EASA Opinion on Ageing Aircraft Structures (Opinion 12/2016)
The European Aviation Safety Agency (EASA) has recently issued Opinion 12/2016 on ageing aircraft structures, and specifically the risk of ageing in the structures of turbine-powered large aeroplanes certified after 1 January 1958. EASA say:
These risks include fatigue of the basic type design, widespread fatigue damage (WFD), corrosion, fatigue of changes and repairs, and continued operation with unsafe levels of fatigue cracking.
Their proposal follows the NPA 2013-07 consultation, which resulted in 674 comments by 48 stakeholders. The Opinion has now gone to the European Commission as part of the rule making process for the 32 EASA Member States. EASA propose to mitigate the risk by amending:
- Regulation (EU) 2015/640 and to include new requirements in its Annex (Part-26 [i.e. CS-26] — ‘Additional airworthiness requirements for operations’). This is to address the issue of ageing aircraft structure for the current fleet and improve the requirements for future aircraft designs, noting that similar provisions to address ageing aircraft safety exist within the US rules for continued airworthiness and safety improvements for large aeroplanes.
- Regulation (EU) No 748/2012, Annex I (Part-21) ‘Certification of aircraft and related products, parts and appliances, and of design and production organisations’ to reference Part-26 in order to ensure that TCHs (type certificate holders) and STCHs (supplementary type certificate holders) address the effects of ageing aircraft structures as part of the approval of type certificates, changes and repairs.
- Regulation (EU) No 1321/2014, Annex I (Part-M) to reference Part-26 in order to ensure that operators address in their maintenance programmes the effects of ageing aircraft structures.
…the proposed ageing aircraft rule will impose new requirements for the current fleet and will include improved requirements for future aircraft designs. The proposal will ensure that Design Approval Holders (DAHs), or applicants for Type Certificates (TCs), Supplemental Type Certificates (STCs), design changes and repair approvals, will produce the necessary data, procedures, instructions and manuals related to ageing structure failures due to corrosion and fatigue and make them available to those who need to comply with them (operators).
Additionally, operators will be required to incorporate these data items into their maintenance programmes while addressing the adverse effects of changes and repairs on each airframe and its associated maintenance requirements.
a) Acceptance of approved data, which satisfied the FAA requirement on ageing aircraft, to comply with Part-26 requirements for ageing aircraft
b) Requirement for damage tolerance inspections (DTIs) and corrosion prevention and control programmes (CPCP) for large aeroplanes below 7,500 lbs of payload or 30 passengers which is not required by the FAA requirements for ageing aircraft
c) No clear provisions to exclude certain aircraft from the Part 26 applicability similar to the exemption process used by the FAA for the ageing aircraft rules.
d) Implementation of a process to ensure the continuing structural integrity programme remains valid throughout the life of the aircraft.
e) Difference with the FAA regarding the definition of the limit of validity (LOV) of the structural maintenance programme.
f) Lack of flexibility for operators regarding means to address the adverse effects repairs and modifications may have on fatigue critical aircraft structure.
g) Burden on STC holders to develop DTI for certain STCs.
Background on Ageing Structure
EASA discuss four threats to ageing structure:
1) Fatigue: Fatigue occurs through the application of cyclic loading and is inevitable in the aluminium alloy materials predominantly used in the current world fleet. The link between loading cycles and fatigue initiation and crack growth establishes an obvious association between fatigue-related ageing and the number of flight cycles/hours accumulated by an aircraft.
2) Continued safe operation of aircraft structures without Damage Tolerance (DT) evaluations: The principle of DT is that although cracks may be initiated due to fatigue and may then subsequently propagate, the process can be understood and managed by inspection, repair and, on a case-by-case basis, a modification. Inspections based upon a DT philosophy are essential to detect cracks before they can affect safety. Testing and analysis is used to determine inspectable defect sizes, crack growth rates, critical crack lengths and the associated residual strength to determine inspection thresholds and intervals.
3) Corrosion: Corrosion in airframes is usually in the form of an electrochemical oxidation of the metal alloy. It is more prevalent in marine and coastal environments due to the high humidity and exposure to salt. Corrosion weakens the material, both reducing the effective cross section and creating locations of stress concentration. To slow down the corrosion rate, careful material selection, surface coatings and other design considerations such as effective drainage are employed. Corrosion combined with fatigue is of the greatest concern.
4) Widespread Fatigue Damage (WFD): WFD in a structure is characterised by fatigue damage originating cracks at multiple locations of size and density, to the extent that the structure no longer maintains its required residual strength. The traditional application of damage tolerance analysis from a single or dual crack origin is not sufficient to preclude WFD. Fatigue cracks related to WFD can grow quickly and interact in such a way that an operator cannot inspect the susceptible structures effectively or often enough to ensure the detection of the cracks before they lead to a structural failure. A separate WFD assessment and the determination of specific maintenance actions are necessary to address WFD adequately.
To preclude WFD, operation of an aircraft should not be allowed beyond a certain point in the life of the airframe, known as the Limit of Validity (LOV) of the structural maintenance programme.
The LoV is the period of time, stated as a number of total accumulated flight cycles or flight hours or both, for which it has been demonstrated that WFD is unlikely to occur in the aircraft structure. The inspections and other maintenance actions and procedures resulting from this demonstration shall be sufficient to prevent catastrophic failure of the aircraft structure.