Flag

A warning device incorporated in certain airborne navigation and flight instruments indicating that:
  1. Instruments are inoperative or otherwise not operating satisfactorily, or
  2. Signal strength or quality of the received signal falls below acceptable values.

Source: FAA Pilot/Controller Glossary (definition from the 2023 edition; current: Change 1 of 7 August 2025).

Also listed as: Flag Alarm.

Related terms

Glossary last updated 2026-10-28.

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