In-flight upset - 154 km west of Learmonth, WA, 7 October 2008,

Table 19: Characteristics of elevator control mechanismsHigh AOA protectionAnti pitch-up compensationControl law Normal law only Normal or alternate lawSpeed Any Mach 0.65 or moreAltitudeAny (must exceed thresholdfor at least 2 seconds whenaircraft below 500 ft)Configuration Any Landing gear retracted, flaps upMaximum authority4° elevator movement (at timeof pitch-downs)Any6° elevator movementHigh angle of attack protectionAerodynamic stall in large aircraft is a potentially dangerous condition and aircraftmanufacturers incorporate design techniques to prevent it. On the A330/340, theFCPCs continually monitored the AOA FCPC input . If the master FCPC detected thatthis value exceeded a predefined threshold (alpha max), then it issued control ordersfor a nose-down elevator movement to reduce the AOA and prevent a stall.High AOA protection was only available when the aircraft was in normal law. IfAOA FCPC input was outside the range of -10° to +30°, the control law reverted fromnormal law to alternate law, and the protection was therefore no longer available. 81In addition, when the aircraft was more than 500 ft above ground level, theprotection was effective immediately; when the aircraft was below 500 ft, it wasonly active after AOA FCPC input exceeded the threshold for 2 seconds or more.The maximum authority or change in elevator movement that could result from thethe high AOA protection varied depending on several factors. The aircraftmanufacturer reported that, at the time of the two in-flight upsets, the maximumauthority was about 4° of elevator movement. The protection would be applied untilthe aircraft’s AOA was reduced below the stall angle.Anti pitch-up compensationAnti pitch-up was a mechanism included in the A330’s control laws to compensatefor a pitch-up 82 tendency at high Mach numbers and high AOA. The compensationwas only available above Mach 0.65 and when the aircraft was in a ‘clean’configuration (that is, with the landing gear and flaps retracted). Its maximumauthority was 6° of elevator movement.8182A number of different conditions could lead to the control law reverting to alternate law. Thecondition that led to the reversion on the 7 October 2008 flight is discussed in section 2.2.2.On a statically stable aircraft, the centre of lift is situated behind the aircraft’s centre of gravity andan increase in AOA would lead to an increase in lift and a restorative tendency to pitch the aircraftnose down. However, at higher Mach numbers and AOAs, it is possible to stall the wing tips. On aswept-wing aircraft, the centre of lift will then move forward, leading to a reduced nose-downreaction to increasing AOA, reduced stability, and a tendency to pitch up.- 77 -