The European Aviation Safety Agency (EASA) issued a safety information bulletin, reminding flight crew of the dangers of low speed at high altitude cruise.
A recently published interim report on a July 2014 accident involving an MD-83 Mali, prompted EASA to issue a bulletin to remind flight crews of some basic flight physics principles to better manage the airplane speed (Mach number) when flying at high altitudes to prevent entry into upset situations such as stall.
Investigators think the MD-83 accident occurred when pressure sensors became obstructed by icing while en route at FL310. Due to the incorrect pressure values, the auto-throttle tried to maintain a lower thrust than was necessary. The aircraft lost speed and developed a tendency to descend. The autopilot, engaged in altitude hold mode, then attempted to raise the nose in order to maintain altitude. The aircraft ultimately stalled.
The bulletin concludes that, when in cruise,
– if continuous Mach decrease cannot be stopped after the maximum available thrust has been applied, and
– if the Mach/airspeed indication can be considered reliable,
flight crews should establish the aeroplane in a reasonable descent to recover the initial targeted Mach. Then after a descent has been initiated, appropriate actions to mitigate the risk of air-to-air collision should be taken (ATC advised, TCAS monitoring etc.). Return to the previous cruise altitude should be initiated only after reaching the optimum Mach for climb and only then in coordination with ATC.
Failure of the flight crew to take the decision to descend in due time will result in a stall with significant altitude loss and a potential loss of control of the aeroplane. The initiation of a controlled descent manoeuvre is the correct action to be taken by the flight crew.