NYC08IA290

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.

On August 26, 2008, about 0635 coordinated universal time (0135 local time), a Boeing 747-209B, N715CK, operated by Kalitta Air as Centurion Air Cargo flight 164, experienced a power loss from the number 3 engine shortly after takeoff. None of the five crewmembers were injured, and there was no damage to the airplane. Night visual meteorological conditions prevailed. The airplane was operating on an instrument flight rules flight plan from Eldorado International Airport (SKBO), Bogota, Colombia, to Miami International Airport (KMIA), Miami, Florida, in accordance with Title 14 Code of Federal Regulations Part 121. Under the provisions of Annex 13 to the Convention on International Civil Aviation, and by mutual agreement, the Colombian Government delegated the incident investigation to the United States. Aeronautica Civil de Colombia designated an accredited representative to the investigation on behalf of the Colombian Government. According to flight crew statements, just after departure, as the landing gear were being retracted, a “loud bang” emanated from the vicinity of the number 3 engine, followed by an excessive exhaust gas temperature (EGT) and “three or four lesser bangs.” The engine was secured “in accordance with the checklist procedure,” fuel was dumped, and the airplane returned to the airport without further incident. A post-flight walk-around inspection of the number 3 engine revealed no visible damage. The airplane was powered by four Pratt and Whitney (P&W) JT9D-7Q engines. The operator reported that the number 3 engine, ESN P702121, had operated 210 hours since overhaul. A Powerplants Group was formed to further examine the engine. According to the Group Chairman's factual report, the number 3 engine was removed from the airplane and shipped to the Kalitta Air engine facility in Oscoda, Michigan for on-wing testing on a Boeing 747-200 test bed. Before engine testing was conducted, the engine was inspected via borescope to determine internal hardware condition and if the engine was capable of being tested safely. Inspection did not reveal any mechanical failures within the engine, or thermal distress in the combustion or turbine sections beyond serviceable limits. Several high pressure compressor blades were found to be nicked or torn beyond engine manual limits and were repaired (blended in-situ) before the engine was run. Fan blade leading edge impressions were also made to evaluate leading edge contour and profile and their impact on engine performance and stability. The engine was tested in accordance with the P&W "Test Number 12" performance evaluation listed in the JT9D-7Q engine maintenance manual to first establish a performance baseline for the engine, and to sequentially test the engine with newly overhauled fan blades installed, then 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 baseline testing of the enginerevealed that the engine did not surge, nor were there any engine exceedences. An engine vane control trim check was performed and revealed that it was within the parameters for most of the engine power range, except for above 90 per cent corrected N2 speed, where it was slightly below the minimum value. It was determined that the results did not significantly impact the engine testing, so the engine was not re-trimmed prior to testing the engine with new fan blades and a new fan case. Based on the range of hardware conditions tested, the tests revealed that fan blade contour and shape had a bigger influence on the engine’s surge margin and low pressure compressor (LPC) operating 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. The results revealed that LPC efficiency and stability further improved above and beyond the improvement from the overhauled fan blades.