AMC E 850  Compressor, Fan and Turbine Shafts

ED Decision 2018/014/R

(1)     General

(a)      A shaft is the system that transmits torque between the disc driving flange or the shaft attachment member of the system that produces power (e.g. turbine) and the system that uses this power (e.g. compressor/fan or driving flange), and for which the mechanical restraints are mainly torsional. This includes any Engine gearbox in that transmission system (for any aircraft gearbox, see paragraph (2)(c) below). The exclusion of discs from this definition of a shaft does not preclude the specification that any Failure thereof should be Extremely Remote.

(b)     Clarification of the terms and probabilities used in CS-E 850 may be found in CS-E 510. The possible shedding of blades is also covered in CS-E 810(b).

(2)     Non-Hazardous Shaft Failures

(a)      Where it is claimed that Hazardous Engine Effects are avoided by ensuring that rotating components are retained substantially in their normal plane of rotation and the control of overspeed is by means of:

—         disc rubbing;

—         blade interference, spragging or shedding;

—         Engine surge or stall;

—         over-speed protection devices,

this may be substantiated by either test or validated  analysis.

(b)     If an applicant elects to demonstrate by test that the consequences of a shaft Failure are non-hazardous, then the test should be performed by initiating the shaft Failure under the worst-case operating conditions within the flight envelope, in any dispatchable configuration, that will maximise the rotor over-speed and the subsequent effects. If it is impractical to fully duplicate the worst-case conditions, an applicant may propose a test under suitably representative conditions to account for the worst case. Those test conditions would need to be submitted to the Agency for acceptance. In addition to the initial rotor speed, other aspects should also be taken into consideration, such as the shaft torque and the relevant Engine pressures and temperatures. Failures that are predicted to occur with a probability of Extremely Remote or less do not need to be taken into account if they meet all the requirements of CS-E 850(b)(2).

         If compliance is not shown by a full Engine test, but instead by a system or component rig test(s), it should be shown that the test(s) is (are) sufficiently representative, in terms of the key characteristics of the shaft Failure and its consequences on all the relevant Engine parts and on the behaviour of subsystems, of the way the Failure would occur on a full Engine.

(c)      If an applicant elects to demonstrate by validated analysis that the consequences of a shaft Failure are non-hazardous, it should be shown that all the likely Failure modes have been identified in the analysis (including the loss of loads caused by a Failure of any gearboxes supplied by the aircraft manufacturer). The Failure analysis should take into consideration the effect of Failures in terms of contact and the loads on the surrounding structure of the Engine and determine whether the affected rotor components are retained substantially in their rotational plane. It should also demonstrate that the structural components, when the loads resulting from the Failure are applied, do not exceed their ultimate stress capability and not to lead to a Hazardous Engine Effect.

         The analysis should be validated against an actual Engine, system or component rig test(s) and/or service events, and it should show a sufficient degree of similarity with the Engine model for which compliance is sought. This similarity should encompass all the relevant aspects of the Failure mechanism and its consequences, such as but not limited to aerodynamics, surge characteristics, engine control logic, rotor speeds and the associated acceleration characteristics, relevant rotor and stator design features, materials, clearances, etc., and should be submitted to the Agency for acceptance.

(3)     Hazardous Shaft Failures

In general, experience has shown that Failures of shafts occur at a rate in excess of Extremely Remote. Consequently, shaft systems should be designed to fail safe as required by CS-E 850(a)(1). However, it is accepted under CS-E 850(a)(3) that, for conventional designs, this is not possible for all parts of a shaft system, but the use of this provision should be strictly limited.

Two possible hazardous effects of shaft Failure should be particularly considered: a release of the complete fan or compressor moving forward and an over-speed of the turbine leading to disc burst.

Industry experience with shaft Failures should be considered under CS-E 850(b)(2)(v). In particular, the following Failure modes have all led to shaft Failures in service:

—          Degradation of a bearing, leading to shaft orbiting and subsequent contact between the shaft and other rotating or static parts;

—          Blade Failure, resulting in an imbalance and rubbing of the shaft on other parts;

—          Corrosion inside the shaft;

—          Fuel flow instability in the Engine Control System inducing a resonance in the shaft;

—          An oil fire around the shaft;

—          Impingement of hot air on the shaft;

—          A bearing Failure;

—          An HCF Failure from a stress concentration feature;

—          A loss of lubrication of a spline.

Further, features such as splines, oil feed holes, couplings, bearing tracks that are integral with the shaft and sealing fins should be shown to be well understood and conducive to well-established and validated stressing techniques.

When the assessment for compliance with CS-E 850(b)(2)(iii) is that a shaft Failure due to the environment can be discounted, the ability to inspect the critical section of a shaft at the defined intervals and the appropriateness of the inspection method should be taken into account. For example, the Failure of a section of a shaft, which could cause Hazardous Engine Effects, in an area which would make inspection of the critical section in accordance with the manual difficult, may not be acceptable.

(4)     Design Assessment

(a)      The following aspects should be included when assessing the causes and probabilities of a shaft Failure -

(i)      The potential for, and possible effects of, undetected material defects;

(ii)     The effects of manufacturing tolerances allowed by the design;

(iii)     Rubbing between any torque-loaded section of the shaft and adjacent surfaces (e.g. other shafts, oil seals, air seals) to the extent that significant over-heating or reduction in strength could occur;

(iv)     The effect on the shaft of a bearing Failure and the desirability of provision (e.g. by maintenance techniques and/or flight instrumentation) for the detection of an incipient bearing Failure. The possibility of isolating the bearing from the shaft and thus increasing the damage tolerance of the system should be considered;

(v)      The effect on the shaft of any likely Engine fire and the necessity for provision of an early warning of any internal fires that may occur;

(vi)     The effect on the shaft of loads which could be transmitted by shock loading resulting from bird strikes, blade Failures, etc.;

(vii)    The effect on the shaft of oscillatory loading, for example, resulting from fuel system oscillations.

(b)     The shaft system should be subjected to the following investigations and/or testing to support the design assessment and the compliance with the objectives of CS-E 850(a).

(i)      Strain gauge or other suitable means of investigation in order to satisfy the vibration survey specifications of CS-E 650 and to ensure that shaft whirling is not present to any significant degree at any likely Engine operating condition.

(ii)     Fatigue evaluation of each shaft in torsional modes, in order to confirm its predicted safe life. An oscillatory torque of a magnitude equal to the maximum envisaged in a representative installation, but not less than ±5% of the normal maximum steady-state torque, should be superimposed on that steady-state torque. In addition, consideration should also be given to any high-frequency vibrations determined from paragraph (4)(b)(i) above and any possible shaft bending.

(iii)     Where necessary, confirmation of stress assumptions by static strength tests.

(iv)     Where necessary, substantiation by test of the design considerations detailed in paragraph (4)(a) above such as to demonstrate that shaft Failure is acceptably remote.

[Amdt No: E/1]

[Amdt No: E/5]