While investigators have yet to ascertain the reason for the loss of control of the Swiftair Boeing MD-83 over Mali, the accident shares several parallels with earlier events in which speed decay in early cruise led MD-80s into a high-altitude stall.
Boeing has previously warned that MD-80s can be vulnerable to speed decay under certain conditions – particularly those in the vicinity of convective weather in warm temperature regions, where thinner air combines with a need for anti-ice protection.
Warm air reduces the lift efficiency of the wing and the ability of aircraft to maintain higher altitudes – especially early in the cruise when the aircraft is heaviest.
Use of engine and airframe anti-ice, to defend against icing phenomena near storm cells, also carries an altitude penalty. Boeing has previously indicated that, for the MD-80's Pratt & Whitney JT8D engine, this penalty can amount to 3,000ft.
Boeing highlighted in a flight operations bulletin in 2002 that, if the selected altitude is unsuitable, the MD-80 might not be able to generate sufficient thrust to maintain the chosen height.
If the airspeed is not monitored carefully, it can bleed away to the point of a stall onset as the autopilot attempts to keep the aircraft at altitude by increasing the angle of attack.
“If the thrust required to maintain level flight is greater than the thrust available, the airplane could decelerate to stall warning before the autopilot disconnects,” says the bulletin, which points out the subtlety of the effect.
Boeing issued the bulletin in response to an incident in which an MD-80 experienced speed decay to the point of stick-shaker activation.
Several similar events have been recorded and the airframer reiterated the risk of high-altitude stall after the fatal loss of a West Caribbean Airways MD-82 over Venezuela in August 2005.
Like the Swiftair jet, the West Caribbean aircraft had been operating a night flight within the Inter-Tropical Convergence Zone, an equatorial area characterised by strong thunderstorm activity.
Venezuelan investigators found that the aircraft, which had its anti-ice protection active, fell “behind the power curve” and was not generating the thrust needed to maintain its altitude of 33,000ft. Over the course of 10min its airspeed decayed and the aircraft entered a stall, and a rapid descent, from which the crew was unable to recover.
Early stall recovery is crucial for the MD-80 because its T-tail and rear-engine design carries the risk of a deep stall, in which the turbulence over a stalled wing renders the elevator ineffective and disrupts the engine airflow.
After the crash of the West Caribbean flight Boeing’s then-chief pilot for flight operations safety, John Spencer, underlined common elements between similar incidents, including the presence of warm, moist air typical of thunderstorms, and aircraft operation at cruise altitudes close to those defined by weight limits.
Crews had not noticed any abnormality beforehand, having not recognised the slow decay of airspeed, and in some cases had mistaken a thrust roll-back, from airflow interruption, for a flame-out.
Investigators have not released the weight details of the Swiftair MD-83 but the 165-seat aircraft, operating for Air Algerie, was transporting 110 passengers and six crew, according to the carrier, and was fuelled for a 1,500nm service from Ouagadougou to Algiers.
French investigation authority BEA has confirmed that the aircraft suffered a gradual decay of airspeed over the space of about 10min, shortly after reaching its cruising altitude of 31,000ft and having skirted around a storm cell.
The inquiry has not confirmed whether the crew activated the anti-ice system, nor has it given much detail on the performance of the powerplants, although BEA chief Remi Jouty briefly mentioned “engine fluctuations”.
BEA has not stated whether the crew received a stall warning as the airspeed dropped to around 160kt, but the cockpit-voice recorder failed to function correctly and is yet to yield any readable information. None of those on board survived the 24 July accident.