AMC1 27.571 Fatigue evaluation of flight structure

ROLLING CONTACT FATIGUE

This AMC supplements FAA AC 27-1B, § AC 27.571 and should be used in conjunction with that AC when demonstrating compliance with CS 27.571.

(a)     Definitions

(1)     Rolling contact fatigue (RCF): a form of fatigue that occurs due to the cyclic strains arising from the loading present during rolling contact between two parts of an assembly, e.g. a bearing race and a rolling element.

Note: For the purposes of this AMC, it also includes combinations of rolling and sliding contact phenomena.

(2)     Integral race: a bearing race that is an integral part of the transmission structural component such as a gear or shaft.

(b)     Explanation

Service experience has shown that RCF can initiate cracks on the surface and below the surface in contact areas structural elements (typically, but not limited to, bearing races and rolling elements and gear teeth) that, in some cases, can propagate to a failure with catastrophic results. It is often assumed that RCF leads first to non-critical partial failures such as micro-pitting and spalling that will be detected before more severe failure modes can develop, such as a complete crack through a part. However, experience has shown that, in some cases, critical failure modes can develop shortly after the occurrence of non-critical partial failures. In such cases, analyses and tests are necessary to demonstrate that sufficient time is available, and the performance of the detection system is adequate to ensure the timely detection to prevent a catastrophic failure.

The certification specifications in CS 27.571 require the identification and fatigue evaluation of the portions of the flight structure, the failure of which could be catastrophic. In order to complete this fatigue evaluation, one of or a combination of the methods proposed in sub-paragraphs (b), (c), (d) and (e) of CS 27.571 should be applied. However, specific characteristics of parts submitted to RCF (e.g. bearings and gears), such as the difficulty to visually inspect the operating nature of these elements, which can lead to mechanical degradation and the impact of RCF, make the application of some of the methods challenging. The procedures of this AMC ensure that the effects of RCF are adequately accounted for in the fatigue evaluations required by CS 27.571.

(c)      Procedure

The fatigue evaluation of the portions of the flight structure, the failure of which could be catastrophic, should include, when applicable, the effect of RCF.

For this purpose, steps should be taken to minimise the risk of crack initiation due to RCF on these components (and in particular for integrated bearings races), by minimising contact pressures, specifying high standards for surface finishes, ensuring good lubrication, guaranteeing cleanliness and maintaining lubricant quality regardless of the fatigue evaluation approach selected. The applicant should verify that the selected allowables are suitable to ensure the integrity of the affected components in the operating conditions (temperature, lubrication, cleanliness, etc.) applicable to their design. Experience has demonstrated that it can be beneficial for bearings to be designed so that the reliability of any integrated race subject to the fatigue evaluation is even higher than the least critical race of the bearing. In this way, degradation of the least critical race can lead to detection of the bearing failure before cracking initiates in the integrated race.

As it is difficult to totally preclude cracking initiated by RCF, a ‘fail-safe evaluation’ is recommended wherever possible, such that cracking of affected structural element(s) is detected prior to its residual strength capability falling below the required levels prescribed in CS 27.571(d)(2). Should fatigue cracks initiate and develop into:

(1)     Partial failure, such as spalling, the applicant should demonstrate that this condition will be detected at an early stage to avoid catastrophic failure due to further fatigue failure, or loss of integrity of the affected part or any surrounding ones; and

(2)     Failure, such as through-cracking of an individual part: the applicant should demonstrate that the remaining structure will withstand service loads and limit or maximum attainable loads (whichever is less) until the failure is detected and damaged components are repaired or replaced to avoid a catastrophic failure.

This demonstration should be performed as appropriate using experience from similar designs, functional tests, structural tests and/or reliable analyses to substantiate that the fail-safe design objective has been achieved, including residual strength demonstration. In addition, the continued safe operation of the affected mechanical system(s) should be ensured for this period considering the potential effect of the failure or partial failure taking into account any pre-existing fatigue damage accrued prior to the failure in the affected component on stiffness, dynamic behaviour, loads and functional performance.

The effectiveness and reliability of means of crack detection for the ‘fail-safe evaluation’, including indirect means of detection such as chip detection systems, and associated instructions for continued airworthiness should be evaluated to show that, if implemented as required, they will result in timely detection and repair or replacement of damaged components. Furthermore, the instructions for continued airworthiness, prescribing the maintenance actions leading up to and following detection of potential failure or partial failure should be substantiated sufficiently to ensure timely repair or replacement of damaged components. The substantiation should consider aspects such as threshold criteria on indicators of means of detection for additional investigative actions and removal from service of the damaged parts, the overall clarity and practicality of the instructions for continued airworthiness and human factors aspects.

In addition to a ‘fail-safe evaluation’, ‘replacement time’ and /or ‘fatigue evaluation’ may be needed in addition to fail-safe evaluation in order to ensure that the assumptions supporting the fail-safety and detection of failure remain valid throughout the operational life of the component.

[Amdt 27/10]