Flag/Flag Alarm
A flag or flag alarm is a warning device on a flight instrument (typically a VOR/ILS CDI or an HSI) that indicates the instrument or navigation signal is unreliable. When the flag appears, the instrument reading should not be trusted for navigation.
When the Flag Appears
- Aircraft is outside the usable range of the navaid
- Aircraft is in a cone of silence directly above the VOR
- Navaid is undergoing maintenance or has failed
- Aircraft electrical power has failed (some flags are power-failure warning devices)
- ILS glideslope or localizer signal has been interrupted
Response to a Flag
A flag on a navigation instrument means: do not use that instrument for navigation. Cross-check with another navaid, GPS, or ATC radar. On an ILS approach, a flag during the critical phase requires an immediate missed approach. Never descend below MDA or continue below DH with a flagged instrument.