Design Intent

 Failure to meet Design Intent is a good description to use for parts that just don't function as intended. The FAA defines "design intent" as part material, geometry, and material surface condition that delivers the form, fit and function required by the part design to meet the service life of the part. Design intent is recognized as including more than those requirements noted on the part drawing or quality control document. If a part meets its "design intent" then the part meets its definition.^1.

Example: When the Beech Sundowner was introduced it had a nasty habit of the engine quitting on short final when the pilot closed the throttle. We, with the help of the carburetor manufacturer, re-jetted the carburetor to correct the problem. As this was a new aircraft we submitted a warranty to Lycoming who denied the warranty as the carburetor was not defective as it was in conformance with the engineering drawing  and therefore nothing was wrong with it!

The carburetor was obviously defective since it did not meet the  "design intent".  The concept of "Design Intent" is a powerful concept that reminds everyone that the part must function properly else it is not safe. We offered to take the engineer for a ride to demonstrate the engine failure on short final but received no response!

Other examples:

• Valve sticking
• Spark plug fouling

Accidents where design intent became an issue:

• United Flight 585
• Lockheed C-130A, N130HP

Caution is advised for maintenance when working on aircraft or components that are being operated outside of the original design intent. Existing maintenance processes may not be adequate. see N130HP.

We have been discussing "Design Intent" from the perspective of failure to perform as expected or required. A subcategory is anomaly.

Anomaly

Anomaly: If you want to place the blame on the pilot. Example "...distracted by the flap anomaly"
Failure: If you want to place blame on the device: Example "a failure of the left outboard foreflap"

Anomaly when applied to surface conditions:
An abnormal surface condition with chemical or physical properties that do not meet design intent. A similar expression is "surface flaw", although flaw implies that the part is not in conformity with the engineering drawing, while "anomaly" as defined by the FAA includes "design intent" such that the part may be in conformance with the engineering drawing, in other words manufactured correctly, but have a surface condition that does not allow the part to function as intended. Anomaly implies a condition left over from manufacturing and not a material degradation, such as corrosion pitting.

Safety Concerns

Manufacturers sometimes approach a design intent failure by not admitting the failure in the existing part but by offering a new and improved replacement part. This tactic reduces safety and places passengers at risk as it implies that the existing product functions with design intent.

Not fully disclosing known risks in EXISTING parts is in itself a safety hazard. "New and Improved" works well in the cereal industry but should be approached with some caution in the aircraft industry.You may prefer the original cereal rather than the new and improved one. The customer may not want to replace a "perfectly good" component with the "new and improved" one if they feel the existing part functions to their satisfaction -- not knowing that it has a design intent flaw.


1. Failure to achieve design intent is an example of "unk-unk". This slang engineering term, thought to have originated at Lockheed, is defined as " something, such as a problem, that has not been and could not have been imagined or anticipated; an unknown unknown."

What we know and what we don't know was best expressed by United States Secetary of Defense, Donald Rumsfield "There are known knowns; there are things we know we know.We also know there are known unknowns; that is to say, we know there are some things we do not know. But there are also unknown unknowns – the ones we don’t know we don’t know."