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Number: ENFS-SB-04-002
Date: 20 May 2004
Subject: Factor of Uncertainty for Unmanned Air Vehicles (UAV)
Background:
There has been debate regarding the use of a reduced factor of uncertainty for UAV’s in
accordance with Joint Service Specification Guide (JSSG) – 2006, paragraph 3.2.12. The
intent of this bulletin is to interpret and clarify the JSSG language in order to provide
guidance for UAV programs.
Discussion:
A 1.5 factor of uncertainty (formerly known as factor of safety) historically has been used to
determine ultimate loads for the design of airframe elements subject to deterministic
structural design criteria. This factor has been accepted in U.S. design practice and has
been fundamental in providing a balance between design efficiency and acceptable risk
levels for aircraft safety with regards to structural integrity. The factor compensates for
various issues that may result from the design, manufacturing and sustainment processes.
These include but are not limited to the following:
Use of nominal tolerances versus allowed minimal blueprint tolerances in structural
analysis.
Use of B-basis material allowables.
Inaccuracies in the structural analysis not uncovered during development and full-
scale testing. This includes untested structure, scale-up effects from component
testing, and testing without environmentally conditioned test articles.
Production variability occurring during standard aerospace industry fabrication,
manufacturing and assembly processes.
Variability inherent during implementation of quality control processes.
Inadvertent pilot commanded load exceedances occurring during operations.
Inherent flight limiter overshoots within vehicle management flight control laws.
Structures Bulletin
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