AC 27-1B, Chg 4 3/31/2014 It is an acceptable procedure for the engine manufacturer, while he is obtaining his type certificate, to accomplish this bench testing to determine the level of tolerance of the FADEC system components to lightning encounter indirect effects. This approach has the advantages that the bench tests are not necessarily required to be repeated when the engine is installed in a different airframe. This recommendation is not meant to add a requirement to the engine manufacturer but to propose a more efficient method of certification. If this tolerance was not determined by the engine manufacturer, the applicant installing the FADEC in a rotorcraft would be expected to furnish this data.
(3) As with any analytical method, it is prudent to include a margin of safety to account for the uncertainties involved in the analytical and testing processes.
Margins account for uncertainty in the compliance method. As confidence in the compliance method increases, the margin can decrease. A factor of two is an acceptable margin for systems with catastrophic failure conditions, if this margin is verified by aircraft test or by analysis supported by aircraft tests. For other verification methods, the margin should be approved by the cognizant certification office.
(4) When an analysis has no associated full-scale vehicle testing to confirm the analysis, the analysis should be very rigorous. Additionally, it should be expected in this situation that this analysis indicates a very large margin of protection.
Many factors must be considered in determining what constitutes an acceptably large margin. The specific additional margin required should be based on an assessment of the inherent uncertainty of a given analysis.
(E) Pass or Fail Criteria. The certification plan should address a pass or fail criteria for the testing and analysis to be performed. The following items should be satisfied to assure acceptable system performance:
(1) No immediate crew action must be required.
(2) Automatic control of the engine cannot be lost for any appreciable period of time. The engine must not be allowed to be out of control for a period of time which will result in a hazard in a worst-case flight condition. Obviously, any rapid, uncontrolled divergence is not acceptable.
(3) No crew action should be required to reset the system. This is not to imply that the system cannot be designed with a manual reset, but the manual reset cannot be used to show compliance to recover from a digital upset.
(4) The resumption of engine control after an upset must be reasonably within the range which existed before the upset.
(5) No critical data can be lost.
(6) After the system recovers, if the performance of the system has been degraded in a noncritical manner which would reduce the capability of the Page F - 24
rotorcraft or the ability of the pilot to cope with adverse operating conditions, then the crew must be alerted to this system degradation.
(v) System Installation Considerations. In most cases, the installation of the system components is a constituent part of the lightning protection. This is particularly true in the use of shielding techniques. If these installation features are required for adequate lightning protection, consideration should be given to ensure that their effectiveness is not derogated in service. ICAs must be provided to the parties who service and operate the rotorcraft to allow them to take actions necessary to ensure the continued effectiveness of the system lightning protection.
c. Failure Analyses.
(1) Power and distribution systems should be analyzed to show compliance with § 27.1309.
(i) One acceptable procedure for documenting the analysis is contained in Society of Automotive Engineers (SAE) Fault/Failure Analysis Procedure ARP 926B, revised June 1, 1997.
(ii) As a minimum, any analysis should consider the effect of failures of components and systems on the capability of the rotorcraft to perform its intended function without hazard.
(iii) The analysis should consider the indication of failure. Those latent failures that occur without indication should be considered in all possible sequences and combinations of additional failures until a positive indication of failure is provided.
(iv) The analysis should consider failure of indirectly related parts of installations which could induce failure in the system being analyzed; for example, the effect of hydraulic fluid sprayed on electrical components as a result of a ruptured hydraulic line. Another example is the result of a ruptured bleed air line and its effect on hydraulic, fuel, or electrical lines or cables.
(v) The Type Inspection Authorization (TIA) should call for specific simulated failures, evaluation of failure detection, failure warning, and performance of the remaining system on the ground and in flight to verify the critical aspects of the failure analysis. The applicant should provide a proposed detailed test procedure for incorporation in the TIA to accomplish this verification. The applicant's proposed tests simulating in-flight failures should be carefully reviewed by both the systems engineer and flight test pilot to assure the flight test crew will not be subjected to hazardous flight.
Where practicable those simulated failures that would be hazardous in flight should be evaluated by ground tests. Analyzed and tested systems (where functioning is required) exhibiting hazards or failing to perform their intended functions under any foreseeable operating conditions must be redesigned to comply with § 27.1309.
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AC 27-1B, Chg 4 3/31/2014
(2) Utilization systems that are required or critical as to performance of intended function or result in rotorcraft hazard upon failure should also be analyzed for failures by the procedures of paragraphs c(1)(i) through c(1) (iv) above. Examples of systems that may be critical are autopilots, hydraulic control systems, navigation and flight instruments on IF-approved rotorcraft, and bleed air systems.
d. Safety Assessment. The international safety community has contributed to several documents providing guidance for conducting safety assessments for civil airborne systems and equipment. "Safety assessment" is terminology commonly used in the aerospace industry and addresses all aspects of fault, failure, or reliability analysis. Depending on the complexity and assessed criticality of the systems or equipment, the safety assessment can vary from a simple analysis which shows that there are no catastrophic, hazardous or severe-major, or major failure conditions, to a Functional Hazard Assessment (FHA) with the various analyses that may be required.
Various analytical techniques may be used to assist the applicant and the FAA/AUTHORITY in conducting the safety assessment for the determination of compliance with the requirements. These analytical techniques are intended to supplement but not replace engineering and operational judgment. A safety assessment can be accomplished by several different methods. One method used in past certifications (Failure Analyses) is addressed in paragraph c. above and is acceptable for showing compliance. Another method is the safety assessment process that contains the FHA addressed herein and is a methodology of general application to show compliance for systems, and all or parts of it may be applied as necessary depending on the complexity of design. The concept of safety assessment is provided in SAE document ARP 4754A,i Guidelines for Development of Civil Aircraft and Systems, and that aspect of this document is endorsed as an acceptable way of showing compliance to the requirements for function and design assurance. SAE Document ARP 4761, "Guidelines and Methods for Conducting the Safety Assessment Process on Civil Airborne Systems and Equipmenti can be used as a guide to perform the different analyses that form the safety assessment. The starting point is a FHA that identifies the assessed criticality of aircraft or systems functions. Further analysis is directed to be performed on the identified sources of these functions (i.e., systems) or part of systems performing these function(s)). One of the most common directed analysis is the reliability analysis.
(1) Functional Hazard Assessment. The FHA is a top down assessment that, on an aircraft level, starts at flight operations and identifies the criticality levels associated with the malfunction or failure of functions. The source of these functions can then be identified after completion of the FHA. There are five criticality levels defined in paragraph f. below, as they pertain to part 27. These level definitions are used in the FHA to decide which level matches the malfunctions or failures under consideration. The bottom-up analysis is then directed to the identified function source.
Reliability analysis is the most common of the bottom-up analysis.
(2) Reliability Analyses. Numerical Reliability Analyses may be developed, on an optional basis, as a continuation of the failure analysis procedure.
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(i) Specific reliability numbers are not shown in § 27.1309. The necessary degree of reliability is a function of the criticality of the system under consideration.
Acceptable sources of component failure rates are (1) military aircraft component failure or fault reports or handbooks, such as the latest revision of MIL-HDBK-217; (2) operator or manufacturer component malfunction or defect records, such as airline component defect records on sufficiently similar component designs; and (3) laboratory life tests.
(A) FLIGHT TIME (Block Time). The time from the moment the rotorcraft first moves under its own power for the purpose of flight until the moment it comes to rest at the next point of landing.
(B) PROBABILITY CLASSIFICATIONS. Five new probability classifications are defined below, although three probability classifications have been used in past certifications. This is not considered a problem as long as the conversion below is applied, and it is recognized that only the divisions within the same range are different. Quantitative ranges are also provided as a common point of reference if numerical probabilities are used. The quantitative ranges given for these classifications are considered to overlap due to the inexact nature of probability estimates. When assessing the acceptability of a failure condition using a quantitative analysis, the numerical ranges given below should normally be interpreted to be the allowable risk for an hour of flight time based on a flight of mean duration for the rotorcraft type.
However, when assessing a function which is used only at a specific time during a flight, the probability of the failure condition should be calculated for the specific time period and expressed as the risk for the flight condition, takeoff, landing, etc., as appropriate.
(1) FREQUENT. (This is the lower part of the range 105 or greater previously applied to the term "PROBABLE".) Frequent events may be expected to occur several times during the operational life of each rotorcraft that is based on a probability on the order of 103 or greater.
(2) REASONABLY PROBABLE. (This is the upper part of the range 10" or greater previously applied to the term "PROBABLE".) Probable events may be expected to occur several times during the operational life of each rotorcraft that is based on a probability on the order of between 103 to 105.
(3) REMOTE. ( The term "REMOTE" is not related to the structural use of the term.) (This is the lower part of the range 109 to 10-5 previously applied to the term "IMPROBABLE.") Remote events are not expected to occur during the total operational life of a random single rotorcraft of a particular type, but may occur during the total operational life of all rotorcraft of a particular type that is based on a probability on the order of between 105 to 107.
(4) EXTREMELY REMOTE. (The term "REMOTE" is not related to the structural use of the term.) (This is the upper part of the range 10° to 10-5 previously applied to the term "IMPROBABLE.") Extremely remote events are not Page F - 27
AC 27-1B, Chg 4 3/31/2014 expected to occur during the total operational life of a random single rotorcraft of a particular type, but may occur during the total operational life of all rotorcraft of a particular type that is based on a probability on the order of between 107 to 10-9.
(5) EXTREMELY IMPROBABLE. (Remains the same for both the three or the five classifications.) Extremely improbable events are so unlikely that they need not be considered to ever occur, unless engineering judgment would require their consideration. A probability on the order of 10 or less is assigned to this classification.
Notes: If a quantitative analysis is used to help show compliance with regulations for equipment which is installed and required only for a specific operating condition for which the rotorcraft is thereby approved, credit may not be taken for the fact that the operating condition does not always exist. Except for this limitation, appropriate statistical randomness of environmental or operational conditions may be considered in the analysis. (However, the particular condition and probability of that condition should be agreed to with the FAA/AUTHORITY.)
The five probability terms defined in paragraph d. (2)(ii)(B) above are intended to relate to an identified failure condition resulting from or contributed to by the improper operation or loss of a function or functions. These terms do not define the reliability of specific components or systems.
Generally, the guidance for Failure Analysis of paragraph c. above, is not required in its entirety for non-IFR rated rotorcraft. The only failure or reliability requirement is that no single failure can result in a hazard to the rotorcraft. This can usually be accomplished by a systems safety assessment that may or may not, depending on complexity and configuration, require a numerical reliability analysis.
e. Documentation. All laboratory tests, ground tests, flight tests, and failure analyses must be documented in sufficient detail to show compliance with § 27.1309 and included in the type design file. Section 21.31(a) provides the regulatory basis for requiring this documentation. If the applicant elects to use a numerical reliability and probability analysis it must also be documented in sufficient detail.
f. Software.
(1) RTCA Document DO-178C, "Software Considerations in Airborne Systems and Equipment Certification," dated December 13, 2011, is the latest standard and is recommended to be used for qualification and subsequent approval of airborne software. See AC 20-115C for guidance on using DO-178C and earlier standards.
DO-178C defines five levels of software (i.e., levels A, B, C, D, and E). The level of software is related to the criticality of the function that may be adversely affected by an error in the software. The criticality categories are as follows:
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(i) Catastrophic - failure conditions that would prevent continued safe flight and landing.
(ii) Hazardous or Severe-Major - failure conditions that would reduce the capability of the aircraft or the ability of the crew to cope with adverse operating conditions to the extent that there would be:
(A) A large reduction in safety margins or functional capabilities.
(B) Physical distress or higher workload such that the flight crew could not be relied on to perform their tasks accurately or completely.
(C) Adverse effects on occupants, including serious or potentially fatal injuries, to a small number of those occupants.
(iii) Major - failure conditions that would reduce the capability of the aircraft or the ability of the crew to cope with adverse operating conditions to the extent that there would be, for example: a significant reduction in safety margins or functional capabilities, a significant increase in crew workload or in conditions impairing crew efficiency, or discomfort to occupants, possibly including injuries.
(iv) Minor - failure conditions that would not significantly reduce aircraft safety, and would involve crew actions that are well within their capabilities. Minor failure conditions may include, for example: a slight reduction in safety margins or functional capabilities, a slight increase in crew workload, such as, routine flight plan changes, or some inconvenience to occupants.
(v) No Effect - failure conditions that do not affect the operational capability of the aircraft or increase crew workload.
(2) The software levels are usually related to the criticality categories. Level A qualified software is the most error-free software and is usually required for functions that could exhibit catastrophic failures. However, additional considerations may moderate this direct relationship and allow some lower level of software qualification for higher function criticality categories. Some of the moderating factors may be architecture of the system or software, redundancy of systems using dissimilar software, hardware or software monitors, and independent function contributions. It is recommended that these practices are carefully employed and prior approval of the methodology should be obtained before the design is pursued. Typically, Level A is required for flight controls where the catastrophic criteria applies. Level B qualification is less than Level A and is employed in some flight controls and required flight instruments, where their malfunction would result in the hazardous or severe major critical category effect on the rotorcraft or its crew and occupants. Level C qualification is less than Level B and is employed in some required flight instruments commensurate with failure category "Major." However, this lower level of qualification may be appropriate for either or both of the higher failure categories if the criteria can be met as Page F - 29
AC 27-1B, Chg 4 3/31/2014 discussed previously for level reduction. Level D qualification is less than Level C and is typically employed for required systems or equipment that do not exhibit the fault potential of the higher categories. Level E qualification is less than Level D and is employed in those systems or equipment that are not required by the regulations and whose malfunction results in no safety effect on the rotorcraft or its crew and occupants.
Examples are entertainment systems, powered seats, etc.
(3) Although a rough correlation exists between software levels and criticality levels that in turn relate to a probability in numbers, these numbers cannot be applied to software to determine reliability obtained by the processes delineated in DO-178C and at this time, have no correlation with probability.
g. Airborne Electronic Hardware (AEH). For airborne complex custom microcoded electronic components assessed as level A, B, or C in the safety assessment, RTCA Document DO-254, "Design Assurance Guidance for Airborne Electronic Hardware," dated April 19, 2000, provides certification guidance for qualification and subsequent approval of these components. Per DO-254, the component will need to be identified as either "simple" or "complex" and tagged to one of the five failure condition classifications. Based on these identifiers, DO-254 provides the guidance to show compliance to § 27.1309. Although RTCA/DO-254 applies specifically to complex custom micro-coded components, applicants are highly encouraged to apply DO-254 to other hardware components up through to the line replaceable unit and systems level.
AC 27.1309A. § 27.1309 (Amendment 27-46) EQUIPMENT, SYSTEMS, AND INSTALLATIONS.
a. Explanation. Amendment 27-46 removed § 27.1309(d) and incorporated a dedicated rule § 27.1316 to better address Electrical and Electronic System Lightning Protection.
b. Procedures. If the certification basis includes § 27.1309 (d), applicants should utilize the previous section of this guidance (section 27.1309 of this AC) to show compliance with protection from lightning effects. If the certification basis includes § 27.1316, applicants should show compliance with protection from lightning effects by referring to the applicable section of this AC (section 27.1316, Electrical and Electronic System Lightning Protection). The paragraphs other than b. (3) of section 27.1309 of this AC are still applicable. This should be coordinated with the appropriate certification office early in the discussions so the path to show compliance with electrical and electronic system lightning protection is clear to the applicant and authority.
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AC 27.1309B. § 27.1309 (AMENDMENT 27-51) EQUIPMENT, SYSTEMS, AND INSTALLATIONS.
a. Explanation. Amendment 27-51 revised § 27.1309 to address advances in technology and eliminate the distinction between multi-engine and single-engine rotorcraft. The rule now includes the same language as § 29.1309. The rule also:
• Revised § 27.1309 (a) to clarify the requirements for performing a failure analysis.
• Replaced § 27.1309 (b) and (c) with language consistent with contemporary industry practices.
b. Types of Equipment. Section 27.1309 covers, but is not limited to, electrical, pneumatic, and hydraulic power sources, distribution, and corresponding utilization systems.
(1) Applicability. Section 27.1309 is intended as a general requirement that is applicable to any equipment or system as installed, in addition to specific systems requirements, except as indicated below.
(i) General. If a specific part 27 requirement predefines systems safety aspects (e.g., redundancy level or criticality) for a specific type of equipment, system, or installation, then the specific part 27 requirement will take precedence. This precedence does not preclude accomplishment of a system safety assessment, if necessary. For example, § 27.695 predefines a required level of redundancy and an implied level of system reliability. However, a system safety assessment approach may still be required to show that the implied system reliability is met and to address assessment of the failure modes.
(ii) Section 27, Subparts B, C, and D. Section 27.1309 does not apply to the design features required by Subparts B, C, and D of part 27 for aspects such as the performance, flight characteristics, structural loads, and structural strength. However, the requirements in § 27.1309 do apply to any equipment and system that the proposed design uses to comply with the requirements of Subparts B, C, and D (e.g., Health Usage Monitoring System certified for maintenance credit and Stability Augmentation System).
(iii) Section 27, Subpart E.
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6/23/23 AC 27-1B, Chg 9 (A) Section 27.1309 does not apply to the type-certificated engine.
However, it does apply to the equipment and systems associated with the engine installation (e.g., electrical power generation, engine displays, transducers, etc.) on the rotorcraft.
(B) Section 27.1309 does not apply to the Rotor Drive Systems (reference § 27.917(b)).
(iv) Section 27, Subpart F. Section 27.1309 does not apply to the loss of function of non-required equipment; however, it does apply to hazards introduced by the installation of that equipment on the rotorcraft and any adverse effects on other systems.
(2) Background.
(i) This guidance describes a method of showing compliance that utilizes the system safety assessment techniques available in the aerospace industry. The safety assessment process described in this guidance can be adopted and applied to equipment, systems, and installations ranging from simple systems to the most critical and most complex systems.
(ii) This guidance is not intended to impose an overly rigorous safety assessment process for those installations involving equipment or systems that are not considered to be complex.
Note: A system is considered to be complex when its operation, failure modes, or failure effects are difficult to comprehend without the aid of analytical methods.
c. Procedures.
(1) Definitions of Probability Classifications.
Note: the following terms were derived from Table 1 - "Failure Conditions Severity as Related to Probability Objectives and Assurance Levels" of SAE ARP 4761.
(1) FREQUENT. The occurrence of any frequent event may be no more often than 103 times during the operational life of each rotorcraft.
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AC 27-1B, Chg 9 6/23/23 (II) PROBABLE. Probable events may be expected to occur on the order of between 10-3 to 10- times during the operational life of each rotorcraft.
(iii) REMOTE. Remote events are not expected to occur a few times during the total operational life of a random single rotorcraft of a particular type, but may occur several times during the total operational life of a number of rotorcraft of a particular type, that are based on a probability on the order of between 10-5 to 10-7.
(iv) EXTREMELY REMOTE. Extremely remote events are not expected to occur during the total operational life of a random single rotorcraft of a particular type, but may occur a few times during the total operational life of all rotorcraft of a particular type, that are based on a probability on the order of between 10-7 to 10-9.
(v) EXTREMELY IMPROBABLE. Extremely improbable events are so unlikely that they need not be considered to ever occur, unless engineering judgment would require their consideration. A probability on the order of 10-° or less is assigned to this classification.
Note: The five probability terms defined in paragraph c(1) above are intended to relate to an identified failure condition resulting from or contributed to by the improper operation or loss of a function or functions. These terms do not define the reliability of specific components or systems.
(2) Definitions of Failure Condition Classifications. Failure Conditions may be classified according to the severity of their effects into one of the following categories (reference Figure AC 27.1309-2).
(i) NO EFFECT. Failure Conditions that would have no effect on safety. For example, Failure Conditions that would not affect the operational capability of the rotorcraft or increase crew workload; however, could result in an inconvenience to the occupants, excluding the flight crew.
(II) MINOR. Failure Conditions that would not significantly reduce rotorcraft safety and that would involve crew actions that are well within their capabilities. Minor Failure Conditions may include, for example, a slight reduction in safety margins or functional capabilities, a slight increase in crew workload, such as routine flight plan changes, or some physical discomfort to occupants.
(iii) MAJOR. Failure Conditions that would reduce the capability of the rotorcraft or the ability of the crew to cope with adverse operating conditions to the extent that there would be, for example, a significant reduction in safety margins or functional capabilities, a significant increase in crew work load or in conditions impairing crew efficiency, physical distress to occupants, possibly including injuries, or physical discomfort to the flight crew.
Page F - 30.3
(iv) HAZARDOUS. Failure Conditions that would reduce the capability of the rotorcraft or the ability of the crew to cope with adverse operating conditions to the extent that there would be:
(A) A large reduction in safety margins or functional capabilities;
(B) Physical distress or excessive workload such that the flight crew's ability is impaired to where they could not be relied on to perform their tasks accurately or completely; or (C) Possible serious or fatal injury to a passenger or a cabin crew member, excluding the flight crew.
Note: Hazardous Failure Conditions can include events that are manageable by the crew by use of proper procedures which, if not implemented correctly or in a timely manner, may result in a Catastrophic Event.
(v) CATASTROPHIC. Failure Conditions that would result in multiple fatalities to occupants, fatalities or incapacitation to the flight crew, or loss of the rotorcraft.
(3) Safety Objective.
(i) The objective of § 27.1309 is to ensure an acceptable safety level for equipment and systems as installed on the rotorcraft. A logical and acceptable inverse relationship must exist between the Average Probability per Flight Hour and the severity of Failure Condition effects, per the requirements of § 27.1309 and as shown in Figure AC 27.1309-1, such that:
(A) Failure Conditions with No Effect on safety may be more frequent than Probable.
(B) Minor Failure Conditions may be Probable.
(C) Major Failure Conditions must be no more frequent than Remote.
(D) Hazardous Failure Conditions must be no more frequent than Extremely Remote.
(E) Catastrophic Failure Conditions must be Extremely Improbable.
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Probable Unacceptable Probability Remote of Failure Extremely Condition Remote Acceptable Extremely Improbable Minor Major Hazardous Catastrophic Severity of Failure Condition Effects FIGURE AC 27.1309-1 Relationship between Probability and Severity of Failure Condition Effects
(ii) The safety objectives associated with Catastrophic Failure Conditions may be satisfied by demonstrating that:
(A) No single failure will result in a Catastrophic Failure Condition; and (B) Each Catastrophic Failure Condition is Extremely Improbable.
(iii) Failure Conditions may be classified according to the severity of their effects to the rotorcraft, occupants, or flight crew as identified in Figure AC 27.1309-2.
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