What happened
On 8 January 2016, a Boeing 737-86N, registration EI-FHG, was performing a commercial passenger flight approaching London Gatwick Airport. While descending through FL130 in light icing conditions, the crew observed that the right-side airspeed indicator had dropped significantly from 250 kt to 115 kt, while the altitude reading differed from the left-side display by 200 ft. This discrepancy triggered an 'IAS and ALT disagree' master caution.
The crew implemented the appropriate procedures from the Quick Reference Handbook (QRH) for unreliable airspeed and disagreed altitude. The commander maintained the autopilot, as the aircraft's attitude and other parameters appeared stable. During the descent, the crew noted that the autothrust was operating with degraded N1 thrust limits. At the flap 1 configuration, the stick shaker activated, though the crew determined the aircraft's ground speed and airspeed were consistent with the left-side instruments, so no recovery maneuvers were required.
During the final stages of the approach, the right-side airspeed and altitude readings returned to alignment with the left-side displays, and the mismatch caution disappeared. The aircraft landed safely without further incident, though an engine control caution appeared after touchdown.
The investigation
The AAIB investigation focused on the pitot-static system and the cause of the erroneous data. The investigation established that the aircraft had been flying through light icing conditions at the time of the discrepancy. Subsequent technical inspections by the operator revealed that the resistance of the first officer's pitot probe heating element was outside of acceptable limits.
Investigators also examined the aircraft's monitoring capabilities. It was noted that there was no cockpit indication of a pitot system malfunction on the window/pitot heat panel during the event. While a manufacturer Service Bulletin existed to modify the heater wiring to trigger a master caution during such failures, the operator had not yet implemented this modification.
Findings
- The erroneous flight data was caused by a partial and transient ice blockage within the first officer's pitot probe.
- This blockage occurred because the heating element in the right-side pitot probe had failed.
- The failure of the heating element allowed ice to accumulate in the probe during the descent through icing conditions.
- The blockage was transient, as the data returned to normal during the final approach, suggesting the ice partially cleared as the aircraft descended.