AAIB investigation to Boeing 737-8AS, EI-DWS
Unreliable IAS and ALT indications, on descent into Liverpool Airport, 9 January 2016
The aircraft was descending towards Liverpool Airport when the crew was presented with erroneous airspeed and altitude information on one of the two cockpit display systems. The crew took appropriate ‘Airspeed and Altitude Unreliable’ actions in accordance with the Quick Reference Handbook (QRH). In the later stages of the descent the airspeed and altitude returned to normal and the aircraft landed without further incident.
Subsequent diagnosis found the left (commander’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 to moderate icing conditions during its descent. Without the protection afforded by the pitot heater, it is likely that an ice restriction developed within the tube thus corrupting pitot pressure data fed into the aircraft system.
A similar event, to a Boeing 737-86N, EI-FHG, is included in this AAIB Bulletin (8/2016).
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Published: 11 August 2016
Date of occurrence: 9 January 2016