Lu, I wasn't suggesting that the airlines design major modifications to deal with airworthiness related matters. I refer to the fact that we receive Service Bulletins from manufacturers and usually initiate action to incorporate those SBs long before the FAA get off their backsides and make any of them mandatory. We also discover potentially unsafe conditions when performing our inspections and draw the manufacturer's attention to them so they can make the necessary design changes.
We are but one small operator out of so many, but we have initiated several corrective modification actions to B757 and B767 aircraft over the past seventeen years. Three of my own design defect reports to Boeing ended in world-wide modification programmes, two of which were mandated by the FAA two years down the road. The point I make is that there is nothing unusual in that; its just what we do for a living.