On 28 February 2006, a Boeing Company 717-200 aircraft, registered VH-NXH, was being operated on a scheduled passenger service from Paraburdoo to Perth, WA. The flight was being conducted under the instrument flight rules (IFR). Onboard the aircraft were two flight crew, four cabin crew and 66 passengers. The aircraft departed Paraburdoo at about 0837 Western Standard Time and was in instrument meteorological conditions (IMC) during the climb.
The stick shaker stall warning system activated soon after the aircraft reached top of climb at Flight Level (FL) 340 and while the aircraft was accelerating to cruise speed. The flight crew did not receive any 'STALL' annunciation on their respective primary flight displays, nor any 'STALL STALL' aural warning or klaxon alert.
The flight crew initiated an immediate on-track descent and advised air traffic services of their requirement to change level. There was an infringement of the relevant procedural separation standards as the aircraft descended through the cruise level of an opposite direction aircraft.
An analysis of the flight recorder data indicated that the activation of the stick shaker was as a consequence of the angle-of-attack sensors becoming static during the climb. The investigation concluded that the immobilisation of the angle-of-attack sensors was consistent with ice restricting the movement of the 'slinger' on which the sensor vane is mounted.
The investigation assessed that the aircraft was not near a stalled condition of flight when the stick shaker warning activated. However, because the angle-of-attack sensors provided input to the aircraft's stall warning system, the immobilisation of those sensors adversely affected the reliability of the aircraft's stall warning system and could have render the automatic stall recovery system inoperative.
As a result of this incident, the aircraft and angle-of-attack sensor manufacturers initiated a detailed design review of the angle-of-attack sensor.
Previously published - Interim Factual Information
Soon after the aircraft reached the planned cruise altitude of FL340, the flight crew observed that the speed indicated on the primary flight display for stick shaker activation was converging towards their current airspeed. The stick shaker activated as the speeds merged and the crew commenced an immediate descent. There was a breakdown in the relevant procedural separation standard as the aircraft descended through the cruise level of an opposite direction aircraft.
Descending through FL290 the speed indicated for stick shaker activation returned to normal and the stick shaker warning ceased. The flight continued to its destination without further incident.
At approximately 0855 Western Standard Time, on 28 February 2006, a Boeing Company 717-200 aircraft, registered VH-NXH, was being operated on a scheduled passenger service from Paraburdoo to Perth, WA. The aircraft was being operated by two flight crew and four flight attendants and carried 66 passengers. The brakes-release weight at Paraburdoo was 44,837 kg.
The weather conditions for the flight were under the influence of a decaying tropical cyclone that had crossed the north-west coast of Western Australia earlier that day. Rain and heavy cloud persisted through most of the region and the aircraft was operating in instrument meteorological conditions. The meteorological forecast indicated a temperature of -39 degrees C at the aircraft's planned cruise altitude.
The flight crew had selected engine anti-ice ON during the climb. The autopilot was engaged and had captured the planned cruise altitude of FL340. The airspeed, body angle and engine power settings were normal for that stage of flight and the aircraft was accelerating normally to cruise speed. During that period, the flight crew detected that, over a period of several seconds, the speed displayed on the primary flight display (PFD) for stick shaker activation (Vss) began converging towards the current indicated airspeed. The speed indicated for Vss appeared to overtake the amber caution foot associated with the flight management computer (FMC) calculated minimum operating speed (Vmin), which appeared to be remaining stationary, rather than moving in conjunction with the Vss indication. The relevant indications on the aircraft's PFD are annotated at Figure 1.
Figure 1: Primary flight display and airspeed tape
The stick shaker 1 warning activated as Vss merged with the current airspeed and then continued to increase and merge with the maximum operating speed (V/MoM/Mmo), with the right edge of the airspeed tape giving the appearance of one continuous red chevron 2 'zipper'. Similar indications were observed on both pilots' PFDs. The crew reported that they did not receive any other cautions, alerts or warnings on the aircraft's engine and alert display.
The flight crew recalled that, although the pitch limit indicator 3 had turned red, indicating that the aircraft was at or near a stalled condition, there was no "STALL" annunciation on the PFD, nor any aural "STALL STALL" warning or klaxon alert. The crew initiated an immediate on-track descent and advised air traffic services (ATS) of their requirement to change level. The stick pusher stall recovery system did not activate and the crew did not identify any secondary indications of an impeding stall, such as aerodynamic buffet or an abnormally high pitch attitude. Although the crew did not detect any evidence of airframe ice on the windscreen or windscreen wiper posts, they selected the airframe anti-ice ON. The crew did not otherwise change the configuration of the aircraft.
The stick shaker continued to operate as the aircraft was descended at approximately 2,000 ft per minute. The flight crew recalled that the speed indicated on the PFD for Vss returned to normal as the aircraft descended through FL290 and that the stick shaker warning ceased at that time.
During the descent, there was a breakdown in the relevant ATS procedural separation standards, as a result of the aircraft descending through the level of an opposite direction aircraft.
The flight crew levelled the aircraft out at FL280 with all of the aircraft's controls and system indications 'normal'. Once the in-flight weather conditions improved, the crew climbed the aircraft to FL300, and the flight landed at Perth without further incident.
Company maintenance engineers performed a built in test equipment check 4 following the aircraft's arrival in Perth. That check confirmed that no faults had been recorded during the occurrence flight and the aircraft was released for service. Subsequent flights were completed without incident.
The aircraft's flight data recorder (FDR) and the electronic recording media for the quick access recorder (QAR) were removed for analysis. Data was also recovered from the non-volatile memory of the aircraft's flight control computers (FCC).
The FDR data indicated that the output from each of the aircraft's angle of attack (AoA) sensors became static (continuously indicating about 4 degrees AoA) passing FL287 on climb, at a total indicated air temperature (TAT) of -4 degrees C. The stick shaker activated approximately 80 seconds after the aircraft had reached FL340, as it was accelerating through a computed airspeed of 258 kts and at a TAT of -10 degrees C. About 14 seconds later the recorded data indicated the commencement of a descent from FL340. The stick shaker indication continued for another 2 minutes 23 seconds, ceasing as the aircraft passed FL288, at a computed airspeed of 308 kts and a TAT of approximately +7 degrees C. Associated with the cessation of the stick shaker warning was the AoA sensors returning to normal operation.
The QAR media was found to contain no recorded data. Examination of that file indicated that the recording media was incorrectly formatted for use in the QAR.
The manufacturer of the aircraft's FCC analysed the contents of each computer's non-volatile memory. That analysis revealed no fault history data for the day of the incident.
On 03 March 2006, an entry was made in the aircraft's maintenance log, reporting the intermittent operation of the air data heat switch 5 annunciator after the system was selected OFF following landing. An indicating globe was replaced and the switch tested serviceable before the aircraft was returned to service.
The aircraft's AoA sensors and air data heat switch were subsequently removed from the aircraft and dispatched for examination by the component manufacturers under the direct supervision of the US National Transportation Safety Board (NTSB).
The investigation is continuing, and will include:
- analysis of the FDR data and the movement of the AoA sensor in the periods immediately preceding and immediately after the sensors were returning a static (not moving) indication
- analysis of results from component testing
- evaluation of input from the NTSB, aircraft and component manufacturers, regarding systems analysis and modes identified for anomalous stick shaker operation.
- The stick shaker is a tactile warning to alert the crew that the aircraft is at or near an aerodynamically stalled condition of flight. It is one of several warning systems designed to alert the crew of that flight condition.
- The red chevron normally indicates airspeeds above VmoMmo and airspeeds below Vss.
- The pitch limit indicator depicts the difference between the aircraft angle of attack (AoA) and stick shaker AoA.
- Examination of fault codes that have been stored for maintenance checkout and fault isolation.
- This switch provides anti-ice protection to the aircraft's pitot-static, AoA and ram air temperature sensor systems.
|Date:||28 February 2006||Investigation status:||Completed|
|Time:||0856 WST||Investigation level:||Complex - click for an explanation of investigation levels|
|Location:||39km NNW EROPA, (IFR)|
|State:||Western Australia||Occurrence type:||Icing|
|Release date:||26 September 2008||Occurrence class:||Environment|
|Report status:||Final||Occurrence category:||Incident|
|Highest injury level:||None|
|Aircraft manufacturer||The Boeing Company|
|Type of operation||Air Transport High Capacity|
|Damage to aircraft||Nil|
|Departure point||Paraburdoo, WA|