Incident Overview
On 9 January 2016, a Boeing 737-8AS (registration EI-DWS) was descending towards Liverpool Airport after an uneventful flight from Alicante. At approximately 1630 UTC, while at flight level 110, the commander's indicated airspeed (IAS) and altitude indications became erratic. The electronic engine control (EEC) displayed an 'altn' caption, indicating alternate thrust mode due to invalid flight condition data. The first officer's and standby instruments remained normal. The aircraft was in instrument meteorological conditions (IMC) with light to moderate icing and total air temperature (TAT) of +8°C.
Crew Actions and Flight Progress
The first officer was the handling pilot, with autopilot B engaged. The erroneous indications did not affect the flight path. Upon noticing erratic autothrust commands shortly after the 'altn' caption, the crew disengaged autothrust and operated manual thrust. They executed the Quick Reference Handbook (QRH) checklists for 'Airspeed Unreliable' and EEC altn, confirming the first officer's and standby instruments as reliable. The aircraft continued descent and entered a hold at FL70, completing two circuits before receiving radar vectors for an ILS approach to Runway 09 at Liverpool John Lennon Airport. Shortly after leaving the hold, the commander's airspeed and altitude indications returned to normal, with no further anomalies during approach and landing. No cockpit indication of a pitot system malfunction was present.
System Description
The Boeing 737 NG series is equipped with pitot probes mounted on the left and right fuselage aft of the radome. An auxiliary probe is on the right forward fuselage, and two elevator pitot probes on the fin. The probes incorporate heating elements as part of the ice and rain protection systems. The left probe serves the commander's air data system; the right probe serves the first officer's. Separate static orifices are fitted for each system.
Investigation Findings
Post-incident diagnosis by the operator identified that the commander's (left) pitot probe heating element had shorted to its casing. The probe, part number 0851HT, had accumulated 17,864 hours and 10,385 cycles. The faulty heater element prevented adequate ice protection, and given the light to moderate icing conditions, a partial ice blockage likely developed in the pitot tube, corrupting pitot pressure data. The partial nature of the blockage explained the erratic indications rather than a complete loss. As the aircraft descended, the blockage alleviated, restoring normal indications. The aircraft manufacturer had issued Service Bulletin 737-30-1070 in April 2014, introducing a wiring modification to trigger a master caution for pitot heater malfunctions; compliance was at the operator's discretion and had been scheduled for the next deep maintenance package. The aircraft was returned to service after replacing the probe.