The Irish Air Accident Investigation Unit (AAIU) issued safety recommendations to Boeing and the FAA following occurrences of undetected pitot heater failures on Boeing 737-800 aircraft.
The recommendations stemmed from an investigation that the AAIU conducted into the causes of a serious incident involving a Boeing 737-800 on January 7, 2012. A Ryanair flight from East Midlands Airport (EGNX), United Kingdom to Riga, Latvia was descending in poor weather conditions with moderate snow when the indicated airspeed (IAS) readings began to diverge. The flight crew decided, following evaluation, that the IAS displayed on the First Officer’s side was incorrect. Airspeed disagreement and other warnings then activated. Following completion of checklists an ILS approach to runway 18 was commenced with Air Traffic Control 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 First Officer’s side and this continued until after the landing.
Subsequent maintenance action found that, although the pitot heater on the First Officer’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.
The operator reported that it had 20 events in 2012 of unannounced pitot heat failures. The life analysis of these failures indicated that the pattern of pitot tube failures showed no trend that would allow determination of the life of pitot probes.
As a result of the investigation, two Safety Recommendations were issued to the Boeing 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 Boeing 737-800.
- AAIU Report 2013-017 (PDF)