Date: 1/19/01 

Initiated By: ANE-110 

AC No: AC 33.76-1 

Change: 

Subject: Bird Ingestion Certification Standards 

1. PURPOSE. This advisory circular (AC) provides guidance and acceptable methods, 

but not the only methods, that may be used to demonstrate compliance with the bird 

ingestion requirements of § 33.76 of the Federal Aviation Regulations, Title 14 of the 

Code of Federal Regulations. Although this AC does refer to regulatory requirements 

that are mandatory, this AC is not, in itself, mandatory. This AC neither changes any 

regulatory requirements nor authorizes changes in or deviations from the regulatory 

requirements. 

2. BACKGROUND. 

 a. This effort was adopted as a part 33 and Joint Aviation Regulations for engines 

(JAR-E) harmonization project and was selected as an Aviation Rulemaking Advisory 

Committee (ARAC) project. 

 b. This AC provides information and guidance that addresses Federal Aviation 

Administration (FAA) type certification standards for aircraft turbine engines with regard 

to bird ingestion. The requirements under § 33.76 reflect recent analysis of the bird 

threat encountered in service by turbine engine powered aircraft. 

3. DEFINITIONS. For the purpose of this AC, the following definitions apply: 

 a. Ingestion. Ingestion is defined as the passage of a bird into the rotating blades of a 

turbine engine. 
AC 33.76-1 1/19/01 

 b. Front of the Engine. The front of the engine is characterized as any part of the 

engine that can be struck by a bird. This includes, but is not limited to, the following 

engine components: 

 (1) inlet mounted components, 

 (2) nose cone, 

 (3) spinner (centerbody) on the fan or compressor rotor, 

 (4) engine inlet guide vane or frame strut assemblies, and 

 (5) any engine protection device. 

 c. Minimum Engine. A minimum engine is defined as a new engine that exhibits the 

type design's most limiting operating parameter(s), with respect to the prescribed bird 

ingestion conditions. These operating parameters include, but are not limited to, power 

or thrust, turbine gaspath temperature, and rotor speed. 

 d. First Stage Rotating Blades. The term "first stage rotating blades" includes the first 

of the exposed stages of any fan or compressor rotor which is susceptible to a bird strike 

or bird ingestion. These first stage rotating blades are considered to be part of the front 

of the engine, as defined in paragraph (3)(b). This definition encompasses ducted, 

unducted and aft fan engine designs. In these latter cases, susceptible blading on multiple 

rotors (e.g., primary and secondary airflow paths) should be considered separately when 

complying with § 33.76. 

 e. Critical Impact Parameter (CIP). A parameter used to characterize the state of 

stress, strain, deflection, twist, or other condition that will result in the maximum impact 

damage to the engine for the prescribed bird ingestion condition. 

 f. Inlet Throat Area. The inlet throat area is the installation limitation on projected 

capture area of the engine inlet nacelle at its minimum inside diameter as established by 

the applicant. 

4. GENERAL. The intent of § 33.76 is to require an applicant to demonstrate that the 

engine is designed and constructed to be structurally and operationally tolerant, to the 

degree specified, following the defined bird ingestion events. 

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1/19/01 AC 33.76-1 

 a. Front of the Engine. The applicant should assess the bird impact to the critical 

parameters of the components at the front of the engine. For example, the ability of the 

spinner to withstand a bird impact should be assessed for the most critical parameters of 

the spinner. This assessment should include bird size, bird velocity, target location, and 

spinner rotational speed. 

 b Artificial Birds. Artificial birds or devices which simulate the mass, shape, density, 

and impact effects of birds, which are adequately validated as equivalent and which are 

acceptable to the Administrator, may be used for the ingestion tests. 

 c. Critical Impact Parameter (CIP). The parameter is generally a function of such 

things as bird mass, bird velocity, fan/rotor speed, impact location, and fan/rotor blade 

geometry. The state of maximum impact damage to the engine is relative to the ability to 

meet the criteria of § 33.76. The CIP for most modern turbofan engines is fan blade 

leading edge stress, although other features or parameters may be more critical as a 

function of operating conditions or basic design. For turboprop and turbojet engines, a 

core feature will most likely be the critical consideration. Regardless of engine design, 

the most limiting parameter should be identified and understood prior to any 

demonstration, as any unplanned variations in controlling test parameters will be 

evaluated for the effect on the CIP and § 33.76 requirements. 

 (1) Example Considerations for Determining the CIP. For turbofan first stage fan 

blades, increasing the bird velocity or bird mass will alter the slice mass, and could shift 

the CIP from leading edge stress to some other highly stressed feature of the blade (e.g., 

blade root). For fan blades with part span shrouds, it may be blade deflection that 

produces shroud shingling and either thrust loss or a blade fracture that could be limiting. 

For unshrouded wide chord fan blades, it may be the trailing edge tip of the blade which 

experiences damage due to an impact induced shock wave traveling through the blade, or 

the twist of the blade in the dovetail that allows it to impact the trailing blade resulting in 

trailing blade damage. 

 (2) CIP Tolerance. For certification tests, the CIP variation should not be greater 

than 10% due to deviations in any test plan controlling parameter. 

 d. Critical Test Parameters. In conducting the analysis or component tests, or both, to 

determine the critical ingestion test parameters, the applicant should consider related 

experience for the type and size of engine being evaluated, with particular attention to the 

types and causes of failures in that related experience. 

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AC 33.76-1 1/19/01 

 e. Engine Tests. Engine tests should be conducted with a fully operational engine 

representative of the type design. The normal functioning of any automatic protective or 

recovery systems not requiring pilot intervention is acceptable (including automatic 

power lever movement). However, any such automatic systems may be required for 

dispatch (e.g., Master Minimum Equipment List) if such functions are necessary to meet 

the requirements of § 33.76. The Applicant may also conduct the test(s) with any 

automatic systems in a functionally degraded state, if this does not constitute a less 

severe test. 

 f. Test Facilities. The object of the test is to cover all the required impact zones. The 

test facility should be appropriately calibrated to ensure that the controlling parameters 

defined by the analysis of the critical conditions (e.g., bird speed, aiming locations) are 

within an acceptable tolerance. This tolerance band should be derived from an analysis 

of the sensitivity of the critical impact parameter to variations in the controlling 

parameters. The band should be such that variation in the most critical impact parameter 

is not more than 10% resulting from any combination of the controlling parameters (See 

paragraph 4. c. above). Also, certain test facilities and installations may affect or reduce 

the stability margin of the engine due to airflow distortion attributed to the close 

proximity of bird gun(s) to the engine inlet. These effects must be identified prior to the 

test. 

 g. Turboprop/Turboshaft Engine Tests. If turboprop or turboshaft engines are tested 

using an alternative load device which could induce different engine response 

characteristics compared to when the engine is coupled with a propeller or as installed in 

the aircraft, the interface with the test facility or other aircraft or propeller systems should 

be monitored during the test. These results should be used for determining how the 

engine would respond in a representative installation, and for ensuring that the engine 

would then comply with the requirements in § 33.76. 

 h. Aircraft/Engine Interface. The Installation Manual required under § 33.5 should 

describe the engine/aircraft interfaces that could be affected by bird ingestion events. Of 

particular interest would be dynamic interactions such as auto surge recovery, auto 

relight, or propeller auto feather. 

 i. Inlet Throat Area. The Installation Manual required under § 33.5 should identify as 

an installation limitation the inlet throat area which was used to determine the quantity 

and 

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1/19/01 AC 33.76-1 

weight of birds for the overall showing of compliance to § 33.76. Section 33.76(a)(2) 

contains the specific requirement for this installation limitation. The applicant should 

take care in determining this value with respect to potential future models or installations, 

which may require a larger number or size of birds or both. Note that the tables of bird 

quantities and weights within § 33.76 are based on inlet throat area, not the inlet highlight 

or engine front flange projected areas. 

 j. Derivative Engines and Major Design Changes. For type certification of derivative 

engine models or major design changes to existing models, the required engine tests 

should be performed under the conditions of § 33.76, unless alternative equivalent 

demonstration evidence acceptable to the Administrator is provided. This substantiation 

evidence may come from the applicant's experience on engines of comparable size, 

design, construction, performance, and handling characteristics, obtained during 

development, certification or operation. Any parametric analysis used as compliance 

substantiation for type certification or for major design change approval, should fall 

within a 10% or less variation in the most critical impact parameter(s) identified for the 

baseline engine certification. The critical impact parameter(s) is often associated with 

impact load at the point of bird and rotor blade contact. This is generally a function of 

bird speed, rotor speed, and blade twist angle. This 10% variation (on the critical impact 

parameter) should not be assumed to be a direct tolerance on the applicants proposed 

changes to takeoff power or thrust ratings themselves. 

 k. Hot Day Corner Point Operation. The intent of § 33.76(a)(1) is to demonstrate, to 

the degree specified, that the engine is designed and manufactured to be structurally and 

operationally tolerant following bird ingestion conditions when operated at this critical 

point of the operating envelope. Compliance may include an actual engine test at hot day 

corner point conditions, or may be based on other representative engine or component 

tests, validated analysis, representative service events, or any combination of these 

acceptable to the FAA. Applicants are encouraged to conduct the bird tests at the hot day 

corner point condition if test facilities allow. Also, tests may be conducted at reasonable 

levels of power/thrust overboost (e.g., operation at P3 limit) to minimize the use for less 

direct methods of showing compliance (e.g., analysis), especially if the proposed engine 

test ambients are less severe then sea-level standard day conditions. If analysis is used to 

show compliance, it should show the ability to predict bird ingestion event outcomes 

relative to the requirements and criteria of § 33.76. The analysis approach must be 

agreed to between the FAA and the applicant, and should be submitted early in the 

program to avoid impacting program schedules. 

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AC 33.76-1 1/19/01 

SECTION 1 

Large Bird Ingestion 

5. GUIDANCE FOR LARGE BIRD INGESTION. 

 a. For the purpose of the § 33.76 test, the complete loss of engine power or thrust 

after ingestion will be accepted. 

 b. The most critical location on the first stage rotating blades may be determined from 

analysis or component tests, or both. Determination of the most critical location to be 

considered should include evidence, where necessary, on: 

 (1) the effect of the bird strike on rotating components, 

 (2) the compressor casing strength, 

 (3) the possibility of multiple blade failures, the strength of the engine structure 

and main shafts relative to the unbalance and excessive torque likely to occur. 

(c) When compliance with the large bird ingestion requirements is proposed to be 

accomplished under 33.76(b)(4), it should be demonstrated that the test(s) conducted 

under 33.94(a) constitutes a more severe demonstration of rotor blade containment and 

rotor unbalance than the large bird ingestion engine test required under 

33.76(a)/(b)(1)&(2). This comparative demonstration should consider, but is not limited 

to, the following results from the required 33.94(a) test(s): 

 (1) Unbalance loads. 

 (2) Torque loads. 

 (3) Surge loads. 

 (4) Containment capability. 

 (5) Fire protection capability. 

 (6) Mound load capability. 

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1/19/01 AC 33.76-1 

Primary engine structure (e.g., rotors, frames, cases, engine/gearbox mounts, etc.) and 

externally mounted components (e.g., oil tank, heat exchangers, etc.) should be 

considered when comparing loads between the two events. Unbalance loads could result 

from rotor material loss or loss of rotor centerline. Torque loads could result from rotorcase interaction or rotor-object impacts. Surge loads could result from engine operating 

instabilities. Axial loads due to rotor-bird impact should also be considered when 

comparing the two events. Also, using 33.76(b)(4) as a compliance method is generally 

predicated on having adequate 33.94(a) test data available, and should focus on 

predicting the engine dynamic response to a large bird ingestion event relative to the 

pass/fail criteria of 33.76(b)(3). 

 (d) The 200 knots ingestion speed for the large bird requirement was selected as the 

optimum speed to accommodate, within a single demonstration, the various critical 

impact parameters (CIP) associated with typical turbofan engine designs currently in 

service. For a specific engine design, however, an applicant may determine that an 

ingestion speed other than 200 knots is more critical when considering the overall criteria 

of § 33.76(b). The applicant may apply for an equivalent safety finding under 

21.21(b)(1) prior to the conduct of the test, and if approved, use a bird speed other than 

200 knots when that speed represents a more conservative or more complete evaluation 

of the proposed design. Use of that alternate speed must then be noted in the certification 

basis as an equivalent level of safety finding under § 21.21(b)(1). 

SECTION 2 

Small and Medium Bird Ingestion 

6. GUIDANCE FOR SMALL AND MEDIUM BIRD INGESTION. 

 a. The applicant should identify the critical target locations for the small and medium 

bird ingestion tests required by § 33.76(c), and consider potential effects of assumed 

installations in the aircraft. After targeting one bird for the most critical exposed 

location, the applicant should target any remaining birds in proportion to the fan face 

area, including the centerbody if applicable, to achieve an even distribution of birds over 

the face of the engine. The even distribution of remaining birds should also include 

consideration of any additional critical locations. Any critical locations not targeted may 

be evaluated separately by analysis or component testing, or both. 

Par 5 Page 7 
AC 33.76-1 1/19/01 

 b. In the tests performed under § 33.76(c), the engine is required to produce at least 

75% of takeoff power or thrust after ingestion of small and medium birds. A momentary 

power or thrust drop below this value may be acceptable as long as the duration does not 

exceed 3 seconds. 

 c. Any analytical means used in place of a test demonstration for evaluating or 

determining compliance with § 33.76, should be validated by evidence based on 

representative tests and should have demonstrated its capability to predict engine test 

results. 

 d. Rig tests may be used to determine if a particular bird size will pass through the 

inlet and into the rotor blades. 

 e. Thrust or power should be measured by a means which can be shown to be 

accurate throughout the test to enable the thrust or power to be set without undue delay 

and maintained to within plus or minus 3% of the specified levels. If a sustained high 

vibration condition exists after the first 2 minutes of operation after the bird ingestion, 

then thrust or power may be varied as a protective measure within plus or minus 3% of 

the specified levels. Alternative load devices of some test facilities such as waterbrakes, 

may be unable to control power within the plus or minus 3% tolerance. This should be 

identified and approved prior to the test. 

 f. Exceedences of engine operating limits are not expected to occur. However, 

exceedences may be permitted to occur only during the first 2 minutes (reference 

§ 33.76(c)(7)(ii)) following the ingestion of the birds in the 20 minute run-on test. Any 

limit exceedence(s) should be recorded, and it should be shown by evidence acceptable to 

the Administrator, that the limit exceedence(s) will not result in an unsafe condition 

(reference § 33.76(c)(10)). This evidence may come from previous test or service 

experience, or analysis thereof. In addition, under such circumstances, the operating 

instructions, installation manual, and maintenance manual should be reviewed to assure 

that appropriate instructions are included within those documents, and that any such 

instructions are appropriately validated. 

 /s/ 

Jay J. Pardee 

Manager, Engine and Propeller Directorate 

Aircraft Certification Service 

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