Sunwing B738 at Toronto on Mar 13th 2011, stick shaker during departure
Last Update: February 28, 2013 / 19:24:01 GMT/Zulu time
Incident Facts
Date of incident
Mar 13, 2011
Classification
Incident
Airline
Sunwing Airlines
Aircraft Type
Boeing 737-800
ICAO Type Designator
B738
Findings as to Causes and Contributing Factors
- A failure in the right pitot-static system caused the output of erroneous airspeed data from the right air data and inertial reference unit. This resulted in erroneous airspeed indications, stall warnings, and for unknown reasons, misleading flight director commands being displayed on the aircraft instruments during take-off and initial climb.
Findings as to Risk
- When an operatorÂ’s proactive and reactive safety management system processes do not trigger a risk assessment, there is an increased risk that hazards will not be mitigated.
- Operators that do not recognize this type of event as a reportable aviation occurrence may not report it, conduct an investigation to further analyze or mitigate the risk, or preserve data from the digital flight data recorder to facilitate an investigation.
- If operators do not thoroughly document aircraft malfunctions, there is an increased risk that aircraft deficiencies will not be completely corrected before the aircraft is returned to service.
- If cockpit and data recordings are not available to an investigation, this may preclude the identification and communication of safety deficiencies to advance transportation safety.
- The acceptance by flight crews and companies of known equipment problems, such as the boom and mask microphones switching problem, could put safety of flight at risk.
Other Findings
- During the transition to safety management systems, Transport Canada must recognize that operators may not always identify and mitigate hazards and adjust its oversight activities to be commensurate with the maturity of the operatorÂ’s safety management system.
The first officer (ATPL, 5000 hours total, 3700 hours on type) was pilot flying, the captain (ATPL, 7500 hours total, 3000 hours ontype) was pilot monitoring. The aircraft was assigned runway 23 for departure from Toronto, the crew decided to perform a reduced thrust takeoff. V1 was determined at 149 knots. The cockpit was prepared for the first officer being pilot flying with the first officer's flight director selected as master.
After the aircraft had lined up the first officer selected TOGA, autothrottle moved the thrust levers to the takeoff setting, the flight directors showed the design target. When the aircraft accelerated through 60 knots the captain's flight director indication commanded 15 degrees nose up. When the captain called 80 knots, the first officer noticed less than 80 knots on his side however attributed the difference to an early call. Both left hand and right hand engine electronic controls switched to soft alternate mode of operation resulting in autothrottle disconnecting at 90 knots. The captain verified proper thrust was set, the takeoff was continued. At 105 KIAS the first officer's flight director commanded 15 degrees nose up. Somewhere between 139 and 149 KIAS the automated V1 call was heard, the first officer noticed his speed was low and detected an airspeed disagree indication. He transferred control to the captain. At 150 KIAS the captain began to provide control inputs, the aircraft began to pitch up at 154 KIAS and became airborne at 166 knots. The aircraft reached a nose up attitude of 15 degrees at 179 KIAS. While climbing through 791 feet AGL the stick shaker activated for an estimated 6 to 8 seconds, the captain at the same time heard a sound similiar to the overspeed clacker in his headset while the first officer did not get that sound, the flight data recorder did not record stick shaker activation or overspeed clacker indication. The flight director, evident by the flight data recorder, however commanded a nose down attitude of 5 degrees below the horizon. The captain, in visual conditions, however determined that the flight director commands were in error and maintained a positive climbing attitude though lowering the nose. At 2000 feet and 189 KIAS the stick shaker activated again for 3 seconds, this time recorded by the flight data recorder too, the airspeed was further increased, flaps and slats cleaned. At 2400 feet MSL the crew began execute the airspeed unreliable airspeed checklist and determined that the left hand air data sources were reliable while the right hand (first officer's) air data sources were not reliable by referring to the standby instruments as well as crosschecking speed, pitch angle and power setting and comparism of captain's and first officer's instruments. The flight director indications on the captain's primary flight display became normal thereafter, however, there were intermittent altitude disagree messages due to the right hand air data computer feeding altitude data exceeding the criteria for a disagree alert. The crew did not declare emergency but requested emergency services on standby due to an overweight landing. The remainder of the flight was uneventful and concluded with a safe landing.
Maintenance personnel checked the aircraft systems but found no anomaly despite three airspeed disagree indications recorded. All pitot tubes were found clean.
2 days prior to the incident flight the aircraft's right hand pitot tube had struck an owl during takeoff from Puerto Vallarta (Mexico) which required the right hand pitot tube to be cleaned. The aircraft had completed 5 uneventful flights since.
In the absence of any definitive finding maintenance replaced the entire right hand pitot system and returned the aircraft to service, there has been no recurrence of the problem. The replaced pitot system was sent in for laboratory analysis which too revealed no defects or anomalies.
The TSB analysed: "In this occurrence, had the aircraft not been in visual conditions, the crew may not have had the visual cues to support its decision not to follow the flight director when it commanded a 5 degree pitch-down attitude at low level, after the system automatically switched to the master flight director at 400 feet above ground level (agl)."
The TSB further analysed the checklists were insufficient stating: "Only after completing the critical initial climb, and dealing with erroneous stall warnings, a possible overspeed warning and misleading flight director commands, could the crew carry out the Quick Reference Handbook (QRH) procedure and confirm that the right airspeed indication was erroneous. The QRH makes no mention of erroneous flight director or stall warning system performance. It does not provide any indication to deselect the flight director on the same side as the erroneous air data, nor does it otherwise caution against the risk of invalid flight director commands. The crew did not use the autopilot and it maintained a flight director selection that resulted in erroneous flight director commands continuing to be displayed. This is because the right flight director was the master which relied on the erroneous right pitot pressure. Continued use of erroneous guidance in adverse weather could seriously compromise flight safety."
The TSB analysed that there had been problems with the captain's headset microphone just before takeoff and again during the flight. The TSB thus concluded that the microphone began to pick up the stick shaker sounds in the cockpit which resulted in the captain receiving a sound similiar to the overspeed clacker, no other explanation was possible.
Incident Facts
Date of incident
Mar 13, 2011
Classification
Incident
Airline
Sunwing Airlines
Aircraft Type
Boeing 737-800
ICAO Type Designator
B738
This article is published under license from Avherald.com. © of text by Avherald.com.
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