AC 29-2C, Chg 4 5/1/2014
(iv) HAZARDOUS or SEVERE-MAJOR. Failure conditions which 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 or Severe-Major 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 which would result in multiple fatalities to occupants, fatalities or incapacitation to the flight crew, or result in loss of 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, as shown in Figure AC 29.1309-1, such that:
(A) Failure Conditions with No Effect on safety may be more frequent than Reasonably Probable.
(B) Minor Failure Conditions may be Reasonably Probable.
(C) Major Failure Conditions must be no more frequent than Remote.
(D) Hazardous or Severe-Major Failure Conditions must be no more frequent than Extremely Remote.
(E) Catastrophic Failure Conditions must be Extremely Improbable.
(ii) The safety objectives associated with Failure Conditions are described in Figure AC 29.1309-2, and appear in Figure AC 29.1309-1, below.
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Probability of Failure Condition Probable Remote Extremely Remote Extremely Improbable Unacceptable Acceptable Minor Major Hazardous Catastrophic Severity of Failure Condition Elfects 5/1/2014 AC 29-2C, Chg 4 FIGURE AC 29.1309-1 Relationship between Probability and Severity of Failure Condition Effects
(iii) 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.
(iv) 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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AC 29-2C, Chg 4 5/1/2014 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 work load which involve 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: This is not intended to imply that the identified software levels are assigned a probability value, but instead, shows a correlation to the Failure Condition Category.
Note 4: Hazardous or Severe-Major 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
(v) The following Figure AC 29.1309-3 provides conversion between the current application of the five failure condition categories as defined in Figure AC 29.1309-2 and the three failure condition categories contained in the present rules.
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Present Rule Qualitative Probability Classification -------- Probable -------- ----------Improbable--------- Extremely Improbable Quantitative Probability Classification >10-5 <10-5 <10-9 Present Rule Failure Condition Category – AC 29 2C & DO-178A Non-essential Essential Critical Current Application of Failure Condition Category No Effect Minor Major Hazardous or Severe - Major Catastrophic FIGURE AC 29.1309-3: Failure Condition Conversion
(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, which ensures that the process and its findings are visible, and readily assimilated. 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 guide 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 guide is one method but not the only method for showing compliance. The safety assessmen t 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 Document ARP 4761 on how to perform a safety assessment. SAE Document ARP 4754A, invoked by AC 20-174, includes additional guidance 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 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 the safety assessment elements to show compliance. Many states of varying complexity exist between these extremes that Page F - 19
AC 29-2C, Chg 4 5/1/2014 may use less than the entire safety assessment process, but require more than a simple statement to show compliance.
(iii) Elements of the safety assessment process are as follows:
• Functional Hazard Assessment (FHA)
• Preliminary System Safety Assessment (PSSA)
•• Fault Tree Analysis (FTA)
• System Safety Assessment (SSA)
•• Failure Mode and Effect Analysis (FMEA)
• Common Cause Analysis (CCA)
•• Zonal Safety Analysis (ZSA)
•• Particular Risk Assessment (PRA)
•• Common Cause Analysis (CMA) (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 Functional Hazard Assessment (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 classifying a function, consider for example, the loss of function, erroneous or misleading, or 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 must be 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, etc.). The product of the FHA is a determination of the failure or malfunction condition category for each system function and identification of the source of the function. Lower level FHAs that focus on the system level hazards reveal detailed information about the specific part of a system that provides the function under evaluation. This more detailed information may narrow the system part to be considered for compliance with enhanced integrity or for additional analysis. However, it must be recognized that there is a limit to the Page F - 20
number of levels that can be relevant in this process. Too many levels can result in the assessment becoming design-dependent rather than the FHA driving the design, as intended.
(C) The FHA is usually based on one of the following methods, as appropriate:
( ) If the system is not complex and its relevant attributes are similar to those of systems us 1 2 ed on other aircraft, the identification and classification may be derived from design, installation, and service experience appraisals that can be shown to be sufficiently similar.
( ) 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.
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.
(D) Choose the means to be used to determine compliance with Section 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 or not 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 or Severe-Major Failure Conditions, a thorough safety assessment is necessary. The applicant should obtain early concurrence of the certification authority on the choice of an acceptable means of compliance.
(E) If an analysis is used as an acceptable means of showing compliance, it should typically include the following information:
(1 2 3 ) A statement of the functions, boundaries, and interfaces of the system.
( ) A list of the parts and equipment of which the system is comprised, including their performance specifications or design standards and development assurance levels if applicable. This list may reference other documents (e.g., Technical Standard Orders (TSOs), manufacturer's or military specifications, etc.).
( ) The conclusions, including a statement of the Failure Conditions and their classifications and probabilities (expressed qualitatively or quantitatively, as appropriate) that show compliance with the requirements of § 29.1309.
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- AC 29-2C, Chg 4 5/1/2014
(4) 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 to the rules; therefore, they must be reviewed by the FAA/AUTHORITY for approval.
(F) Certification Maintenance Requirements (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. 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.
Such tolerances are acceptable because the uncertainty described in the “Note” below is accounted for as discussed in this section. 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, should be approved by the FAA/AUTHORITY having cognizance over the type design that relates to the system and its installation.
Note: It is recognized that, for various reasons, component failure rate data are not precise enough to enable accurate estimates of the probabilities of Failure Conditions. This results in some degree of uncertainty, as indicated by the expression “on the order of” in the descriptions of the quantitative probability terms that are provided in paragraph b.(1) above. When calculating the probability of each Failure Condition, this uncertainty should be accounted for in a way that does not compromise safety.
(G) Assess the analyses and conclusions of multiple safety assessments to ensure compliance with the requirements for all rotorcraft level Failure Conditions.
(H) Prepare compliance statements, maintenance requirements, and flight manual requirements.
(I) 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 Page F - 22
are not being used. The probability of failure for those systems must 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, which 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/AUTHORITY.
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AC 29-2C, Chg 4 5/1/2014 Certification Requirements Maintenance Requirements Flight Manual Requirements Rotorcraft Level Functional Hazard Assessment Assessment of Rotorcraft Level System Safety Assessment System Safety System Level FHA’s Assessments Analyses FIGURE AC 29.1309-4: Safety Assessment Process Overview Page F - 24
Note: FHA may be based on a design and installation appraisal for these systems.
Conduct Functional Hazard Assessment Is There a Safety Effect? No Yes Yes No Yes No Verify by Design and Installation Appraisal.
Is the Failure Condition Minor?
Is the System and Installation Similar to a Previous Design?
Develop Similarity Analysis and Obtain Approval from FAA/Authority * Conduct Qualitative or Quantitative Analysis, or both.
* Note: Catastrophic and Hazardous/ Severe-major 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 - 25
AC 29-2C, Chg 4 5/1/2014
(5) Assessment Methods Design Appraisal Installation Appraisal . Various methods for assessing the causes, severity, and probability of Failure Conditions are available to support experienced engineering and operational judgment. Some of these methods are structured. The various types of analysis are based on either inductive or deductive approaches. Probability assessments may be qualitative or quantitative. Descriptions of some types of analys is are provided below and in Document ARP 4761.
(i) . This is a qualitative appraisal of the integrity and safety of the system design.
(ii) . This is a qualitative appraisal of the integrity and safety of the installation. Any deviations from normal, industry -accepted installation practices, such as clearances or tolerances, should be evaluated, especially when appraising modifications made after entry into service. Hazards introduced by the installation of the system should also be evaluated.
(iii) Failure Modes and Effects Analysis (FMEA).
(A) This is a structured, inductive, bottom-up analysis, which is used to evaluate the effects on the system and the rotorcraft of each possible element or component failure. When properly formatted, it will aid in identifying latent failures and the possible causes of each failure mode. Document ARP 4761 provides methodology and detailed guidelines that may be used to perform this type of analysis. An FMEA could be a piece part FMEA or a functional FMEA. For modern microcircuit based line replaceable units (LRUs) and systems, an exhaustive piece part FMEA is not practically feasible with the present state of the art. In that context, a FMEA may be more functional than piece part oriented. A functional oriented FMEA can lead to uncertainties in the qualitative and quantitative aspects, which can be compensated for by more conservative assessment, such as:
(1) assuming all failure modes result in the Failure Conditions of interest,
(2) careful choice of system architecture, or
(3) taking into account the experience lessons learned on the use of similar technology.
(B) Specific reliability numbers are not shown in § 29.1309. 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 MIL-HDBK-217C; (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.
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(iv) Fault Tree or Dependence Diagram Analysis Markov Analysis Common Cause Analysis (CCA) . Structured, deductive, topdown analyses which are used to identify the conditions, failures, and events that would cause each defined Failure Condition. They are graphical methods of identifying the logical relationship between each particular Failure Condition and the primary element or component failures, other events, or combinations thereof that can cause it. A failure modes and effects analysis may be used as the source document for those primary failures or other events.
(v) . A Markov model (chain) represents various system states and the relationships among them. The states can be either operational or nonoperational. The transitions from one state to another are a function of the failure and repair rates. Markov analysis can be used as a replacement for fault tree and dependence diagram analysis, but it often leads to more complex representation, especially when the system has many states. It is recommended that Markov analysis be used when fault tree or dependence diagrams are not easily usable, namely to take into account complex transition states of systems which are difficult to represent and handle with classical fault tree or dependence diagram analysis.
(vi) . The acceptance of adequate probability of Failure Conditions is often derived from the assessment of multiple systems based on the assumption that failures are independent. Therefore, it is necessary to recognize that such independence may not exist in the practical sense and specific studies are necessary to ensure that independence can either be assured or deemed acceptable.
CCA identifies common causes for faults or malfunctions. Once identified, the source of the common cause is either eliminated from the design, the common cause or the system design is made to be tolerant of the related fault or malfunction, or the common cause or system design combination is found to be acceptable due to the failure or malfunction condition category it represents. The potential for failures or malfunctions due to common causes is inherent in designs that provide multiple functions reliant on common hardware or common software. This is also true for systems that produce related functions and share a common installation area. The installation area may represent a threat from several sources such as Electro-Magnetic Interference (EMI), mechanical hazards, and environmental influences. The Common Cause Analysis is sub-divided into three areas of study:
(A) Zonal Safety Analysis (ZSA). This analysis has the objective of ensuring that the equipment installations within each zone of the rotorcraft are at an adequate safety standard with respect to design and installation standards, interference between systems, and maintenance errors. The ZSA examines the physical zone of the rotorcraft, in which the system under consideration is installed, to ensure that the surrounding equipment or appliance installations do not compromise the system independence requirements. Mechanical failures that might generate fragments that could damage the system under evaluation are an example of the type of event that would be considered in a zonal analysis.
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