While descending towards Riga, in poor weather conditions with moderate snow, the indicated airspeed (IAS) readings began to diverge. The Flight Crew decided, following evaluation, that the IAS displayed on the First Officer’s (F/O) side was incorrect. Airspeed disagreement and other warnings then activated. Following completion of checklists an ILS approach to Runway (RWY) 18 was commenced with Air Traffic Control (ATC) actively monitoring the aircraft. During the approach both the autopilot and autothrottle disconnected and the approach was continued hand flown. During the later stages of the approach the stall warning (stick shaker) activated on the F/O’s side and this continued until after the landing.
Subsequent maintenance action found that, although the pitot heater on the F/O’s side had failed due to a short circuit, the pitot heater failure warning had not activated because the design of the warning system may not detect failures of this nature.
As a result of the Investigation, two Safety Recommendations are issued to the aircraft’s Manufacturer regarding the design of the pitot heater failure warning system and the guidance provided to flight crew. A further Safety Recommendations is issued to the Federal Aviation Administration (FAA) regarding the Failure Modes and Effects Analysis (FMEA) of the B737-800.