CS 25.361 Engine and auxiliary power unit torque
ED Decision 2009/017/R
(See AMC 25.361)
(a) For engine installations:
(1) Each engine mount, pylon and adjacent supporting airframe structures must be designed for the effects of –
(i) A limit engine torque corresponding to take-off power/thrust and, if applicable, corresponding propeller speed, acting simultaneously with 75% of the limit loads from flight condition A of CS 25.333(b);
(ii) A limit engine torque corresponding to the maximum continuous power/thrust and, if applicable, corresponding propeller speed acting simultaneously with the limit loads from flight condition A of CS 25.333(b); and
(iii) For turbo-propeller installations only, in addition to the conditions specified in subparagraphs (a)(1)(i) and (ii), a limit engine torque corresponding to take-off power and propeller speed, multiplied by a factor accounting for propeller control system malfunction, including quick feathering, acting simultaneously with 1 g level flight loads. In the absence of a rational analysis, a factor of 1·6 must be used.
(2) The limit engine torque to be considered under sub-paragraph (1) must be obtained by:
(i) for turbo-propeller installations, multiplying mean engine torque for the specified power/thrust and speed by a factor of 1·25.
(ii) for other turbine engines, the limit engine torque must be equal to the maximum accelerating torque for the case considered.
(3) The engine mounts, pylons, and adjacent supporting airframe structure must be designed to withstand 1g level flight loads acting simultaneously with the limit engine torque loads imposed by each of the following conditions to be considered separately:
(i) sudden maximum engine deceleration due to a malfunction or abnormal condition; and
(ii) the maximum acceleration of the engine.
(b) For auxiliary power unit installations:
The power unit mounts and adjacent supporting airframe structure must be designed to withstand 1g level flight loads acting simultaneously with the limit torque loads imposed by the following conditions to be considered separately:
(1) sudden maximum auxiliary power unit deceleration due to malfunction or abnormal condition or structural failure; and
(2) the maximum acceleration of the auxiliary power unit.
[Amdt 25/8]
AMC 25.361 Engine and auxiliary power unit torque
ED Decision 2009/017/R
CS 25.361(a)(1) is applicable to all engine installations, including turbo-fans, turbo-jets and turbo-propellers, except CS 25.361(a)(1)(iii) which applies only to turbo-propeller installations.
CS 25.361(a)(2)(i) - “Mean engine torque” refers to the value of the torque, for the specified condition, with any dynamic oscillations removed.
CS 25.361(a)(3)(i) - Examples are; high power compressor surges, blade tip rub during manoeuvres, small and medium bird encounters, or combinations of these events.
CS 25.361(a)(3)(ii) and (b)(2) - As an example, the term “maximum acceleration” is taken to be that torque seen by the engine mounts under a runaway of the fuel metering unit up to its maximum flow stop.
[Amdt 25/8]
CS 25.362 Engine failure loads
ED Decision 2009/017/R
(See AMC 25.362.)
(a) For engine mounts, pylons and adjacent supporting airframe structure, an ultimate loading condition must be considered that combines 1g flight loads with the most critical transient dynamic loads and vibrations, as determined by dynamic analysis, resulting from failure of a blade, shaft, bearing or bearing support, or bird strike event. Any permanent deformation from these ultimate load conditions should not prevent continued safe flight and landing.
(b) The ultimate loads developed from the conditions specified in paragraph (a) are to be:
(1) multiplied by a factor of 1.0 when applied to engine mounts and pylons; and
(2) multiplied by a factor of 1.25 when applied to adjacent supporting airframe structure.
[Amdt 25/8]
AMC 25.362 Engine Failure Loads
ED Decision 2009/017/R
1. PURPOSE. This AMC describes an acceptable means for showing compliance with the requirements of CS 25.362 “Engine failure loads”. These means are intended to provide guidance to supplement the engineering and operational judgement that must form the basis of any compliance findings relative to the design of engine mounts, pylons and adjacent supporting airframe structure, for loads developed from the engine failure conditions described in CS 25.362.
2. RELATED CS PARAGRAPHS.
a. CS-25:
CS 25.361 “Engine and auxiliary power unit torque”
CS 25.901 “Powerplant installation”
b. CS-E:
CS-E 520 “Strength”
CS-E 800 “Bird strike and ingestion”
CS-E 810 “Compressor and turbine blade failure”
CS-E 850 “Compressor, Fan and Turbine Shafts”
3. DEFINITIONS. Some new terms have been defined for the transient engine failure conditions in order to present criteria in a precise and consistent manner in the following pages. In addition, some terms are employed from other fields and may not necessarily be in general use. For the purposes of this AMC, the following definitions should be used.
a. Adjacent supporting airframe structure: Those parts of the primary airframe that are directly affected by loads arising within the engine.
b. Ground Vibration Test: Ground resonance tests of the aeroplane normally conducted for compliance with CS 25.629, “Aeroelastic stability requirements.”
c. Transient failure loads: Those loads occurring from the time of the engine structural failure, up to the time at which the engine stops rotating or achieves a steady windmilling rotational speed.
d. Windmilling engine rotational speed: The speed at which the rotating shaft systems of an unpowered engine will rotate due to the flow of air into the engine as a result of the forward motion of the aeroplane.
4. BACKGROUND.
a. Requirements. CS 25.362 (“Engine failure loads”) requires that the engine mounts, pylons, and adjacent supporting airframe structure be designed to withstand 1g flight loads combined with the transient dynamic loads resulting from each engine structural failure condition. The aim being to ensure that the aeroplane is capable of continued safe flight and landing after sudden engine stoppage or engine structural failure, including ensuing damage to other parts of the engine.
b. Engine failure loads. Turbine engines have experienced failure conditions that have resulted in sudden engine deceleration and, in some cases, seizures. These failure conditions are usually caused by internal structural failures or ingestion of foreign objects, such as birds or ice. Whatever the source, these conditions may produce significant structural loads on the engine, engine mounts, pylon, and adjacent supporting airframe structure. With the development of larger high-bypass ratio turbine engines, it became apparent that engine seizure torque loads alone did not adequately define the full loading imposed on the engine mounts, pylons, and adjacent supporting airframe structure. The progression to high-bypass ratio turbine engines of larger diameter and fewer blades with larger chords has increased the magnitude of the transient loads that can be produced during and following engine failures. Consequently, it is considered necessary that the applicant performs a dynamic analysis to ensure that representative loads are determined during and immediately following an engine failure event.
A dynamic model of the aircraft and engine configuration should be sufficiently detailed to characterise the transient loads for the engine mounts, pylons, and adjacent supporting airframe structure during the failure event and subsequent run down.
c. Engine structural failure conditions. Of all the applicable engine structural failure conditions, design and test experience have shown that the loss of a fan blade is likely to produce the most severe loads on the engine and airframe. Therefore, CS 25.362 requires that the transient dynamic loads from these blade failure conditions be considered when evaluating structural integrity of the engine mounts, pylons and adjacent supporting airframe structure. However, service history shows examples of other severe engine structural failures where the engine thrust-producing capability was lost, and the engine experienced extensive internal damage. For each specific engine design, the applicant should consider whether these types of failures are applicable, and if they present a more critical load condition than blade loss. In accordance with CS-E 520(c)(2), other structural failure conditions that should be considered in this respect are:
— failure of a shaft, or
— failure or loss of any bearing/bearing support, or
— a bird ingestion.
5. EVALUATION OF TRANSIENT FAILURE CONDITIONS
a. Evaluation. The applicant’s evaluation should show that, from the moment of engine structural failure and during spool-down to the time of windmilling engine rotational speed, the engine-induced loads and vibrations will not cause failure of the engine mounts, pylon, and adjacent supporting airframe structure. (Note: The effects of continued rotation (windmilling) are described in AMC 25-24).
Major engine structural failure events are considered as ultimate load conditions, since they occur at a sufficiently infrequent rate. For design of the engine mounts and pylon, the ultimate loads may be taken without any additional multiplying factors. At the same time, protection of the basic airframe is assured by using a multiplying factor of 1.25 on those ultimate loads for the design of the adjacent supporting airframe structure.
b. Blade loss condition. The loads on the engine mounts, pylon, and adjacent supporting airframe structure should be determined by dynamic analysis. The analysis should take into account all significant structural degrees of freedom. The transient engine loads should be determined for the blade failure condition and rotor speed approved per CS-E, and over the full range of blade release angles to allow determination of the critical loads for all affected components.
The loads to be applied to the pylon and airframe are normally determined by the applicant based on the integrated model, which includes the validated engine model supplied by the engine manufacturer.
The calculation of transient dynamic loads should consider:
— the effects of the engine mounting station on the aeroplane (i.e., right side, left side, inboard position, etc.); and
— the most critical aeroplane mass distribution (i.e., fuel loading for wing-mounted engines and payload distribution for fuselage-mounted engines).
For calculation of the combined ultimate airframe loads, the 1g component should be associated with typical flight conditions.
c. Other failure conditions. As identified in paragraph 4(c) above, if any other engine structural failure conditions, applicable to the specific engine design, could result in higher loads being developed than the blade loss condition, they should be evaluated by dynamic analysis to a similar standard and using similar considerations to those described in paragraph 5.b., above.
6. ANALYSIS METHODOLOGY.
a. Objective of the methodology. The objective of the analysis methodology is to develop acceptable analytical tools for conducting investigations of dynamic engine structural failure events. The goal of the analysis is to produce loads and accelerations suitable for evaluations of structural integrity. However, where required for compliance with CS 25.901 (“Powerplant installation”), loads and accelerations may also need to be produced for evaluating the continued function of aircraft systems, including those related to the engine installation that are essential for immediate flight safety (for example, fire bottles and fuel shut off valves).
b. Scope of the analysis. The analysis of the aircraft and engine configuration should be sufficiently detailed to determine the transient and steady-state loads for the engine mounts, pylon, and adjacent supporting airframe structure during the engine failure event and subsequent run-down.
7. MATHEMATICAL MODELLING AND VALIDATION
a. Components of the integrated dynamics model. The applicant should calculate airframe dynamic responses with an integrated model of the engine, engine mounts, pylon, and adjacent supporting airframe structure. The model should provide representative connections at the engine-to-pylon interfaces, as well as all interfaces between components (e.g., inlet-to-engine and engine-to-thrust reverser). The integrated dynamic model used for engine structural failure analyses should be representative of the aeroplane to the highest frequency needed to accurately represent the transient response. The integrated dynamic model consists of the following components that must be validated:
— Airframe structural model.
— Propulsion structural model (including the engine model representing the engine type-design).
b. Airframe Structural Model and Validation
(1) An analytical model of the airframe is necessary in order to calculate the airframe responses due to the transient forces produced by the engine failure event. The airframe manufacturers currently use reduced lumped mass finite element analytical models of the airframe for certification of aeroelastic stability (flutter) and dynamic loads. A typical model consists of relatively few lumped masses connected by weightless beams. A full aeroplane model is not usually necessary for the engine failure analysis, and it is normally not necessary to consider the whole aircraft response, the effects of automatic flight control systems, or unsteady aerodynamics.
(2) A lumped mass beam model of the airframe, similar to that normally used for flutter analysis, is acceptable for frequency response analyses due to engine structural failure conditions. However, additional detail may be needed to ensure adequate fidelity for the engine structural failure frequency range. In particular, the engine structural failure analysis requires calculating the response of the airframe at higher frequencies than are usually needed to obtain accurate results for the other loads analyses, such as dynamic gust and landing impact. The applicant should use finite element models as necessary. As far as possible, the ground vibration tests normally conducted for compliance with CS 25.629 (“Aeroelastic stability requirements”) should be used to validate the analytical model.
(3) Structural dynamic models include damping properties, as well as representations of mass and stiffness distributions. In the absence of better information, it will normally be acceptable to assume a value of 0.03 (i.e., 1.5% equivalent critical viscous damping) for all flexible modes. Structural damping may be increased over the 0.03 value to be consistent with the high structural response levels caused by extreme failure loads, provided it is justified.
c. Propulsion Structural Model and Validation
For propulsion structural model and validation, see AMC 25-24.
[Amdt 25/8]
CS 25.363 Side load on engine and auxiliary power unit mounts
ED Decision 2003/2/RM
(a) Each engine and auxiliary power unit mount and its supporting structure must be designed for a limit load factor in a lateral direction, for the side load on the engine and auxiliary power unit mount, at least equal to the maximum load factor obtained in the yawing conditions but not less than –
(1) 1·33; or
(2) One-third of the limit load factor for flight condition A as prescribed in CS 25.333(b).
(b) The side load prescribed in sub-paragraph (a) of this paragraph may be assumed to be independent of other flight conditions.
CS 25.365 Pressurised compartment loads
ED Decision 2016/010/R
(See AMC 25.365)
For aeroplanes with one or more pressurised compartments the following apply:
(a) The aeroplane structure must be strong enough to withstand the flight loads combined with pressure differential loads from zero up to the maximum relief valve setting.
(b) The external pressure distribution in flight, and stress concentrations and fatigue effects must be accounted for.
(c) If landings may be made with the compartment pressurised, landing loads must be combined with pressure differential loads from zero up to the maximum allowed during landing.
(d) The aeroplane structure must be strong enough to withstand the pressure differential loads corresponding to the maximum relief valve setting multiplied by a factor of 1·33, omitting other loads.
(e) Any structure, component or part, inside or outside a pressurised compartment, the failure of which could interfere with continued safe flight and landing, must be designed to withstand the effects of a sudden release of pressure through an opening in any compartment at any operating altitude resulting from each of the following conditions:
(1) The penetration of the compartment by a portion of an engine following an engine disintegration.
(2) Any opening in any pressurised compartment up to the size Ho in square feet; however, small compartments may be combined with an adjacent pressurised compartment and both considered as a single compartment for openings that cannot reasonably be expected to be confined to the small compartment. The size Ho must be computed by the following formula:
where,
Ho = maximum opening in square feet, need not exceed 20 square feet.
P = 
As = maximum cross sectional area of the pressurised shell normal to the longitudinal axis, in square feet; and
(3) The maximum opening caused by aeroplane or equipment failures not shown to be extremely improbable. (See AMC 25.365(e).)
(f) In complying with sub-paragraph (e) of this paragraph, the fail-safe features of the design may be considered in determining the probability of failure or penetration and probable size of openings, provided that possible improper operation of closure devices and inadvertent door openings are also considered. Furthermore, the resulting differential pressure loads must be combined in a rational and conservative manner with 1 g level flight loads and any loads arising from emergency depressurisation conditions. These loads may be considered as ultimate conditions; however, any deformation associated with these conditions must not interfere with continued safe flight and landing. The pressure relief provided by the intercompartment venting may also be considered.
(g) Bulkheads, floors, and partitions in pressurised compartments for occupants must be designed to withstand conditions specified in sub-paragraph (e) of this paragraph. In addition, reasonable design precautions must be taken to minimise the probability of parts becoming detached and injuring occupants while in their seats.
[Amdt 25/18]
AMC 25.365(e) Pressurised compartment loads
ED Decision 2003/2/RM
The computed opening size from 25.365(e)(2) should be considered only as a mathematical means of developing ultimate pressure design loads to prevent secondary structural failures. No consideration need be given to the actual shape of the opening, nor to its exact location on the pressure barrier in the compartment. The damage and loss of strength at the opening location should not be considered.
A hazard assessment should determine which structures should be required to withstand the resulting differential pressure loads. The assessment of the secondary consequences of failures of these structures should address those events that have a reasonable probability of interfering with safe flight and landing, for example failures of structures supporting critical systems. For this assessment the risk of impact on the main structure from non critical structures, such as fairings, detached from the aircraft due to decompression need not be considered.
CS 25.367 Unsymmetrical loads due to engine failure
ED Decision 2003/2/RM
(a) The aeroplane must be designed for the unsymmetrical loads resulting from the failure of the critical engine. Turbo-propeller aeroplanes must be designed for the following conditions in combination with a single malfunction of the propeller drag limiting system, considering the probable pilot corrective action on the flight controls:
(1) At speeds between VMC and VD, the loads resulting from power failure because of fuel flow interruption are considered to be limit loads.
(2) At speeds between VMC and VC, the loads resulting from the disconnection of the engine compressor from the turbine or from loss of the turbine blades are considered to be ultimate loads.
(3) The time history of the thrust decay and drag build-up occurring as a result of the prescribed engine failures must be substantiated by test or other data applicable to the particular engine-propeller combination.
(4) The timing and magnitude of the probable pilot corrective action must be conservatively estimated, considering the characteristics of the particular engine-propeller-aeroplane combination.
(b) Pilot corrective action may be assumed to be initiated at the time maximum yawing velocity is reached, but not earlier than two seconds after the engine failure. The magnitude of the corrective action may be based on the control forces specified in CS 25.397(b) except that lower forces may be assumed where it is shown by analysis or test that these forces can control the yaw and roll resulting from the prescribed engine failure conditions.
ED Decision 2005/006/R
The structure supporting any engine or auxiliary power unit must be designed for the loads, including gyroscopic loads, arising from the conditions specified in CS 25.331, CS 25.341, CS 25.349, CS 25.351, CS 25.473, CS 25.479, and CS 25.481, with the engine or auxiliary power unit at the maximum rpm appropriate to the condition. For the purposes of compliance with this paragraph, the pitch manoeuvre in CS 25.331(c)(1) must be carried out until the positive limit manoeuvring load factor (point A2 in CS 25.333(b)) is reached.
[Amdt 25/1]
CS 25.373 Speed control devices
ED Decision 2005/006/R
If speed control devices (such as spoilers and drag flaps) are installed for use in en-route conditions:
(a) The aeroplane must be designed for the symmetrical manoeuvres and gusts prescribed in CS 25.333, CS 25.337, the yawing manoeuvres in CS 25.351, and the vertical and lateral gust and turbulence conditions prescribed in CS 25.341(a) and (b) at each setting and the maximum speed associated with that setting; and
(b) If the device has automatic operating or load limiting features, the aeroplane must be designed for the manoeuvre and gust conditions prescribed in sub-paragraph (a) of this paragraph, at the speeds and corresponding device positions that the mechanism allows.
[Amdt 25/1]