U.S. Department 

of Transportation 

Federal Aviation 

Administration 

Advisory 

Circular 

Subject: Engine Failure Loads Date: D R A F T 

Initiated By: ANM-115 

AC No: 25.362-X 

1. PURPOSE. This advisory circular (AC) describes an acceptable means for showing 

compliance with the requirements of Title 14, Code of Federal Regulations (14 CFR) 

25.362, Engine failure loads. Section 25.362 specifies the engine failure dynamic load 

conditions that apply to the engine mounts, pylons, and adjacent supporting airframe 

structure. 

2. APPLICABILITY.

2.1 The guidance provided in this document is directed to airplane and engine 

manufacturers, modifiers, foreign regulatory authorities, and Federal Aviation 

Administration (FAA) transport airplane type certification engineers and their 

designees. 

2.2 The material in this AC is neither mandatory nor regulatory in nature and does not 

constitute a regulation. While these guidelines are not mandatory, they are derived 

from extensive FAA and industry experience in determining compliance with the 

relevant regulations. These means are issued, in the interest of standardization, for 

guidance purposes and to outline a method that has been found acceptable in showing 

compliance with the standards set forth in the rule. If, however, we become aware of 

circumstances that convince us that following this AC would not result in compliance 

with the applicable regulations, we will not be bound by the terms of this AC, and we 

may require additional substantiation or design changes as a basis for finding 

compliance. 

2.3 The material in this AC does not change or create any additional regulatory 

requirements, nor does it authorize changes in, or permit deviations from, existing 

regulatory requirements. 

2.4 Except in the explanations of what the regulations require, the term “must” is used in 

this AC only in the sense of ensuring applicability of this particular method of 

compliance when the acceptable method of compliance described in this AC is used. 
XX/XX/XX D R A F T AC 25.362-X 

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3. RELATED 14 CFR REGULATIONS.

3.1 Part 25 Sections. 

 Section 25.361, Engine and auxiliary power unit torque. 

 Section 25.629, Aeroelastic stability requirements. 

 Section 25.901, Installation. 

3.2 Part 33 Sections. 

 Section 33.23, Engine mounting attachments and structure. 

 Section 33.65, Surge and stall characteristics. 

 Section 33.94, Blade containment and rotor unbalance tests. 

4. 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 AC, the following definitions 

should be used: 

4.1 Adjacent Supporting Airframe Structure. Those parts of the primary airframe that 

are directly affected by loads arising within the engine. 

4.2 Ground Vibration Test. Ground resonance tests of the airplane normally conducted 

for compliance with § 25.629, Aeroelastic stability requirements. 

4.3 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. 

4.4 Windmilling 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 airplane. 

5. BACKGROUND.

5.1 Requirements. Section 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 transient dynamic loads resulting from the specified engine 

structural failure conditions. The objective is to ensure that the airplane 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. 
XX/XX/XX D R A F T AC 25.362-X 

3 

5.2 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, the applicant must perform 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 must be sufficiently 

detailed to characterize the transient loads for the engine mounts, pylons, and adjacent 

supporting airframe structure during the failure event and subsequent run-down. 

5.3 Engine Structural Failure Conditions.

5.3.1 Of all the applicable engine structural failure conditions, design and test experience 

have shown that the loss of a blade is likely to produce the most severe loads on the 

engine and airframe. Therefore, § 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. Examples are failure of a shaft, failure or loss of any bearing or 

bearing support, and bird ingestion. Therefore, § 25.362 requires that these failure 

conditions be considered. When evaluating bird ingestion, the bird weight and quantity 

requirements specified in § 33.76 should be used. 

5.3.2 Other engine failure conditions may include partial blade loss and bird strike in which 

the engine continues to rotate at high speed with significant imbalance before being 

shut down. If an engine contains fused or frangible bearing supports, and if the engine 

can run on with a partial blade loss just below the fuse release threshold, significant 

loads can develop before the engine is shut down. Depending on system modal 

characteristics, the run-on loads in some parts of the system may exceed the transient 

loads during a design full blade loss. Therefore, if an engine contains fused or 

frangible bearing supports, the applicant should assess the condition of engine run-on 

with a partial blade loss just below the fuse release threshold. 
XX/XX/XX D R A F T AC 25.362-X 

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6. EVALUATION OF TRANSIENT FAILURE CONDITIONS.

6.1 Evaluation.

6.1.1 The applicant’s evaluation should show that, from the moment of engine structural 

failure and during rundown to the time of windmilling rotational speed, the 

engine-induced loads and vibrations will not cause failure of the engine mounts, pylon, 

or adjacent supporting airframe structure. Note that the effects of continued rotation 

(windmilling) are described in AC 25-24, Sustained Engine Imbalance. 

6.1.2 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. 

6.2 Blade Loss Condition.

6.2.1 The applicant should determine loads on the engine mounts, pylon, and adjacent 

supporting airframe structure 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, as specified in § 33.94, 

and over the full range of blade release angles to allow determination of the critical 

loads for all affected components. 

6.2.2 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. 

6.2.3 The calculation of transient dynamic loads should consider the effects of the engine 

mounting station on the airplane (i.e., right side, left side, inboard position, etc.) and the 

most critical airplane mass distribution (i.e., fuel loading for wing-mounted engines and 

payload distribution for fuselage-mounted engines). 

6.2.4 For calculation of the combined ultimate airframe loads, the 1g component may be 

associated with typical flight conditions. 

6.3 Other Failure Conditions. As identified in paragraph 5.3 of this AC, the other engine 

structural failure conditions specified in § 25.362 must also be evaluated by dynamic 

analysis to a similar standard and using similar considerations to those described in 

paragraph 6.2 of this AC. 

7. ANALYSIS METHODOLOGY.

7.1 Objective of the Methodology. The objective of the analysis methodology is to 

develop acceptable analytical tools for conducting investigations of dynamic engine 
XX/XX/XX D R A F T AC 25.362-X 

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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 § 25.901 regarding powerplant installation, loads and accelerations 

may also need to be produced for evaluating the continued function of aircraft systems 

related to the engine installation that are essential for immediate flight safety (for 

example, fire bottles and fuel shut off valves). 

7.2 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 rundown. 

8. MATHEMATICAL MODELING AND VALIDATION.

8.1 Components of the Integrated Dynamic Model. The applicant should calculate 

airframe dynamic responses with an integrated model of the engine, engine mounts, 

pylon, and airframe structure. The integrated dynamic model used for engine structural 

failure analyses should be representative of the airplane 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). 

8.2 Airframe Structural Model and Validation.

8.2.1 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 airplane 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. 

8.2.2 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 

§ 25.629, Aeroelastic stability requirements, should be used to validate the analytical 

model. 
XX/XX/XX D R A F T AC 25.362-X 

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8.2.3 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. 

8.3 Propulsion Structural Model and Validation.

8.3.1 The airframe and engine manufacturers should mutually agree upon the definition of 

the model, based on test and experience. 

8.3.2 Engine manufacturers construct various types of dynamic models to determine loads 

and to perform dynamic analyses on the engine rotating components, its static 

structures, mounts, and nacelle components. Dynamic engine models can range from a 

centerline two-dimensional (2D) model, to a centerline model with appropriate 

three-dimensional (3D) features, such as mount and pylon, up to a full 3D finite 

element model with explicit time integration used to capture the physics of short 

duration impact events such as blade loss and bird strike. 

8.3.3 These models typically include all major components of the propulsion system, such as 

the nacelle intake, fan cowl doors, thrust reverser, common nozzle assembly, all 

structural casings, frames, bearing housings, rotors, and a representative pylon. 

Gyroscopic effects are included. The models provide for 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). 

8.3.4 Features that should be modeled specifically for blade loss analysis typically include 

fan imbalance, component failure and wear, rubs (blade-to-casing, and intershaft), and 

resulting stiffness changes. Manufacturers whose engines fail the rotor support 

structure by design during the blade loss event should also evaluate the effect of the 

loss of support on engine structural response. 

8.3.5 The model should be validated based on vibration tests and results of the blade loss test 

required for compliance with § 33.94, giving due allowance for the effects of the test 

mount structure. The model should be capable of accurately predicting the transient 

loads from blade release through rundown to steady state. In cases where compliance 

with § 33.94 is granted by similarity instead of test, the model should be correlated to 

prior experience. For compliance with § 25.362, the engine model, once validated, 

should be modified to include the influence of representative adjacent supporting 

airframe structure. 

8.3.6 Validation of the engine model static structure, including the pylon, is achieved by a 

combination of engine and component tests that include structural tests on major load 

path components. The adequacy of the engine model to predict rotor critical speeds 

and forced response behavior is verified by measuring engine vibratory response when 

imbalances are added to the fan and other rotors. Vibration data are routinely 
XX/XX/XX D R A F T AC 25.362-X 

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monitored on a number of engines during the engine development cycle, thereby 

providing a solid basis for model correlation. 

8.3.7 Correlation of the model against the § 33.94 blade loss engine test is a demonstration 

for which the model accurately predicts initial blade release event loads, any rundown 

resonant response behavior, frequencies, potential structural failure sequences, and 

general engine movements and displacements. To enable this correlation to be 

performed, instrumentation of the blade loss engine test should be used (e.g., use of 

high-speed cinema and video cameras, accelerometers, strain gauges, continuity wires, 

and shaft speed tachometers). This instrumentation should be capable of measuring 

loads on the engine attachment structure. 

END