On the matter of the terminology of aeronautical structures survivability
Authors: Efimov V. V. 
Published: October 2019

Aim. The paper examines the existing definitions of survivability and damage tolerance (operational survivability) of aeronautical structures. An attempt is made to unambiguously define the survivability of aeronautical structures that can subsequently be extended to an aircraft as a whole and other complex technical items. The primary goal of this paper is to clearly distinguish between dependability and survivability. In order to ensure efficient operation and flight safety, an aircraft must possess airworthiness, a comprehensive characteristic of an aircraft that is defined by the implemented design principles and solutions and that allows performing safe flights under expected conditions and under the established methods of operation. The expected operating conditions are described in the Aviation Regulations – Airworthiness Requirements. Despite the fact that compliance with the Airworthiness Requirements ensures a sufficiently high level of flight safety, the most vital structural components are designed in such a way as to remain operable even under extreme conditions beyond the expected operating conditions. But dependability cannot be responsible for operability outside the expected operating conditions. Conclusion suggests itself that under extreme conditions beyond the expected operating conditions operability is to be ensured by another property, i.e. survivability.

Methods. This research was conducted using the logical and probabilistic approaches. The author examined literary sources primarily dedicated to the matters of dependability and survivability of aeronautical structures, as well as other complex technical items. In order to ensure an optimal understanding of the differences and correlation between the concepts of dependability and survivability, the probabilistic approach was used.

Results. Upon the analysis of literary sources, survivability was defined as the property of an item to retain in time the capability to perform the required functions under extreme conditions beyond the expected operating conditions under the specified methods of maintenance, storage and transportation. Additionally, the paper proposes the definition of damage tolerance (operational survivability) as the property of an item to retain in time the capability to perform the required functions under extreme conditions beyond the expected operating conditions depending on the methods of maintenance, storage and transportation. The probabilistic approach to the delimitation of the concepts of dependability and survivability of aeronautical structures was examined using the known indicator of operating efficiency of a transport aircraft that is represented as the mathematical expectation of the efficiency indicator. An aircraft may be either in the expected operating conditions or in extreme conditions beyond the expected operating conditions. No third option exists. Then, the sum of the probabilities of an aircraft encountering such conditions must be equal to one. The probability of no-failure can be calculated by means of the probability of the contrary event, i.e. the probability of failure that can be represented as the product of the probability of an aircraft encountering certain operating conditions and the probability of failure in such conditions. For the case of extreme conditions beyond the expected conditions the well-known concepts of perishability and vulnerability with the author’s improvements can be used.

Conclusions. A definition of survivability was obtained that is clearly different from the concepts of dependability and fail-safety. Additionally, the concept of damage tolerance (operational survivability) was proposed that was introduced similarly to the previously introduced concept of operational dependability.