General Engineering Question 2

[debodine]

While this question is not strictly limited to Aircraft Engineering, since that is what I have been doing for years I wanted to asks my peers.

Some times when I review an engineering drawing to determine if it meets applicable standards (FAA, company, industry), I will find that an engineer has made a notation for some process of "Manipulate per Spec XYZ, Method A or Method B. Method B is preferred". The same will sometimes occur with material as in "Make from Material A or Material B. Material A is preferred."

I require engineers who need my review of their data to remove preference requirements. I do this because:

1. Engineering's job as I see it, at least in our industry to create FAA approved engineering data, is to determine what meets design intent.

If I put a note on a drawing similar to "Manipulate per Spec XYZ, Method A or Method B.", that means I have determined that both Method A and Method B meet the design intent and will result in a product that is in FAR compliance. If I put a note on a drawing similar to "Make from Material A or Material B", that means I have determined that both Material A and Material B meet the design intent and will result in a product that is in FAR compliance. I believe that is where engineering should stop.

2. When the manager of the department tasked with completing this task must choose between approved options, it is their opportunity to look at the current industry, company and market conditions to determine which of the two approved methods best fits their need for cost control, schedule adherance, inventory and manpower utilization, and any other factors that might need to be considered in choosing the method.

I believe that for engineering to specify a preference results in one of two typical outcomes:

A. The department manager who must choose between options will default to the engineering preference, possibly resulting in wasted resources (buying preferred material when in stock will do, ordering items to complete the preferred method when items are already on hand to complete the non-preferred method, etc.) I doubt this happens too often, but it is possible so I listed it.

B. The department manager will ignore the engineering preference callout because they have a whole host of conditions to consider when making the choice. Some or all of those conditions may have changed radically since the engineer decided to establish a "preference".

To my mind we have already in place supply chain procedures for establishing what is most cost effective for the project from any given set of options. And if scenario B is the common scenario (and I think it must be or the department manager charged with making the choice is not doing their job) then why put any preference on the engineering drawing?

I am open to your comments, including correcting me if I am missing anything truly important that would drive adding an engineering preference to any aircraft engineering drawing.

 

[debodine]

While the last few posters make excellent points regarding the value of establishing options on an engineering drawing, the discussion has morphed into a discussion of how restrictive to be in adding or not adding options on an engineering drawing. That means we have moved away from the original point which was not about whether or not to add options. The original point was did specifying an engineering preference amongst the established options add value to the drawing.

For the benefit of all I want to emphasize that I personally, and the company I work for take the position that a well done job of engineering research and design will result in as many acceptable options as possible being specfied on the drawing within the time alloted for research. We consider it good practice to make the extra effort to define multiple options to give all customers of our data (both internal and external) the maximum flexibility possible.

So to very briefly restate the OP, here is the root of my OP in three points.

1. Engineering drawings often have options between acceptable materials and processes...the more the better for all users of the data.

2. Non-engineering issues will almost always be the factors that decide which option to use. [Which material is already in inventory or can be obtained on schedule cheaper than the other options? Which process can be performed with the least expensive tooling and/or skilled resource, or the least amount of expensive chemicals? Which version of the part can be fabricated without using our milling machine which is currently down for repair?]

3. Since non-engineering factors will almost always determine which option is chosen, that means the engineer at the time of creating the options will very rarely know how those non-engineering factors will add up to at the time the decision is made regarding which option to choose.

My conclusion: While my peers on this forum have succeeded in convincing me that adding a preference on an engineering drawing will generally cause no harm, I still believe it is a waste of time. When an engineer specifies a preference, the liklihood of the preference actually being followed is nil unless coincidentally all of the non-engineering factors add up to that same choice. If the combination of factors don't add up that same choice, then the department doing the choosing (usually supply chain) will ignore the engineering preference, and rightly so.

Based on the good information provided to me by this forum I have decided if other persons in my company really believe some preference should be listed on our drawing, I will let it go and not fight it. However, I will explain to them my reasoning why I think they are wasting engineering's time in making the request since I remain convinced it adds no value to the drawing.

Again thanks to all for sharing your wisdom and experience with me on this forum. Unless someone believes they have a case that will instruct me how putting an engineering preference amongst acceptable options listed on an engineering drawing can add value and therefore I should encourage rather than discourage the practice, I think we should consider letting this thread come to an end.

Best regards to all for your participation and instruction!

 

[WKTaylor]

debodine...

For what it is worth...

"In Taylor's world" here are the subtitles for "preferred" and "optional".

Preferred. Use for significant quantity buys thru the long-term, where cost, schedule and long-term availability will never be in question. ONLY current generation parts/materials/processes are specified under preferred.

Optional. Use for short term needs, such as one-off repairs/mods/prototypes, etc where the preferred part/material/process may not be readily available; or where the "preferred" is NOT really the end-user [customer] preference. Many "options" are NOT company specified and-are/or-will-be superceded by other specs and/or production will be phased-out in the next few years.

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[debodine]

Wil:

I can certainly see the logic in those definitions. I will discuss this with our company managers to see if we might benefit from adopting similar formal definitions in our system. Though I am not management, they are usually open to ideas that can add value no matter where they come from. Being human, they certainly betray a preference for ideas developed among themselves, but they do a pretty decent job of listening to good ideas.

Thanks!

 

[KENAT]

While these considerations are valid, I still cant convince myself they belong on a formal Engineering drawing (at least to ASME Y14.5).

If they're functionally acceptable then from an engineering drawing point of view they're alternates, there is no functional 'engineering' reason to prefer one or the other.

That said other factors are significant and if not recorded on the drawing, then where?

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[Sparweb]

Hi debodine,

I don't think I ever voluntarily put "option A or B" on a drawing, in any form, unless the client I'm consulting for has explained to me that they need two different ways of getting the job done. For internal production drawings, the process will be defined as X at the outset and approved that way.

Options usually get put on later, as a revision, once the drawing has been "out there" and exposed to realities of production or installation, and come back with a problem (unobtainable materials, for example). It happens to all of us.

That said, I sometimes say "minimum" with a dimension or a material spec, just so I don't box the user into something too specific.

STF