Stall warnings in high capacity aircraft: The Australian context 2008 ...

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ATSB – AR-2012-172 Meteorological conditions Stall warnings are not necessarily only associated with operations in poor weather (IMC). Flight conditions at the time of a stall warning were reported in 100 of the 169 genuine stall warnings that were reported to the ATSB between 2008 and 2012, and more than half of these (55 per cent) occurred in VMC. The graph below shows that there was a stark difference in the phase of flight a stall warning was most likely to occur in VMC versus IMC. Figure 4: When genuine stall warnings occur, by flight conditions Genuine stall warnings reported during initial climb all involved VMC flight. About 40 per cent of those stall warnings occurred when the flight crew was retracting high-lift devices. Most other occurrences involved activation due to momentary loss of airspeed in wake turbulence or due to a wind change when aircraft were at pitch angles of about 10°. There were only two occurrences reported where an aircraft was banking on initial climb and was affected by turbulence, leading to a wing loading (g) increase. Three occurrences were stick shaker activations, and coincided with a checklist item to retract the flaps. The effects of increased stall speed (and stall warning activation speed) when the wing load factor increases is discussed later in this report. In climb and descent, stall warnings that occurred in VMC were normally due to a wind gust or turbulent conditions causing an airspeed fluctuation while the aircraft was in a high angle of attack condition, or was slowing down to approach a hold point. In most of these cases, the stall warning was as a result of a sudden change in wing loading (g), as the aircraft were operating at an airspeed (generally V Ref + 80 kts) which was well above the stall warning speed, V SW (V Ref is about 8 to 10 per cent higher than V SW ). In IMC conditions, stall warnings in cruise were very common. All were associated with turbulence during the cruise, causing either airspeed or altitude fluctuations. In 9 of 21 stall warnings where the aircraft was in cruise in IMC, the aircraft was in a turn while deviating around weather when the fluctuations occurred, increasing wing (g) loading. Of those stall warnings that occurred on climb or descent in IMC, many involved autopilot (autothrottle) disengagement due to unsatisfactory speed control in turbulence, or due to a change in wind direction. Most stall warnings on approach and landing in IMC occurred during turbulence from windshear, or when manoeuvring around showers or convective cloud on approach. Many of these cases happened during a turn, and were associated with a sudden drop in airspeed and a resulting increase in wing (g) loading. Approach and landing stall warnings were, however, more common in VMC. In VMC, they also occurred in turns where turbulence or windshear affected the aircraft, but there were more occurrences where the flight crew allowed the airspeed to bleed off too much on approach and fall below V SW . Sometimes, this was due to a wind change in turbulent conditions, but in other cases it was due to increased workload on approach or task distraction (such as monitoring for traffic). › 14 ‹
ATSB – AR-2012-172 What conditions were aircraft in when the stall warning occurred? Genuine stall warnings reported to the ATSB in the 2008 to 2012 period (169 reports) were reviewed to identify common precursor events, aircraft configurations, or flight profiles that allowed the margin between actual airspeed and V S to reduce to a point where stall warning system activated. In 154 of these occurrences, enough information was reported to determine what was happening before and after the stall warning (in terms of the aircraft’s flight profile, attitude, configuration, crew control inputs, and any weather effects). Figure 5 below shows that most of these stall warnings (81 per cent) occurred in association with a flight disturbance (turbulence, windshear, a gust or change in wind direction). Sixty-five per cent of these occurrences involved a Boeing 747 or 767 aircraft. Figure 5: Precursor conditions to genuine stall warning system activations reported to the ATSB, 2008 to 2012 Other common precursors were increases in the aircraft’s pitch angle or airspeed rate of change, operations in the low speed range, autopilot or autothrottle initiated changes to pitch angle or power setting to maintain a particular glideslope or altitude, or aircraft attitudes that increase wing load factor (bank angle above 20°). › 15 ‹
Page 1 and 2: Stall warnings in high capacity air
Page 3 and 4: Safety summary Why the ATSB did thi
Page 5 and 6: ATSB - AR-2012-172 Context Stall wa
Page 7 and 8: ATSB - AR-2012-172 Background What
Page 9 and 10: ATSB - AR-2012-172 A situation like
Page 11 and 12: ATSB - AR-2012-172 Safety analysis
Page 13 and 14: ATSB - AR-2012-172 operation of the
Page 15 and 16: ATSB - AR-2012-172 Boeing 717 stick
Page 17: ATSB - AR-2012-172 the flaps when t
Page 21 and 22: ATSB - AR-2012-172 Figure 6 below s
Page 23 and 24: ATSB - AR-2012-172 altitude. This a
Page 25 and 26: ATSB - AR-2012-172 • minor passen
Page 27 and 28: ATSB - AR-2012-172 In Australia, de
Page 29 and 30: ATSB - AR-2012-172 Summary Stall wa
Page 31 and 32: ATSB - AR-2012-172 Appendix A Worst
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Page 35 and 36: ATSB - AR-2012-172 Sources and subm
Page 37 and 38: ATSB - AR-2012-172 Terminology used

Stall warnings in high capacity aircraft: The Australian context 2008 ...

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