Practical Engineering

[imp]

The engineer who designs things often is responsible for choosing, specifying, purchasing, and using many types of often complex devices which may be designed and built by others. Therefore, he or she must also be something of a fortune-teller to predict suitability of others' products from a standpoint of life- that is, others' product might cause premature failure of the engineer's end product.

 

Often, experience and guts will be all-important. If the engineer works for a company which sells the things he designs, cost-consciousness is the prevalent theme. Unfortunately, cheapness designs-in early failure. On the other hand, if the engineer designs things for USE by his/her own company, assuming no undue cost restraints, he or she can come up with things which last almost forever.

 

The "bottom line" of the thread is: How do you believe airplane builders are able to design and produce, competitively, products which are as safe as we know them to be, when automotive builders, for example, competitively build products which are far less inherently unsafe than airplanes? imp

[Spyman]

Hi imp, first I must say that I am an old fashioned guy, so if possible I always try to make my own designs to last "forever".

 

But cheap designs with short lifetime can have a more accurate predictive lifetime, so when safety is involved it is more safe to have parts with a good predictive lifetime and change them before their failure.

 

It could also be more econimically even if the price of five cheap parts is higher than one which function five times longer because the failure itself also have a cost, the loss of expected production and a failure can damage other equipment, material and human lifes.

 

In airplanes a lot of parts have to be checked on regular intervals and some are replaced even if they seems to still function when their predicted lifetime is shorter than next flight. It's expensive but they have to guarantee the travellers some safety otherwise nobody would use them.

 

With cars there is a totally different ballgame, the owner himself needs to ensure his own and others safety, but there is no danger if for example the engine would failure and I think that in all modern cars the breaksystem is doubled to allow for driving until failure. A lot of people can't and won't pay for changing parts that might function for another year or two, so since there is no danger to continue to drive until a failure happens, (might be unconvinient though), they, like me, will keep on driving until the old car breaks down.

 

Automotive builders only have to make the parts last for the warranty time and a big part of the business is that they can earn a lot on replacement parts. Sometimes I even get the impression that some parts are engineered to failure to force the owner to buy a ridiculously expensive replacement part.

[Klaynos]

In Europe and the US there are regulations on design specifications of aircraft. And in most countries an airframe requires a license to fly in their airspace, this kind of thing forces companies to allow their engineers to not be as competitive as they could be.

[imp]

A few examples of Engineering dilemmas:

The first Boeing 747 airplanes NEEDED 2, as I recall, hydraulic pumps to adequately power control surfaces, etc., of a given horsepower- therefore the Engineering decision was to provide each aircraft with 5!

 

A piece of production equipment I designed for use in my company calculated out to need a 1.3 horsepower drive motor. I specified 5 hp, despite additional cost. The motor was still "loafing" along after running over ten years. I could not have done it that way, if we were attempting to sell the machine.

 

The plant built in Henderson, Nevada in the 1940's to make magnesium for use in aircraft was ENTIRELY fitted with only 3-phase electric motors, down to fractional horsepower for small fans. Someone did not like single-phase motors, at all!

 

Our plant in Indiana was required to curtail all use of natural gas in winter; 2 propane tanks of 30,000 gallon capacity provided the substitute energy. As the Facilities Engineer, I was faced with a real problem when a sudden cold snap dropped temperatures low enough to begine freezing the liquid propane leaving the tanks. I learned that gas suppliers routinely blend BUTANE with the propane, as it was then cheaper (the customer usually was unaware of this practice). Butane freezes at a higher temperature than propane. The recommended fix? Install a "propane heater" which burned propane in order to allow delivery into the plant. I refused to authorize it, and instead convinced the natural gas supplier to allow emergency use when the temperature threatened to close down operations. An interesting game, Engineering! imp