AVCODES |
| 821 | UJ | Almasria Universal Airlines | ICAO: LMU, Callsign: Almasria, | ET | AVCODES |
| 822 | UJ | USA Jet Airlines, Inc. | ICAO: JUS, Callsign: Jet USA, Designator U7 recalled by IATA | USA | AVCODES |
| 823 | UJC | Ultimate Jetcharters, Inc. | IATA: UE, Callsign: Ultimate, Name changed to Ultimate Jetcharters LLC dba Ultimate Air Shuttle. IATA designator changed from UI. | USA | AVCODES |
| 824 | UJC | Ultimate Jetcharters, Inc. | IATA: UI, Callsign: Ultimate, IATA designator changed to UE. | USA | AVCODES |
| 825 | UJC | Ultimate Jetcharters LLC dba Ultimate Air Shuttle | IATA: UE, Callsign: Ultimate, Name changed from Ultimate Jetcharters, Inc. IATA designator changed from UI. | USA | AVCODES |
| 826 | UJEVE | Ulrich Joh. Evers GmbH & Co. KG | | DE | VKM |
| 827 | UJIL | US JESCO INTERNATIONAL LTD INC | | | SCAC4 |
| 828 | UJR | Universal Jet Rental de Mexico, S.A. de C.V. | Callsign: Universal Jet, | MEX | AVCODES |
| 829 | UJR | Universal Jet Rental de Mexico | | | ICAO1 |
| 830 | UJS | Eurojet Service | | RUS | AVCODES |
| 831 | UJSD | UNIFORMES JULIAN SA DE CV | | | SCAC4 |
| 832 | UJT | Universal Jet Aviation, Inc. | Callsign: Uni-jet, Name changed to Journey Aviation, LLC | USA | AVCODES |
| 833 | UJT | Universal Jet Aviation | | | ICAO1 |
| 834 | UJUI | UJU INC | | | SCAC4 |
| 835 | UJX | AtlasJet Ukraine | IATA: UH, Callsign: Atlas Ukraine, IATA Prefix: 500, Ceased operations. | UA | AVCODES |
| 836 | UK | KLM uk Ltd. | ICAO: UKA, Callsign: Ukay, IATA Prefix: 130, Name changed to Buzzaway Limited. Name changed from Air Uk Ltd. | GB | AVCODES |
| 837 | UK | Air Uk Ltd. | ICAO: UKA, Callsign: Ukay, IATA Prefix: 130, Name changed to KLM uk Ltd. | GB | AVCODES |
| 838 | UK | British Island Airways Ltd. (BIA) | Callsign: Britisland, IATA designator changed to KD | GB | AVCODES |
| 839 | UK | Buzzaway Limited | ICAO: UKA, Callsign: Ukay, IATA Prefix: 130, Name changed from KLM uk Ltd. Application for AOC withdrawn. Name changed from Air Uk Ltd. | GB | AVCODES |
| 840 | UK | TATA SIA Airlines Limited t/a Vistara | ICAO: VTI, Callsign: Vistara, IATA Prefix: 228, Joint venture between India's Tata Sons and Singapore Airlines. | IND | AVCODES |
| 841 | UKA | KLM uk Ltd. | IATA: UK, Callsign: Ukay, IATA Prefix: 130, Name changed to Buzzaway Limited. Name changed from Air Uk Ltd. | GB | AVCODES |
| 842 | UKA | Air Uk Ltd. | IATA: UK, Callsign: Ukay, IATA Prefix: 130, Name changed to KLM uk Ltd. | GB | AVCODES |
| 843 | UKA | Buzzaway Limited | IATA: UK, Callsign: Ukay, IATA Prefix: 130, Name changed from KLM uk Ltd. Application for AOC withdrawn. Name changed from Air Uk Ltd. | GB | AVCODES |
| 844 | UKA | Aero Flightops UK Limited | IATA: FO, Callsign: Black Jet, | GB | AVCODES |
| 845 | UKA | Buzzaway Limited | | | ICAO1 |
| 846 | UKFI | UNIK FORWARDING INC | | | SCAC4 |
| 847 | UKI | UK International Airlines Ltd | Callsign: Khaliq, ICAO Callsign changed to Magenta | GB | AVCODES |
| 848 | UKI | UK International Airlines Ltd | Callsign: Magenta, ICAO Callsign changed from Khaliq | GB | AVCODES |
| 849 | UKI | UK International Airlines | | | ICAO1 |
| 850 | UKIC | UR-KI INC | | | SCAC4 |
| 851 | UKKX | United Carbon Co. | 7/1930-1/1941 | | NAKINA |
| 852 | UKKX | United Carbon Co. | Apr-38 | | NRHS |
| 853 | UKKX | UNITED CARBON CO. | | | PWRC |
| 854 | UKL | PJSC Ukraine-Airalliance | Callsign: Ukraine Alliance, IATA Prefix: 271, Founded in 1992. | UA | AVCODES |
| 855 | UKL | Air UK Leisure Ltd. | IATA: MV, Callsign: Leisure, Name changed to Leisure International Airways on 01 May 1997. | GB | AVCODES |
| 856 | UKL | Ukraine Air Alliance | | | ICAO1 |
| 857 | UKM | Ukrainian Mediterranean Airlines Ltd. | IATA: UF, Callsign: Ukraine Mediterranee, IATA Prefix: 828, | UA | AVCODES |
| 858 | UKM | Ukraine Mediterranean Airlines Ltd t/a UM Air Airlines | IATA: UF, Callsign: Ukraine Mediterranee, IATA Prefix: 828, Ceased operation during 2018. | UA | AVCODES |
| 859 | UKM | UM Airlines | IATA: UF | | ICAO1 |
| 860 | UKMI | ULJANIK SHIPMANAGEMENT INC | | | SCAC4 |
| 861 | UKN | Ukraine Air Enterprise | Callsign: Enterprise Ukraine, | UA | AVCODES |
| 862 | UKN | Ukraine Air Enterprise | | | ICAO1 |
| 863 | UKO | Yukos | Callsign: Yukos Bulgaria, Name changed to Naftex Bulgaria Holding | BG | AVCODES |
| 864 | UKO | Naftex Bulgaria Holding | Callsign: Naftex Bulgaria, Name changed from Yukos. Ceased operations. | BG | AVCODES |
| 865 | UKP | UK Home Office | Callsign: Police, | GB | AVCODES |
| 866 | UKP | United Kingdom Home Office | | | ICAO1 |
| 867 | UKR | Air Ukraine | IATA: 6U, Callsign: Air Ukraine, IATA Prefix: 891, Founded in 1992. Suspended operations in 2002. AOC revoked in 2004. | UA | AVCODES |
| 868 | UKR | Air Ukraine | IATA: 6U | | ICAO1 |
| 869 | UKRU | SETTI | | UA | BIC |
| 870 | UKS | Ukrainian Cargo Airways | IATA: 6Z, Callsign: Cargotrans, IATA Prefix: 516, Designator 6Z* recalled by IATA | UA | AVCODES |
| 871 | UKS | Ukrainian Cargo Airways | IATA: 6Z | | ICAO1 |
| 872 | UKT | Atlantic Bridge Aviation Ltd. | Callsign: Cloud, | GB | AVCODES |
| 873 | UKT | UKTUS, Aircompany | Callsign: Uktus, | RUS | AVCODES |
| 874 | UKTC | UK TRUCKING LLC | | | SCAC4 |
| 875 | UKTL | UNITED KING TRANSPORT LTD | | | SCAC4 |
| 876 | UKTN | UNIK TRANSPORTATION LLC | | | SCAC4 |
| 877 | UKU | Second Sveridlovsk Air Enterprise | Callsign: Pyshma, | RUS | AVCODES |
| 878 | UKU | Second Sverdlovsk Air Enterprise | | | ICAO1 |
| 879 | UKW | Lviv Airlines | IATA: 5V, Callsign: Ukraine West, IATA Prefix: 062, Ceased operations. Founded in 1992. | UA | AVCODES |
| 880 | UKW | Lviv Airlines | IATA: 5V | | ICAO1 |
| 881 | UKZ | Zoom Airlines Ltd | IATA: ZX, Callsign: Flyzoom, | GB | AVCODES |
| 882 | UL | SriLankan Airlines Limited | ICAO: ALK, Callsign: Srilankan, IATA Prefix: 603, Formerly Air Lanka Limited. | CL | AVCODES |
| 883 | UL | Air Lanka Limited | ICAO: ALK, Callsign: Air Lanka, IATA Prefix: 603, Name changed to Srilankan during 1998. IATA designator changed from UJ | CL | AVCODES |
| 884 | UL | Lansa S.r.l. | | HN | AVCODES |
| 885 | ULA | Ultraair, LLC | Callsign: Ultrajet, | USA | AVCODES |
| 886 | ULA | Zuliana de Aviacion .C.A. | IATA: OD, Callsign: Zuliana, Designator OD recalled by IATA | YV | AVCODES |
| 887 | ULAC | ULTRA AIR CARGO INC | | | SCAC4 |
| 888 | ULB | Ulba | Callsign: Uskam, | KZ | AVCODES |
| 889 | ULC | Airlanka Cargo Limited | Callsign: Airlanka Cargo, | CL | AVCODES |
| 890 | ULC | Albinati Aviation Ltd. | Callsign: Albi, | M | AVCODES |
| 891 | ULCE | U L C EXPRESS | | | SCAC4 |
| 892 | ULCH | ULCH TRANSPORT LIMITED | | | SCAC4 |
| 893 | ULCN | USA LOGISTIX CORPORATION | | | SCAC4 |
| 894 | ULCU | UNICO LOGISTICS CO LTD | | ROK | BIC |
| 895 | ULCU | UNICO LOGISTICS | | | SCAC4 |
| 896 | ULCW | UNICO LOGISTICS CO LTD | | | SCAC4 |
| 897 | ULCX | Unit Load Car Corp. | added 7/1959; 4/1961-4/1970; eliminated 10/1974 | | NAKINA |
| 898 | ULCX | Unit Load Car Corp. | added 7/59; 4/61-4/70; eliminated 10/74 | | NRHS |
| 899 | ULCX | UNIT LOAD CAR CORP. | | | PWRC |
| 900 | ULCZ | ULRIZH CEMENT CO. | | | PWRC |
