French and Malian investigators are advising a mandatory change to Boeing MD-80 flight manuals to highlight the insidious effects of engine icing.
The recommendation follows the fatal loss of a Swiftair MD-83 in July last year, which has been attributed to an undetected speed decay triggered by inaccurate engine pressure ratio data.
Investigators believe the crew did not activate the anti-icing system for the pressure probes on the nose cones in the MD-83’s Pratt & Whitney JT8D engines.
The probes became obstructed by ice as the aircraft – which had departed Ouagadougo in Burkina Faso – diverted around a region of convective weather shortly after reaching a cruise altitude of 31,000ft.
As a consequence the probes relayed an overestimate of the engine pressure ratio and the aircraft responded by reducing thrust in order to bring the figure within authorised limits. The decreasing speed went unnoticed until the aircraft began to stall.
French investigation authority BEA and the Malian commission of inquiry have recommended that MD-80 flight manuals be modified to draw crews’ attention to the risks associated with engine probe icing during cruise, even in the absence of other visible evidence of ice accretion.
“As of today documents such as the [flight manual] do not contain specific procedures to allow crews, on the basis of indicated engine parameters, to quickly bring to light a situation with inconsistent [engine pressure data] resulting from obstruction of the [engine probes],” the inquiry states.
“Detection of this inconsistency would enable crews to react before the [aircraft] dangerously approached a stall situation.”
It adds that pilots should be provided with a means to identify quickly, and remedy, an erroneous pressure-ratio reading.