Appendix 5 PSE, FCS, and WFD-susceptible structure

(a)Overview Four key terms used when showing compliance to the damage tolerance and fatigue requirements of CS-25 and EASA guidance for the continued structural integrity of ageing aircraft in AMC 20-20 are: ‘principle structural element (PSE)’, ‘fatigue critical structure (FCS)’, ‘widespread fatigue damage (WFD)-susceptible structure’ and ‘design detail point (DDP)’. This Appendix provides clarification on the intended meanings of these terms and how they relate to each other.

(b)Principal structural element (PSE)

(1)The term ‘principal structural element (PSE)’ is defined in this AMC as follows: ‘Principal structural element (PSE)’ is an element that contributes significantly to the carrying of flight, ground or pressurisation loads, and whose integrity is essential in maintaining the overall structural integrity of the aeroplane.

(2)While this definition does not specifically address the fatigue susceptibility of the structure, or environmental or accidental damage, it is intended to address the majority of the structure that must be evaluated according to CS 25.571. CS 25.571(a) states the following: ‘This evaluation must be conducted for each part of the structure that could contribute to a catastrophic failure’.

(3)Examples of PSEs are found in paragraph 7(f) of this AMC.

(4)The above reinforces the notion that the identification of PSEs should be based solely on the importance of the structure to assure the overall aeroplane integrity.

(5)Paragraph 7(f) of this AMC provides guidance for identifying PSEs. Many manufacturers use this list as a starting point for their list of Fatigue Critical Structure (FCS). CS 25.571(b) is intended to address all structure that could contribute to a catastrophic failure resulting from fatigue, environmental and accidental damage, and, therefore, may include some structure that is not considered FCS. Nevertheless, all PSE should be considered when developing a list of FCS.

(6)The definitions used by applicants to identify PSEs have not been consistent among applicants and, in some cases, among models produced by the same applicant. The lack of standardisation of the usage and understanding of the term ‘PSE,’ and the resulting diversity that exists between type design PSE lists, led authorities to introduce the new term ‘Fatigue Critical Structure (FCS)’ in the ‘Ageing Aircraft Requirements and Guidance Material’.

(c)Fatigue Critical Structure (FCS)

(1)‘Fatigue critical structure (FCS)’ is defined as aircraft structure that is susceptible to fatigue cracking, which could contribute to a catastrophic failure. Fatigue critical structure also includes structure which, if repaired or modified, could be susceptible to fatigue cracking and contribute to a catastrophic failure. Structure is most often susceptible to fatigue cracking when subjected to tension-dominated repeated loads during operation. Such structure may be part of the baseline structure or part of a modification. ‘baseline structure’ means structure that is designed under the original type certificate or amended type certificate for that aircraft model (i.e. the as-delivered-aeroplane model configuration).

(2)Fatigue critical structure is generally a subset of principal structural elements, specifically those elements that are susceptible to fatigue damage. The exception may be a DDP that is susceptible to fatigue and, although not part of a PSE, could result in catastrophic failure if it were to fail (e.g. an undercarriage door hinge has been categorised by some TCHs as a DDP and FCS, when its failure would lead to loss of the door and the door could impact the aircraft with catastrophic results. In this case the door was not classified as a PSE because the TCH had not considered the door to contribute significantly to carrying flight, ground or pressurisation loads. Considering further aspects of the PSE definition now adopted, it might be claimed that the door is not essential to maintain the overall integrity of the aircraft, i.e. the aircraft may be safe without it. However, due to the need to identify all detail design points and FCS whose failure could cause catastrophic failure of the aircraft it is in any case subject to the fatigue and damage tolerance requirements.)

(d)Detail design points (DDP) ‘Detail design point’ is an area of structure that contributes to the susceptibility of the structure to fatigue cracking or degradation such that the structure cannot maintain its load carrying capability, which could lead to a catastrophic failure.

(e)Widespread fatigue damage (WFD)-susceptible structure

(1)‘Widespread fatigue damage (WFD)’ is the simultaneous presence of cracks at multiple structural locations, which are of sufficient size and density such that the structure no longer meets the residual strength requirements of CS 25.571(b).

(2)‘Multiple site damage (MSD)’ and ‘Multiple element damage (MED)’ are conditions that, with no intervention, can lead to WFD. The term ‘WFD-susceptible structure’ refers to areas of structure that, under normal circumstances, could be expected to eventually develop MSD and/or MED cracks, which could lead to WFD.

(3)Although not explicitly stated, structure susceptible to WFD cannot be inspected reliably to preclude WFD. Unless a flight cycles and/or flight hours limit is placed on an aeroplane, modifications may be needed to preclude WFD. Structure susceptible to WFD is a subset of FCS.

[Amdt 25/19]