A-Basis and B-Basis Concepts
What A-basis and B-basis mean, how they are derived, when each is required and what the engineer needs to know about statistical allowables.
What is it?
A-basis and B-basis are statistical allowable values defined in MMPDS and CMH-17. A-basis is the value that 99% of the population exceeds with 95% confidence. B-basis is the value that 90% of the population exceeds with 95% confidence. They require multiple specimens from multiple heats and product forms to establish.
How they are derived
The allowable is calculated from a population of test specimens. The number of specimens, heats and lots determines whether A-basis or B-basis can be established. A-basis requires at least 100 specimens from at least 10 heats or lots (approximately). B-basis requires fewer. The statistical method accounts for sample size, distribution shape and confidence level.
When to use each
- A-basis: required for primary structure where failure would result in loss of the vehicle or catastrophic failure
- B-basis: acceptable for fail-safe or redundant structure where load can redistribute after failure
- S-basis: used when insufficient data exists for A-basis or B-basis — the specification minimum
- The certification basis defines which is required — the engineer does not choose arbitrarily