My thanks to everyone who has responded to this subject. Let me just set the record straight on one point. Over the last 18 months or so we have had about 5 or 6 engines surge. In every case we carried out the standard checks/replacements and if we could not fix it we pulled the engine. We did not continue operation with a surging engine.
We have been working with our AMC and they in turn with RR Indianapolis to fix this problem. As has already been suggested in earlier posts the most likely engine to surge is one that demonstrates a very high power assurance margin. I can confirm that has been the case with our engines.
RR are also of the opinion that over the years the various efforts to extract more power from this engine has resulted in a deterioration of the stall/surge margin. The "fix" (at least supposedly for our engines), is to adjust the diffuser vanes and change the airflow through them. This will reduce the power but also eliminate the surging. At the same time RR have developed some new test cell procedures which are supposed to identify an engine which is likely to surge.
It seemed that we were finally getting to grips with this but just a couple of days ago I fitted an engine which came out of another helicopter and had been runing perfectly. The engine was run on a test cell at an AMC to confirm power and also went through the surge testing with no problem. The first run we did with it in the helicopter we had a surge. Right now we are going over the same old ground of component replacement. Again this was a powerful engine, +14Q at 780 T5 and N1 of 102.7%
It really throws me why this engine should now decide to surge on this airframe when it was okay on the previous and in the cell. I will keep you informed of the outcome.
By the way, we have operated these engines for about 16 years and all of a sudden we get this big problem right out of the blue.