Originally Posted by
Buster15
As an ex Production Concession and ex DDR signatory, you raise some interesting points.
Regarding Concessions, I agree with your concern and in an ideal world they would not be necessary. However, for a concession to be approved, it has to go through a rigorous engineering assessment to ensure that Fit/Form/Function or appearance would not be affected and that the part would performance normally.
Regarding DDR's, this process is primarily to allow the customer/operator to request the Design Authority to make an assessment on the suitability of a run part or a part in-service to be either refitted to one of their modules or engines or to be classified as serviceable.
It was primarily intended to obviate in service logistical problems mainly at (the old) ML 2/3/4. The DDR process is also used to develop in service acceptance standards where satisfactory evidence can be used to relax certain limits. So as you can see, DDR's are there for the benefit of the customer and have a significant affect on cost of ownership.
Regarding repaired engines returned from RR it is again the customs decision as to whether they wish to use the DDR process and as with Concessions, the customer can agree or disagree with their use.
While you may be correct that many engines feature such parts you are not correct when you say that these parts do not conform or reflect the design intent. For them to be approved engineering must be able to confirm that they do not adversity affect the design intent. Should that not be the case the customer will be consulted as to whether they are prepared to accept them.
Lastly I would most certainly not agree with your statement regarding fitting 'any old rubbish'. In my practical experience this is not only not the case but any part that would fit your description would be and are scrapped.
Having worked on a number of collaborative engines, all the engine companies involved operate with almost identical processes so this is not unique to RR.
As a former DDR signatory on all Bristol engines myself, I know whereof I speak. Your statement of the rules pertaining to DDRs is correct, however, those rules are not allways followed. This is probably not the time or place to cite examples which would support my statements, however, since the introduction of MRMS type contracts where RR bear the cost of repairs not the customer, the DDR system is definitelyy not working in the customers favour. My point is that it is my hope that UK MoD will ensure that future contracts either remove or severely restrict their use beyond the existing concurrence system.