History of Flight
During descent and approach to London Gatwick Airport in light icing conditions, the crew of Boeing 737-86N, registration EI-FHG, observed the right-side airspeed indications decrease from 250 kt to 115 kt and the altitude differ from the left-side indication by 200 ft. This was immediately followed by an 'IAS and ALT disagree' master caution. The crew initiated the 'IAS Disagree, ALT Disagree and Airspeed Unreliable' checklist actions as per the Quick Reference Handbook (QRH), confirming the left-side indications were correct and the right-side at fault. The commander chose not to disconnect the autopilot, being content with the aircraft attitude and system parameters. The descent continued from FL130 into a normal approach. At FL70, the crew noted that the autothrust was not receiving 'N1 limiting' values and was using degraded N1 thrust limits from the Flight Management Computer. After selecting flap 1, the stick shaker activated, but attitude and N1 were within limits, and GPS ground speed corroborated airspeed, so no recovery action was taken. During the final stage of the approach, the first officer's airspeed indicator rose to 220 kt and altitude rose to 300 ft, in agreement with left-side displays, and the 'IAS and ALT disagree' caution cleared. The aircraft landed without further incident, though the Electronic Engine Control (EEC) altn1 caution illuminated after landing but required no action. There was no cockpit indication of pitot system malfunction throughout the incident.
System Description
The Boeing 737 NG series, including EI-FHG, are fitted with pitot probes mounted on the left and right of the fuselage just aft of the radome, an auxiliary probe on the right side of the forward fuselage, and two 'elevator pitot probes' on the fin. The probes incorporate heating elements as part of the aircraft ice and rain protection systems. The left-side probe is referred to as the commander's probe, and the right-side as the first officer's probe. There is also a static plate with separate orifices for each air data system.
Analysis
About the time of the incident, the commander noted entering light icing conditions, observing its 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's responses to the crew's actions confirmed a blockage within the first officer's pitot probe as the probable cause. The partial recovery of airspeed and altitude indications suggested only a partial blockage, which alleviated as the aircraft descended. The operator's fault diagnosis found the first officer's pitot heater resistance out of limits. The probe (Part No 0851HT) had accrued 20,232 hours and 11,526 cycles. The aircraft manufacturer had issued Service Bulletin (SB) 737-30-1070 in April 2014, introducing a minor modification to the pitot probe heater wiring to ensure a master caution appears in the event of a heater malfunction; compliance was at the operator's discretion and was under consideration.
Conclusion
The erroneous airspeed and altitude indications were likely caused by partial and transient ice blockage of the first officer's pitot probe while in light icing conditions, resulting from failure of the pitot probe heating element. The partial icing conclusion is supported by the self-clearing of the blockage during final approach, when the IAS and ALT information returned to normal.