On a drawing there is a flag note next to a hole with diameter, position, and perpedicularity callouts . The flag note reads “Key Characteristic per AS9103”. I’ve seen many clever ways to nnotate a Key Characteristic, be it a flag note, image of a house key, etc. But what is this Key Characteristic and how might a designer help someone understand what is important?

AS9103 defines a Key Characteristic for a part as “those selected geometrical, material properties, functional and/or cosmetic features, which are measurable, whose variation control is necessary in meeting Customer requirements and enhancing Customer Satisfaction.”

The “functional…features” apply to most drawings and Key Characteristics, but what is really intended to be done about this important feature? The definition leaves the intent quite vague. Many believe a characteristic is a dimension of a feature, and some believe it’s the whole feature. In the example above, the hole has 3 dimensions: diameter, position, and perpendicularity. Admit it. You’re thinking “Seriously? .0008?? How can I meet a Cpk of 1.33 on .0008?”

Is the perpendicularity dimension even included in the Key Characteristic? Because of AS9103 ambiguity, it is difficult to determine the intent of the designer by merely seeing a flag note annotating a Key Characteristic. As a manufaturer, it would be safe to say that the entire feature and all its dimensions are considered the Characteristic, but because there’s room for interpretation, the designer might not get what he or she wants.

There are a few options to define the engineering intent, but they all lead to furthering the requirement. As a designer, question your intended audience. As a manufacturer, question your engineering. Between controlling features or just its select dimensions, it doesn’t matter who is right. If more than one person can come to a different conclusion, define the ambiguity.