To: Blacksheep
but I couldn't let his suggestion that the manufacturer always knows best, go unchallenged. I'm sure that Lu will be the first to agree that there are times when he reflects upon his work and worries if he got it right. Manufacturers don't always get it right and those in the Tech Services side of the business [like me for instance] earn our living from picking up the pieces and sweeping up after them.
This what I said.
“The point is that the manufacture does not always get it right nor do the technicians from the airline”.
From a design standpoint the pilots that fly a new aircraft are in fact test pilots because the aircraft is not always 100% and the structural engineers that design a fix are the same ones that design the aircraft. In the issuance of a new airframe to the flying public the manufacturer will issue all types of maintenance manuals one of which is the structural repair manual. This particular manual is constantly evolving because they can not include every repair that might be encountered. When a non-covered repair is designed it will be included in the next update of the manual. These types of repairs must be expedited because of the loss of revenue for the airline. A mistake might be made in their stress calculations but I believe in the case of the repair on the Japanese 747 pressure bulkhead the repair was well thought out but it was poorly implemented by the Japanese technicians and their error was not detected by the Boeing reps.
I have challenged many engineers relative to the reliability or maintainability of a design only to be rebuffed or receiving a lame excuse relative to the impact on the schedule or the cost of making the change and when the aircraft entered into service the identified problems manifested themselves. Cases in point are the A-310, the Apache, the V-22 and the EH-101 to name a few.
Here is an example of working on a Boeing contract. I did the FMECA and the reliability analysis on the Truck positioner used on the 767 landing gear. I requested information from Boeing on a pressure relief valve that they had designed and they refused to provide the information saying that it was proprietary. That left a big hole in the analysis. The designer made the Truck positioner in accordance with the stress levels provided by Boeing. When placed in test at Boeing the truck positioner would fail structurally. Boeing called in the designer and the stress engineer from the German Company that built the Truck positioner and they worked for over a month at Boeing to determine the cause of the failure. A Boeing stress engineer looked at the Germans’ calculations and asked why they were using the stress levels that were in their calculations for the design. He told them that the stress levels were much higher. When asked the Boeing engineer indicated that their initial calculations were incorrect but it was determined that they never relayed the new stress levels to the German company.