On 28 February 2006, a Boeing 717-200 being operated by National Jet for Qantas Link on a domestic scheduled passenger flight from Paraburdoo to Perth, Western Australia in day Instrument Meteorological Conditions (IMC) experienced an activation of the stall protection system just after the aircraft had levelled at a cruise altitude of FL340. The response of the flight crew was to initiate an immediate descent without either declaring an emergency or obtaining ATC clearance and, as a result, procedural separation against opposite direction traffic at FL320 was lost. The 72 occupants were uninjured and the aircraft was undamaged.
An Investigation was carried out by the Australian Transport Safety Bureau (ATSB). It was established that the stick shaker had activated 90 seconds after the aircraft had begun to accelerate to cruise speed after levelling at FL340. There had been no ‘STALL’ annunciation on either PFD or any ‘STALL STALL’ aural warning or warning horn. It was reported that the activation had occurred as the indicated stall speed on both PFDs had merged with the current airspeed and that this indicated stall speed had continued to increase until it merged with the Mmo so that the right edge of the airspeed tape was continuously red and white edged. There was no corresponding master warning on the Engine and Alert Display (EAD) and the stick pusher had not operated. In response, an immediate on track descent had been initiated and ATC advised of a requirement for a lower level. In response, ATC advised of opposite direction traffic. The stick shaker continued to operate as descent at approximately 2500 fpm continued and it was noted that the indicated stall speed on the PFDs had returned to normal as the aircraft descended through FL290 with the stick shaker ceasing at the same time with the aircraft then levelled at FL280.
The flight crew had initiated an immediate on-track descent and advised air traffic services of their requirement to change level. The infringement of the prescribed procedural separation standards against opposite direction traffic at FL320, about which ATC had provided traffic information, was a direct consequence of the decision to begin and continue descent without clearance.
An analysis of Flight Data Recorder (FDR) data showed that the stick shaker activation had been caused by the angle-of-attack sensors becoming static during the climb and it was concluded that this had been consistent with ice restricting the movement of the ‘slinger’ on which the sensor vanes are mounted. Whilst it was clear that there had not been a near-stall episode, it was noted that whilst the sensor vanes were not free to move, the stall protection system had effectively been inoperative.
The weather conditions prior to the event were reviewed. It was established that it had been raining heavily during the turn round at Paraburdoo and that IMC had been encountered soon after departure with moderate turbulence and areas of rain until passing about FL200 although aircraft weather radar had not indicated any thunderstorms or areas of heavy precipitation on track and there was no recollection of any ice accumulating on the aircraft wiper blades, the usual indication of flight in icing conditions. However, the Bureau of Meteorology concluded that conditions may have been conducive to the formation of supercooled water droplets.
It was considered by the Investigation that:
“The almost simultaneous immobilisation of the separate and independent sensor vanes suggested that each was affected by the same in-flight condition. Furthermore, the immobilisation occurred within 1 minute of the total indicated air temperature (TAT) decreasing below 0° C and continued until a short time after the TAT increased above 0° C during the descent. That, and the nature of the in-flight immobilisation of the angle-of-attack sensors, was consistent with ice having physically restricted the movement of each sensor’s unheated slinger and faceplate assembly, before melting with increasing TAT during the descent.”
As a result of this event, Boeing and the OEM of the angle-of-attack sensor initiated a detailed design review of the component. This was advised to have consisted of an examination of the original certification process for the sensor, it’s electrical characteristics, the operability of the vane and case heater and the mechanical design of the vane assembly and slinger-faceplate assembly. It was noted by the Investigation that whilst the vanes themselves were heated, the ‘slingers’, the vane shafts and the face plates mounted flush on the fuselage were not.
The Investigation findings were formally stated as follows:
- The aircraft’s angle-of-attack sensors immobilised during the climb at a relatively low airspeed and high angle-of-attack. That sensor immobilization was probably the consequence of ice restricting the movement of the sensors’ vane-slinger-shaft assembly.
- The flight crew’s decision to commence an immediate descent in response to the activation of the stick shaker made an infringement of the separation standards unavoidable.
- The immobilisation of the angle-of-attack sensors adversely affected the reliability of the aircraft’s stall warning system and could render the automatic stall recovery system inoperative.
- The aircraft was not near a stalled condition of flight when the stick shaker warning activated.
The Final Report: Aviation Occurrence Investigation – AO-2006-154 Final of the Investigation was published on 26 September 2008.
No Safety Recommendations were made but the fact that a detailed design review of the angle-of-attack sensor initiated as a result of the investigated incident by Boeing in conjunction with the sensor manufacturer was continuing was noted.