AAIB investigation to Boeing 737-86N, EI-FHG
Unreliable airspeed and altitude indications, on approach to London Gatwick Airport, 8 January 2016
Summary:
The aircraft was on approach to London Gatwick when the crew was presented with erroneous airspeed and altitude information on one of the two cockpit display systems. The crew took appropriate ‘IAS Disagree, ALT Disagree and Airspeed Unreliable’ actions in accordance with the Quick Reference Handbook (QRH). In the final stage of the approach the airspeed and altitude returned to normal and the aircraft landed without further incident.
Subsequent diagnosis found the right (first officer’s) pitot probe to have a fault with its ice and rain protection heating element. Ordinarily this would not have affected the operation of the probe but the aircraft had encountered light icing conditions during its descent. Without the protection afforded by the pitot heater, it is likely that an ice restriction developed within the tube, corrupting pitot pressure data fed into the aircraft system.
A similar event, to a Boeing 737-8AS, EI-DWS, is included in AAIB Bulletin (8/2016).
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