Originally Posted by
ShyTorque
If they're anything like the door locating/locking pins on A109s, there is a loud bang, a rush of air, your ears pop and you hope all you're going to need is clean underwear. But they are factory parts.....
Just because they're certificated with all the expense involved, it doesn't mean to say they're up to the job.
The point is that if the consequence of failure would be "bad stuff" it is necessary to ensure that the part is made from known materials by a known process in a controlled environment. Aeroplane parts are operated much closer to structural limits than non-aeroplane parts because weight is always an issue.
In (for example) a car the engine mountings are bolted down with parts that have structural margins of well over 200%, so they are pretty insensitive to small variations in material, dimensions and manufacturing process. But a typical aviation part has a structural margin of 10-20% because the extra weight of more generous margins is unaffordable, so small variations in strength are very significant.
Look back through the accident reports where structural failures were a cause and you'll see it over and over again - parts with incorrect heat treatment, or features that were outside limits, or the wrong alloys. Assuring that these things are within spec costs money - often more than the cost of making the part (in much the same way that assuring a bandage is sterile costs more than making the bandage, since people are fixated with medical analogies). Perhaps you should ask Tim Lancaster whether the cost of ensuring bolts were the correct dimensions is a worthwhile expense, because nothing bad ever happened to flight 5390, did it?
It has been suggested that helicopter voltage regulators are a rip-off because they fail more often than the ones in cars, which is bunkum. A helicopter environment is much harsher in terms of temperature variation, vibration, shock etc etc and in most cases the unit will be run much closer to its electrical limits for far more of the time as well (because of the weight thing).
Everything you put on an aeroplane must (by law) have known design characteristics that have been verified by analysis, analogy or physical testing. Their supply and storage must be controlled and documented (as a matter of law). All of that costs money - a lot of money. Aeroplanes are built in tiny quantities (compared to say cars or washing machines) and so these costs form a much larger proportion of the end-price than for non-aviation parts. This isn't a matter of being "ripped off" - it's just a matter of complying with the law in a world where killing people results in large legal bills.
PDR