Norwegian International B738 at London on Jan 8th 2016, unreliable airspeed

Last Update: August 11, 2016 / 16:37:52 GMT/Zulu time

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Incident Facts

Date of incident
Jan 8, 2016

Classification
Report

Airline
Norwegian Air International

Flight number
D8-2859

Departure
Stockholm, Sweden

Destination
London Gatwick, United Kingdom

Aircraft Registration
EI-FHG

Aircraft Type
Boeing 737-800

ICAO Type Designator
B738

A Norwegian Air International Boeing 737-800, registration EI-FHG performing flight D8-2859 from Stockholm (Sweden) to London Gatwick,EN (UK) with 78 passengers and 6 crew, was descending towards London Gatwick Airport when the right hand airspeed indications dropped from 250 to 115 KIAS and the altimeters differed by 200 feet immediately followed by a master caution for IAS and ALT disagree. The crew worked the "IAS Disagree, ALT Disagree, Unreliable Airspeed" checklists and confirmed the left hand indication was correct while the right hand one was false. The captain (50, ATPL, 13,385 hours total, 9,985 hours on type) decided to not disconnect the autopilot being content with the aircraft's attitude and system parameters and continued the descent from FL130 to a normal approach. Descending through FL070 the flight crew noticed that autothrust was not getting N1 limiting values. After selecting flaps 1 the stick shaker activated, although attitude and N1 were within limits, GPS ground speed and airspeed corroborated, hence the crew took no recovery action. During the final stages of the approach the first officer's airspeed rose to 220 KIAS and the altimeter increased 300 feet getting in line with the left hand indications, the IAS and ALT disagree indications disappeared. The crew continued for a normal and safe landing. After landing the EEC ALTN caution activated.

The AAIB released their bulletin reporting that at no time during the flight a fault indication for the pitot system appeared. The AAIB concluded the probable cause of the occurrence was:

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

The AAIB analysed:

About the time of the incident the commander noted they had entered light icing conditions observing the effect on the windscreen. When the ‘IAS and ALT disagree’ caution illuminated he concluded the cause to be a frozen pitot probe or static port. The aircraft responses to the various actions taken by the crew confirmed that a blockage within the first officer’s pitot probe was the probable cause. Although the airspeed and altitude information was unreliable, there was not a complete loss of information which suggests only a partial blockage of the probe which alleviated as the aircraft continued its descent.
Incident Facts

Date of incident
Jan 8, 2016

Classification
Report

Airline
Norwegian Air International

Flight number
D8-2859

Departure
Stockholm, Sweden

Destination
London Gatwick, United Kingdom

Aircraft Registration
EI-FHG

Aircraft Type
Boeing 737-800

ICAO Type Designator
B738

This article is published under license from Avherald.com. © of text by Avherald.com.
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