Unreliable Airspeed describes a situation where the airspeed indicators on an aircraft are providing inaccurate or unreliable readings.

Unreliable airspeeds can be recognised by:

• different airspeed indications between ASIs by 5 knots or more
• unexpected aircraft behaviour and attitudes
• buffeting
• aircraft system warnings
• Comparing with GPS speed indications

Ways to prevent such situations include:

• Inspect all pitot heads and static ports during the pre-flight inspection.
• Use pitot/static heat.
• Understand the relationship between the ADC, the autopilot and flight director. Disconnect the latter two, plus the autothrust.
• Monitor primary flight path parameters (pitch attitude, thrust setting and IAS) during periods of potential icing.
• Be aware of normal attitudes and power/thrust settings for the various phases of flight.
• Keep away from the low- and high-speed ends of the flight envelope.
• Revert to safe default parameters for pitch attitudes and thrust settings.
• Remain in VMC, if possible.
• Advice GPS speed indications.