Serious incident Boeing 737-8AS (WL) EI-DWS,
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ASN Wikibase Occurrence # 189336
 
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Date:Saturday 9 January 2016
Time:16:30
Type:Silhouette image of generic B738 model; specific model in this crash may look slightly different    
Boeing 737-8AS (WL)
Owner/operator:Ryanair
Registration: EI-DWS
MSN: 33625/2472
Fatalities:Fatalities: 0 / Occupants: 179
Aircraft damage: None
Category:Serious incident
Location:Liverpool-John Lennon International Airport (LPL/EGGP) -   United Kingdom
Phase: En route
Nature:Passenger - Scheduled
Departure airport:Alicante Airport (ALC/LEAL)
Destination airport:Liverpool-John Lennon International Airport (LPL/EGGP)
Investigating agency: AAIB
Confidence Rating: Accident investigation report completed and information captured
Narrative:
After an uneventful flight from Alicante (FR9887) the aircraft had commenced its descent into Liverpool. At FL110 the commander’s indicated airspeed (IAS) and altitude (ALT) indications became erratic followed by an altn caption on the electronic engine control (the altn caption indicates that the engine EEC is in alternate thrust setting mode in this case due to invalid
flight condition data).
The first officer’s and the standby instruments were unaffected and appeared normal. The aircraft was in light to moderate icing conditions, in IMC, with a Total Air Temperature (TAT) of +8ºC. At the time the first officer was the handling pilot and autopilot B was engaged. The erroneous indications had no effect on the aircraft flight path. The crew observed that shortly after the altn caption illuminated the autothrust made erratic thrust commands so they disengaged the autothrust and flew manual thrust for the remainder of the flight. The QRH checklist for ‘Airspeed Unreliable’ and EEC altn was actioned and it was confirmed that the first officer’s and standby instruments were reliable data sources. The aircraft continued its descent and took up a hold at FL70 where two circuits were flown before radar vectors to the ILS Runway 09 (LPL) were resumed. Shortly after leaving the hold as the aircraft descended, the commander’s airspeed and altitude indications returned to normal and there were no further erroneous readings during the approach and landing. There was no cockpit indication of a pitot system malfunction throughout the incident.

Conclusion
The erroneous airspeed and altitude indication was likely to have been caused by partial and transient ice blockage of the commander’s pitot probe whilst in light to moderate icing conditions. This was as a result of the failure of the pitot probe heating element. The partial icing conclusion is supported by the fact that the blockage alleviated itself in the descent
when the IAS and ALT information returned to normal.

Sources:

AAIB

Accident investigation:
cover
  
Investigating agency: AAIB
Report number: 
Status: Investigation completed
Duration:
Download report: Final report


Revision history:

Date/timeContributorUpdates
17-Aug-2016 14:05 harro Added

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