What happened
On 7 January 2012, a Boeing 737-800, registration EI-DHI, was descending toward Riga Airport, Latvia, in weather conditions characterized by moderate snow. During the descent from 6,000 ft, the flight crew observed that the indicated airspeed readings on the two primary flight displays began to diverge. The airspeed on the First Officer's side was decreasing rapidly, while the Captain's side showed an increasing speed.
Following the detection of the airspeed disagreement, the crew performed necessary checklists and stabilized the aircraft at 4,000 ft. During this period, several other issues emerged, including an altitude disagreement and an engine warning related to the Electronic Engine Control (EEC) entering alternate mode. As the aircraft continued its approach, the autopilot and autothrottle disconnected, forcing the crew to fly the aircraft manually.
During the later stages of the approach, as the crew selected flaps and decelerated, the stall warning (stick shaker) activated on the First Officer's side. This warning persisted through the landing. Despite the distracting nature of the stick shaker and the unreliable airspeed indications, the crew successfully landed the aircraft without any injuries to the 140 passengers or 6 crew members.
The investigation
The AAIU investigation focused on the technical cause of the airspeed divergence and the failure of the cockpit warning systems. Maintenance inspections following the incident revealed that the First Officer's pitot probe heater had suffered a short circuit. The probe, which had been in service for over 23,000 hours, showed signs of corrosion and damage to the nose tip.
Investigators examined why the crew was not alerted to the heater failure. They found that while the pitot heaters were switched on, the failure warning lights did not illuminate. This was because the electrical short allowed the heater to continue drawing current, which prevented the system from detecting the malfunction. The investigation also noted that the operator had experienced multiple similar unannounced pitot heater failures across its fleet.
Findings
- The primary cause of the airspeed disagreement was the failure of the First Officer's pitot probe heater.
- The aircraft's pitot heater failure warning system was unable to detect and announce the heater failure because the short circuit allowed the system to continue drawing current.
- The activation of the stick shaker during the approach was a secondary consequence of the unreliable airspeed data.
- The existing Failure Modes and Effects Analysis (FMEA) for the aircraft did not fully account for the consequences of unannounced pitot heater failures, such as subsequent stall warnings.