Engine surge problem

[mirabeau]

Got an engine surge problem on a B727 200 RE centre engine JT8D -17.
The engine went for a workshop visit to check the High press compressor discs for an AD. some discs were replaced.
During the first flight the engine surged at FL 350 during climb. Able to operate without surge by applying bleeds to that engine.
Workshop then replaced the nozzle guide vanes, but engine surged again at high alt when throttling back.
Another workshop visit and the low press C1 blades were replaced. Air test failed again with surge at FL 380 throttling back to cruise power.

All sense lines, bleed lines and intake duct checked already.

ANYONE OUT THERE WITH ANY IDEAS ?

[DC-ATE]

Is it only at FL350 and above? Maybe too high for the weight? High angle of attack has caused problems with the center engine. At least that's what I heard in the waaaaay back years.

[Mad (Flt) Scientist]

Well, on the assumption that the engine didn't do this pre-shop visit, one would tend to be suspicious of the changes made during the shop visit. Maybe there's a rotor with excessive tip clearance (God knows how) and it's screwing things up in the HP compressor.

Incidentally, is the surge being"fixed" by opening bleeds in the LP or HP; that might tend to point the blame at part of the engine.

Other than that, was anything else changed? Fuel control system? Though if they've had the engine on and off the a/c a couple of times its hard to believe a common mistake was made in stallation .... if the same team did it each time ....

[galaxy flyer]

OTOH

At my old and deceased airline, the placard on some planes said, "bleed #2 about FL310", below which F/Es would note, in pencil, "below FL280", then "FL250", and so on

GF