Subject: INITIAL MAINTENANCE 

INSPECTION (IMI), 14 CFR §33.90, TEST 

FOR TURBINE ENGINES 

Date: 3/5/04 

Initiated By: ANE-110

AC No: 33.90-1 

Change: 

1. PURPOSE. This advisory circular (AC) provides guidance and acceptable methods, but not 

the only methods, for demonstrating compliance with the test requirements of §33.90 of Title 14 

of the Code of Federal Regulations (14 CFR §33.90), Initial maintenance inspection. The 

information provided in this AC replaces the guidance in paragraph 61, §33.90 IMI, of AC 33-

2B, Aircraft Engine Type Certification Handbook. 

2. APPLICABILITY. 

 a. The guidance provided in this document is directed to engine manufacturers, modifiers, 

Federal Aviation Administration (FAA) engine type certification engineers, and FAA designees. 

 b. This material is neither mandatory nor regulatory in nature and does not constitute a 

regulation. It describes acceptable means, but not the only means, for demonstrating compliance 

with the applicable regulations. The FAA will consider other methods of demonstrating 

compliance that an applicant may elect to present. Terms such as “should,” “shall,” “may,” and 

“must” are used only in the sense of ensuring applicability of this particular method of 

compliance when the method of compliance in this document is used. While these guidelines are 

not mandatory, they are derived from extensive FAA and industry experience in determining 

compliance with the relevant regulations. On the other hand, if the FAA becomes 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 as the basis for finding compliance. 

 c. This material does not change, create any additional, authorize changes in, or permit 

deviations from existing regulatory requirements. 

3. RELATED REGULATION. 14 CFR §33.90. 
AC 33.90-1 3/5/04 

4. DEFINITIONS. For the purposes of this AC, the following definitions apply: 

 a. Initial Maintenance Inspection (IMI). IMIs are those inspections specified in the 

instructions for continued airworthiness (ICA) submitted under §33.4 that are considered 

necessary to determine the serviceability of the engine. IMIs may be required by the type 

certificate (TC) holder in the ICA Airworthiness Limitations Section or may be recommended at 

certain intervals. 

 b. IMI Intervals. IMI intervals are the maximum hours or cycles that an engine or engine 

module should be operated in service before a hardware serviceability IMI is conducted on 

engine static and rotating assemblies, systems, and controls. 

NOTE: The results of this test run should support the specified engine IMI 

interval or module/system-level IMI intervals, as required or recommended by the 

applicant for the type design. Other sources of information may also play a role 

in IMI interval determination. The results of the §33.90 testing should also 

support the IMI methods and serviceability limits specified in the ICA.

 c. Engine Flight Cycle. An engine flight cycle is the predicted average flight profile of 

engine parameters and effects representative of the way the engine is expected to operate in 

service. 

 d. Overhaul. Overhaul refers to the process of disassembling, cleaning, inspecting, repairing 

as necessary, reassembling, and testing for approval for return-to-service within the 

specifications of the manufacturer’s overhaul data. 

 e. Serviceable. An engine part, component, or assembly is serviceable if it is in a physical 

condition acceptable for continued service operation in accordance with the ICA submitted under 

§33.4. 

5. BACKGROUND. Section 33.90 requires the applicant to perform a test run for each engine 

model for which a new TC is required. The test run must simulate the conditions under which 

the engine is expected to operate in service, including start-stop cycles that are typical of 

expected service use. The primary purpose of this test requirement is to help establish any entryinto-service (EIS) IMI intervals for that type design. The engine must remain in a serviceable 

condition between the required or recommended maintenance inspections, or fixed overhaul 

periods. Therefore, this AC provides guidance on test methods and procedures, test pass/fail 

criteria, and EIS IMI or overhaul requirements or recommendations. The applicant should also 

consider the suitability of manual serviceability limits as part of this overall evaluation. 

6. CONDUCT OF IMI TESTS. 

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3/5/04 AC 33.90-1 

 a. IMI Test Cycle Assessment. The applicant should provide an assessment of expected 

service operating conditions as part of the overall test plan. In the assessment, the applicant 

should show that the proposed test cycle represents the expected engine flight cycle regarding 

established power/thrust ratings, reverse thrust usage, component stress and temperature, exhaust 

gas temperature (EGT), vibration, and cycle/operating time cumulative damage or other critical 

factors. For multiple aircraft applications, the applicant should show that the test cycle 

adequately represents all identified or anticipated installations. Two types of test cycles have 

been used in the past to demonstrate the IMI interval(s). 

 (1) Full Cycle Test. This method requires the engine to be run through the exact 

sequences of thrust or power settings for the period of time identified in the engine flight cycle. 

One complete cycle of a full cycle test involves the exact number of operating hours as a typical 

engine flight cycle and includes engine start and shutdown. 

 (2) Accelerated Severity Cycle Test. This method is structured to provide a rigorous test 

of engines (or engine parts) whose durability is primarily affected by cyclic operation. This type 

of test may vary the time at various thrust or power settings and the sequence of thrust or power 

selections from that of the engine flight cycle. Determining the relationship between the 

accelerated severity cycle test and the full flight cycle involves a complex analysis of stress, 

temperature, and resulting life for each affected part of the engine. The accelerated severity 

cycle test may include several major stress cycles during a given portion of overall engine test, 

resulting in a small number of engine hours in comparison to the number of engine flight cycles 

demonstrated. The accelerated severity cycle test is generally not considered ideal for 

substantiating those engine parts whose durability is primarily affected by hours of operation 

rather than by cycles. For those cases, other test or service experience data may be required to 

substantiate the IMI intervals when using this test method. The accelerated severity cycle test 

should include engine start and shutdown. 

 (3) Combination tests using full cycles and accelerated severity cycles have also been 

used. 

 b. Test Engine Configuration. Section 33.90 specifies compliance by test demonstration 

using an engine that substantially conforms to its final type design. 

 (1) The applicant should consider other hardware items not normally part of the engine 

type design (for example, thrust reverser, air starter, engine build-up (EBU) hardware) for 

inclusion in the test. For information on EBU hardware or installation configuration, the 

applicant should consult the installer. 

 (2) The test should also be performed with the engine in a typical installed configuration to 

the maximum extent possible, and with representative airframe accessories and interfaces 

connected and operating. The typical accessory loads and bleed air extraction that would be 

experienced during the engine flight cycle should be scheduled throughout the test to the 

maximum extent possible. 

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AC 33.90-1 3/5/04 

 (3) For turboprop applications, the test engine should have a representative propeller 

installed. The applicant should incorporate applicable design features such as propeller braking 

and APU-mode operation into the test cycle. 

 (4) For turboshaft applications, the test engine output shaft should be loaded so as to 

simulate the appropriate characteristics of the main rotor of the intended installation. Potential 

main rotor characteristics include, but are not limited to, inertia and torsional vibration. 

 c. Test Parameters. The test may be conducted at sea level conditions if the test effectively 

reproduces the conditions of power/thrust, stress, component temperature, EGT, and unbalance 

vibration that would be experienced during an Engine Flight Cycle.

 d. Test Duration. The total number of test cycles and overall test duration should be 

equivalent to the anticipated initial on-wing life for the new engine model in a typical 

installation. At a minimum, the applicant will need to run a suitable number of cycles to 

establish the IMI interval(s) and to show that a type design engine remains in a serviceable 

condition between required maintenance inspections. 

 e. Pass/Fail Criteria. The engine type design will have met the requirements of §33.90 when 

the post-test IMI hardware condition results support the proposed intervals. The following 

should be considered:

 (1) Over the test duration and using normal maintenance practices contained in the ICA’s, 

the engine can deliver rated takeoff thrust or power for a sea-level hot day corner point operating 

condition without exceeding any operating limitations. Further, the engine is free of surge or 

stall or other significant anomalies when operated in accordance with the operating instructions 

provided in support of §33.5. 

 (2) Following the test a teardown inspection should show that each engine part conforms

to the type design and is eligible for continued operation in service in accordance with the ICA

information submitted for compliance with §33.4. Hardware may be found serviceable if 

appropriate inspections or limitations are included within the ICA. 

 (3) Note that an alternative plan to replace a pre-Type Certificate full engine teardown 

inspection may be accepted by the FAA. Such a plan should include appropriate installed engine 

inspections and checks, along with a full engine teardown inspection prior to entry-into-service. 

Generally, this alternative should be supported by an extended number of test cycles completed 

before the full teardown. This type of proposal should be coordinated with the FAA well in 

advance of testing.

 (4) The §33.90 certification documentation should identify those parts of the engine that 

will have specific IMI requirements or recommendations. A formal plan should exist to include 

this information, including life limits, inspections, intervals, and serviceability criteria, in the 

ICA in accordance with the existing provisions of §33.4. 

 f. Determination of Time/Cycle Intervals for Initial Maintenance Inspections. 

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3/5/04 AC 33.90-1 

 (1) For a successful full cycle test, the applicant may use the full number of cycles and full 

number of hours demonstrated during the test as the IMI interval. 

 (2) For a successful accelerated severity cycle test, the applicant may use the full number 

of cycles for those engine parts for which the test cycle was shown to be equal to or more severe 

than the assumed engine flight cycle. 

 (3) The applicant would not normally be allowed to extrapolate to an interval in excess of 

those hours/cycles actually demonstrated. However, it may be acceptable to consider credit for 

additional hours if the cycle used (for example, accelerated severity cycle) involved operation at 

high thrust settings for durations well in excess of those of the engine flight cycle. This 

approach requires caution, as some parts of the engine will wear as a function of time at load, 

rather than from low cycle fatigue, and life extrapolation based on material property data alone is 

imprecise at best. Under these circumstances, it may be necessary to draw from other engine, 

component, or subassembly tests for supporting evidence. 

7. FIXED ENGINE OVERHAUL PERIOD. If the engine is not intended to be covered by a 

structured inspection program, the TC holder may recommend a fixed overhaul period as the 

equivalent of an IMI. Under this approach, the applicant should conduct the §33.90 engine test 

in a similar manner to that described in paragraph 6 of this AC, and the test results should 

support the desired fixed overhaul period. 

//signed by JJP on 3/5/04// 

Jay J. Pardee 

Manager, Engine and Propeller Directorate 

Aircraft Certification Service 

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