(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 27-1B, Chg 9 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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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.
(i) FREQUENT. The occurrence of any frequent event may be no more often than 10 3 times during the operational life of each rotorcraft.
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AC 27-1B, Chg 9
(ii) PROBABLE. Probable events may be expected to occur on the order of between 10-3 to 10-5 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 10-9 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.
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AC 27-1B, Chg 9
(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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AC 27-1B, Chg 9 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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AC 27-1B, Chg 9 FIGURE AC 27.1309-2 Failure Condition Categories and Probability Definitions Table for Failure Condition Categories and Probability Definitions Effect on rotorcraft No effect on operational capabilities or safety Slight reduction in functional capabilities or safety margins Significant reduction in functional capabilities or safety margin Large reduction in functional capabilities or safety margins (Note 4) Loss of rotorcraft Effect on occupants excluding flight crew Inconvenience Physical discomfort Physical distress, possibly including injuries Serious or fatal injury to a passenger or a cabin crew member (Note 2) Multiple Fatalities Effect on flight crew No effect on flight crew Slight increase in workload that involves crew actions well within crew capabilities such as routine flight plan changes Physical discomfort or a significant increase in workload or in conditions impairing crew efficiency Physical distress or excessive workload impairs ability to perform tasks accurately or completely Fatalities or incapacitation DO-178C Software Level (Note 3)
E D C B A Failure Condition Category No Effect Minor Major Hazardous or Severe- Major Catastrophic Qualitative Probability Frequent Reasonably Probable Remote Extremely Remote Extremely Improbable Quantitative Probability No probability requirement 10-3 (Note 1) 10-5 10-7 10-9 Note 1: A numerical probability range is provided here as reference. The applicant is not required to perform a quantitative analysis, or substantiate by such an analysis, that this numerical criterion has been met for Minor Failure Conditions.
Note 2: This is true if it can be shown that the given failure condition can be contained to a fatal injury of one occupant only.
Note 3: AC 20-174, Development of Civil Aircraft and Systems, provides guidance on using SAE ARP4754A as an acceptable method for establishing a development assurance process.
Note 4: 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.
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AC 27-1B, Chg 9
(4) Safety Assessment Process Overview.
(i) When showing compliance with § 27.1309, the considerations covered in this guidance should be addressed in a methodical and systematic manner. This section of the AC is provided primarily for the use by applicants who are not familiar with the various methods and procedures generally used in the industry to conduct safety assessments. This section and Figures AC 27.1309-4 and AC 27.1309-5 are not intended to be certification checklists, and they do not include all of the information provided in this guidance. This section contains one method, but not the only method, for showing compliance. The safety assessment process is a structured method of general applicability for showing compliance with the regulation. Other methodologies may be used to show compliance, but the safety assessment process is the only structured process that is defined, and these defined objectives would be the logical criteria applied to any other means of showing compliance. More detailed guidance can be found in SAE ARP4761 on how to perform a safety assessment. SAE ARP4754A, discussed in AC 20-174, includes additional information on how the safety assessment process relates to the system development process.
(ii) The safety assessment process contains several parts that may be necessary as a whole or in part, depending on the criticality or complexity of the system under consideration. The rigor of assessment and analysis performed is also dependent on the system criticality or complexity. At the extremes, some systems may be simple enough such that the entire safety assessment can be performed by observation and compliance shown by a simple statement. Complex and higher criticality systems may require application of all of the safety assessment elements to show compliance. Many states of varying complexity exist between these extremes that may use less than the entire safety assessment process but require more than a simple statement to show compliance. For purposes of the FHA, an autorotation is not considered continued safe flight and landing.
(iii) Typical elements of the safety assessment process are as follows:
Functional Hazard Assessment (FHA) Preliminary System Safety Assessment (PSSA) o Fault Tree Analysis (FTA) System Safety Assessment (SSA) o Failure Mode and Effect Analysis (FMEA) Common Cause Analysis (CCA) o Zonal Safety Analysis (ZSA) o Particular Risk Assessment (PRA) o Common Cause Analysis (CMA) Page F - 30.7
AC 27-1B, Chg 9 (A) Define the system and its interfaces, and identify the functions that the system is to perform. Determine whether or not the system is complex, similar to systems used on other aircraft, or conventional. Where multiple systems and functions are to be evaluated, consider the relationships between multiple safety assessments.
(B) Identify and classify Failure Conditions. All relevant applicantengineering organizations, such as systems, structures, propulsion, and flight test, should be involved in this process. This identification and classification may be done by conducting an FHA, which is a top-down assessment. The FHA starts at the rotorcraft operational level to identify possible hazards to the rotorcraft, its occupants, and flight crew. When establishing the hazard classification of a function, consider for example, the known loss of the function, the erroneous behavior or misleading information provided by the function, and degradation of the function as possible hazards. The hazards are classified using the five failure condition categories found in Figure AC 27.1309-2. The identified Failure Conditions are then evaluated with respect to the intended operations and combinations of operations for which certification of the rotorcraft is sought (e.g., VFR, IFR, CAT A, IFR plus CAT A). SAE ARP4761 provides guidance for determining when and under what conditions multiple failures should be considered.
(C) If the system is complex, it is necessary to systematically postulate the effects on the safety of the aircraft and its occupants resulting from any possible failures, considered both individually and in combination with other failures or events.
(D) Choose the means to be used to determine compliance with § 27.1309. The depth and scope of the analysis depends on the types of functions performed by the system, the severity of the associated Failure Conditions, and whether the system is complex (see Figure AC 27.1309-5). Once the Failure Conditions are determined to be Major Failure Conditions, experienced engineering and operational judgment, design and installation appraisals, and similarity consideration for previously installed systems may be acceptable. This may be sufficient in itself or in conjunction with qualitative analyses or selectively used quantitative analyses. For Catastrophic and Hazardous Conditions, a thorough safety assessment is necessary. The applicant should obtain early concurrence from the FAA on the choice of an acceptable means of compliance.
(E) Include a description that establishes correctness and completeness and traces the work leading to the conclusions. This description should include the basis for the classification of each Failure Condition (e.g., analysis or ground, flight, or simulator tests). It should also include a description of precautions taken against common-cause failures, provide any data such as component failure rates and their sources and applicability, support any assumptions made, and identify any required flight crew or ground crew actions, including any Candidate Certification Maintenance Requirements (CCMRs), which are also referred to as Certification Check Requirements (CCRs). Certification Maintenance Requirements (CMRs) are not the preferred method for showing compliance with the rules; therefore, they must be reviewed by the FAA for approval. Any CMRs must be included in the airworthiness Page F - 30.8
AC 27-1B, Chg 9 limitations section of the instructions for continued airworthiness in the maintenance manual, as required by § 27.1529 and Appendix A to Part 27.
Note: CMRs may be needed to help show compliance with § 27.1309 for significant latent failures. Rational methods, which usually involve quantitative analyses or relevant service experience data, should be used to determine CMR intervals. It is recognized that, for various reasons, component failure rate data is not precise enough to accurately estimate the probabilities of Failure Conditions. This results in some degree of uncertainty, as indicated by the expression “on the order of” in the definition of the quantitative probability terms in paragraph c(1) above. These intervals should have reasonable tolerances so that CMRs can be performed concurrently with other maintenance, inspection, or check procedures not required by design for compliance with § 27.1309. Any tolerance ability should be built into the interval that is included in the airworthiness limitation section of the maintenance manual.
If CMRs are used, they and their intervals and tolerances, and any post-certification changes or procedures provided in the type design for a rotorcraft owner or operator to make such changes, must be approved by the FAA office having cognizance over the type design that relates to the system and its installation, as required by §§ 21.31,
21.95, and 21.97.
(F) When assessing the acceptability of a failure condition using a quantitative analysis, the given numerical range 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 that is used only at a specific time during a flight, the probability of the failure condition should be calculated for the specific period and expressed as the risk for the flight condition, takeoff, landing, etc., as appropriate. This is only true for those systems that cannot fail undetected when they are being used. The probability of failure for those systems should be calculated from that value based on the extended period of not only the active operation, but also the exposure time for the latent failure.
Note: If a quantitative analysis is used to help show compliance with the regulations for equipment or systems that are installed and only required for a specific operating condition for which the rotorcraft is thereby approved, credit is usually not allowed due to the fact that the operating condition does not always exist. If an applicant does request that such credit be allowed, they must obtain the approval of the FAA.
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AC 27-1B, Chg 9 Certification Maintenance Requirements Requirements Flight Manual Requirements Rotorcraft Level Functional Assessment of Rotorcraft Hazard Assessment Leven System Safety Assessment System Level FHAs System Safety Assessments Analyses FIGURE AC 27.1309-4: Safety Assessment Process Overview Note: FHA may be based on a design and installation appraisal for these systems.
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