Loss of power in two engines during initial climb, and a resulting near-stall, led to the fatal crash of a Kalitta Air Boeing 747-200F moments after departure from the Colombian capital Bogota.
Although the four Pratt & Whitney engines had functioned normally during the take-off roll from Runway 31R, the outboard right-hand powerplant suffered a persistent surge on rotation.
As the freighter became airborne it struggled to achieve a stable climb. The crew did not keep the airspeed above the threshold for an engine-out ascent and the stick-shaker activated, warning of a stall risk and prompting the pilot to reduce the angle of attack.
Although the airspeed rose, the 747 also lost height and was flying just 400ft above the ground. An attempt to increase pitch generated a second stick-shaker warning, and the pilot responded by pushing the throttles forward to maximise thrust in a bid to gain airspeed and altitude.
But about 20s later the other outboard engine, on the left wing, also lost power after a failure in the low-pressure turbine, which led the powerplant to shed parts.
The precise reason for the disintegration could not be determined, said Colombia's Grupo de Investigacion de Accidentes in its final report into the 7 July 2008 accident.
But it stated that the failure began in the third stage of the turbine, and probably involved the loss of guide vanes or section of air seal.
With two engines malfunctioning, the aircraft was unable to sustain flight in its configuration. It began to experience problems with a third engine, the inboard left-hand JT9D, which repeatedly surged. The remaining engine functioned normally.
The aircraft came down northwest of the airport, at Madrid, Cundinamarca, and was completely destroyed on impact. Two people on the ground were killed but, remarkably, the 747's eight occupants survived.
Investigators have battled to explain not only the double engine failure but also the subsequent poor flight performance of the aircraft. Simulation and analysis showed the 747-200F's expected climb capability with two engines inoperative should have been better than that demonstrated by the ill-fated jet. The simulated flights indicated a 200ft/min better climb rate over the space of 3min.
Even though analysts examined various weight, balance and operational scenarios, the inquiry report said: "The reasons for the performance degradation of the crashed aircraft could not be clearly determined from the available data."
Inspection of the outboard right-hand engine, the first to show signs of problems, revealed no abnormal pre-impact wear or damage. But the inquiry report said the engine, installed six months before the crash, was not operating efficiently because of such factors as "excessive" blade-tip clearance and Bogota airport's high altitude. It put the net loss at 5.8%.
Colombian investigators also found "discrepancies" between the aircraft's maintenance records and manufacturers' maintenance manuals which "may have affected the engine performance".