What happened
On 9 January 2016, a Boeing 737-8AS, registration EI-DWS, was performing a commercial passenger flight from Alicante to Liverpool. While descending through FL110, the crew noticed that the commander's airspeed and altitude readings had become erratic. Simultaneously, an 'altn' caption appeared on the electronic engine control (EEC), and the autothrust system began making irregular thrust commands.
At the time of the incident, the aircraft was operating in instrument meteorological conditions (IMC) and encountering light to moderate icing. While the commander's instruments were providing incorrect data, the first officer's instruments and the standby systems remained stable. The crew responded by disengered the autothrust, switching to manual thrust, and following the 'Airspeed Unreliable' and EEC procedures outlined in the Quick Reference Handbook (QRH). After entering a holding pattern at FL70, the aircraft's flight path remained unaffected. As the descent continued, the erroneous readings eventually returned to normal, allowing the aircraft to complete the approach and land at Liverpool Airport without further incident. There were no injuries and no damage to the aircraft.
The investigation
The AAIB investigation focused on why the commander's instruments failed while the first officer's instruments remained functional. Investigators examined the aircraft's pitot probe system, which includes heating elements designed to prevent ice and rain buildup.
Post-flight diagnostics performed by the operator revealed that the heating element within the commander's (left) pitot probe had shorted to its casing. This electrical fault meant the probe was not receiving the necessary heat to prevent ice accumulation. The investigation also noted that there was no cockpit warning indicating a pitot heat malfunction during the flight. While a manufacturer service bulletin existed to modify the wiring so that a master caution would trigger during such a failure, the operator had not yet implemented this modification on this specific aircraft, having scheduled it for a future maintenance period.
Findings
- The primary cause of the erratic flight data was a partial and transient ice blockage within the commander's pitot probe.
- This blockage was made possible by a failure in the pitot probe's heating element, which had shorted to the probe casing.
- The presence of light to moderate icing conditions during the descent facilitated the formation of the ice restriction.
- The blockage was temporary, as the indications returned to normal once the aircraft descended into warmer air.