MOC VTOL.2250(c) No catastrophic effect from structural single failures in the Category Enhanced

The following method is accepted for compliance with VTOL.2250(c) in the Category Enhanced for structural elements and parts:

(a)     To demonstrate that no single failure has catastrophic consequences per design, a Safety Assessment should be performed that includes the following steps:

(1)     a complete and comprehensive list of structural elements or parts and their interfaces should be provided;

(2)     the functions that the structural elements or parts perform should be identified; and

(3)     a safety assessment should be performed to identify all structural elements and parts for which failure could lead to Catastrophic consequences. All reasonably anticipated and conceivable failure modes should be taken into account without considering mitigation means, and all the stages of flight and operating conditions should be considered.

(4)     The conclusions of the Safety Assessment should demonstrate the non-catastrophic classifications of all single failures and thereby show direct compliance with VTOL.2250 (c).

(5)     If any single failure is identified that can lead to a catastrophic consequence:

(i)      a structural redesign or vehicle re-configuration should be considered.

(ii)     For simply loaded static elements⁽¹⁾ that are not involved in a system function, if redesign or reconfiguration is impractical or adds excessive design complexity that would impair the overall safety objective, it should be demonstrated that catastrophic consequences from any single failure are extremely improbable applying a combination of the compensating provisions in accordance with paragraph (b).

Note⁽¹⁾: Simply loaded static elements are typically airframe components. Elements that are high cycle fatigue loaded, rotating and/or complexly loaded such as control surfaces.

(b)     Structural Failure Rate

For structural elements or parts and failure modes identified in (a)(5)(ii), if a quantitative assessment is not directly feasible, an acceptable combination of compensating provisions should be implemented that provides sufficient confidence to achieve the safety objective and is appropriate to address the failure mode that could result in catastrophic consequences.

In addition, the framework outlined below may be used to determine the Structural Failure Rate for the MOC VTOL.2240(d) assessment, if a quantitative assessment is not directly feasible.

It should address each of the three following aspects (1) to (3), for which a non-exhaustive list of examples is provided below for each aspect:

(1)     Design Robustness:

(i)      Larger static safety margins

(ii)     Thorough proven understanding of the load distribution

(iii)     Natural frequencies far from the forcing frequencies

(iv)     Larger fatigue (safe life) margins

(v)      Damage tolerance demonstration of larger damages

(vi)     Low complexity of the design and a limited number of failure modes

(vii)    Design values based on a statistical A-basis (99% probability with 95% confidence) as a minimum

(2)     Quality of the part

(i)      Identification of key manufacturing parameters and processes that are strictly controlled, the modification of which require OEM validation.

(ii)     Regular material batch testing throughout the life of the element or part

(iii)     Non-destructive tests (NDT) / Destructive tests (DT) of one sample from every batch throughout the life of the element or part

(iv)     Non-destructive acceptance test of every article

(v)      Process control specimens or witness coupons

(vi)     Special assembly procedures or functional tests to avoid maintenance errors

(vii)    Sensitivity to production process variability is low or is taken into account in the design

(3)     In-Service Continued Structural Robustness

(i)      Regular non-destructive inspections (NDI)

(ii)     Limited repairs permitted without TC Holder support

(iii)     End of flight reports of relevant parameters, for example, vibration, loads, deflection, temperature, acoustic emission

(iv)     Active in-flight monitoring with pilot notification

(v)      In-Service Monitoring to verify the health and operating conditions and the effectiveness of design and maintenance provisions, as well as other procedures, throughout the life of the type design., refer to MOC VTOL.2240(e)

(vi)     Health and Usage Monitoring System (HUMS), refer to MOC VTOL.2240(e)

(vii)    Notification required to the TC Holder of any unusual or unexpected wear or deterioration of parts in service

For some elements the determination of the failure rate could be more appropriately determined using other cycles, such as flight cycles or centrifugal force cycles. A conservative spectrum should then be used to convert the structural failure rate into probability per flight hour.

(c)      In the safety assessment in (a)(3) of this MOC related to bearings, as a minimum and when applicable, the following failure modes of bearings should be considered:

(1)     rupture of one or several of the bearing constituents

(2)     partial or complete seizure of the bearing

(3)     advanced spalling of bearings races or rolling elements

(4)     advanced wear of bearing rings, rolling elements or cages

(5)     loss of bearing preload

(6)     permanent deformation