Originally Posted by
Donkey9871
Admittedly we only have one UK manufacturer for aero engines but I do hope that if future aircraft are going to be equipped with RR engines the contract ensures that the use of concessions and DDRs is prohibited. UK MoD pays a lot for the engines they have in their aircraft and the price is the same irrespective of how many concessions and DDRs RR apply to them. I very much doubt thare are any engines currently in service that actually comply with original design specification. RR churn out any old rubbish and apply concessions and DDRs to cover the deficiencies/defects. Supposedly overhauled engines have parts that don't conform to the limits routinely installed. Engines returned to RR for repair are routinely returned to service with defects that have not been rectified. New build engines frequently have concessions applied to non-conforming parts and DDRs incorrectly applied to new parts. Unfortunately the contracts to supply engines permit these behaviours.
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.