Section 33.74 generally does not consider certain inherently hazardous basic
06/23/2016 AC 33.74/92-1B basic nonhazardous, in-flight shutdown (IFSD) cause, such as fuel starvation.
2.1.3.3 Inherently Hazardous Basic Engine Failures.
Section 33.74 generally does not consider certain inherently hazardous basic engine failures, such as main rotor structure failures (disks, spacers, seals, and shafts. etc.), unless those failures are considered initially nonhazardous; or would likely result in sustained continued rotation of the engine.
Failure of such components usually results in a limited period of rotation after the initial event.
2.1.3.4 Rotor Locking Device Failure.
Section 33.74 generally does not consider rotor locking device failure scenarios, since the rotor locking device is expected to perform its intended function per the requirements of § 33.92. Refer to paragraph 2.2 for guidance.
2.1.3.5 Failed Airframe Structure or Airframe Foreign Object Source Ingestion.
Section 33.74 does not consider ingestion of failed airframe structures (for example, slat/flap or radome parts); or significant airframe foreign object sources (for example, lavatory ice), if beyond the scope of part 33 type certification.
2.1.3.6 Effect of Continued Rotation on Other Aircraft Structures, Systems, or Flight Crew.
Section 33.74 does not apply to the effects of engine continued rotation on other aircraft structures; other aircraft systems; and the flight crew.
Operating Conditions. 2.1.4 2.1.4.1 General.
You should determine the flight conditions that could occur during continued rotation operations. This determination should include, but is not limited to, the criteria described in the following subparagraphs (subparagraph 2.1.4.2 through subparagraph 2.1.4.5 ).
2.1.4.2 Maximum Exposure Time for Continued Rotation for Individual Event.
Determination of the maximum exposure time for continued rotation must consider all exposure time scenarios regardless of expected probability, and therefore: must not be based on the probability of occurrence as a function of flight phase; should consider special operations, such as ETOPS, and must cover the maximum diversion time of the airplane during such operations; must address the maximum diversion time approved for the engine, if the engine is approved for Early ETOPS eligibility in accordance 2-2 06/23/2016 AC 33.74/92-1B with § 33.201; and should assume a minimum diversion time of one hour for all non-ETOPS aircraft applications.
2.1.4.3 Turbine Rotor(s) Rotational Speeds within One-Engine Inoperative Flight Envelope.
Determination of turbine rotor(s) rotational speeds within the one-engine inoperative flight envelope should take into account significant flight phases (takeoff, climb, cruise, descent, approach, and landing). It should also consider the effect of engine damage on continued rotation rotor speeds.
2.1.4.4 Unbalance Levels.
Determination of unbalance levels must consider the initial event and subsequent damage, as they apply to the failure condition that you are evaluating.
2.1.4.5 Engines Certificated for Use on Supersonic Aircraft.
You must show compliance for the expected duration at supersonic and supersonic-to-subsonic transition flight conditions. This is in addition to duration of continued rotation for the remainder of flight at subsonic speeds.
Pass/Fail Criteria. 2.1.5 Section 33.74 defines the pass/fail criteria. The compliance determination should consider both the initial event and the duration of the continued rotation.
Compliance Considerations. 2.1.6 2.1.6.1 General.
You should take into consideration the criteria in the following subparagraphs (subparagraph 2.1.6.2 through subparagraph 2.1.6.7 ) when you develop your analysis technique.
2.1.6.2 Fire Hazards.
Protection against fire is especially important during extended continued rotation periods. Continued rotation of the main rotors, coupled with high unbalance levels, could produce high stress levels in flammable-fluid- conveying parts at sub-idle natural frequencies. Failure of such parts could result in leakage of hazardous quantities of flammable fluids and increased fire hazard. For example, in some engine designs, continued rotation will result in continued main engine oil flow (no shutoff means) with the potential for leakage if certain oil system parts fail. Continued rotation could also result in “rubbing” of titanium rotor and stator components with the potential for a titanium fire and increased fire hazard.
You should assess the applicable continued rotation conditions against the fire protection requirements of § 33.17 and § 33.75. The fire protection assessment should consider both the initial event and the duration of the continued rotation.
2-3 06/23/2016 AC 33.74/92-1B 2.1.6.3 Fatigue Assessment.
You should perform a fatigue assessment for an installed engine over the assumed diversion profile. This assessment should account for transient exposure to peak vibrations, such as the initial event. This assessment should also account for sustained exposure to vibration loads over the maximum continued rotation period. Average material properties may be used.
For each component you evaluate, you should show the accumulated fatigue damage to be less than or equal to the fatigue damage to failure of the component, so that the conditions of § 33.75(g)(2)(i) through (vi) do not occur over the continued rotation period. The fatigue assessment should consider both the initial event and the duration of the continued rotation.
2.1.6.4 Loads.
You should determine loads on the engine structure by test, validated analysis, or both. You should determine the steady and vibratory loads for the significant operating conditions noted in paragraph 2.1.4 of this AC, and take into consideration: the range of continued rotation frequencies for the various failure events; continued rotation periods; aircraft accelerations; and ambient temperature variation.
Load determination should consider both the initial event and the duration of the continued rotation.
2.1.6.5 Electrical Discharge.
You should consider the potential hazard of electrical discharge from ignition exciters that, by design, continue to build a charge during continued rotation. The objective is to prevent ignition sources within the engine nacelle that may affect maintenance personnel after flight completion.
2.1.6.6 Analysis Methodologies.
Your analysis techniques must be able to provide sufficient detail to determine the continued rotation loads on the installed engine. You must adequately validate these techniques by demonstrating the capability to predict dynamic results for the identified continued rotation conditions.
Before you use an analysis to produce new certification compliance data to represent new design features or new operating environments or operating characteristics, it must be calibrated and validated using data from one of, or a combination of, the following: prior certification test; field data; component tests; and/or acceptable component test or laboratory test.
You must examine any component or laboratory test or tests used to produce new data for quality, applicability to the new product being certified, and adherence to your established engineering and testing standards, or to accepted industry standards.
You may need to use conservative assumptions regarding installation to cover multiple installations.
2-4 06/23/2016 AC 33.74/92-1B 2.1.6.7 Engine Modeling.
Any models you use should be validated; contain sufficient detail to accurately conduct transient and steady state analyses; include all major engine static and rotating components; and provide for representative connections at the engine-aircraft interfaces.
Rotor Locking Device (14 CFR 33.92). 2.2 General. 2.2.1 A rotor locking device is defined as any flight crew-actuated, engine type design component or system that prohibits engine rotor rotation after engine shutdown.
Activation of the rotor locking device stops and prevents continued rotation of the engine rotor(s) during flight, when the engine is not operating. You may incorporate a rotor locking device into the engine type design, or as an aircraft installation requirement.
Incorporation of a rotor locking device complies with the safety objective of § 33.74 (refer to paragraph 2.1 ). Incorporation of a rotor locking device must satisfy the operational and endurance test requirements identified in § 33.92, when the engine is subjected to the environmental conditions that result in the maximum torque. The assessment of the maximum torque should consider both damaged and undamaged engine rotors.
14 CFR 33.92 applies to turbine engines installed in airplanes and rotorcrafts that incorporate a means to stop and lock the rotor(s) to prevent continued rotation.
Compliance to § 33.92 must be demonstrated by test, the means to stop and lock the rotor if continued rotation is prevented.
Reliability. 2.2.2 Because the rotor locking device is not expected to be used frequently, you should show that under normal engine operating conditions, the device will not deteriorate beyond serviceable limits or fail to perform the intended function.
Overall reliability, as related to continued rotation related failure modes, should be consistent with § 33.75(a)(3) requirements for protection against hazardous effects.
Design Criteria. 2.2.3 You should ensure that the rotor locking device is designed so that the flight crew may lock (to stop rotation) and unlock (to initiate engine restart attempts) the device in flight, as required. You should also evaluate the effect an uncommanded activation of the rotor locking device in flight can have on continued safe flight and landing of the aircraft. Significant effects should be explained in the Installation and Operating Instructions required by § 33.5. Finally, you should evaluate the various environmental threat effects (for example, rain, hail, icing, and bird ingestion) on rotor locking device performance over the engine operating envelope. Significant effects should be explained in the Installation and Operating Instructions required by § 33.5.
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APPENDIX A
06/23/2016 AC 33.74/92-1B APPENDIX A APPENDIX A. ADVISORY CIRCULAR FEEDBACK FORM A-1