Ryanair B738 at Riga on Jan 7th 2012, right pitot heating failed without indication, stick shaker, flaps overspeed
Last Update: December 19, 2013 / 16:04:01 GMT/Zulu time
The Irish AAIU released their final report concluding the probable cause of the incident was:
Failure of the First Officerâ€™s pitot probe heater.
- Failure of the pitot probe heater warning circuit to detect and annunciate the heater failure.
The AAIU analyzed that the crew "correctly diagnosed the problem and the outcome was a safe landing" although the crew could not identify an obvious cause for the speed and altitude disagreements and the right hand engine electronic control's alternate mode.
The AAIU analyzed that the pitot probe heating can fail without announciation if the probe is partially heated due to a short, that does not trigger the circuit breaker or the probe drawing insufficient current to prompt the circuit breaker trip and sufficient current to not trigger the monitoring device. In either case the pitot probe is not protected from icing without the crew being aware of this.
The AAIU analyzed: "Activation of the stall warning stickshaker during a final approach, following other unanticipated failures such as the autothrottle and autopilot self-disconnecting, would be particularly disconcerting in the circumstances where the flight crew is aware that an IAS is unreliable, although there is no obvious cause. While the Manufacturerâ€™s QRH contained checklists for both IAS disagreement and airspeed unreliable, the checklists do not contain any caution regarding the known issue of some pitot heater failure modes not resulting in activation of the pitot heater failure warning. Furthermore, no guidance is given regarding further operational consequences that may be the consequence of a faulty airspeed indication, such as spurious stall warnings."
Three safety recommendations were released as result of the investigation.
Metars at the time of occurrence:
METAR EVRA 071850Z 18006KT 9999 -RASN OVC005 01/01 Q1001 R18/290158 NOSIG=
METAR EVRA 071920Z 19006KT 9999 -RASN OVC005 01/01 Q1001 R18/290158 NOSIG=
METAR EVRA 071950Z 17005KT 9999 -SNRA OVC005 01/01 Q1001 R18/290158 NOSIG=
This article is published under license from Avherald.com. © of text by Avherald.com.
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