Boeing 747-209B N715CK incident near Bogota, CO

Probable cause

The combination of fan blade edge contour, variable blade trim, and the high altitude, which resulted in an engine surge.

NTSB analysis

Just after takeoff from a high-altitude airport, as the landing gear were being retracted, a “loud bang” emanated from the vicinity of the No. 3 turbofan engine. The exhaust gas temperature rose, followed by three or four lesser bangs, all of which indicated an engine stall/surge. The engine was secured in accordance with the checklist procedure, fuel was dumped, and the airplane returned to the airport without further incident. On-wing test bed testing first established a performance baseline for the engine, which was then tested with newly overhauled fan blades installed, and subsequently tested with a newly overhauled fan case installed in order to assess the effects of changing fan blade geometry and fan blade-to-fan case clearance. Initial testing of the engine did not result in an engine surge or any engine exceedences. An engine vane control trim check revealed that it was within the parameters for most of the engine power range, except for above the 90 percent corrected N2 speed, where it was slightly below the minimum value. Because the results did not significantly impact testing, the engine was not re-trimmed prior to testing the engine with new fan blades and a new fan case. Tests revealed that fan blade contour and shape had a bigger influence on the engine’s surge margin and low pressure compressor (LPC) stability line than the fan blade-to-fan case clearance variations within the range of test conditions. Fan blades that were within the engine manual limits – rounder leading edges with little to no erosion and proper chord length – had improved LPC efficiency and engine stability. The last engine test was conducted with the compressor variable vanes re-trimmed open, and within limits at the high speed range, with the results indicating further improvement in LPC efficiency and stability. While no single variable likely resulted in the engine surge, several variables, including blade edge contour, variable blade trim and high altitude, likely combined to produce the stall/surge.

Text from the NTSB. Open the NTSB docket for NYC08IA290 for the full record.