If the primary air data system is unreliable, what should be used to maintain critical flight data?

• A. Standby instruments and follow procedure.
• B. Switch to relying on outside visual cues alone.
• C. Use GPS altitude and airspeed exclusively.
• D. There is no alternative; continue as planned.

Answer: (A)

When the primary air data system is unreliable, you rely on standby instruments that provide essential flight information independently from the main system. These instruments—such as the standby attitude indicator, standby airspeed, and standby altimeter (often with a vertical speed indicator)—are powered separately so you can maintain control of the aircraft even if the primary sensors are giving erroneous data. Following the established procedure for unreliable air data guides you to set and verify critical values, maintain safe airspeed and altitude, and keep the airplane properly pitched and configured. Relying on outside visual cues alone isn’t enough in instrument conditions, since they don’t give precise, dependable data. GPS altitude and airspeed aren’t designed to substitute for the primary or standby air data sources in real time, so they can’t safely replace the backup instruments.