| U | Unspecified |
| U | Upper (airway designator) |
| U | UNICOM |
| UAA | Upper Advisory Area |
| UAC | Upper Area Control |
| UACC | Upper Area Control Centre |
| UAD | Upper Advisory Route |
| UAR | Upper Air Route |
| UDA | Upper Advisory Service Area |
| UDR | Upper Advisory Route |
| UFN | Until Further Notice |
| UHF | Ultra High Frequency (300 - 3000 MHz) |
| UIC | Upper Flight Information Centre |
| UIR | Upper Flight Information Region |
| UNCT'L | Uncontrolled |
| UNICOM | Aeronautical Advisory Service |
| UNIL | Unlimited |
| UNLGTD | Unlighted |
| UNREL | Unreliable |
| U/S | Unserviceable |
| USAF | US-Air Force |
| USB | Upper Sideband |
| USCG | US Coast Guard |
| USG | US Gallon |
| USN | US-Navy |
| USRA | Upper Airspace Special Rules Area |
| UTA | Upper Control Area |
| UTC | Co-ordinated Universal Time |
| UWY | Upper Airway |
