Narrative:

I just received word that there may have been engine damage from compressor stalls on one of my previous flights. The following describes the events as I recall them.at around 500 to 1;000 AGL after takeoff we heard a sudden 'pop'. It sounded and felt a lot like what a compressor stall feels like in the simulator; so that is what we decided it must have been. I was the pilot flying; so I asked for the autopilot; and then pulled up the secondary engine instruments page; so I could see what was what. At that time; we had no unusual indications at all. Further; it was just after sunset; and another aircraft was on the runway behind us; waiting for take off clearance. We did not see any flash of fire; as compressor stalls often have; nor did the other flight crew or tower mention anything. So; we were wondering what had happened when we received another 'pop' about 30 seconds later.again; this was not a continuous popping or shaking; and no unusual engine indications existed; other than #4 engine had about 1;500 pph higher ff than all others. For some reason; I felt that if it was a stall; it must be #3; since there was no yaw but it sounded like it came from the right side. So; I cracked #3 back about a knob width. There was no recurrence; and all engine indications remained normal. After we cleaned up; I asked the first officer and the relief pilot to work the compressor stall checklist (actually; engine surge or stall). It really did not apply; since the engine was not stalling and all indications were normal at that time. The crew agreed that it could have been a stall; but that all indications were ok now; and we really had no idea what engine; if any; had been affected. It could have been a load shift or bird strike. I sent the relief pilot; with an O2 bottle; to check for load shifts after he suggested that. Everything appeared normal. After he returned; I got up and debriefed our jumpseater; an md-11 captain. I asked him what he thought it could have been; and if he had seen any flash of fire or what not. He said nothing seemed strange; other than the two 'pops' and he saw no indications. He asked if we could inspect the engine visually. So; I returned to the seat and sent the relief pilot back down to the main deck to look out the inspection windows. He said he had very clear views of the engines; and they appeared to be just fine. I asked the relief pilot to call the company on satcom and phone conference with maintenance control to see if they had any guidance. Maintenance control said that they had no indications of any problems. The flight crew; dispatch and maintenance all agreed that the engines were operating normally; and we all suspected a compressor stall but had no solid evidence. We all agreed it was safe to continue. In the end; it appears #3 had stalled; due to faulty vsv [variable stator vane] scheduling; according to a mechanic I talked to after landing.

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Original NASA ASRS Text

Title: A B747-400 flight crew endeavored to determine the source and severity of two 'pop' sounds heard during initial climb. The lack of any evidence of the source convinced them; in concert with Dispatch and Maintenance resources to continue the flight to its destination. Later sources believe them to have been compressor stalls; perhaps due to improper Variable Stator Vane scheduling.

Narrative: I just received word that there may have been engine damage from compressor stalls on one of my previous flights. The following describes the events as I recall them.At around 500 to 1;000 AGL after takeoff we heard a sudden 'pop'. It sounded and felt a lot like what a compressor stall feels like in the simulator; so that is what we decided it must have been. I was the pilot flying; so I asked for the autopilot; and then pulled up the secondary engine instruments page; so I could see what was what. At that time; we had no unusual indications at all. Further; it was just after sunset; and another aircraft was on the runway behind us; waiting for take off clearance. We did not see any flash of fire; as compressor stalls often have; nor did the other flight crew or Tower mention anything. So; we were wondering what had happened when we received another 'pop' about 30 seconds later.Again; this was not a continuous popping or shaking; and no unusual engine indications existed; other than #4 engine had about 1;500 PPH higher FF than all others. For some reason; I felt that if it was a stall; it must be #3; since there was no yaw but it sounded like it came from the right side. So; I cracked #3 back about a knob width. There was no recurrence; and all engine indications remained normal. After we cleaned up; I asked the First Officer and the Relief Pilot to work the compressor stall checklist (actually; engine surge or stall). It really did not apply; since the engine was not stalling and all indications were normal at that time. The crew agreed that it could have been a stall; but that all indications were OK now; and we really had no idea what engine; if any; had been affected. It could have been a load shift or bird strike. I sent the Relief Pilot; with an O2 bottle; to check for load shifts after he suggested that. Everything appeared normal. After he returned; I got up and debriefed our jumpseater; an MD-11 Captain. I asked him what he thought it could have been; and if he had seen any flash of fire or what not. He said nothing seemed strange; other than the two 'pops' and he saw no indications. He asked if we could inspect the engine visually. So; I returned to the seat and sent the Relief Pilot back down to the main deck to look out the inspection windows. He said he had very clear views of the engines; and they appeared to be just fine. I asked the Relief Pilot to call the company on SATCOM and phone conference with Maintenance Control to see if they had any guidance. Maintenance Control said that they had no indications of any problems. The flight crew; Dispatch and Maintenance all agreed that the engines were operating normally; and we all suspected a compressor stall but had no solid evidence. We all agreed it was safe to continue. In the end; it appears #3 had stalled; due to faulty VSV [Variable Stator Vane] scheduling; according to a mechanic I talked to after landing.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.