Atlas/ Glossary/ FLAG Abbreviation FLAG. FLAG - A warning device incorporated in certain airborne navigation and flight instruments indicating that: Source FAA Pilot/Controller Glossary → Related — Abbreviation. ABEAM ABEAM ABORT ABORT — To terminate a preplanned aircraft maneuver AFM AFM — airplane flight manual AGL AGL — above ground level ALS ALS — approach light system APU APU — auxiliary power unit