AC 29-2C, Chg 4 5/1/2014 strike). Section 29.610 contains some methods which may be utilized for less complex mechanical systems; however, a great deal of difficulty will be experienced in trying to use these criteria to demonstrate that a very complex avionic system complies with § 29.1309(a). These avionic systems usually only require protection from indirect effects of lightning and therefore a method such as that outlined in paragraph b.(9)(iii) above is recommended. This method may be readily adapted to other avionic systems performing critical functions. Also, this identifies an acceptable quantification of the expected airborne environment. The next step involves expanding the fault/failure analysis to determine if the malfunctioning of several “essential” systems in relation to other systems would result in a hazard to a category B rotorcraft or preclude the continued safe flight and landing of a category A rotorcraft. If groups of functions are so identified, sufficient lightning protection should be provided to prevent a hazardous malfunction situation on category B rotorcraft or provide those conditions which prevent continued safe flight and landing on a category A rotorcraft are extremely improbable.
In performing this part of the analysis, attention should be given to the fact that many of the required equipment, systems, and installations may fail simultaneously with other required equipment, systems, and installations and result in a reduction of the capability of the rotorcraft but still not result in a catastrophe. An example of required equipment for which the simultaneous failure of all the required equipment is catastrophic is a failure which results in a total loss of attitude display for IFR certified rotorcraft operating in instrument meteorological conditions. Note that the analysis which is utilized to demonstrate that these failures are extremely improbable should have the encounter with a worst-case lightning strike as a given event (i.e., probability is unity). Additionally, for a category A rotorcraft, an autorotation is not considered continued safe flight and landing.
AC 29.1309A. § 29.1309 (Amendment 29-53) EQUIPMENT, SYSTEMS, AND INSTALLATIONS.
a. Explanation Procedures . Amendment 29 -53 removed § 29.1309(h) and incorporated a dedicated rule § 29.1316 to better address Electrical and Electronic System Lightning Protection.
b. . If the certification basis includes § 29.1309 (h), applicants should utilize the previous section of this guidance (section 29.1309 of this AC) to show compliance with protection from lightning effects. If the certification basis includes § 29.1316, 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 (iv) of section 29.1309 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 29.1309B. § 29.1309 (AMENDMENT 29-59) EQUIPMENT, SYSTEMS, AND INSTALLATIONS.
a. Explanation. Amendment 29-59 revised § 29.1309 to address advances in technology and eliminate the distinction between Category A and Category B rotorcraft. The rule also:
Revised section § 29.1309 (a) to clarify the requirements for performing a failure analysis.
Replaced section § 29.1309 (b) with language consistent with contemporary industry practices.
Revised the alerting requirements of section § 29.1309 (c) to remove the term “warning,” which has been interpreted as requiring a red level alert, when the intent was to notify the crew of all required annunciations.
Removed sections § 29.1309 (e), (f), and (g) dealing specifically with electrical systems, as they are covered by other regulatory sections.
(1) Applicability
(i) General Section 29, Subparts B, C, and D . Section 29.1309 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.
. If a specific part 29 requirement predefines systems safety aspects (e.g., redundancy level or criticality) for a specific type of equipment, system, or installation, then the specific part 29 requirement will take precedence. This precedence does not preclude accomplishment of a system safety assessment, if necessary. For example, § 29.695 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 29.1309 does not apply to the design features required by Subparts B, C, and D of part 29 for aspects such as the performance, flight characteristics, structural loads, and structural strength. However, the requirements in § 29.1309 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 29, Subpart E.
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(A) Section 29.1309 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 29.1309 does not apply to the Rotor Drive Systems (reference § 29.917(b)).
(iv) Section 29, Subpart F. Section 29.1309 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.
b. 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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(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 to 10 .
(v) EXTREMELY IMPROBABLE -9 . 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 -9 . 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 b(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
(i) NO EFFECT MINOR . Failure Conditions may be classified according to the severity of their effects into one of the following categories (reference Figure AC 29.1309-2).
. 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) . 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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(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.
CATASTROPHIC. Failure Conditions that would result in multiple fatalities to occupa
(v) nts, fatalities or incapacitation to the flight crew, or loss of the rotorcraft.
(3) Safety Objective.
(i) The objective of § 29.1309 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 § 29.1309 and as shown in Figure AC 29.1309-1, 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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Probability of Failw-e Condition Probable Remote Extremely Remote Extremely Improbable Unacceptable Acceptable Minor Major Hazardous Catastrophic Severity of Failw-e Condition Effects 6/23/23 AC 29-2C, Chg 9 FIGURE AC 29.1309-1 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 the following Figure AC 29.1309-2.
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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 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.
FIGURE AC 29.1309-2 Failure Condition Categories and Probability Definitions Page F - 40.6
(4) Safety Assessment Process Overview.
(i) When showing compliance with § 29.1309, the considerations covered in this guidance should be addressed in a methodical and systematic manner. This section of the AC (Safety Assessment Process Overview) is provided primarily for the use of 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 29.1309 -4 and AC 29.1309-5 are not intended to be certification checklists, and they do not include all 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 ARP 4761 on how to perform a safety assessment. SAE ARP 4754A, discussed 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 - 40.7
(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 applicant-engineering organizations, such as systems, structures, propulsion, and flight test, should be involved in this process. This identification and classification may be done by conducting a 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
29.1309-2. 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 ARP 4761 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 § 29.1309.
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 29.1309-5). 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 limitations section of the instructions for continued airworthiness in the maintenance manual, as required by Page F - 40.8
§ 29.1529 and Appendix A to Part 29.
Note: CMRs may be needed to help show compliance with § 29.1309 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 b.(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 § 29.1309. 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 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 which 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.
Page F - 40.9
Cert.iflcaUon Requirements Rotorcreft Level functional Hazard Assessment System Level FHAs Note: FHA may bo based on a design and installation appraisal for these ayatema.
Analyses Maintenance Requir-emenls flight Manual Requirements Assessment of Rotorcnifl Leven System Safety Assessment System Safety Assessments 6/23/23 AC 29-2C, Chg 9 FIGURE AC 29.1309-4: Safet y Assessment Process Overview Page F - 40.10
Note: FHA may be based on a design and installation appraisal for these systems.
Conduct Functional Hazard Assessment Is there a Safety Effect?
Yes Is the Verify by Design and Failure Condition Installation Appraisal.
Minor?
No Is the System Develop Similarity Analysis and and Installation similar to Obtain Approval from Previous Design? FAA No *Conduct Qualitative or Quantitative Analysis or both.
*NOTE: Catastrophic failure conditions will likely require both qualitative and quantitative analysis, depending on system complexity.
FIGURE AC 29.1309-5: Depth of Analysis Flowchart Page F - 40.11